A COMPOSITION FOR ADDITIVE MANUFACTURING

CROSS REFERENCE

[0001] The present application claims priority to Australian provisional application no. 2021903325, filed 18 October 2021, the entire contents of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The invention relates to a composition and method for additive manufacturing, in particular for metallic additive manufacturing. The invention also relates to a method for producing the composition, and alloys and printed parts produced from the composition. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND

[0003] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0004] Additive manufacturing (AM), which creates metallic parts by successive addition of material, is shaping the future of manufacturing. However, owing to the layer-wise build fashion of AM, metallic alloys produced by AM often develop spatially dependent phases through solid- state phase transformations under the intrinsic thermal cycling of the process. Such inhomogeneous phase distribution can lead to non-uniform mechanical properties in parts produced according to the AM process, presenting substantial risks to applications involving multiaxial stress states.

[0005] Unlike conventional manufacturing processes, such as casting and forging, AM builds the metallic part up layer by layer via melting the feedstock (such as powder or wire) with a high energy source (for example, a laser, electron beam or plasma arc). This unique feature of AM processes is a double-edged sword. On the one hand, it offers the possibility of producing desirable shapes, micro structures and properties which otherwise cannot be achieved by using conventional routes. On the other hand, the steep thermal gradient, high cooling rate in conjunction with the complex thermal history typically encountered during AM make only a few commercial alloys suitable for AM processes. Despite being well fabricated by AM with a very high density, these alloys often do not mechanically perform to the best of their capacities, because they have been designed and optimized for conventional manufacturing routes. In particular, metallic alloys produced by AM often feature highly inhomogeneous microstructures - such as coarse columnar grains and spatially dependent phases along the building direction - which develop during solidification and subsequent solid-state phase transformations. Although promoting grain refinement during solidification can effectively eliminate the columnar grains and thereby reduce the mechanical inhomogeneity, the solidified microstructures are usually replaced or supplemented by the solid-state phase transformation products that inevitably exhibit a spatial distribution due to the spatially variable thermal cycling. In fact, these phases and their morphology as well as distribution may dominate the final mechanical properties. However, the spatial distribution of phases remains a long-standing challenge in achieving uniform mechanical properties by AM.

[0006] Although post- AM heat treatment can be performed to homogenize the microstructure in AM produced parts, this treatment lengthens the production cycle and, more importantly is generally unable to completely eliminate the mechanical inhomogeneity due to the site- specific thermal history in the as-fabricated part.

[0007] Accordingly, there is a need to produce alternative feedstocks for the AM process that are capable of producing alloys having homogenous microstructures

[0008] It is an object of the present invention to overcome or ameliorate one or more the disadvantages of the prior art, or at least to provide a useful alternative.

SUMMARY OF THE INVENTION

[0009] The inventors of the present application have surprisingly discovered that compositions based on titanium - 6% aluminium - 4% vanadium (Ti-6A1~4V, in weight percent), and including titanium particles which are preferably “commercially pure”, and a beta stabiliser, are able to be subjected to an AM process to produce alloys which may have improved mechanical properties when compared with Ti-6A1-4V alloys formed by AM. Adding titanium particles and a beta stabiliser appears to facilitate the decomposition of a’ -martensite during the printing process, thereby potentially completely or substantially eliminating anisotropy and inhomogeneity which would otherwise ordinarily be present in printing of Ti-6A1-4V parts.

[00010] Without wishing to be bound by theory, it is believed that the addition of a combination of a beta- stabilising element, including, but not limiting to, iron, molybdenum, chromium, niobium, vanadium, manganese, cobalt, and nickel, and dilution with additional Ti may result in improvements in mechanical properties (for example, tensile strength and/or ductility) and in property homogeneity with minimal defects that may ordinarily arise in AM. [00011] In particular embodiments, the mechanical properties may be further improved by addition of one or more interstitial solutes, such as oxygen and nitrogen.

[00012] The high-density crack-free Ti alloys that are exemplified herein are a significant advance in the art as it is now possible to achieve high strength and high ductility in Ti alloys, while minimising or eliminating defects. Accordingly, the modified alloys of the invention may enable a more widespread use of AM in industry.

[00013] It is a further advantage of certain embodiments of the invention that relatively inexpensive raw materials may be utilised to achieve these effects, that there may be few if any modifications required to the AM process that is commonly used, that the feedstock production process may be relatively straightforward, and/or that only relatively minor additions of a betastabilising element (i.e., beta stabiliser) may be required.

[00014] As discussed further below, and by way of example, high-density crack-free Ti alloys have been printed which are isotropic, homogenous, and have controllable properties of yield strength ranging from about 830 MPa to 1250 MPa together with elongation from about 10% to 30%, which is comparable with the best performance reported in the literature for both 3D printed and conventionally manufactured Ti-6A1-4V parts, without the need for any pre- or posttreatments. The alloys of the invention may be produced at a slight added cost over the relatively inexpensive Ti-6A1-4V, but may be suitable replacements for current commercial Ti based alloys that are, by comparison, very expensive.

[00015] The alloys of the invention may have many applications, for example in aerospace and defence. The advance in the art as disclosed herein is anticipated to enable more widespread adoption of AM of alloys based on Ti-6A1-4V. It has also been surprisingly found that alloys of the invention may display improved electrical properties.

[00016] In a first aspect of the invention there is provided a composition for additive manufacturing, the composition comprising:

T1-6A1-4V particles;

Ti particles; and a beta stabiliser.

[00017] The following options may be used in conjunction with the first aspect, either individually or in any combination.

[00018] In certain embodiments, the composition further comprises an interstitial solute element. The interstitial solute element may provide high interstitial solid solution strengthening. In certain specific embodiments, the interstitial solute element is selected from the group consisting of oxygen, hydrogen, carbon, nitrogen, boron, and combinations thereof.

[00019] The beta stabiliser (i.e., beta- stabilising element) may be any element except for Al, Zr, Sn, O, N and C. In certain embodiments, the beta stabiliser (i.e., beta- stabilising element) comprises an element selected from the group consisting of: iron, molybdenum, chromium, niobium, vanadium, manganese, cobalt, and nickel. In certain specific embodiments, the beta stabiliser (i.e., beta-stabilising element) comprises, or is iron.

[00020] In certain embodiments, the beta stabiliser (or beta- stabilising element) is an element within a compound, or complex, or is a component of a particle, for example, an oxide, carbide, boride, or nitride particle. For example, if the beta stabiliser is iron, it may be in the form of iron oxide in the composition. In the case where the composition also comprises an interstitial solute element, the interstitial solute element and beta stabiliser (or beta- stabilising element) may be in the same compound, complex, or particle.

[00021] In certain embodiments, the composition comprises oxide, carbide, boride, or nitride particles. In certain specific embodiments, the oxide, carbide, boride, or nitride particles comprise the beta stabiliser. In certain specific embodiments, the oxide, carbide, boride, or nitride particles are oxide particles. In certain specific embodiments, the oxide particles are oxide nanoparticles.

[00022] The oxide, carbide, boride, or nitride particles may comprise, for example, molybdenum oxide, vanadium oxide, niobium oxide, tantalum oxide, manganese oxide, iron oxide, chromium oxide, cobalt oxide, nickel oxide, copper oxide, molybdenum carbide, vanadium carbide, niobium carbide, tantalum carbide, manganese carbide, iron carbide, chromium carbide, cobalt carbide, nickel carbide, copper carbide, molybdenum nitride, vanadium nitride, niobium nitride, tantalum nitride, manganese nitride, iron nitride, chromium nitride, cobalt nitride, nickel nitride, copper nitride, and combinations thereof. In certain specific embodiments, the oxide, carbide, boride, or nitride particles comprise iron (III) oxide.

