AN ALLOY Introduction The present invention relates to a complex concentrated alloy (CCA) designed for high temperature applications. The alloy has a unique combination of a high ductility, high temperature mechanical strength and high producibility. In an embodiment this is achieved in combination with a low density and/or reasonable alloy cost. Conventional alloys for high mechanical strength at high temperature usually rely upon a main principal component. Alloys with very high melting points are often based upon the refractory elements, molybdenum, tungsten, niobium and tantalum. Although these alloys have very high melting points and high strength they do tend to experience significant deterioration in mechanical properties as temperatures approach alloy melting point. To address this challenge there has been recent development activity in the field of complex concentrated alloys (CCAs) and refractory complex concentrated alloys (RCCAs). However the alloys which have currently been proposed do not achieve the desired combination of high melting point, high temperature strength, high ductility and ease of producibility, combined with low density and reasonable cost to reach commercial application in fields such as gas turbine technology, jet turbine technology and rocket propulsion. Examples of alloys which have been previously researched are shown in Table 1. (Miracle et al., 2011; Senkov, Woodward and Miracle, 2014; Gorr et al., 2017; Muller et al., 2019; Ge et al., 2020; Sheikh et al., 2020) Table 1 Alloy (at%) Al Cr Mo Nb Ta Ti V W Zr RHEA1 ^{0.0 0.0 20.0 20.0 20.0 0.0 20.0 20.0 0.0} RHEA2 ^{0.0 0.0 25.0 25.0 25.0 0.0 0.0 25.0 0.0} RHEA3 ^{20.0 20.0 20.0 0.0 20.0 20.0 0.0 0.0 0.0} RHEA4 ^{15.4 7.7 0.0 15.4 15.4 46.2 0.0 0.0 0.0} RHEA5 ^{16.7 16.7 16.7 16.7 16.7 16.7 0.0 0.0 0.0} RHEA6 ^{10.0 0.0 0.0 20.0 16.0 30.0 4.0 0.0 20.0} RHEA7 ^{16.7 0.0 16.7 16.7 16.7 16.7 16.7 0.0 0.0} CN109252082 discloses MoNb1.5Hf0.5Zr0.5Ti1.5 as a high temperature structural alloy. This alloy has an entropy of only 12.512 meaning that the alloy is susceptible to the formation of intermetallic phases. Indeed in one embodiment a kind of multi-component alloyed (MoNb1.5Hf0.5Zr0.5Ti1.5)-(W0.4Al0.2Cr0.2C0.2) refractory high-entropy alloy is disclosed. This alloy results in the formation of refractory metal carbides in the alloy. The amount of carbon in that alloy is 3.3 at%. The present invention provides an alloy consisting of, in atomic percent: aluminium: 19.45% or less; zirconium: 16.7% or less; vanadium: 12% or less; tungsten: 35.0% or less; chromium: 15.0% or less; iron: 6.5% or less; nickel: 15.0% or less; titanium: 40.0% or less; tantalum: 35.0% or less; niobium: 35.0% or less; molybdenum: 35.0% or less; cobalt: 20.0% or less; manganese: 35.0% or less; copper: 15.0% or less; hafnium: 10.0% or less; rhenium: 35.0% or less; platinum: 15.0% or less; palladium: 15.0% or less; rhodium: 20.0% or less; ruthenium: 25.0% or less; iridium: 20.0% or less; silicon: 5.0% or less; and incidental impurities less than 1 at% in sum; wherein the following equations are satisfied: ^^ ^^ ^^ 13.38 where ^^ _^ is the atomic fraction of element ^^ in the alloy and ^^ ^^ ^^ _^ is the number of valence electrons associated with an atom of element ^^ and R is the universal gas constant. Such an alloy has good producibility as well as high temperature strength and ductility. In an embodiment the following equation is fulfilled: ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ 8.0 ≥ ^ ^^ _^ ^^ _^ ^ where ^^ _^ is the atomic fraction of element ^^ in the alloy and ^^ _^ is the density for pure element ^^. Such an alloy has reduced density. In an embodiment the following equation is fulfilled: 2200 preferably 2400 ≤ ∑ _^ ^^ _^ ∙ ^^ _^^ more preferably 2500 m _{ore preferably 2600} m _{ore preferably 2700} m _{ore preferably 2800} most preferably 2900 where ^^ _^ is the atomic fraction of element ^^ in alloy bulk composition and ^^ _^^ is the melting point of pure element ^^. Such an alloy has high melting pint and as a result high temperature strength and creep resistance. In an embodiment the following equation is fulfilled: where ^^ _^ is the atomic fraction of element ^^ in alloy bulk composition. Such an alloy has higher entropy and so avoids intermetallic phase formation which leads to increased ductility. In an embodiment the alloy includes rhenium in an amount of 35% or less provided A _Re ≤ (35 - A _Ta ) + (35 - A _W ) + (20 - A _Nb ) and where A _Re , A _Ta , A _W and A _Nb are the amounts in atomic percent of rhenium, tantalum, tungsten and niobium respectively. Such an alloy has lower cost and lower density. In an embodiment the alloy includes 19.0 at% or less aluminium, preferably 15.0 at% or less aluminium, more preferably 10.0 at% or less aluminium, even more preferably 5.0 at% or less aluminium, most preferably 1.0 at.% or less aluminium. Such an alloy has improved producibility. In an embodiment the alloy includes 16.0 at% or less zirconium, preferably 15.0 at% or less zirconium, most preferably 14.0 % or less zirconium, preferably 13.0 at.% or less zirconium, more preferably 12.4 at% or less zirconium, most preferably 5.0 at% or less zirconium. Such an alloy has reduced risk of intermetallic formation and so improved ductility. In an embodiment the alloy includes 10.0 at% or less vanadium, preferably 7.3 at% or less vanadium, more preferably 4.3 at% or less vanadium, most preferably 1.0 at.% or less vanadium. Such an alloy has reduced risk of intermetallic formation and so improved ductility. In an embodiment the alloy includes 10.0 at% or less palladium, preferably 5.0 at% or less palladium, more preferably 1.5 at.% or less palladium, more preferably 1.0 at.% or less palladium, most preferably 0.5 at% or less palladium. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 15.0 at% or less iridium, preferably 9.0 at% or less iridium, preferably 8.0at% or less iridium, more preferably 1.0 at.% or less iridium, preferably 0.5 at.% or less iridium, more preferably 0.1at.% or less iridium. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 15.0 at% or less rhodium, preferably 8.0 at%, more preferably 6.0 at% or less rhodium, more preferably 1.0 wt.% or less rhodium, even more preferably 0.5 at.% or less rhodium, most preferably 0.1 at.% or less rhodium. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 2.0 at% or more iridium, preferably 5.0 at.% or more iridium, more preferably 12.0 at.% or more iridium, more preferably 17.0 at.% or more iridium. Such an alloy has a higher melting point and also improved resistance to environmental attack. In an embodiment the alloy includes 5.0 at% or more rhodium, preferably 10.0 at.% or more rhodium, more preferably 15.0 at.% or more rhodium. Such an alloy has improved corrosion resistance combined with a high melting point and low density. In an embodiment the alloy includes 2.0 at% or more palladium, preferably 5.0 at.% or more palladium, more preferably 8.0 at.% or more palladium. Such an alloy has increased entropy and so improved high temperature strength and creep resistance. In an embodiment the alloy includes 3.0 at% or more platinum, preferably 7.0 at.% or more platinum, more preferably 12.0 at.% or more platinum. Such an alloy has improved corrosion resistance and also may be particularly compatible with the alloy when coated in a platinum containing protective layer. In an embodiment the alloy includes 10.0 at% or less platinum, preferably 5.0 at% or less platinum, more preferably 1.5 at% or less platinum, more preferably 1.0 at.% or less platinum, more preferably 0.5 at.% or less platinum. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 34.0 at% or less tungsten, preferably 30.0 at.% or less tungsten, more preferably 29.0 at% or less tungsten, more preferably 25.0 at% or less tungsten, more preferably 20.0 at% or less tungsten, even more preferably 15.0 at% or less tungsten, most preferably 10.0 at.% or less tungsten. Such an alloy has reduced density. In an embodiment the alloy includes 2.0 at% or more vanadium, preferably 5.0 at% or more vanadium, preferably 8.0 at% or more vanadium. Such an alloy has improved high temperature strength and creep resistance and good ductility, achieved without large increases in density or a reduction in melting point, whilst being low enough to reduce the risk of intermetallic phase formation. In an embodiment the alloy includes 2.0 at% or more tungsten, preferably 5.0 at% or more tungsten, preferably 8.0 at% or more tungsten, more preferably 11.0 at% or more tungsten. Such an alloy will have increased melting temperature and so increased strength and creep resistance. In an embodiment the alloy includes 6.5 at% or less iron, preferably 3.0 at% or less iron, most preferably 1.0 at.% or less iron. Such an alloy will have reduced likelihood of intermetallic phase formation. In an embodiment the alloy includes 3.0 at% or less silicon, preferably 1.0 at% or less silicon, more preferably 0 at% silicon. Such an alloy has improved toughness resulting from a lower chance of silicide formation. In an embodiment the alloy includes 1.0 at% or more silicon, preferably 2.0 at% or more silicon, more preferably 3.0 at% or more silicon. Such an alloy has improved strength. In an embodiment the alloy includes 8.0 at% or less nickel, preferably 3.0 at% or less nickel, more preferably 1.0 at% or less nickel. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 35.0 at% or less titanium, preferably 34.0 at% or less titanium, more preferably 33.0 at% or less titanium, more preferably 32.0 at% or less titanium, even more preferably 25.0 at% or less titanium. Such an alloy will have reduced likelihood of intermetallic phase formation. In an embodiment the alloy includes 5.0 at% or more titanium, preferably 8.0 at% or more titanium, more preferably 13.0 at% or more titanium, most preferably 20.0 at% or more titanium, more preferably 30.0 at.% or more titanium. Such an alloy has reduced density. In an embodiment the alloy includes 32.8 at% or less tantalum, preferably 29.0 at% or less tantalum, more preferably 25.0 at% or less tantalum, more preferably 22.4 at% or less tantalum, more preferably 20.0 at.% or less tantalum, more preferably 15.0 at.% or less tantalum, preferably 10.0 at.% or less tantalum. Such an alloy has reduced cost and density. In an embodiment the alloy includes 2.0 at% or more tantalum, preferably 5.0 at% or more tantalum, preferably 10.0 at% or more tantalum, more preferably 15.0 at% or more tantalum, most preferably 20.0 at% or more tantalum. Such an alloy has increased high temperature strength. In an embodiment the alloy includes 30.0 at% or less niobium, preferably 25.0 at% or less niobium, more preferably 20.0 at.% or less niobium, more preferably 18.0 at.% or less niobium, more preferably 12.0 at.% or less niobium, more preferably 10.0 at.% or less niobium. Such an alloy reduces the ability of intermetallics to form, particularly when high levels of chromium are present. In an embodiment the alloy includes 4.0 at% or more ruthenium, preferably 11.0 at% or more ruthenium, more preferably 18.0 at.% or more ruthenium, more preferably 22.0 at% or more ruthenium. Such an alloy has increased melting temperature and so achieves high strength and reasonable density and cost. In an embodiment the alloy includes 18.0 at% or less ruthenium, preferably 10.0 at% or less ruthenium, more preferably 8.5 at.% or less ruthenium, more preferably 1.0 at% or less ruthenium. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 5.0 at% or more niobium, preferably 8.0 at% or more niobium, more preferably 10.0 at.% or more niobium, preferably 12.0 at.% or more, most preferably 18.0 at.% or more. Such an alloy has increased melting temperature resulting in higher high temperature strength and creep resistance and also some advantages for certain types of processing. In an embodiment the alloy includes 20.0 at% or less molybdenum, preferably 16.0 at% or less molybdenum, preferably 15.0 at% or less molybdenum, preferably 14.0 at% or less molybdenum, more preferably 12.0 at% of less molybdenum, even more preferably 7.0 at% or less molybdenum. Such an alloy has increased resistance to oxidation. In an embodiment the alloy includes 2.0 at% or more molybdenum, preferably 4.0 at% or more molybdenum, more preferably 6.0 at% or more molybdenum, more preferably 8.0 at.% or more and even more preferably 10.0 at.% or more molybdenum and most preferably 12.0 at.% or more molybdenum. Such an alloy has increased melting temperature and so improved high temperature strength and creep resistance. In an embodiment the alloy includes 0.5 at% or more nickel, preferably 2.0 at% or more nickel, more preferably 4.0 at% or more nickel. Such an alloy has increased entropy (leading to increased producibility and strength and creep resistance) without significant reduction in melting point or density. In an embodiment the alloy includes 0.5 at% or more iron, preferably 2.0 at% or more iron, more preferably 4.0 at% or more iron. Such an alloy has increased entropy and also reduced density. In alloys with high chromium content, printability is also improved. In an embodiment the alloy includes 1.0 at% or more hafnium, preferably 3.0 at% or more hafnium, more preferably 5.0 at% or more hafnium. Such an alloy is preferred because of the oxygen scavenging properties of hafnium and also its effect in reducing the ductile to brittle transition temperature. In an embodiment the alloy includes 2.0 at% or more tungsten, preferably 5.0 at% or more tungsten, more preferably 8.0 at% or more tungsten, more preferably 11.0 at% or more tungsten. Such and alloy has increased melting temperature and so higher high temperature strength and creep resistance. In an embodiment the alloy includes 1.0 at% or more aluminium, preferably 4.0 at% or more aluminium, more preferably 7.0 at% or more aluminium, more preferably 10.0 at.% or more aluminium. Such an alloy has reduced VEC and entropy and reduced density. In an embodiment the alloy includes 2.0 at% or more zirconium, preferably 4.0 at% or more zirconium, more preferably 6.0 at% or more zirconium, more preferably 8.0 at% or more zirconium, more preferably 10.0 at% or more zirconium, most preferably 12.0 at.% or more zirconium. Such an alloy has increased ductility and lower density. In an embodiment the alloy includes 15.0 at% or less cobalt, preferably 10.0 at% or less cobalt, more preferably 7.0 at.% or less cobalt. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 0.5 at% or more cobalt, preferably 1.0 at% or more cobalt, more preferably 2.0 at% or more cobalt, even more preferably 3.0 at% or more cobalt, most preferably 4.0 at% or more cobalt. Such an alloy is less likely to form intermetallic phases, at little detriment to cost, density or melting point. In an embodiment the alloy includes 10.0 at% or less copper, preferably 5.0 at% or less copper, more preferably 2.0 at.% or less copper. Such an alloy achieves lower VEC and so has improved ductility. In an embodiment the alloy includes 0.5 at% or more copper, preferably 1.0 at% or more copper, more preferably 2.0 at% or more copper, more preferably 3.0 at.% or more copper. Such an alloy has increased entropy, resulting in better phase stability and strength and creep resistance. In an embodiment the alloy includes 0.5 at% or more chromium, preferably 2.0 at% or more chromium, more preferably 4.0 at% or more chromium. Such an alloy has improved printability, particularly in combination in the presence of iron. In an embodiment the alloy includes 0.5 at% or more manganese, preferably 2.0 at% or more manganese, more preferably 4.0 at% or more manganese. Such an alloy has increased entropy, resulting in better phase stability and strength and creep resistance, and low density. In an embodiment the alloy includes 30.0 at% or less manganese, preferably 23.0 at% or less manganese, more preferably 15.0 at% or less manganese, most preferably 1.0 at.% or less manganese. Such an alloy has lower VEC, resulting in improved ductility. In an embodiment the alloy includes 3.0 at% or more rhenium, preferably 5.0 at% or more rhenium, more preferably 8.0 at% or more rhenium, more preferably 15.0 at% or more rhenium, more preferably 20.0 at% or more rhenium, most preferably 25.0 at% or more rhenium. Such an alloy has increased high temperature strength and creep resistance. In an embodiment the alloy includes 28.0 at.% or less rhenium, preferably 22.0 at.% or less rhenium, more preferably 20.0 at% or less rhenium, more preferably 15.0 at% or less rhenium, more preferably 14.5 at% or less rhenium, more preferably 10.0 at% or less rhenium, more preferably 5.0 at% or less rhenium, more preferably 1.0 at.% or less rhenium. Such an alloy achieves lower cost. In an embodiment the alloy includes 5.5 at% or less hafnium, preferably including 1.0 at.% or less hafnium. Such an alloy has reduced cost. In an embodiment the following equation is fulfilled: ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ Such an alloy has improved ductility. In an embodiment the following equation is satisfied: A _W + 0.8A _Ta ≤ 43 preferably A _W + 0.8A _Ta ≤ 34 more preferably A _W + 0.8A _Ta ≤ 29 most preferably A _W + 0.8A _Ta ≤ 20 where ^^ _்^ ^^ ^^ ^^ ^^ _^ are the atomic percent of tantalum and tungsten, respectively, in the alloy. Such an alloy has reduced density. In an embodiment the following equation is satisfied: A _Re + 0.67A _Ta ≤ 35 preferably ARe + 0.67ATa ≤ 28 more preferably A _Re + 0.67A _Ta ≤ 22 most preferably A _Re + 0.67A _Ta ≤ 15 where ^^ _ோ^ ^^ ^^ ^^ ^^ _்^ are the atomic percent of rhenium and tantalum, respectively, in the alloy. Such an alloy has reduced density. In an embodiment the following equation is satisfied: A _Al + A _Zr + A _Ti + A _Si + A _Hf ≥ 16.5 preferably A _Al + A _Zr + A _Ti + A _Si + A _Hf ≥ 31.5 where ^^ _^^ , ^^ _^^ , ^^ _ௌ^ , ^^ _ு^ ^^ ^^ ^^ ^^ _்^ are the atomic percent of aluminium, zirconium silicon, hafnium and titanium respectively in the alloy. Such an alloy has increased ductility. In an embodiment the following equation is satisfied: A _V + 2A _Ta ≤ 40 preferably A _V + A _Ta ≤ 20 preferably AV + ATa ≤ 15 preferably A _V + 2A _Ta ≤ 20 where A _V and A _Ta are the amounts in atomic percent of vanadium and tantalum respectively. Such an alloy has reduced cost. In an embodiment the alloy includes 14.0 at% or less chromium, preferably 13.0 at% or less chromium, more preferably 7.7 at% or less chromium, even more preferably 7.0 at% or less chromium, most preferably 2.0 at% or less chromium. Such an alloy has reduced chance of intermetallic formation and so increased ductility. In an embodiment the following equation is satisfied: 15 ≤ A _W + A _Ta + A _Mo + A _Nb + A _Re preferably 25 ≤ A _W + A _Ta + A _Mo + A _Nb + A _Re more preferably 0.6A _Al + 25 ≤ A _W + A _Ta + A _Mo + A _Nb + A _Re even more preferably 1.2A _Al + 30 ≤ A _W + A _Ta + A _Mo + A _Nb + A _Re most preferably 0.8A _Al + 47 ≤ A _W + A _Ta + A _Mo + A _Nb + A _Re where A _W , A _Al , A _Ta , A _Mo , A _Re and A _Nb are the amounts in atomic percent of tungsten, aluminium, tantalum, molybdenum, rhenium and niobium respectively. Such an alloy has increased melting temperature and thereby increased high temperature strength and creep resistance.