[00023] The composition may be a solid or in the form of a liquid. In certain embodiments, the composition may be in the form of a suspension. In certain embodiments, the composition may be in the form of a sol. In certain embodiments, the composition is in the form of a powder, e.g., a solid powder.

[00024] In certain embodiments, the composition comprises a substantially uniform dispersion of the Ti-6A1-4V particles; Ti particles; and oxide, carbide, boride, or nitride particles, wherein the Ti-6A1-4V particles and the Ti particles each have at least a partial coating of the oxide, carbide, boride, or nitride particles. In this context, “substantially uniform” means that the Ti- 6A1-4V particles; Ti particles; and oxide, carbide, boride, or nitride particles are substantially homogenously distributed in the composition. It may mean, for example, that any 1 cm ³ sample of a 1000cm ³ composition will have the same concentration (within about ±5%, ±10%, or ±20%) of each of the three components as the bulk (1000cm ³ ) composition. In this context, “a partial coating of the oxide, carbide, boride, or nitride particles”, means that both the Ti and Ti-6A1-4V particles have at least a portion of their outer surface, e.g. at least about 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% of their outer surface area coated with the oxide, carbide, boride, or nitride particles. In certain embodiments, at least about 20% of the outer surface of each Ti and Ti-6A1-4V particle is coated with oxide, carbide, boride, or nitride particles.

[00025] The composition may comprise from about 10 wt.% to about 90 wt.% Ti particles, or from about 15 wt.% to about 85 wt.%, about 20 wt.% to about 80 wt.%, about 25 wt.% to about 75 wt.%, about 10 wt.% to about 80 wt.%, about 10 wt.% to about 75 wt.%, about 20 wt.% to about 90 wt.%, or about 25 wt.% to about 90 wt.% Ti particles. It may, for example, comprise about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90 wt.% Ti particles. The Ti particles may comprise about 90 wt.% or more Ti, or about 95 wt.% or more, about 97 wt.% or more, about 98% or more, about 99 wt.% or more, about 99.5 wt.% or more, or about 99.7 wt.% or more Ti. In certain embodiments, the Ti particles comprise at least about 99% by weight of Ti.

[00026] The Ti particles may be any shape or size. They may be, for example a regular shape, such as a substantially spherical shape, or they may be an irregular shape. The Ti particles may have an average diameter of from about 5 nm to about 5000 pm, or from about 50 nm to about 5000 pm, about 100 nm to about 5000 pm, about 500 nm to about 5000 pm, about 1 pm to about 5000 pm, about 5 pm to about 5000 pm, about 10 pm to about 5000 pm, about 10 pm to about 1000 pm, or about 10 pm to about 100 pm. They may have, for example, an average diameter of about 5, 10, 11, 12, 15, 20, 50, 100, 200, or 500 nm, or about 1, 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 500, 1000, 2000 or 5000 pm. In certain embodiments, the Ti particles have an average diameter of from about 10 pm to about 100 pm.

[00027] The composition may comprise from about 10 wt.% to about 90 wt.% Ti-6A1-4V particles, or from about 15 wt.% to about 85 wt.%, about 20 wt.% to about 80 wt.%, about 25 wt.% to about 75 wt.%, about 10 wt.% to about 80 wt.%, about 10 wt.% to about 75 wt.%, about 20 wt.% to about 90 wt.%, or about 25 wt.% to about 90 wt.% Ti-6A1-4V particles. It may, for example, comprise about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90 wt.% Ti-6A1-4V particles. In certain embodiments, the Ti-6A1-4V particles consist essentially of: from about 5.5 to about 6.75 wt.% Al; from about 3.5 to about 4.5 wt.% V; from about 0 to about 1 wt.% in total of one or more elements which are not V, Al, or Ti; and the balance Ti.

[00028] The Ti-6A1-4V particles may be any shape or size. They may be, for example a regular shape, such as a substantially spherical shape, or they may be an irregular shape. The Ti-6A1-4V particles may have an average diameter of from about 5 nm to about 5000 pm, or from about 50 nm to about 5000 pm, about 100 nm to about 5000 pm, about 500 nm to about 5000 pm, about 1 pm to about 5000 pm, about 5 pm to about 5000 pm, about 10 pm to about 5000 pm, about 10 pm to about 1000 pm, or about 10 pm to about 100 pm. They may have, for example, an average diameter of about 5, 10, 11, 12, 15, 20, 50, 100, 200, or 500 nm, or about 1, 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 500, 1000, 2000 or 5000 pm. In certain embodiments, the Ti-6A1-4V particles have an average diameter of from about 10 pm to about 100 pm. In certain embodiments, the Ti particles and Ti-6A1-4V particles have an average diameter of from about 10 pm to about 100 pm.

[00029] The composition may comprise from about 0.005 wt.% to about 20 wt.% oxide, carbide, boride, or nitride particles, or from about 0.01 wt.% to about 20 wt.%, about 0.02 wt.% to about 10 wt.%, about 0.05 wt.% to about 10 wt.%, about 0.05 wt.% to about 5 wt.%, about 0.1 wt.% to about 5 wt.%, about 0.1 wt.% to about 10 wt.%, about 0.1 wt.% to about 15 wt.%, or about 0.1 wt.% to about 20 wt.% oxide, carbide, boride, or nitride particles. It may, for example, comprise about 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 15, or 20 wt.% oxide, carbide, boride, or nitride particles. The oxide, carbide, boride, or nitride particles may comprise about 90 wt.% or more oxide, carbide, boride, or nitride, or about 95 wt.% or more, about 97 wt.% or more, about 98 wt.% or more, about 99 wt.% or more, about 99.5 wt.% or more, or about 99.7 wt.% or more oxide, carbide, boride, or nitride. For example, in the case where the oxide, carbide, boride, or nitride particles are iron oxide particles, the iron oxide particles may comprise about 90 wt.% or more iron oxide, or about 95 wt.% or more, about 97 wt.% or more, about 98 wt.% or more, about 99 wt.% or more, about 99.5 wt.% or more, or about 99.7 wt.% or more iron oxide.

[00030] The oxide, carbide, boride, or nitride particles may be any shape. They may be, for example a regular shape, such as a substantially spherical shape, or they may be an irregular shape. In certain embodiments, the oxide, carbide, boride, or nitride particles are nanoparticles. The oxide, carbide, boride, or nitride particles may have an average diameter of from about 0.5 nm to about 1000 nm, or from about 1 nm to about 1000 nm, about 1 nm to about 500 nm, about 2 nm to about 500 nm, about 2 nm to about 100 nm, about 2 nm to about 20 nm, or about 2 nm to about 10 nm. They may have, for example, an average diameter of about 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 20, 50, 100, 200, 500, or 100 nm. In certain embodiments, the oxide, carbide, boride, or nitride particles have an average diameter of about 20 nm or less. In certain embodiments, the oxide, carbide, boride, or nitride particles have an average diameter of about 10 nm or less.

[00031] In certain specific embodiments, the composition comprises: from about 20 wt.% to about 80 wt.% Ti-6A1-4V particles; from about 20 wt.% to about 80 wt.% Ti particles; and from about 0.005 wt.% to about 5 wt.% oxide, carbide, boride, or nitride particles, optionally from about 0.05 wt.% to about 5 wt.% oxide, carbide, boride, or nitride particles.

[00032] In certain specific embodiments, the composition comprises: from about 25 wt.% to about 75 wt.% Ti-6A1-4V particles; from about 25 wt.% to about 75 wt.% Ti particles; and from about 0.1 wt.% to about 0.5 wt.% iron (III) oxide nanoparticles, optionally from about 0.25 wt.% to about 0.5 wt.% iron (III) oxide nanoparticles.