The term “consisting of” is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100 atomic percent. Unless stated otherwise, all amounts are given in atomic percent (at%). A _i is atomic percent of element i in the alloy and x _i is atomic fraction of element i in the alloy. The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which: Figures Figure 1 is plots of Co, Cu, Ni, Ir, Pd, Pt, Rh and Ru on the y axes vs VEC on the x- axis for all alloys in the space defined in table 2 with no further restrictions; Figure 2 shows zirconium concentration and whether or not Al-Zr intermetallics are present; Figure 3 shows iron concentration and whether or not Fe-Ti intermetallics are present; Figure 4 plots density of alloys as a function of W and Ta content. Lines are drawn for density ≤11, ≤10, ≤9 and ≤8 g/cm ³ in figures 4a, 4b, 4c and 4d respectively. The alloy space is the broadest elemental space as defined in Table 10 and also including the constraints VEC≤5.0 and entropy≥13.38; Figure 5 plots density of alloys as a function of Re and Ta content. Lines are drawn for density ≤11, ≤10, ≤9 and ≤8 g/cm ³ in figures 5a, 5b, 5c and 5d respectively. The alloy space is the elemental space as defined in the broadest elemental space as defined in Table 10 and also including the constraints VEC≤5.0 and entropy≥13.38; Figure 6 plots VEC on the y axis against Al equivalent (sum of Al, Zr, Ti, Hf and Si). The alloys plotted are those falling within claim 1 and also including the constraints VEC≤5.0, entropy≥13.38; Figure 7 plots melting temperature as a function of tungsten equivalent (W _eq , sum of tungsten, tantalum, molybdenum, niobium and rhenium) vs at% aluminium. The alloy space is the broadest elemental space as defined in Table 10 and also including the constraints VEC≤5.0 and entropy≥13.38. Figures 7a-e plot alloys with a melting point of greater than 2200, 2400, 2500, 2600 and 2800 K respectively, for all alloys; Figure 8 shows the density vs melting point trade-off for all alloys falling within the broadest elemental range defined in Table 10 and also including the constraints VEC≤5.0 and entropy≥13.38; Figure 9 is plots of Ir, Rh, Pd, Pt, Re and Ru on the y axes vs cost on the x-axis for all alloys falling within the broadest elemental range defined in Table 10 and also including the constraints VEC≤5.0 and entropy≥13.38; and Figure 10 shows cost vs Ta and V concentrations. The alloy space is the broadest elemental space as defined in Table 10 and also including the constraints VEC≤5.0 and entropy≥13.38 ABD design steps Traditionally, high performance alloy materials have been designed through empiricism. Thus their chemical compositions have been isolated using time consuming and expensive experimental development, involving small-scale processing of limited quantities of material and subsequent characterisation of their behaviour. The alloy composition adopted is then the one found to display the best, or most desirable, combination of properties. In the field of CCA’s and RCCA’s the removal of a principal element results in an extremely large number of possible alloying combinations, therefore modelling techniques provide novel insight that can be applied to isolate improved alloys. A modelling-based approach used for the isolation of optimised CCA’s is described here, termed the “Alloys-By-Design” (ABD ^® ) method. This approach utilises a framework of computational materials models to estimate design relevant properties across a very broad compositional space. In principle, this alloy design tool allows the so-called inverse problem to be solved; identifying optimum alloy compositions that best satisfy a specified set of design constraints. The first step in the design process is the definition of an elemental list along with the associated upper and lower compositional limits. The compositional limits for each of the elemental additions considered in this invention – referred to as the “alloy design space” - are detailed in Table 2. The starting point is that at least three different elements should be present (though at least four elements are likely to be needed to ensure high enough entropy) meaning that each element can be present up to 35 at%. Titanium was allowed to increase up to 40 at% because of its importance in reducing valence election concentration (VEC), described below. The only elements which reduce VEC are aluminium, zirconium, silicon, hafnium and titanium. However excessive amounts of aluminium can lead to reduction in processibility due to it’s very low melting point relative to refractory elements. Zirconium is limited in order to reduce the formation of intermetallic phases. Silicon and hafnium also form detrimental secondary phases. Thus titanium is the only of these elements which can be used in very high amounts to reduce VEC. If a very high entropy alloy is desired, the amount of titanium can be reduced to 35.0 at% or less. Certain elements are restricted below 35.0 at% based on an understanding on their likely effect in practice, particularly the promotion of intermetallic formation. In RCCA systems, the high entropy effect promotes the formation of a solid solution with lower intermetallic fraction than would be expected from the constituent binary and ternary subsystems. Most solidification pathways for processing RCCA alloys tend to result in microsegregation. In regions where this effect is most severe, e.g. interdendritic regions, elemental concentrations may be extreme enough to reduce entropy below the critical value for microstructural stabilisation. This increases likelihood of intermetallic formation. To ensure alloys encompassed by this invention retain their resistance to intermetallic formation, even upon segregation-inducing solidification, additional restrictions are placed on elements most prone to secondary phase formation. These include chromium, hafnium, zirconium, iron and vanadium. Based on the XRD and EBSD measurements of prior art alloys, compositions containing both chromium and niobium are prone to formation of C14 laves phases, based on the Cr ₂ Nb stoichiometry, when chromium content is 18 at% or higher. (Ma and Zhang, 2012; Senkov et al., 2013; Chen et al., 2016). The introduction of a 0.1 volume fraction of this phase to an alloy has been shown reduce the plastic strain limit of an HEA system by 50%. (Ma and Zhang, 2012). In order to maximise alloy ductility, the formation of this phase should be minimised. The phase diagram for the Cr-Nb systems shows chromium in niobium has a maximum solubility limit of 15 at% at 1650°C, beyond which substantial Cr ₂ Nb forms (Massalski and Okamoto, 1990). Therefore a maximum chromium content of 15.0 at% or lower is permitted in this invention. More preferably chromium will be lower than 14.0 at%, or even more preferably lower than 13.0 at%, to reduce supply of chromium atoms for intermetallic formation. An alloy containing 7.7 at% chromium and 15.4 at% niobium was found, by XRD and ESBD, to form no Cr ₂ Nb phase (Sheikh et al., 2020). It is therefore even more preferable for chromium in this invention to be 7.7 at% or lower. The solubility of chromium in niobium drops even further as temperature drops, to 7.0 at% at 1300°C, and 2.0 at% at 1000°C. It is therefore more preferable for chromium to be 7.0 at% or lower, and even more preferable to be 2.0 at% or lower, because this will increase resistance to Cr ₂ Nb formation in a wider temperature range. In an embodiment no chromium is present except at incidental impurity level. Hafnium is not a principal element in the composition space due to being difficult to source, but it’s ability to scavenge oxygen and lower ductile-to-brittle transition temperature make it able to impart useful properties on a RCCA (Tsakiropoulos, 2022). Based on the Hf-Nb phase diagram, above 10.0 at% hafnium, it will promote the formation of second, hafnium-rich, phase. It is therefore required for hafnium to be present in levels of 10.0 at% or lower. The high temperature niobium alloy, C103, has composition of Nb-10Hf-1Ti (wt%), which is equivalent to 5.5 at% hafnium. It is therefore preferable for hafnium to be present in levels of 5.5 at% or lower. It has been found experimentally that such an alloy has a suitable microstructure and has low cost. Preferably hafnium is present at levels of 1.0 at.% or less. The presence of zirconium is known to lead to the formation of intermetallics, with aluminium (Soni et al., 2018; Tsai et al., 2019). Intermetallic formation can be reduced by limiting the concentration of zirconium. Tsai et al., 2019 report Zr intermetallic formation at 16.7 at% Zr. On this basis, zirconium is limited to 16.7 at% or lower to reduce AlZr-based phase formation. It is preferable for 16.0 at% or lower, more preferably 15.0 at% or lower, even more preferably 14.0 at% or lower or even 13.0 at% or lower. The presence of iron is known to lead to the formation of Fe ₂ Ti intermetallic with titanium (Tsai et al., 2019). This phase is very brittle, reducing alloy ductility if present in the microstructure (Wang et al., 2007; Zhou et al., 2007). On the basis of reported alloys, Fe2Ti can be reduced by iron content being lower than 15.4 at%. Based on the vanadium-tantalum phase diagram it is thermodynamically favourable for C14 and C15 laves phase to form when vanadium concentration exceeds 12.0 at% (Bale et al., 2016) ^. It is therefore required that vanadium concentration be restricted to 12.0 at% or lower. It is more preferable for vanadium content to be even lower, such as 6.0 at% or less or even 1.0 at% or less, to further reduce the availability of vanadium atoms for intermetallic formation. In one embodiment, where the alloy is extremely resistant to intermetallic formation, it is preferred for vanadium to be absent except at incidental impurity level. It is desirable for alloys of this invention to be easily processible through melt- based methods, sinter-based methods, a combination of both, or other methods. These are the same approaches adopted for production of commercially available refractory and nickel alloys (e.g. C103, FS-85, IN718, CMSX-4). Desirably the RCCAs in this invention can be easily substituted into existing processes. Alloy production by any method requires individual raw elements to be combined. When high levels of aluminium (lowest melting point in the design space) are combined with high levels of very high melting point refractory elements (e.g. W, Re, Ta, Nb, Mo) difficulties arise in achieving uniform alloying. The high strength of the RCCAs in this invention means machining can be time-consuming and expensive. They are thus best-suited to near-net-shape processing methods, including, but not limited to, casting, additive manufacture (AM) and hot isostatic pressing (HIP). For casting, e.g. through arc-melting, aluminium melts significantly earlier during heating than the refractory elements, and for alloys with particularly high aluminium content, the temperature of the melt cannot necessarily be raised to high enough temperatures for elements such as tungsten to melt fully. This leads to unmelted pockets of material, requiring remelting or scrapping. The EIGA atomisation process is the preferred method of producing the feedstock powders for AM and HIP. It was found through powder production trials that combining high levels of refractory elements with high levels of aluminium leads to ineffective sintering during EIGA electrode production. This is because the temperature required for effective sintering of refractory elements (>1200°C) is typically higher than the melting point of aluminium (660°C). One alloy for which this issue was encountered is RHG2E (22Al9Zr5W5Cr35Ti5Ta19Nb at%). As these alloys are primarily intended for use either in vacuum, inert atmosphere, or with an engineered coating for oxidation protection, a continuous alumina scale is not required to form. It is therefore permitted for aluminium to be lower than 20.0 at%. To avoid the melting and sintering issue, this invention requires aluminium to be 19.45 at% or lower, preferably 19.0 at% or lower, more preferably 15.0 at% or lower, more preferably 10.0 at% or lower, even more preferably 5.0 at% or lower. In one embodiment, it is preferably for aluminium to be absent from the alloy altogether except at the level of incidental impurities. The main refractory elements in this invention (Mo, Nb, Re, Ta, Ti, V, W and Zr) have a low solubility for silicon (3, 5, 10, 5, 5, 7, 4, 0 at%, respectively). Beyond the solubility limit, silicide phases, e.g. M ₃ Si, can form. At low volume fractions, these silicide phases can increase yield strength substantially. However, the silicides fracture in a brittle manner, so a high volume fraction or continuous network of the phase through a microstructure are severely detrimental to fracture toughness. To avoid excessive formation of silicides, Si concentration should be kept below 5.0 at%, as this corresponds to the limits for Nb, Ta and Ti. Preferably silicon will be 3.0 at% or lower, even more preferably 1.0 at% or lower, most preferably 0 at% (e.g. at the level of incidental impurities), as this will remove any possibility of very brittle silicides forming. Table 2: Alloys design space in at% searched using the “Alloys-by-Design” method. Element (at%) Min Max Al 0.0 19.45 Co 0.0 35.0 Cr 0.0 15.0 Cu 0.0 35.0 Fe 0.0 15.4 Hf 0.0 10.0 Ir 0.0 35.0 Mn 0.0 35.0 Mo 0.0 35.0 Nb 0.0 35.0 Ni 0.0 35.0 Pd 0.0 35.0 Pt 0.0 35.0 Re 0.0 35.0 Rh 0.0 35.0 Ru 0.0 35.0 Si 0.0 5.0 Ta 0.0 35.0 Ti 0.0 40.0 V 0.0 12.0 W 0.0 35.0 Zr 0.0 16.7 The selection of alloys relies upon calculating merit indices for the alloys within the alloy composition space (as set out in Table 2). Examples of these merit indices include entropy (which relates to the propensity for the elements within the alloy to form a solid solution structure and so relates to strength and creep resistance), valence electron concentration (which relates to the resistance to intermetallic phase formation and good ductility), melting point index, density and cost. In the second stage, the calculated merit indices over the whole space defined in table 2 are compared with limits for required behaviour for configurational entropy and valence electron concentration and these design constraints are considered to be the boundary conditions to the problem. All compositions which do not fulfil the boundary conditions are excluded. At this stage, the alloy space will be reduced in size. In the third stage the dataset of remaining compositions was analysed and example compositions which have favourable merit indices compared to prior art alloys were prepared. Further limits to the alloy scope were made as a result of experimental observations. Merit index descriptions The example merit indices are now described. The first merit index is the entropy index. A sufficiently high entropy of mixing for solid solution enhances stability of the solid solution, relative to the formation of intermetallic phases. This stability often extends to elevated temperatures and can increase ease of processing. Entropy is given by where ^^ _^ is the atomic fraction of element ^^ in the alloy and R is the universal gas constant. The solid solution structure of RCCAs is stabilised by the high entropy effect, which simplifies the microstructure significantly, compared with what would be expected of the constituent binary and ternary systems. Entropy of RCCAs is dominated by configurational entropy. There exist a number of competing definitions of the threshold of configurational entropy at which an alloy is considered a high-entropy- alloy, benefitting from the high entropy effect. The first definition is for configurational entropy of a value of 1.5R (12.47) or greater, where R is the universal gas constant, with value 8.3145 J K ^-1 mol ^-1 . (Murty et al., 2019) (Murty et al., 2019) A further definition is for configurational entropy of a value of ln(5)R (13.38) or greater, based on an alloy composed of five or more principal elements in equiatomic ratios (Yeh, 2013). Some studied HEA compositions have been 5 element equiatomic, including but not limited to RHEA1 and RHEA3 in Table 1. These compositions have been reported to benefit from the high entropy effect, and their configurational entropy is thus a promising starting point from which to improve. (Miracle et al., 2011; Gorr et al., 2017) Thus the present invention requires a configurational entropy of 13.38 or more. To further improve these properties, this invention more preferably requires entropy equal to or greater than 13.5 to promote even greater stability of the solid solution as well as increase lattice distortion and sluggish diffusion, which increase strength and creep resistance, respectively. As can be seen from table 7 below, alloys with configurational entropies of 13.8 and above have been designed and these are preferred. In an embodiment the alloy is substantially single phase, namely a single matrix in which all elements are in solid solution. That is, the alloy is substantially intermetallic phase (e.g. carbide, oxide, boride etc) free, with a maximum volume percent (measured by optical microscopy) of 5% or less of intermetallic phase (typically up to 2% carbide, up to 1% oxide and 1-2% boride). CN109252082 discloses that carbide phase is detected by XRD analysis indicating that the volume fraction of carbide is above 10% (10% is the typical lower limit of what is detectable by XRD analysis). The second merit index is the valence electron concentration index. The Valence Electron Concentration describes the number of electrons in the outer electron shell of an atom given by where ^^ _^ is the atomic fraction of element ^^ in the alloy bulk composition and ^^ ^^ ^^ _^ is the valence electron concentration of element ^^. (Miracle and Senkov, 2017) A low VEC is indicative of an alloy which is predominantly BCC phase resulting in an alloy with high ductility. The number of electrons in the outer shell is given in table 3 below. Valence Electron Concentration is reported to be a good predictor of crystal structure in high entropy alloys (Tsai et al., Criterion for sigma phase formation in Cr- and V-containing high-entropy alloys. Mater. Res. Lett. 2013,1, 207-212), where VEC < 6.87 corresponds to predominantly BCC phase, sigma-free alloys. It has also been reported (Sheikh et al., 2016) that further restriction of VEC can promote intrinsic ductility in single-phase BCC high entropy alloys. Brittle behaviour was absent for VEC < 4.60. In this invention, additional elemental restrictions (on Cr, Zr and Fe) were used to limit formation of specific intermetallics, improving alloy ductility. The power of these restrictions is shown by the following examples. NbMoCrTiAl was experimentally observed to form Cr ₂ Nb laves phase, and has 20 at% chromium, and VEC of 4.8. (Muller et al., 2019) Another alloy, RHG3F has a similar VEC, 4.9, but significantly lower chromium content, 5 at%. It was experimentally observed to be laves-free. For this reason, the VEC requirement can be relaxed to VEC≤5.00 when intermetallic-forming elements, such as Cr, Zr and Fe are limited as in the present invention. A VEC of 5.00 or lower is the design requirement for the present invention. It is preferable for VEC to be 4.90 or lower, more preferably 4.80 or lower, even more preferably 4.70 or lower. And even lower VEC such as 4.60, 4.40 or even 4.20 is preferred in one embodiment. The third merit index is the melting point index. For high temperature application, melting of the alloy should be avoided, so a high melting point is required. In