[00033] The weight ratio of Ti particles to Ti-6A1-4V particles in the composition may be from about 20:1 to about 1:20, or from about 15:1 to about 1:15, about 10:1 to about 1:10, about 5:1 to about 1:5, or about 2:1 to about 1:2. It may be, for example, about 20:1, 15:1, 10:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:10, 1:15, or 1:20. In certain embodiments, the weight ratio of Ti particles to Ti-6A1-4V particles in the composition is from about 1:5 to about 5:1.

[00034] The weight ratio of oxide, carbide, boride, or nitride particles to Ti-6A1-4V particles in the composition may be from about 1:2000 to about 1:5, or from about 1:1000 to about 1:10, about 1:500 to about 1:10, about 1:500 to about 1:20, or about 1:100 to about 1:20. It may be, for example, about 1:2000, 1:1000, 1:500, 1:100, 1:80, 1:70, 1:60, 1:50, 1:40, 1:30, 1:20, 1:10, or 1:5. In certain embodiments, the weight ratio of oxide, carbide, boride, or nitride particles to Ti- 6A1-4V particles in the composition is from about 1:500 to about 1:20.

[00035] In certain embodiments, the composition may comprise one or more additional components. The one or more additional components may comprise a flowing agent, and anticaking agent, and/or a carrier. The carrier may be a solvent, e.g. an organic solvent. In certain embodiments, the solvent may be water.

[00036] In certain embodiments, the composition is a feedstock for a 3D printer. The composition comprises a mixture of particles of Ti-6A1-4V, Ti and a beta stabiliser.

[00037] In certain embodiments, the additive manufacturing is 3D printing. In certain embodiments, it is a powder bed fusion additive manufacturing process. [00038] In a second aspect of the invention there is provided a method for preparing a composition for additive manufacturing, the method comprising the steps of: forming a mixture of charged particles of Ti 6Al 4V and Ti; and contacting the mixture with oppositely charged particles to form at least a partial coating of the oppositely charged particles on the particles of Ti-6A1-4V and Ti, thereby producing said composition; wherein the oppositely charged particles comprise a beta- stabilising element.

[00039] The following options may be used in conjunction with the second aspect, either individually or in any combination.

[00040] In certain embodiments, the oppositely charged particles are oxide, carbide, boride, or nitride particles.

[00041] The oxide, carbide, boride, or nitride particles; beta stabilising elements (or beta stabilisers); Ti particles; Ti-6A1-4V particles; and/or composition may be as hereinbefore described with respect to the composition according to the first aspect.

[00042] In certain embodiments, the Ti-6A1-4V and Ti particles are negatively charged, and the oxide, carbide, boride, or nitride particles are positively charged.

[00043] In certain embodiments, the Ti-6A1-4V and Ti particles are positively charged, and the oxide, carbide, boride, or nitride particles are negatively charged.

[00044] The charged Ti-6A1-4V particles; Ti particles; and/or oxide, carbide, boride, or nitride particles may be inherently charged. Alternatively, they may be charged, or may have their inherent surface charge modified, by the inclusion of a surface charge modifying agent on an outer surface thereof. The surface charge modifying agent may be a polycationic or polyanionic material. For example, in the case where the Ti-6A1-4V and Ti particles are negatively charged, the surface modifying agent may be a polyanionic material. The polyanionic material may be a polyanionic polymer, for example, PSS, hyaluronic acid, or polyacrylic acid. Alternatively, in the case where the Ti-6A1-4V and Ti particles are positively charged, the surface modifying agent may be a polycationic material. The polycationic material may be a polycationic polymer, for example, PDDA, or poly(2-(trimethylamino)ethyl methacrylate).

[00045] In certain embodiments, the surface charge of the Ti-6A1-4V particles; Ti particles; and/or oxide, carbide, boride, or nitride particles may be modified through successive treatments with surface charge modifying agents having alternate charges. For example, the surface charge of Ti-6A1-4V particles and/or Ti particles if inherently positive, may be first modified to be negative by the addition of a polyanionic surface charge modifying agent, and subsequently modified to be positive by the addition of a polycationic surface charge modifying agent.

[00046] In certain embodiments, the method further comprises measuring the surface charge of the Ti-6A1-4V particles; Ti particles; and oxide, carbide, boride, or nitride particles, and modifying the surface charge of the Ti-6A1-4V particles; Ti particles; and/or oxide, carbide, boride, or nitride particles, such that the Ti-6A1-4V particles and Ti particles have substantially the same surface charge (i.e. both are positively charged, or both are negatively charged), and are oppositely charged to the oxide, carbide, boride, or nitride particles.

[00047] In certain specific embodiments, the method comprises adding a polyanionic surface modifying agent to the Ti-6A1-4V particles so as to provide a negative surface charge thereto. In certain embodiments, the method further comprises a subsequent treatment of the negative surface charged Ti-6A1-4V particles with a polycationic surface modifying agent so as to provide a positive surface charge thereto. In certain embodiments, the method further comprises a further treatment of the positive surface charged Ti-6A1-4V particles with a polyanionic surface modifying agent so as to provide a negative surface charge thereto. In certain specific embodiments, the polycationic surface modifying agent is PDDA, and the polyanionic surface modifying agent is PSS.

[00048] In certain specific embodiments, the method comprises adding a polycationic surface modifying agent to the Ti particles so as to provide a positive surface charge thereto. In certain embodiments, the method further comprises a subsequent treatment of the positive surface charged Ti particles with a polyanionic surface modifying agent so as to provide a negative surface charge thereto. In certain specific embodiments, the polycationic surface modifying agent is PDDA, and the polyanionic surface modifying agent is PSS.

[00049] In certain embodiments, the mixing step is conducted in a liquid, which is subsequently substantially removed to produce the composition in a powder form. The liquid may comprise water and/or one or more organic solvents.

[00050] In certain embodiments, the Ti-6A1-4V particles are mixed with a polyanionic surface modifying agent (e.g. PSS) in a first liquid (e.g. water). The mixture may be washed with water to remove excess surface modifying agent, before adding Ti particles and a polycationic surface modifying agent (e.g. PDDA) in a second liquid (e.g. water). The mixture may be further washed with water to remove excess surface modifying agent, before adding a polyanionic surface modifying agent (e.g. PSS) in a third liquid (e.g. water). The mixture may be further washed with water to remove excess surface modifying agent, before adding oxide, carbide, boride, or nitride particles (e.g. iron oxide particles) in a fourth liquid (e.g. water). The fourth liquid may subsequently be removed to form the inventive composition.

[00051] The concentration of the surface modifying agent in the first, second, and/or third liquid may be from about 0.05 mg/mL to about 200 mg/mL, or from about 0.05 mg/mL to about 100 mg/mL, about 0.05 mg/mL to about 50 mg/mL, about 0.1 mg/mL to about 200 mg/mL, about 0.

5 mg/mL to about 100 mg/mL, about 1 mg/mL to about 100 mg/mL, about 1 mg/mL to about 50 mg/mL, or about 1 mg/mL to about 20 mg/mL. It may be, for example, about 0.05, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 120, 150, or 200 mg/mL.

[00052] The method may further comprise a drying step to substantially remove the liquid from the composition. The drying step may include a heating step. The heating may be performed at a temperature of greater than about 100, 150, 200, 250, 300, 350, or 400 °C for a period of greater than 1, 2, 3, 5, 8, 10, 12, or 24 hours. The moisture content of the composition may be less than about 5 wt. %, 2 wt. %, 1 wt. %, 0.5 wt. %, 0.2 wt. %, or 0.1 wt. %. It may be, for example, about 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, or 5 wt. % moisture.

[00053] The method of the second aspect may produce the composition according to the first aspect. The composition of the first aspect may be produced by the method according to the second aspect.