addition, high temperature strength and creep resistance is best when melting point is high. Melting point T _m is given by ^^ ^^ = ∑ _^ ^^ _^ ∙ ^^ _^^ where ^^ _^ is the atomic fraction of element ^^ in alloy bulk composition and ^^ _^^ is the melting point of pure element ^^. Table 3 gives the melting points of the elements in the alloy. The fourth merit index is density. The density, ρ was calculated using a simple rule of mixtures, where ρ _୧ , is the density for a given element and x _i is the atomic fraction of the alloy element. Table 3 gives the densities of the elements involved. The fifth merit index is cost. In order to estimate the cost of each alloy a simple rule of mixtures was applied, where the atomic fraction of the alloy element, x _i , was multiplied by the current (2021) raw material cost for the alloying element, c _i , in USD/mol. The estimates assume that processing costs are identical for all alloys, i.e. that the product yield is not affected by composition. Table 3 gives the costs of all elements involved. Table 3 E ^{lement (at%)} _{^^ ^^ ^^^} ^{^^} _{^^ ^^^ ^^^} Al 3 933 2.7 0.0 Co 9 1768 8.9 1.9 Cr 6 2136 7.2 0.5 Cu 11 1358 9.0 0.4 Fe 8 1811 7.9 0.0 Hf 4 2504 13.3 160.6 Ir 9 2720 22.7 10735.3 Mn 7 1519 7.5 0.1 Mo 6 2896 10.2 3.8 Nb 5 2742 8.6 6.8 Ni 10 1728 8.9 0.8 Pd 10 1828 12.0 5267.8 Pt 10 2042 21.1 5423.3 Re 7 3459 21.0 666.6 Rh 9 2236 12.5 15127.1 Ru 8 2607 12.4 1061.2 Si 4 1683 2.3 0.0 Ta 5 3293 16.7 55.2 Ti 4 1843 4.5 0.5 V 5 2183 6.1 18.9 W 6 3695 19.3 6.5 Zr 4 2128 6.5 3.3 The ABD method described above was used to isolate alloy compositions which met the required entropy and VEC criteria described above and which showed promising other characteristics as defined by the third to fifth merit indices. The composition limits defined in Table 2 apply. Cobalt, copper, nickel, iridium, palladium, platinum, rhodium and ruthenium have the highest VEC of the elements in Table 2. Figure 1 is a series of plots showing the alloys in the space of table 2. Plotted on the y-axis is the amount of certain elements and on the x-axis the VEC. As can be seen, in order to achieve a VEC of 5.00 or less these elements need to be kept below the following concentrations in the alloy to ensure ductility: A _Co ≤ 20.0; A _Cu ≤ 15.0; A _Ni ≤ 15.0; A _Ir ≤ 20.0; A _Pd ≤ 15.0; A _Pt ≤ 15.0; A _Rh ≤ 20.0; A _Ru ≤ 25.0 Where A _Co , A _Cu , A _Ni , A _Ir , A _Pd , A _Pt , A _Rh , A _Ru , are the atomic percent of cobalt, copper, nickel, iridium, palladium, platinum, rhodium and ruthenium in the alloy, respectively. It is even more preferable to further reduce the content of the elements, to reach even lower VEC levels. Thus copper is preferably kept at a level of 10.0 at% or less, or more preferably 5.0 at% or lower or even more preferably 2.0 at% or less copper. Cobalt is kept at 15.0 at% or less in a preferred embodiment and at 10.0 at% or less in a further preferred embodiment and most preferably is 7.0 at% or less. For the same reason nickel is preferably kept low, for example to 8.0 at% or less or 3.0 at% or less or even to 1.0 at% or less nickel. Iridium and rhodium are preferably kept at a level of 15.0 at% or less, or more preferably 8.0 at% or lower or even 1.0 at% or lower. Palladium and platinum are preferably kept at level of 10.0 at% or less, or more preferably 5.0 at% or lower or 1.0 at% or lower to keep costs low. Ruthenium is preferably kept at a level of 18.0 at% or less, or more preferably 10.0 at% or lower. In one embodiment, removing all of these high-VEC elements (e.g. to levels of incidental impurities) is preferred. The modelling results indicate that for Al, Cr, Fe, Hf, Mn, Mo, Nb, Re, Si, Ta, Ti, V, W and Zr, VEC ≤ 5.00 is achievable throughout the ranges of table 2 and so these were assumed to be allowable. For alloy design, the limits in table 2 therefore apply, unless reduced amounts are required to avoid intermetallics. This invention does not depend on growth of an oxide for oxidation protection, so the Pilling Bedworth ratio of the alloy is not used as a design criterion. Due to this, rhenium, which has a high elemental Pilling Bedworth ratio, is permitted up to 35.0 at%. Despite this, the high cost and density of rhenium mean it is preferable for content to be lower than 35 at%, and substitute for other refractory elements in small quantities. Rhenium has a higher melting point than Ta and Nb, and similar melting point to W. Due to this, rhenium can impart similar high temperature strength on an alloy, so can substitute for Ta, W and Nb. Therefore, 35 at% or less of rhenium is permitted, provided A _Re ≤ (35 - A _Ta ) + (35 - A _W ) + (35 - A _Nb ) and where A _Re , A _Ta , A _W and A _Nb are the amounts in atomic percent of rhenium, tantalum, tungsten and niobium respectively. Rhenium is very difficult to source, resulting is a high elemental cost, it also has high density. For this reason, it is preferable for rhenium content to be 20.0 at% or less, more preferably 15.0 at% or less, or most preferably 10.0 at% or less or even 5.0 at% or less. To achieve particularly high melting point, a minimum rhenium content of 3.0 at % or even 5.0 at% or more is preferred, at the expense of cost and density, as described above. Previously researched alloys from Table 1 are reprinted in Table 4 for ease of reference. Example alloys which fulfil all requirements above are presented in Table 5, whilst comparative examples are presented in Table 6. The merit index values for all benchmark alloys, examples and comparative examples are presented in Table 7. As can be seen, it is possible to fulfil composition requirements stipulated in Table 10, but not fulfil entropy and VEC requirements. For example, RHEA8 and RHEA9 both fall within the composition set out in Table 10, but have VEC higher than 5.0, so are not encompassed by the invention. Additionally RHEA10, RHEA11 and RHEA12 also fall within the composition limits, but their entropy is lower than 13.38, so the alloys are not encompassed by the invention.

Table 4 Alloy ( _at%) ^{Al Co Cr Cu Fe Hf Ir Mn Mo Nb Ni Pd Pt Re Rh Ru Si Ta Ti V W Zr} RHEA1 ^{0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 0.0 20.0 20.0 0.0} RHEA2 ^{0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.0 25.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.0 0.0 0.0 25.0 0.0} RHEA3 ^{20.0 0.0 20.0 0.0 0.0 0.0 0.0 0.0 20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 20.0 0.0 0.0 0.0} RHEA4 15.4 0.0 7.7 0.0 0.0 0.0 0.0 0.0 0.0 15.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.4 46.2 0.0 0.0 0.0 RHEA5 16.7 0.0 16.7 0.0 0.0 0.0 0.0 0.0 16.7 16.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 16.7 16.7 0.0 0.0 0.0 RHEA6 10.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 16.0 30.0 4.0 0.0 20.0 RHEA7 16.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 16.7 16.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 16.7 16.7 16.7 0.0 0.0 Table 5 Z ^r 13.0 1 ^3.0 RHG3F 2.9 0.0 5.0 0.0 0.0 0.0 0.0 0.0 10.5 21.9 0.0 0.0 0.0 4.8 0.0 0.0 0.0 10.5 23.8 0.0 8.6 12.4 RHG3B 0.0 5.0 0.0 0.0 0.0 0.0 0.0 12.4 21.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.6 25.7 0.0 12.4 10.5 RHG3D ^{1.9 0.0 5.0 0.0 0.0 0.0 0.0 0.0 12.4 20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.6 27.6 0.0 12.4 12.4}

RHG3J ^{0.95 0.0 5.0 0.0 0.0 0.0 0.0 0.0 18.1 12.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 14.3 20.0 10.5 6.7 12.4} RHG3M ^{10.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 9.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 23.0 31.0 11.0 11.0 5.0} RHG3N ^{16.0 0.0 5.0 0.0 5.0 0.0 0.0 0.0 5.0 25.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 34.0 10.0 0.0 0.0} RHG3O ^{12.0 0.0 5.0 10.0 0.0 0.0 0.0 0.0 0.0 15.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 10.0 27.0 5.0 0.0 16.0} RHG3P ^{12.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 0.0 15.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.0 29.0 5.0 0.0 4.0} RHG3Q ^{4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 25.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 33.0 0.0 0.0 8.0} RHG3R ^{16.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 18.0 10.0 0.0 16.0} RHG3S ^{16.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 32.0 0.0 5.0 12.0} RHG3T 4.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 35.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 15.0 10.0 10.0 16.0 RHG3U 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 35.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.0 12.0 5.0 10.0 8.0 RHG3V 8.0 0.0 5.0 0.0 0.0 0.0 0.0 0.0 10.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 35.0 22.0 0.0 15.0 0.0 RHG3W 8.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.0 23.0 0.0 25.0 4.0 RHG3X 8.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 0.0 0.0 0.0 0.0 10.0 0.0 5.0 15.0 33.3 0.0 0.0 8.4 RHG3Y 0.0 0.0 0.0 0.0 0.0 10.0 0.0 0.0 0.0 10.0 0.0 0.0 0.0 5.0 0.0 0.0 0.0 35.0 3.3 0.0 20.0 16.7 RHG3Z 16.7 0.0 0.0 0.0 0.0 10.0 0.0 0.0 0.0 0.0 0.0 0.0 10.0 5.0 0.0 0.0 0.0 30.0 6.6 0.0 5.0 16.7 RHG3AA ^{0.0 0.0 0.0 0.0 0.0 10.0 9.0 0.0 0.0 20.0 0.0 0.0 0.0 5.0 0.0 0.0 0.0 15.0 31.7 0.0 0.0 9.4} RHG3AB ^{16.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.0 0.0 0.0 0.0 30.0 0.0 0.0 0.0 0.0 25.0 0.0 5.0 8.4} RHG3AC ^{0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 0.0 0.0 0.0 0.0 0.0 11.5 0.0 25.0 36.0 0.0 10.5 12.5} RHG3AD ^{4.2 0.0 0.0 0.0 0.0 10.0 0.0 0.0 0.0 5.0 0.0 15.0 0.0 0.0 0.0 0.0 5.0 5.0 39.1 0.0 0.0 16.7}

Table 6 Alloy ( _at%) ^{Al Co Cr Cu Fe Hf Ir Mn Mo Nb Ni Pd Pt Re Rh Ru Si Ta Ti V W Zr} RHG2E ^{22.0 0.0 5.0 0.0 0.0 0.0 0.0 0.0 0.0 19.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 35.0 0.0 5.0 9.0} RHG3C ^{20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 18.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 9.0 23.0 11.0 14.0 5.0} RHG3L ^{20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 17.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 9.0 29.0 11.0 9.0 5.0} RHEA8 ^{10.0 0.0 10.0 0.0 0.0 0.0 0.0 0.0 0.0 20.6 15.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 34.4 10.0 0.0 0.0} RHEA9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20.0 20.0 0.0 20.0 0.0 RHEA10 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 28.6 28.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 28.6 0.0 0.0 14.3 RHEA11 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 33.3 33.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 33.3 0.0 0.0 0.0 RHEA12 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.0 25.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 25.0 25.0 0.0 0.0 0.0

Table 7 Alloy S _config ρ T _m VEC Cost RHEA1 13.38 12.2 2962 5.40 18.3 RHEA2 11.53 13.7 3157 5.50 18.1 13.38 8.3 2240 4.80 12.0 11.79 6.9 2135 4.31 9.8 14.91 8.3 2328 4.84 11.2 13.78 7.5 2264 4.30 11.8 RHEA7 14.91 8.1 2336 4.68 14.3 RHG3K 14.22 9.4 2603 4.78 11.3 RHG3E 14.19 9.2 2600 4.82 9.4 RHG3F 16.76 9.7 2594 4.94 40.8 RHG3B 15.97 9.3 2566 4.88 8.0 RHG3D 15.71 9.3 2574 4.88 8.0 RHG3J 16.95 9.3 2580 4.97 12.4 RHG3N 14.83 9.4 2453 4.55 16.4 RHG3M 14.02 6.0 2056 4.59 4.0 RHG3O 16.18 7.1 2069 4.98 8.2 s RHG3P 14.82 7.5 2079 4.83 10.6 e RHG3Q 13.84 7.6 2365 4.96 5.7 E RHG3R 14.71 7.9 2265 4.34 14.9 RHG3S 13.87 8.2 2281 4.29 13.0 RHG3T 15.25 8.6 2470 4.91 8.4 RHG3U 14.14 11.0 2810 4.95 18.3 RHG3V 14.28 11.7 2742 4.92 21.2 RHG3W 13.69 12.0 2788 4.97 16.3 RHG3X 14.71 7.9 2254 4.77 1522.8 RHG3Y 14.23 14.2 3009 5.00 71.3 RHG3Z 15.78 12.3 2440 4.98 609.2 RHG3AA 14.88 10.7 2526 4.95 1025.7 RHG3AB 13.71 10.7 2454 4.99 201.8 RHG3AC 13.38 10.5 2614 4.99 137.5 RHG3AD 14.66 7.5 2065 4.96 810.1 RHG2E 13.99 6.5 2054 4.22 4.9 s _e RHG3C 15.41 8.3 2287 4.46 9.5 l _p m RHG3L 15.03 7.8 2218 4.52 8.6 a _x E RHEA8 13.87 6.3 2018 5.31 3.7 e _v ⁱ _t RHEA9 13.38 11.8 2914 5.20 14.6 a _r ^a _p RHEA10 11.24 7.6 2472 4.86 3.7 m o C RHEA11 9.13 7.8 2524 5.00 3.7 RHEA12 11.53 10.0 2719 5.00 16.6 The RHG3 alloys were manufactured and their properties investigated. On the basis of experimental results, the following further limitations are imposed on the inventive alloy. Figure 2 maps zirconium concentration with AlZr intermetallic phase formation. The alloys plotted correspond to those in Table 8, and are combination of those from Metals 2019 article and alloys manufactured and tested experimentally by the inventors. This invention encompasses alloys with zirconium content lower than 16.7 at%, as this avoids regions know to form AlZr intermetallics. RHG3J, RHG3M and RHG3F all contain both Al and Zr, but Al-Zr intermetallics were not observed. It is therefore preferable for the alloys encompassed by this invention to contain less than or equal to 12.4 at% zirconium (i.e. a level lower than RHG3J and RHG3F, both of which are AlZr intermetallic free), or even 5.0 at% or less zirconium. Table 8

Alloy ( _at.%) ^{Al Cr Fe Mn Mo Nb Ni Re Ta Ti V W Zr} Tsai 3 16.7 16.7 0.0 0.0 0.0 16.7 0.0 0.0 0.0 16.7 16.7 0.0 16.7 Tsai 5 20.0 0.0 0.0 0.0 10.0 20.0 0.0 0.0 10.0 20.0 0.0 0.0 20.0 Tsai 6 20.0 0.0 0.0 0.0 0.0 20.0 0.0 0.0 20.0 20.0 0.0 0.0 20.0 Tsai 7 22.2 0.0 0.0 0.0 0.0 11.1 0.0 0.0 0.0 33.3 22.2 0.0 11.1 Tsai 8 20.0 0.0 0.0 0.0 0.0 20.0 0.0 0.0 0.0 20.0 20.0 0.0 20.0 Tsai 9 27.3 0.0 0.0 0.0 0.0 18.2 0.0 0.0 0.0 18.2 18.2 0.0 18.2 RHG3J 1.0 5.0 0.0 0.0 18.1 12.4 0.0 0.0 14.3 20.0 10.5 6.7 12.5 RHG3F 2.9 5.0 0.0 0.0 10.5 21.9 0.0 4.8 10.5 23.8 0.0 8.6 12.5 RHG3M 10.0 0.0 0.0 0.0 0.0 9.0 0.0 0.0 23.0 31.0 11.0 11.0 5.0 RHG2D 22.0 5.0 0.0 0.0 0.0 19.0 0.0 0.0 5.0 35.0 0.0 5.0 9.0 RHG2E 22.0 5.0 0.0 0.0 0.0 19.0 0.0 0.0 5.0 33.0 0.0 7.0 9.0 Figure 3 maps iron concentration with Fe ₂ Ti intermetallic phase formation. The alloys plotted correspond to those in Table 9, and are a combination of those from the Metals 2019 article and alloys manufactured and tested experimentally by the inventors. As stated earlier in this document, keeping iron content lower than 15.4 at% avoids regions know to form Fe ₂ Ti intermetallic. This requirement was tightened further to encompass alloys containing iron content of 6.5 at% or lower, based on experimental testing of RHG1K. RHG1H and RHG1K both contain iron and titanium, but Fe-Ti intermetallics were not observed. RHG1K is therefore the alloy with highest iron content not to form Fe ₂ Ti intermetallics, whilst still containing titanium. It is therefore necessary for the alloys encompassed by this invention to contain less than or equal to 6.5 at% iron, or even 3.0 at% or less iron to further reduce supply of iron atoms. ( _at.%) ^{Zr Zhou 1 15.4 15.4 15.4 0.0} _15.4 ^{0.0 0.0 0.0} _{15.4 23.1 0.0} ^0.0 _0.0 P ^{i 1 16.7 0.0 16.7} _{16.7 16.7} ^{0.0 0.0 0.0} _{16.7 16.7 0.0} ^0.0 _0.0 ^{Wang 1 0.0 16.7 16.7} _{16.7 16.7} ^{0.0 0.0 0.0} _{16.7 16.7 0.0} ^0.0 _0.0 ^{Wang 2 0.0 17.2 17.2} _{17.2 17.2} ^{0.0 0.0 0.0} _{17.2 13.8 0.0} ^0.0 _0.0 ^{Kunce 1 0.0 0.0 16.7} _{0.0 16.7} ^{0.0 0.0 0.0} _{16.7 16.7 16.7} ^0.0 _16.7 RHG1K 29.7 0.0 3.2 0.0 6.5 3.1 25.0 0.0 0.0 17.4 6.2 8.3 0.5 RHG1H 3.6 0.2 1.6 0.0 0.2 0.3 21.2 17.2 0.0 2.6 0.0 53.2 0.0 On exposure to oxygen at high temperature vanadium is likely to form a vanadium- based oxide such as V ₂ O ₅ . This oxide species can form a low melting point eutectic which accelerates corrosion. (Esmaily et al., 2020) The alloys in this invention allows vanadium to be present in an amount of 12.0 at% or less and can operate in a vacuum, such as space, an inert atmosphere, or with an engineered coating system to impart oxidation and corrosion resistance. Such coatings include, but are not limited to: platinum, iridium, rhodium, ruthenium, palladium, or alloys thereof; disilicides; MAX phase (Ti2Al1C1); HfC; ZrC; IrHf; R512E (Si-20Cr-20Fe); or combinations of above. These coatings reduce the formation of vanadium oxides by blocking diffusion of vanadium to oxygen-exposed surfaces, or oxygen to vanadium-containing material. An example of a candidate coating is pure platinum, diffusion bonded to the RCCA. A diffusion-couple experiment between WTaNbVMo and pure platinum was conducted, and it was found that after 100 h at 1200°C anneal, vanadium diffused through the 100 µm thick platinum layer. Even with a coating, it is possible V ₂ O ₅ can form, and cause corrosion issues. When the alloy substrate surface is exposed directly to oxygen at high temperature, experiment has shown that vanadium content should be lower than 7.0 at%, to avoid V ₂ O ₅ formation (e.g. 20 h at 1300°C). The platinum coating imparts additional resistance to this oxidation, with experiment showing vanadium concentration is reduced from 20 at% within the substrate, to 14 at% at coating-air boundary. This is a concentration reduction to 70% of the bulk alloy, due to the platinum coating. It is therefore preferred that vanadium concentration in the alloy be 10.0 at% or lower (7.0 × ^{^^^} ^ _^ ), to avoid V ₂ O ₅ formation. Preferably the alloy contains 7.3 at% or less vanadium, more preferably 4.3 at% or less vanadium. Five alloys invented, manufactured and tested experimentally were found to have excellent performance: RHG3B, RHG3D, RHG3E, RHG3F and RHG3K. These alloys all exhibit very high Vickers hardness, 628, 570, 445, 617 and 523 HV, respectively. Under compression testing, all five alloys exceeded 1500 MPa strength. RHG3B and RHG3F exceeded 4% compressive ductility, a good value for RCCA. RHG3K and RHG3E exceeded 14%, which is excellent for RCCAs. RHG3D exceeded 2000 MPa compressive strength, and reached the load-limit of the machine before fracture, so a ductility value was not recorded, but strength was exceptional. All alloys were observed to be free of intermetallic phases in SEM. RHG3K was found to be particularly well-suited to additive manufacture, exhibiting low solidification and solid-state cracking throughout a broad laser parameter window. Due to displaying an excellent combination of strength, ductility and printability, RHG3K was also tested using hot hardness up to 600°C, where it measured an equivalent strength of 1090 MPa, similar to CM247 (1140 MPa) and far exceeding FS-85 (500 MPa) and C103 (430 MPa) at the same temperature, using the same test method. Following the trend in decay of hot hardness, a predicted high temperature strength value of 700 MPa is expected at 1300°C. It is desirable to minimise the density. This is done by controlling the content of additions of elements with high atomic mass, particularly tungsten, tantalum and rhenium. The relationship between the levels of tungsten and tantalum and predicted alloy density is shown in Figures 4a-d. A maximum target alloy density of 11 g/cm³ or less, to be competitive with literature RCCA, MoNbTaTiVW, is achieved when: A _W + 0.8A _Ta ≤ 43 at% Where A _W and A _Ta are the content of tungsten and tantalum in the alloy, respectively, in atomic percent. Preferably, the alloy density of 10 g/cm³ or less, to be lower than FS85, is achieved when: A _W + 0.8A _Ta ≤ 34 at% This requires A _^ to be less than 34.0. More preferably the alloy density is 9 g/cm³ or less, to be lower than C103, as this results in lighter parts which require less creep strength when used in rotating applications. This is achieved when: A _W + 0.8A _Ta ≤ 29 at% This requires A _^ to be less than 29.0. Most preferable maximum alloy density is 8 g/cm³ or less, to be lower than CMSX-4, corresponding to: A _W + 0.8A _Ta ≤ 20 at% This requires A _Ta and A _^ to be less than 25.0 and 20.0, respectively. The relationship between the levels of rhenium and tantalum and predicted alloy density is shown in Figures 5a-d. A maximum target alloy density of 11 g/cm³ or less, to be competitive with literature RCCA, MoNbTaTiVW, is achieved when: A _Re + 0.67A _Ta ≤ 35 at% Where A _Re and A _Ta