[00054] In a third aspect of the invention there is provided a method for additive manufacturing using the composition according to the first aspect.

[00055] The following options may be used in conjunction with the third aspect, either individually or in any combination.

[00056] The method may be a 3D printing method. It may be a powder bed fusion additive manufacturing process. It may use a laser, or an electron beam. The composition described herein according to the first aspect may be a feedstock for the additive manufacturing process.

[00057] In certain embodiments, the method does not comprise a post-formation heating step. As used herein, the term “post-formation heating step”, means an additional heating step after an additive manufactured part has been formed. That is any heating step which occurs after the step in which the final layer of the additive manufactured part is fused.

[00058] In a fourth aspect of the invention there is provided use of a composition according to the first aspect for additive manufacturing. [00059] The additive manufacturing may be a 3D printing process, for example, a powder bed fusion additive manufacturing process. The composition as described herein according to the first aspect may be a feedstock for the additive manufacturing process.

[00060] The use of the fourth aspect may incorporate the method according to the third aspect. The method according to the third aspect may be used in the use according to the fourth aspect.

[00061] In a fifth aspect of the invention there is provided a 3D printed part made from the composition according to the first aspect, or prepared according to the method of the third aspect.

[00062] The following options may be used in conjunction with the fifth aspect, either individually or in any combination.

[00063] In certain embodiments, the 3D printed part has substantially isotropic mechanical properties. In other words, if the 3D printed part is aligned on x-y-z cartesian coordinates, where the x-y plane is the build plane of a powder bed fusion additive manufacturing process, and z is the build direction of the process, then the 3D printed part has substantially the same mechanical properties along each of the x-, y-, and z-directions. In certain embodiments, the mechanical properties in each of the x-, y-, and z-directions are within ±20%, ±10%, ±5%, ±2%, or ±1% of each other.

[00064] In certain embodiments, the surface mechanical properties of the part are substantially the same as, or are identical to, the bulk mechanical properties of the part.

[00065] In certain specific embodiments, the mechanical properties are tensile strength and/or ductility.

[00066] In a sixth aspect of the invention there is provided an alloy which consists or consists essentially of: from about 1.1 to about 5.4 wt.% Al; from about 0.7 to about 3.6 wt.% V; from about 0.005 to about 4.8 wt.% Ml, optionally from about 0.01 to about 4.8 wt.% Ml; from about 0.01 to about 4.8 wt.% M2; from about 0 to about 1 wt.% impurities; and the balance Ti; wherein Ml is one or more element selected from the group consisting of H, B, C, N, and O; and M2 is one or more beta-stabilising element.

[00067] The following options may be used in conjunction with the sixth aspect, either individually or in any combination. [00068] The beta- stabilizing element may be the beta stabilizing element, or beta stabiliser as described hereinbefore with respect to the composition according to the first aspect. In certain specific examples the beta- stabilizing element is Fe.

[00069] The alloy may comprise less than about 1 wt.% impurities, or less than about 0.7, 0.5, 0.4, 0.3, 0.2, or 0.1 wt.% impurities.

[00070] In certain embodiments, the alloy consists or consists essentially of: from about 1.4 to about 5 wt.% Al; from about 0.9 to about 3.5 wt.% V; from about 0.005 to about 0.5 wt.% O, optionally from about 0.1 to about 0.5 wt.% O; from about 0.1 to about 0.6 wt.% Fe, optionally from about 0.2 to about 0.6 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti.

[00071] In certain specific embodiments, the alloy consists or consists essentially of: about 1.5 wt.% Al; about 1.1 wt.% V; about 0.3 wt.% O; about 0.3 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti.

[00072] In certain specific embodiments, the alloy consists or consists essentially of: about 3.1 wt.% Al; about 2.1 wt.% V; about 0.3 wt.% O; about 0.3 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti.

[00073] In certain specific embodiments, the alloy consists or consists essentially of: about 3.1 wt.% Al; about 2.1 wt.% V; about 0.4 wt.% O; about 0.5 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti. [00074] In certain specific embodiments, the alloy consists or consists essentially of: about 4.6 wt.% Al; about 3.0 wt.% V; about 0.3 wt.% O; about 0.4 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti.

[00075] In certain specific embodiments, the alloy consists or consists essentially of: about 4.8 wt.% Al; about 3.2 wt.% V; about 0.4 wt.% O; about 0.5 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti.

[00076] The alloy of the sixth aspect may be formed from the composition of the first aspect. The composition of the first aspect may be used to form the alloy of the sixth aspect.

[00077] The alloy of the sixth aspect may be produced according to the method of the third aspect. The method of the third aspect may produce the alloy of the sixth aspect.

[00078] The 3D printed part of the fifth aspect may be comprised of the alloy of the sixth aspect. The alloy of the sixth aspect may be a material of the 3D printed part of the fifth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[00079] Figure 1 shows a comparison of microstructures and tensile properties of Ti-6A1-4V and an example newly developed alloy (25Ti-0.25O) according to the invention which was fabricated using L-PBF: a) Schematic of the L-PBF process and the intrinsic thermal cycles that different locations of the fabricated part undergo. b) SEM-BSE micrographs showing spatially dependent phases in Ti-6A1-4V along the building direction (BD) (also see Fig. 5b). Note that the top surface is predominantly composed of acicular a’ martensite. The lower region shows a partial decomposition of a’ martensite due to more thermal cycles. It can be seen that a’ martensite, a phase and thin |3 film are presented, as marked with white arrows. The bottom region exhibits a well-defined lamellar (a+p) microstructure. Scale bar: 2 pm. c) Tensile engineering stress-strain curves of Ti-6A1-4V along the vertical and horizontal directions. Insert: schematic of preparation of vertical and horizontal tensile specimens from the as-built parts. The horizontal tensile specimens are marked from Hl to H6 along the building direction. d) SEM-BSE micrographs showing homogeneous lamellar (a+ ) microstructure in the example newly developed 25Ti-0.25O alloy (see also Fig. 5d). The well-defined lamellar (a+P) microstructure can be observed from the top surface to the bottom region. Scale bar: 2 pm. e) Tensile engineering stress-strain curves of the 25Ti-0.25O alloy along the vertical and horizontal directions. Insert, the preparation of the tensile specimens was the same as that of Ti-6Al-4V.

[00080] Figure 2 shows an example feedstock preparation and characterization: a) Schematic of feedstock preparation process via the surface engineering approach. b), c) The FC2O3 suspension before (b) and after (c) the addition of processed Ti-6A1-4V and CP-Ti powders as shown in a). d) Comparison of feedstocks prepared by mechanical mixing and the surface engineering approach. e), f) SEM and EDS images showing the homogeneous distributions of both CP-Ti and FC2O3 in the powder feedstock for the designed 50Ti 0.250 alloy. Scale bars, 200 pm (e, ei, ei), 20pm (f, fi, fi).

[00081] Figure 3 shows mechanical properties of the example newly developed alloys fabricated by L-PBF according to the invention: a) Tensile engineering stress- strain curves along the vertical and horizontal directions, indicating uniform tensile properties. b) The elongation to failure of Ti-6A1-4V and the newly developed alloys along the vertical and horizontal directions. The significant data deviation of Ti 6Al 4V from the line indicates a high degree of scattering in ductility. The error bars represent the standard deviation of the mean. c) Comparison of the tensile properties of the newly developed alloys and those of Ti-6A1-4V (and Ti-6A1-4V based composite) fabricated by L-PBF (both in as-built and heat-treated states), EB-PBF and DED.