and the content of rhenium and tantalum in the alloy, respectively, in atomic percent. Preferably, the alloy density is 10 g/cm³ or less, to be lower than FS85, is achieved when: A _Re + 0.67A _Ta ≤ 28 at% This requires A _ୖ^ to be less than 28.0. More preferably the alloy density is 9 g/cm³ or less, to be lower than C103, as this results in lighter parts which require less creep strength when used in rotating applications. This is achieved when: A _Re + 0.67A _Ta ≤ 22 at% This requires A _Re and A _^ୟ to be less than 22.0 and 32.8, respectively. Most preferable maximum alloy density is 8 g/cm³ or less, to be lower than CMSX-4, corresponding to: A _Re + 0.67A _Ta ≤ 15 at% This requires A _Re and A _^ୟ to be less than 15.0 and 22.4, respectively. Of the elements in the invention design space, aluminium, zirconium, silicon, hafnium and titanium are the only constituents with an elemental VEC lower than the target of 5.0 for the alloy. A minimum quantity of these elements is required to meet the VEC target of 5.0 or lower as well as the entropy target as shown in Figure 6. When the design space is restricted in that way the requirement for these lower VEC elements is: A _Al + A _Zr + A _Si + A _Hf + A _Ti ≥ 16.5 at% In an embodiment, it is preferred that aluminium, zirconium, silicon, hafnium and titanium levels are increased to levels even higher than those above, to reach even lower VEC, making ductility even better. For example, to achieve a VEC of 4.6 or lower, the following is required: A _Al + A _Zr + A _Si + A _Hf + A _Ti ≥ 31.5 at% The alloy of the invention is designed to have good ductility, but also good mechanical strength and creep resistance at elevated temperature. Strength correlates with both entropy (as mentioned earlier) and melting point, meaning melting point should be maximised. In addition, creep rate becomes most severe at homologous temperatures increased beyond 0.6, where homologous temperature is defined as: ^ _{^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^ ^^,} ^{^^} ^ _{^ > 0.6 ^} Where ^^ is the environmental temperature and ^^ _^ is the melting point. In order to reduce the homologous temperature, and thus the creep rate, ^^ _^ can be maximised. This can be achieved by maximising the melting temperature index (defined earlier). Preferred minimum target melting temperature index is 2200K, or even 2400K, and even above 2500K and 2600K and these are more preferred limits. Alloys with a melting temperature index of 2700K or more, and even 2800K (or 2900K) or more are also shown to be possible and in one embodiment are preferred. These correspond to a minimum quantity of refractory elements in the alloy, see Figure 7, defined by a W-equivalent, where ^^ _^ , ^^ _்^ , ^^ _ெ^ , ^^ _ே^ , ^^ _ோ^ are the concentration of tungsten, tantalum, molybdenum, niobium and rhenium in the alloy in atomic percent: For melting point index to exceed 2200K, 2400K, 2500K, 2600K and 2800K, W _eq must roughly exceed 15, 25, 25, 35 and 50, respectively, and the following equations should be obeyed (derived through Figure 7a-e). More accurately, for melting point index greater than: 2200K, ^^ _^^ ≥ 15 2400K, ^^ _^^ ≥ 25 2500K, ^^ _^^ ≥ 0.6 2600K, ^^ _^^ ≥ 1.2 ^^ _^^ + 30 2800K, ^^ _^^ ≥ 0.8 ^^ _^^ + 47 A trade-off also exists between melting point and density, shown in Figure 8. As the melting point lower limit is raised, the lowest achievable density is also raised. Similarly, as density upper limit is lowered, the maximum achievable melting point is lowered. Balancing these properties is part of the invention because high density induces higher stresses during rotation, but reducing density corresponds to a reduction in melting point, which will lower the strength and creep strength, reducing the ability of the alloy with withstand the loading. This invention does not include a cost restriction (that is, the elemental selection is made independent of cost). Costs fluctuate with demand and availability of elements. A number of high-value applications will benefit from the high temperature capability of some of the high value elements in the design space, such as the platinum group metals. For these applications, due to a combination of large budget and small quantities, allowable alloy cost is unlimited. In some applications, such as glass fibre production, the alloys encompassed by this invention will compete with other high-PGM content alloys. For these, it will be preferable for cost to be restricted such that it is lower than 1000 USD/mol. As shown in Figure 9a and 9b this leads to a requirement that: A _Ir < 9.0; A _Rh < 6.0 Where A _Ir and A _Rh are the atomic percent of iridium and rhodium in the alloy, respectively. For higher volume industrial applications such as gas turbines and aerospace, it will be preferable for alloy cost to compete with popular refractory alloys such as C103 and FS85. As the alloys encompassed by this invention have superior performance than conventional alloys, an increased alloy cost is justified. In this embodiment it will be preferable for cost to be restricted such that it is lower than 100 USD/mol. As shown in Figure 9c-f this leads to a requirement that: A _Pd < 1.5; A _Pt < 1.5; A _Re < 14.5; A _Ru < 8.5; In this embodiment iridium and rhodium are limited to 0.5 at% or lower due to their very high cost. In an embodiment, the alloys encompassed by this invention can exhibit similar or better performance than common refractory alloys, but at an even lower cost. It is desirable for the present invention to be competitive on a cost basis with existing refractory and nickel-based superalloys. Alloy cost in USD/mol is desirably less than 14, preferably less than 12, more preferably less than 10. It is even more preferable to be even lower cost than popular commercial alloys. In this embodiment, an alloy cost lower than 8 is preferred, corresponding to an cost lower than FS85, C103 and CMSX-4, preferably elemental cost will be lower than 6.5 and more preferably it will be lower than 5. In the composition space where cost is competitive with commercial high temperature alloys, vanadium and tantalum dominate total alloy cost. A _V and A _Ta are the atomic percent of vanadium and tantalum in the alloy, respectively. As shown in Figures 10a-e, for cost index lower than the following values: 14, ^^ _்^ ≤ 20 12, ^^ _^ + 2 ^^ _்^ ≤ 40 This requires A _^ୟ to be less than 20. 10, ^^ _^ + ^^ _்^ ≤ 20 8, ^^ _^ + ^^ _்^ ≤ 15 This requires A _^ୟ to be less than 15.0 at%, but arranging for Ta to be present in 16.0 at% or less does result in a relatively low cost alloy. 6.5, ^^ _^ + 2 ^^ _்^ ≤ 20 This requires A _^ୟ to be less than 10.0. In this lowest-cost embodiment iridium and rhodium are permitted only in quantities of 0.1 at% or lower, palladium and platinum are permitted in quantities of 0.5 at% or lower, and rhenium and ruthenium are permitted in quantities of 1.0 at% or lower. Aluminium has the lowest VEC and density of any element in the design space. Small additions also result in a substantial increase in solid solution strength due to its large atomic radius. (Lin et al., 2015) For these reasons, a lower limit of 1.0 at%, 4.0 at%, or even 7.0 or 10.0 at% is preferable. Whilst increased zirconium content can lead to the formation of zirconium-based intermetallics, it’s addition is also beneficial in lowering Valence Electron Concentration, increasing ductility and reduction in density. For these reasons, an increased zirconium content of 2.0 at% or more or even 4.0 at% or more is preferred. Even higher levels lead to further improvements so that a level of 6 at% or more or even 8.0 or 10.0 at% or more is most preferred if high ductility and low density are desired. Higher levels of zirconium, say 12.0 at% or more or even 13.0 at% or more lead to alloys with superior ductility and lower density. The addition of vanadium is beneficial for raising configuration entropy without detrimentally affecting the other merit indices due to its moderate valence electron concentration (described above), low density and high melting point. In low levels vanadium’s unique combination of high melting point and low density can simultaneously improve high temperature strength capability, and reduce rotational loads, respectively. For these reasons, increasing the vanadium content may be preferred. Thus vanadium at levels of 2.0 at% or more, preferably 5.0 at% or more, or even 8.0 at% or more, is preferred. The high density of tungsten means it can be sensible to reduce its content to 30.0 at% or lower, preferably 25.0 at% or lower, more preferably 20.0 at% or lower, even more preferably 15.0 at% or lower. In an embodiment, to keep density low, tungsten is present in an amount of 10.0 at% or less. However, tungsten has the highest melting point of any element in the invention, meaning increasing the content can result in a large temperature and strength capability increase. Thus preferred lower limits for the amount of tungsten are 2.0 at% or even 5.0 at% or even 8.0 at%. At 11.0 at% or more tungsten high temperature strength is superior, and this is preferred if that property is desired. The addition of hafnium is beneficial in small quantities due to its oxygen scavenging abilities and effect of lowering the ductile-to-brittle transition temperature. Thus, preferred lower limits for the amount of hafnium is 1.0 at% or higher, preferably 3.0 at% or higher, more preferably 5.0 at% or higher. Iron has a moderate impact on properties such as melting point, not greatly improving or deteriorating performance. Increasing iron reduces density, and having small additions of a larger number of elements allows higher alloy entropy to be achieved, stabilising the microstructure. It’s addition, particularly in conjunction with chromium, can also improve printability during