[00082] Figure 4 shows APT characterization of the example newly developed alloys: a), b) 3D reconstruction of Fe distribution in the samples with different FC2O3 addition levels (a, 50-0.250, b, 50-0.500). Scale bars, 50 nm. c), d) Proximity histograms across the ft phase in a (marked with dashed black cycle) and b (marked with black dashed rectangle) showing enrichment (Fe and V) and depletion (O and Al) of alloying elements in the /> phase. e) The element partitioning ratio K ¹ (which is defined by Cp/C _a , where Cp is the concentration inside the enriched features and C _a outside these regions) of Fe, V, Al and O derived from the APT data. Note that high partitioning ratios suggest a strong accumulation of an element in these regions, while K ¹ < 1 indicates elemental depletion. It can be seen that Fe shows much higher partitioning rates than V. The error bars denote the standard deviation of the mean.

[00083] Figure 5 shows microstructures of Ti-6A1-4V and an example newly developed 25Ti-0.25O alloy at different magnifications: a) SEM-BSE micrographs of different locations in the Ti-6A1-4V part along the building direction (BD). Scale bar: 10 pm. b) Higher magnification of the selected regions in a. The height of the L-PBF produced part was 40 mm. The distance between two characterization locations was about 7 mm. Scale bar: 4 pm. c) SEM-BSE images of different locations in the 25Ti-0.25O alloy part. Scale bar: 10 pm. d) Higher magnification of the selected regions in c. Scale bar: 4 pm.

[00084] Figure 6 shows micro-CT characterization of the grip and gauge regions of Ti-6A1-4V tensile specimens after tensile testing: a), b) Micro-CT characterization of the grip and gauge regions of the Ti-6A1-4V tensile specimens tested along the vertical and horizontal direction. The horizontal specimen is Hl in Figure 1c. The scanning resolution is 4 pm. In the grip region, visible pores are mainly distributed in the edge area. This is attributed to the manufacturer’s default processing parameters (that is, laser power of 100 W and scanning speed of 450 mm s ^-1 ) used to create the borders of Ti-6A1-4V parts. It should be noted that such pores did not affect the final tensile results. Under the refined processing conditions, only a few micropores were detected in other areas, indicating a very high density of the Ti-6A1-4V parts in the as-built state. While in the gauge region, a number of pores were found near the fracture surface, particularly in the central region. Such porosity seemed unlikely to be produced during AM, because they do not distribute in other areas that are far from the fracture surface, but rather were induced in the deformation process. It is well documented that a flat fibrous zone of a fracture surface is typically associated with pore nucleation, growth and coalescence. c) Micro-CT characterization of the grip region of Ti- 6 Al- 4V horizontal tensile specimen at a higher scanning resolution of 2 pm. No porosity was detected under the present resolution limit.

[00085] Figure 7 shows SEM images of the fracture surfaces of Ti-6A1-4V tensile specimens: a) The fracture surface of the vertical tensile specimen. Scale bar: 500 pm. b), c) Higher magnification images taken from the selected regions in a and b, respectively. The vertical specimen exhibited typical ductile fracture features, with a flat fibrous zone in the centre and a peripheral shear lip. Scale bars: 100 pm (b) and 20 pm (c). d) The fracture surface of the horizontal specimen (Hl in Figure 1c), showing a less ductile fracture with a limited reduction of area. Scale bar: 500 pm. e) Higher magnification of the fracture surface (taken from the selected region in d) showing considerable quasi-cleavage facets (marked with white arrows). Similar fracture features have been observed in other study on additively manufactured Ti-6A1-4V by L- PBF and may result from the crack propagation along the long martensite colony. Scale bar: 100 pm. f) Numerous dimples at a higher magnification of the region in the selected region in e. Scale bar: 20 pm.

[00086] Figure 8 shows EBSD characterization of Ti-6A1-4V specimens tested along the vertical and horizontal directions: a) EBSD inverse pole figure (IPF) of a’ and/or a phases with a hexagonal close-packed (HCP) crystal structure. Scale bar: 100 pm. b) The reconstructed prior-p grain structure of the vertical tensile specimen. Scale bar: 100 pm. c) EBSD inverse pole figure (IPF) of a’ and/or a phases with a hexagonal close-packed (HCP) crystal structure. Scale bar: 100 pm. d) The reconstructed prior-P grain structure of the horizontal tensile specimen (Hl in Figure 1c). Scale bar: 100 pm.

[00087] Figure 9 shows micro-CT characterization of the grip and gauge regions of tested tensile specimens of the example newly developed alloys: a), b) The vertical (a) and horizontal (b) tensile specimens of 75Ti-0.25O. c), d) The vertical (c) and horizontal (d) tensile specimens of 50Ti-0.250. e), f) The vertical (e) and horizontal (f) tensile specimens of 25Ti-0.25O.

It shows that there are a few pores at the edges of the grip region, due to a default processing parameter used in the part borders. This is similar to that observed in the Ti-6A1-4V specimens.

[00088] Figure 10 shows SEM images of the fracture surfaces of 25Ti-0.25O tensile specimens: a) The fracture surface of the vertical tensile specimen. Scale bar: 500 pm. b), c) Higher magnification images taken from the selected regions in a and b, respectively. Scale bars, 100 pm (b) and 20 pm (c). d) The fracture surface of the horizontal tensile specimen. Scale bar: 500 pm. e), f) Higher magnification images taken from the selected regions in d and e, respectively. Scale bars: 100 pm (e) and 20 pm (f).

Unlike Ti-6A1-4V, the newly developed 25Ti-0.25O exhibited essentially similar fracture features in both the vertical and horizontal tensile specimens. It can be seen that both specimens showed a significant reduction of area and numerous dimples at a higher magnification, indicating a typical ductile fracture. Additionally, the horizontal specimen did not show any quasi-cleavage facets, as observed in the case of Ti-6A1-4V.

[00089] Figure 11 shows EBSD characterization of 25Ti-0.25O specimens tested along the vertical and horizontal directions: a) EBSD inverse pole figure (IPF) of HCP a phase. Scale bar: 100 pm. b) The reconstructed prior-P grain structure of the vertical tensile specimen. Scale bar: 100 pm. c) EBSD inverse pole figure (IPF) of HCP a phase. Scale bar: 100 pm. d) The reconstructed prior-P grain structure of the horizontal tensile specimen. Scale bar: 100 pm. [00090] Figure 12 shows microstructures of the example 50Ti-0.250 alloy in the as-fabricated part and geometrically complex component: a) SEM-BSE images of different locations in the part with simple geometry. Scale bar: 10 pm. b) Higher magnification of the selected region in a. The distance between two characterization locations is about 7 mm. Scale bar: 2 pm. c) Components with complex geometry produced by L-PBF. Scale bar: 20 pm. d) The model showing the locations of microstructural examination of the component with smaller size in c. e) Lamellar (a+P) microstructures in different locations marked in d. Scale bar: 5 pm.

DEFINITIONS

[00091] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting.

[00092] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[00093] Unless the context clearly requires otherwise, throughout the description and the claims, the terms “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. For example, a composition, mixture, process or method that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, mixture, process or method.

[00094] The transitional phrase “consisting of’ excludes any element, step, or ingredient not specified. If in the claim, such would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase “consisting of’ appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

[00095] The transitional phrase “consisting essentially of’ is used to define a composition, process or method that includes materials, steps, features, components, or elements, in addition to those literally disclosed, provided that these additional materials, steps, features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting essentially of" occupies a middle ground between "comprising" and "consisting of".

[00096] Where applicants have defined an invention or a portion thereof with an open-ended term such as "comprising", it should be readily understood that (unless otherwise stated) the description should be interpreted to also describe such an invention using the terms "consisting essentially of' or "consisting of." In other words, with respect to the terms “comprising”, “consisting of’, and “consisting essentially of’, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of’ or, alternatively, by “consisting essentially of’.

[00097] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”.

[00098] The terms “predominantly”, “predominant”, and “substantially” as used herein shall mean comprising more than 50% by weight, unless otherwise indicated.

[00099] As used herein, with reference to numbers in a range of numerals, the terms "about", "approximately" and "substantially" are understood to refer to the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to + 1 % of the referenced number, most preferably -0.1 % to +0.1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth.

[000100] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention. [000101] As used herein, the term “beta- stabilising element” or “beta stabiliser” means an element which is known to stabilise the beta phase (body centred cubic) form of titanium. Beta stabilising elements or beta stabilisers include, but are not limited to, the following elements: Mo, Cd, Cr, Cu, Si, Nb, Ta, V, Fe, Mn, Co, Ni, and Pd.

[000102] As used herein, the term “interstitial solute” means an element which may provide interstitial solid solution strengthening to commercially pure titanium and titanium alloys. Interstitial solutes may include O, N, H, B and C.

[000103] As used herein, the term “impurity” or “impurities” with respect to an alloy or composition according to the invention, refer to the total weight percentage of any non-specified elements in the composition or alloy. For example, if a composition is specified as consisting essentially of from 10 wt.% to 90 wt.% Ti, from 10 wt.% to 90 wt.% Ti-6A1-4V, from 0.5 wt.% to 1 wt.% iron oxide, and from 0 wt.% to 1 wt.% impurities, then this means that the total of any elements that are not Fe, O, Ti, V, or Al is from 0 wt.% to 1 wt.% in the composition.

ABBREVIATIONS

[000104] AM: additive manufacturing; APT: atom probe tomography; BD: building direction; BSE: backscattered electron; CBS: concentric backscattered; CP-Ti: commercially pure (> 99wt.%) titanium; DED: directed energy deposition; DICTRA: Diffusion-Controlled Transformation; EP-PBF: electron beam-based powder bed fusion; EBSD: electron backscatter diffraction; EDS: energy dispersive X-ray spectroscopy; FIB: focused ion beam; FVM: finite volume method; HCP: hexagonal close-packed; ICP-AES: inductively coupled plasma atomic emission spectroscopy; IHT: intrinsic heat treatment; IPF: inverse pole figure; IVAS: integrated visualization and analysis software; LbL: layer-by-layer; L-PBF: laser powder bed fusion; Micro-CT: microfocus computed tomography; PDDA: poly(diallyldimethylammonium chloride); PSS: poly(sodium 4-styrene sulfonate); SEM: scanning emission microscopy; Ti-6A1-4V: titanium alloy comprising 5.5-6.75 wt.% aluminium, 3.5-4.5 wt.% vanadium, 0-1 wt.% impurities, and the balance titanium; TiGen: Titanium Genome.

[000105] Preferred features, embodiments and variations of the invention may be discerned from the following Examples which provides sufficient information for those skilled in the art to perform the invention. The following Examples are not to be regarded as limiting the scope of the preceding Summary of the Invention in any way.

EXAMPLES

Ti-6A1~4V [000106] Ti-6A1-4V - the workhorse in the titanium industry - was selected as the representative material. During the AM process such as L-PBF (Fig. la), after the first layer was fused, Ti 6Al 4V underwent solid-state — > a’ transformation due to the high cooling rate in the AM process. As the successive layers were added, the acicular a’ martensite that was initially formed decomposed to the lamellar (a + 0) microstructure under extensive thermal cycles (Fig. la) that act as an intrinsic heat treatment (IHT). As a consequence, the microstructures of Ti-6A1-4V fabricated by L-PBF are known to feature spatially dependent phases along the building direction, with acicular a’ martensite on the top surface whereas partially or fully stabilized lamellar (a + 0) microstructure forming in the lower regions.

[000107] Such a graded phase distribution was also confirmed in this work (Fig. lb, Figs. 5a and 5b). Compared with the lamellar (a + 0) microstructure, acicular a’ martensite generally resulted in inferior ductility because of its inability to resist crack initiation. To reveal the influence of phase inhomogeneity on the mechanical property, tensile testing of the L-PBF fabricated Ti-6A1-4V specimens were performed along both vertical and horizontal directions at room temperature (see Methods). The as-fabricated Ti-6A1-4V exhibited similar strength but highly scattered ductility along both directions (Fig. 1c). In particular, the tensile engineering strain corresponding to the horizontal direction was spatially dependent, ranging from 9.4% to 17.6%. It is known that porosity and columnar grains are the most common fingerprints for mechanical inhomogeneity. Note that in this work we have refined the AM processing parameters, with the aim of minimising porosity and breaking up the columnar prior-0 grains. Microfocus computed tomography (Micro-CT), scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) characterizations suggest that neither porosity nor columnar grains are likely to be responsible for the highly scattered ductility observed here (Figs. 6, 7 and 8). Note that the gauge regions of the vertical tensile specimens only covered the same layer range in the middle of the parts, whereas those of the horizontal tensile specimens spanned from the bottom region to the top surface (Fig. 1c). This, coupled with microstructural observation (Fig. lb), revealed that the spatially dependent phase distribution was the most plausible major cause of such mechanical inhomogeneity.

Inventive composition

[000108] Here, based on Ti-6A1-4V, the inventors of the present application have engineered phase stability by promoting in-situ martensite decomposition during AM by modifying the alloy chemical composition. The concentration of V - which exhibits slow partitioning ratio in the 0 phase - was decreased through a large addition of commercially pure titanium (CP-Ti), and small amounts of Fe - that shows stronger partitioning tendency in the P phase - were introduced in the form of iron oxide (FC2O3) nanoparticles.

[000109] This modification resulted in significant Fe redistribution during fabrication and effectively encouraged a’ martensite decomposition. In sharp contrast to Ti-6A1-4V (Figs, lb and 1c), the newly developed alloy - for example, the one with 25 wt% CP-Ti and 0.25 wt% Fe2Os additions (hereafter referred to as 25Ti-0.25O, where Ti denotes CP-Ti and O represents Fe2O3. The other alloys are denoted in the same way) at a similar strength level to Ti-6A1-4V - exhibited homogeneous lamellar (a+P) microstructures across the fabricated part that resulted in uniform tensile properties in both the vertical and horizontal directions (Figs. Id and le).

Feedstock preparation

[000110] A powder feedstock was prepared for L-PBF. Conventional mechanical mixing has been used in standard feedstock preparation, but often suffers from blending inhomogeneity due to the agglomeration of additive particles. In this work, a surface engineering approach was used to synthesize the Fe2O3-doped titanium feedstock (Methods and Fig. 2a). This approach was based on a layer-by-layer (LbL) assembly technology. However, here a functional multilayer deposition was not used, but instead an alternating adsorption process was used to induce stable charged surfaces of both Ti-6A1-4V and CP-Ti powders, which facilitated the adhesion of Fe2O3 particles. Zeta potential measurement were performed to determine the surface charges of titanium powers and FC2O3 particles for the adsorption sequence (Methods and Fig. 2a). In contrast to mechanically mixed powders which exhibited dramatic agglomeration of FC2O3 particles even at the macroscale (Fig. 2d), the feedstock prepared by the surface engineering approach showed substantially uniform distribution of FC2O3 particles on the surface of individual titanium powder, as evidenced by SEM together with energy-dispersive X-ray spectroscopy (EDS) mapping (Figs. 2e and 2f).