laser powder bed fusion additive manufacture. For these reasons, increasing iron levels to 0.5 at% or more or 2 at% or more or even 4.0 at% or more is preferred. Nickel has moderate melting point and density, so it’s additional is beneficial for ensuring entropy of the alloy increased with small additions, without severe detriment to the other properties. For this reason, increasing nickel levels to 0.5 at% or more or 2.0 at% or more or even 4.0 at% or more is preferred. Additions of titanium to an alloy have a substantial impact on reducing density. It is preferable to increase titanium levels to 5.0 at% or more for this reason. Even larger amounts of titanium can be beneficial in certain applications and in some embodiments the alloy has 8.0 at% or more titanium or even 13.0 at% or more or 20.0 at% or more titanium. A superior alloy in terms of density is available if titanium is present in an amount of 30.0at% or more. Titanium is able to form a number of intermetallics, including Fe ₂ Ti, discussed above. Titanium is so beneficial for alloy properties due to low VEC, low density and moderate melting point, that it is preferable to limit other elements (e.g. Fe) in order to control intermetallics, instead. Nevertheless, reducing titanium content will add additional control over the microstructure. For this reason, in some embodiments of the invention, it is preferable to limit titanium to 34.0 at% or less, or more preferably 33.0 at% or less, even more preferably 32.0 at% or less. In some circumstances, where a large quantity of other elements which form intermetallics with titanium are present, even 25.0 at% or less is preferred. Tantalum is costly due to difficulty sourcing the metal from conflict-free regions. This, in addition to having a high density, mean it is preferable to reduce levels in this invention. Tantalum significantly increases high temperature strength due to it’s melting point, so higher levels are preferred, for example 2.0 at% or more, preferably 5.0 at% or more or even 8.0 at% or more or 10.0 at% or more. Levels of 14.0 at% or more or 15.0 at% or more or even 20.0 at% or more are preferred where high strength is desired. Niobium additions raise melting point, improving high temperature strength. Whilst the effect is not as strong as some other elements, such as tungsten, this is an advantage for some alloy processing methods, such as arc melting, where extremely high melting point elements may be difficult to incorporate into the alloy melt. Thus in some embodiments niobium is present in an amount of 5.0 at% or more or 8.0 at% or more and most preferably 10.0 at% or more. Niobium is preferably one of the required elements and so is present in an amount of 12.0 at% or more or even 18.0 at% or more. Niobium is also able to form intermetallics with chromium, as discussed above. Limiting chromium to control intermetallic formation is favoured but reducing niobium may be beneficial in some embodiments. In these cases niobium content is 30.0 at% or less, preferably 26.0 at% or less or 25.0 at% or less, preferably 24.0 at% or less, more preferably 20.0 at% or less. The addition of molybdenum can be beneficial in increasing alloy high temperature strength, by raising melting point. For this reason, increasing molybdenum levels to 2.0 at% or more, for example, can be beneficial. In some embodiments molybdenum is present in an amount of 4.0 at% or more or even 6.0 at% or more. Molybdenum is preferably one of the required elements and so is present in an amount of 8.0 at% or more or 10.0 at% or more and most preferably 12.0 at% or more. In high quantities, molybdenum forms a volatile oxide species, MoO ₃ , when oxidised. In the case of minor coating system failure, through a scratch or chip the volatilisation of MoO ₃ could lead to a minor failure becoming catastrophic, leading the delamination of the coating, and severely reducing the effective strength of a component. For this reason it is preferable for molybdenum content to be 20.0 at% or lower or even 16.0 at% or lower or even 15.0 at% or lower. Even more preferably molybdenum is present in an amount of 14.0 at% or lower, or even 12.0 at% or less, even 7.0 at% or less. Cobalt has a moderate impact on density, cost and melting point. Small additions result in an increase in entropy, stabilising microstructure. For these reasons, increasing cobalt content to 0.5 at% or more, for example, can be beneficial. In some embodiments cobalt is present in an amount of 1.0 at% or more to take advantage of these effects. Even more preferably cobalt is present in an amount of 2.0 at% or more, or even 3.0 at% or more. Most preferably cobalt is present in an amount of 4.0 at% or more. Chromium has a relatively low density and it’s addition, particularly in conjunction with iron, can also improve printability during laser powder bed fusion additive manufacture. For these reasons, increasing chromium levels to 0.5 at% or more or 2.0 at% or more or even 4.0 at% or more is preferred. Small additions of copper result in an increase in entropy, stabilising microstructure. For these reasons, increasing copper content to 0.5 at% or more, for example, can be beneficial. In some embodiments copper is present in an amount of 1.0 at% or more to take advantage of these effects. Even more preferably copper is present in an amount of 2.0 at% or more, or even 3.0 at% or more. Manganese has a relatively low density, and it’s addition can increase entropy, stabilising microstructure. For these reasons, increasing manganese content to 0.5 at% or more or 2.0 at% or more or even 4.0 at% or more is preferred. It also has a moderately high VEC, so more embodiments of this invention in which a very low VEC value is required, or in which high quantities of low VEC elements are not tolerated, manganese quantities should be limited. It is thus preferred for manganese to be 30.0 at% or lower, more preferably 23.0 at% or lower, even more preferably 15.0 at% or lower. Preferably manganese is present in an amount of 1.0 at% or less to keep the VEC low. Silicon can greatly improve strength through small additions, as long as high silicide fractions can be avoided. In this embodiment silicon levels of 1.0 at% or higher are preferred, more preferably 2.0 at% or higher, even more preferably 3.0 at% or higher. Iridium has the highest melting point of the PGMs, similar to niobium, but is highly unreactive, meaning it can reduce the likelihood of attack on the alloy in the scenario where the coating is breached. It is thus preferred for iridium to be 2.0 at% or higher, more preferably 5.0 at% or higher. In an embodiment where cost is unlimited, iridium of 12.0 at% or higher, or even 17.0 at% or higher is preferred. Ruthenium has very high melting point, almost as high as iridium, but significantly lower density and lower cost. It is thus preferred for ruthenium to be 4.0 at% or higher, preferably 11.0 at% or higher, more preferably 18.0 at% or higher, even more preferably 22.0 at% or higher. Rhodium has a unique combination of high melting point, excellent corrosion resistance, but moderately low density. It is thus well-suited as an addition to the alloy, preferably 5.0 at% or higher, more preferably 10.0 at% or higher, even more preferably 15.0 at% or higher. Palladium is an element with the ability to increase entropy through small additions without significantly upsetting other properties. This is due to possessing a moderate melting point and relatively low density. It is therefore preferred for palladium to be 2.0 at% or higher, preferably 5.0 at% or higher, more preferably 8.0 at% or higher. Platinum is a beneficial addition in small quantities due to it’s combination of corrosion resistance and good compatibility with PGM diffusion barriers (e.g. pure platinum). For these reasons, 3.0 at% or higher is preferred, 7.0 at% or higher is more preferably, and 12.0 at% or higher is even more preferable. Rhenium is the element with the second highest melting point of any in the space, meaning it has the ability to increase high temperature strength and creep resistance with only small additions. It therefore performs a similar role to tungsten, but enables higher entropy by substituting for existing tungsten content in an alloy. It is thus preferred for rhenium to be 3.0 at% or higher, more preferably 8.0 at% or higher, even more preferably 15.0 at% or higher. In an embodiment where cost is not a concern, 20.0 or 25.0 at% or higher can even be preferred. While the alloy can include some incidental impurities, these should be limited to 1.0 at% in sum to reduce the chance of forming an excessive amount of intermetallics. In particular, in an embodiment the amount of carbon should be below 1.0 at% which should be easily achievable using normal manufacturing techniques. In this way the alloy can be made substantially free of refractory metal carbide. In an embodiment the alloy is substantially carbon free. Summary of composition limits Table 10 Most Most _{Element (at%) Min Max} ^Preferable Preferable M _in Preferable Preferable Max Min Max Al 0.0 19.45 0.0 5.0 0.0 1.0 Co 0.0 20.0 0.0 7.0 0.0 1.0 Cr 0.0 15.0 0.0 7.0 0.0 1.0 Cu 0.0 15.0 0.0 5.0 0.0 1.0 Fe 0.0 6.5 0.0 1.0 0.0 1.0 Hf 0.0 10.0 0.0 1.0 0.0 1.0 Ir 0.0 20.0 0.0 1.0 0.0 1.0 Mn 0.0 35.0 0.0 1.0 0.0 1.0 M _{o 0.0 35.0 8.0 20.0} ^{10 or} 16.0 or 14.0 ^{Nb 0.0 35.0 10.0 25.0 12.0} 24.0 or O _{r 18.0} 20.0 Ni 0.0 15.0 0.0 1.0 0.0 1.0 Pd 0.0 15.0 0.0 1.5 0.0 1.0 Pt 0.0 15.0 0.0 1.5 0.0 1.0 Re 0.0 35.0 0.0 5.0 0.0 1.0 Rh 0.0 20.0 0.0 1.0 0.0 1.0 Ru 0.0 25.0 0.0 1.0 0.0 1.0 Si 0.0 5.0 0.0 1.0 0.0 1.0 _{Ta 0.0 35.0 5.0 20.0} ^{8.0 or} 1 _{4.0 16.0} Ti 0.0 40.0 20.0 32.0 30.0 32.0 V 0.0 12.0 0.0 1.0 0.0 1.0 W 0.0 35.0 5.0 15.0 8.0 10.0 Zr 0.0 16.7 10.0 15.0 12.0 14.0

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