Mechanical properties

[000111] A series of titanium alloys were produced by tuning CP-Ti and/or Fe2O3 addition levels and tensile testing of the newly developed alloys was performed under identical conditions as Ti-6A1-4V feedstock (see Methods). In contrast to Ti-6A1-4V (Fig. le), a striking mechanical response of the newly developed alloys was a uniform ductility in both vertical and horizontal directions (Figs. 3a and 3b). Further, a change of strength-ductility combinations did not accompany a loss of a uniform mechanical response. Moreover, by simply tuning the CP-Ti and/or Fe2O3 addition levels, the inventors have surprising discovered that it is possible to tailor mechanical properties of the inventive alloys over a broad range, with a yield strength from 831.4 ± 2.7 MPa to 1,220.8 ± 6.5 MPa and an elongation to failure from 26.7 ± 0.6% to 13.7 + 0.9% (Fig. 3c). For example, a very high strength can be achieved-which is comparable to, or even higher than, those reported for Ti-6A1-4V fabricated by L-PBF and L-PBF plus heat treatment (L-PBF + HT) - yet still with higher ductility (for example, 25Ti-0.500 and 25Ti-0.25O alloys in Fig. 3c). Besides, by either decreasing FeiCh or increasing CP-Ti addition levels (that is, 50Ti-0.500, 50Ti-0.250 and 75Ti-0.25O alloys), the inventors have surprising discovered that the resulting alloys have ductility higher than 20%, which is double the minimum requirement of Ti-6A1-4V recommended for critical structural applications (that is, 10%). Overall, the tensile properties of the inventive alloys not only profoundly outperformed those of the conventionally manufactured Ti-6A1-4V (for example, mill-annealed, and solution-treated and aged), but also significantly extended the current strength-ductility limit reported for Ti-6A1-4V and Ti-6A1-4V based composite by L-PBF (including with additional heat treatment), and directed energy deposition (DED) and electron beam-based powder bed fusion (EB-PBF) (Fig. 3c).

Microstructural and elemental characterizations

[000112] To better understand the exceptional uniform mechanical properties of the alloys produced, detailed microstructural and elemental characterizations were performed (see Methods). The Micro-CT (Fig. 9), SEM (Fig. 10) and EBSD (Fig. 11) characterizations ruled out the possibilities of porosity and columnar grain as potential sources of mechanical inhomogeneity in the newly developed alloys. Moreover, unlike Ti-6A1-4V, the newly developed alloys showed uniform lamellar (a+P) microstructures from the bottom region to the top surface (Figs. 5c and 5d, Figs. 12a and 12b). Thus, in the new alloys the most plausible major origin of mechanical inhomogeneity as observed in Ti-6A1-4V had been eliminated.

[000113] Compositional analysis using atom probe tomography (APT) was also performed on the new alloys (see Methods). These experiments showed that increasing FezCL doping level in the feedstock led to the transition of discontinuous film to a continuous and relatively thick P phase (Figs. 4a and 4b). The element partitioning ratio K ¹ , which was derived from APT proximity histograms (Figs. 4c and 4d), showed that Fe had much higher partitioning ratios than V (Fig. 4e), indicating much stronger partitioning tendency of Fe in the P phase. In addition, the concentrations of Fe in the P phase were in the range of 6.5-8 6 at.% These values were close to those reported for post- AM heat-treated Ti-6A1-4V, indicating that Fe may reach its equilibrium state in the P phase. The elemental analysis further supported that the faster partitioning ratio of Fe allowed for the acceleration of martensite decomposition during fabrication.

[000114] Promoting in-situ a’ martensite decomposition through processing control offers a pathway to high-performance Ti-6A1~4V alloys. To further demonstrate the applicability of this approach, geometrically complex components having different sizes (Figs. 12c and 12d) were fabricated from the inventive feedstock compositions. Microstructural examination verified that homogeneous lamellar (a + p) microstructures rather than acicular a’ martensite formed even in the downscaled part (Fig. 12e). This confirms that the inventive approach enables the fabrication of geometrically complex components with uniform (a+P) microstructures.

Methods

Feedstock preparation

[000115] Ti-6A1-4V ELI (Grade 23, SLM Solutions Group AG, Germany) and CP-Ti (Grade 1, Advanced Powders and Coatings, Canada) powders used had a spherical shape with a particle size range of 20-63 pm. The iron (III) oxide (FeiCh) (Sigma- Aldrich, Germany) particles had a particle size of less than 5 pm. The unit price of Fe^CF particles was much lower than the prices of the Ti-6A1-4V and CP-Ti powders.

[000116] To prepare the feedstock for L-PBF, surface charges of Ti-6A1-4V, CP-Ti and FeiC were first measured in deionized water by using a Malvern Zetasizer (ZS90, Malvern Instruments, UK). It was found that Ti-6A1-4V powders yielded a high positive zeta potential of 53.50 ± 0.54 mV while the measured zeta potential of FciCh particles was relatively low, with a value of 9.12 ± 0.18 mV. Conversely, CP-Ti powders were negatively charged with a zeta potential of -17.97 ± 1.94 mV. The adsorption sequence was based on the zeta potential measurement. Because Ti-6A1-4V and CP-Ti powders were oppositely charged (Fig. 2a), the positively charged Ti-6A1-4V powders were first mixed in the solution containing 8 mg/mL poly(sodium 4-styrene sulfonate) (PSS, average molecular weight: M _w = ~ 70,000, powder, Aldrich) which acted to negatively charge the surface. A monolayer of the polyanion was adsorbed and thus the surface charges of Ti-6A1-4V powders were reversed. After being rinsed in deionized water (the aim of rinsing was to remove the loosely adsorbed PSS from the powders), Ti-6A1-4V powders together with CP-Ti powders (both showed a negative charge) were immersed in a positively charged 8 mg/mL poly(diallyldimethylammonium chloride) (PDDA, average molecular weight: M _w = 200,000 - 350,000, 20 wt% in H2O, Aldrich) solution. Again, the adsorption of PDDA monolayer led to the reversal of surface charge.

[000117] This process was repeated so that both Ti-6A1-4V and CP-Ti powders were negatively charged due to the adsorption of PSS monolayer (Fig. 2a). Finally, the rinsed Ti-6A1-4V and CP-Ti powders were mixed in a red-coloured FC2O3 suspension. After stirring for 30 minutes, sedimentation took place - the powers that showed the red colour settled down at the bottom of the beaker and left a clear layer of liquid above (Fig. 2b) - indicating successful dispersion of FciCh particles in the titanium powders. After removal of the liquid, the powders were dried in the oven for 10 hours. Following the feedstock preparation, the powder mixtures were characterized by scanning emission microscopy (SEM, JEOL JSM-6610, Japan) equipped with energy dispersive X-ray spectroscopy (EDS).

[000118] For purpose of comparison, powder mixtures were also prepared by mechanical mixing. Ti-6A1-4V and CP-Ti powders with 0.5 wt.% FeiCE particles were mixed using a Tubular shaker mixer (Willy A. Bachofen AG, Switzerland) for 60 minutes.

Additive manufacturing

[000119] Laser powder bed fusion was performed on an SLM®125HL machine (SLM Solutions Group AG, Germany) equipped with a 1060 nm wavelength IPG fibre laser (max laser power of 400 W and laser spot size of 70 pm - 100 pm). Prior to L-PBF, the titanium substrate plate was preheated up to 200°C under a high-purity Ar atmosphere. L-PBF was carried out when the oxygen level was reduced to below 0.02 vol.%. Parameter optimization was performed by using Ti 6Al 4V powders, with the aim of achieving a very high density and interrupting the columnar grains. The “meander” scanning strategy with initial 45° scanning angle and 67° rotation between each layer was adopted. The building time of each layer (exposure time and coating time) was kept constant at 15 seconds. The refined processing parameters were 350W laser power, 1400 mm s ^-1 scanning speed, 30 pm layer thickness and 120 pm hatch spacing.

[000120] For both Ti-6A1-4V and the newly developed alloys, titanium parts with dimension of 40 mm (length) x 10 mm (width) x 40 mm (height) were built with a 2 mm supporting structure on a 50 mm x 50 mm titanium substrate plate. For each composition, two titanium parts were simultaneously built on the substrate plate so that the thermal histories of these parts encountered during AM were essentially the same. The chemical compositions of the newly developed alloys were measured by using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and are listed in Table 1.

Table 1 Measured chemical compositions (in wt.%) of as-received Ti-6A1-4V powder, CP-Ti powder, and the example newly designed alloy parts.

Material Al V Fe O N C H Ti

Ti- 6A1- 4V 6.48 4.06 0.21 0.08 0.01 0.02 0.003 Bal.

CP-Ti / / 0.04 0.18 0.01 0.02 0.002 Bal.

75Ti-0.25O 1.54 1.07 0.30 0.32 0.056 0.02 0.005 Bal.

50Ti-0.250 3.08 2.05 0.33 0.30 0.040 0.01 0.006 Bal.

50Ti— 0.500 3.08 2.06 0.48 0.42 0.063 0.02 0.006 Bal.

25Ti- 0.250 4.62 3.03 0.38 0.28 0.060 0.02 0.006 Bal. 25Ti-0.500 4.80 3.20 0.50 0.40 0.062 0.02 0.008 Bal.

Mechanical testing

[000121] For tensile testing, dog bone-shaped tensile specimens with gauge dimension of 10 mm (length) x 2.5 mm (width) x 2 mm (thickness) were machined from the as-built parts along both the vertical and horizontal directions by electrical discharge machining (Fig. 1c). The specimens were carefully marked to keep track of their location on the as-built parts. Prior to tensile testing, the tensile specimens were mechanically polished down to 4000 grit size to eliminate the postmachining surface roughness. Room-temperature tensile tests were carried out on an electromechanical universal testing machine (Model 5584, Instron Inc., USA) equipped with a 10 kN load cell at a constant strain rate of 0.001 s ^-1 . The strain evolution of tensile specimens was tracked using an Instron AVE2 non-contacting video extensometer (Instron Inc., USA) with a data rate of 490 Hz and a resolution of 0.5 pm in the axial dimension. Six specimens were tested for each group. Following tensile testing, the fracture surface was analysed using a JEOL ISM-6610 SEM.

Microstructure characterization

Microfocus computed tomography

[000122] Microfocus computed tomography (Micro-CT) was performed on the tensile specimens after tensile testing using a Micro-CT system (diondo d2, Germany), with a spatial resolution of 4 pm. Micro-CT characterization was carried out on the grip and gauge regions of the specimens, which allowed for observation of the porosity distribution in both the as built and post-testing states. Selected region of the horizontal specimen was also characterized at a spatial resolution of 2 pm.

Scanning electron microscopy

[000123] For phase analysis, samples were cut from different locations along the building direction of the as-fabricated parts and were mechanically polished using Struers OP-S suspension containing 20 vol.% of H2O2 for 30 minutes without any chemical etching. The micro structure was characterized in backscattered electron (BSE) mode using a FEI Scios Dual Beam system (Thermo Fisher Scientific Inc., USA) equipped with a concentric backscattered (CBS) detector under 3 kV accelerating voltage, 1.6 nA probe current and 5.5 mm working distance.

Electron backscatter diffraction [000124] Samples for EBSD characterization were ground and mechanically polished using Struers OP-S suspension containing 20 vol.% H2O2 for 30 minutes. Final electropolishing was carried out at room temperature using a voltage of 20 V for 240 seconds in Struers electrolyte A3. EBSD characterization was performed on a SEM (JEOL JSM-7800F, Japan) with a step size of 0.3 pm. The EBSD data was analysed using OIM Analysis 7.3 software. The prior-P grain structure was reconstructed using the ARPGE software package.

Atom probe tomography

[000125] The elemental distributions were analysed by atom probe tomography (APT) using a local electrode atom probe CAMECA LEAP 4000X SI. Samples for APT characterization were prepared using a Zeiss Auriga dual-beam focused ion beam (FIB, Carl Zeiss Microscopy, Germany), with standard FIB lift-out procedures. The data was acquired under a high vacuum of 2xl0 ^-11 torr, at a specimen temperature of 40 K, a pulse repetition rate of 200 kHz, and UV laser energy of 40 pJ. The APT data was reconstructed using CAMECA integrated visualization and analysis software (IV AS 3.8.2) (CAMECA Scientific Instruments, USA), with tip profile method in reference to the SEM image of each tip.

Conclusion

[000126] The inventors of the present application have designed and additively manufactured a series of titanium alloys that possess exceptional tensile properties without notable mechanical inhomogeneity. The typical phase inhomogeneity associated with thermal cycling inherent to AM can be eliminated by tailoring the phase stability through rational alloy design, assisted by computational thermodynamics and kinetics.

[000127] Without being bound by theory, the inventors postulate that the self-stabilization alloy design strategy used herein may enable the elimination of phase inhomogeneity by transforming the metastable, undesired phase - which is commonly observed in the top surface of laser powder bed fusion (L-PBF) produced Ti-6A1-4V parts - into stabilized, mechanically favourable phases like those in the lower regions. The inventors postulate that these improved alloy properties may be because of the decrease of V content and introduction of small amounts of iron (Fe) with much higher tracer diffusivity, which may allow for stronger elemental partitioning during AM, thereby promoting in-situ phase decomposition. This may result in the combination of two exceptional mechanical responses: uniform mechanical properties due to homogeneous micro structures, and enhanced strength-ductility balance resulting from desirable constituent phases. The exceptional, uniform mechanical properties make these newly developed alloys stand out among Ti-6A1-4V and Ti-6A1-4V based composites made by AM. [000128] In the examples set forth herein, FC2O3 particles were used as a trace additive for the inventive alloys, because it is inexpensive, and its red colour can serve as an indicator for the feedstock preparation process (as described above). However, a person of skill in the art will understand that any suitable compound comprising a beta-stabiliser or beta-stabilising element, for example any oxide, carbide, boride, or nitride of a beta-stabilising element, could be used as a replacement for the iron oxide used in the examples set forth.

[000129] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. In particular, features of any one of the various described examples may be provided in any combination in any of the other described examples. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.

[000130] Disclosed herein are the following forms:

1. A composition for additive manufacturing, the composition comprising:

Ti-6A1-4V particles;

Ti particles; and a beta stabiliser.

2. The composition of form 1, further comprising an interstitial solute element.

3. The composition of form 2, wherein the interstitial solute element is selected from the group consisting of oxygen, hydrogen, carbon, nitrogen and boron.

4. The composition of any one of forms 1 to 3, wherein the beta- stabiliser comprises an element selected from the group consisting of: iron, molybdenum, chromium, niobium, vanadium, manganese, cobalt, and nickel.

5. The composition of any one of forms 1 to 4, wherein the beta- stabiliser comprises, or is iron.

6. The composition of any one of forms 1 to 5, comprising oxide, carbide, boride, or nitride particles.

7. The composition of form 6, wherein the oxide, carbide, boride, or nitride particles comprise the beta stabiliser.

8. The composition of form 6 or 7, wherein the oxide, carbide, boride, or nitride particles comprise iron (III) oxide. 9. The composition of any one of forms 6 to 8, wherein the oxide, carbide, boride, or nitride particles have an average diameter of about 20 nm or less.

10. The composition of any one of forms 6 to 9, where the weight ratio of oxide, carbide, boride, or nitride particles to Ti-6A1-4V particles is from about 1:500 to about 1:20.

11. The composition of any one of forms 6 to 10, which comprises a substantially uniform dispersion of the Ti-6A1-4V particles; Ti particles; and oxide, carbide, boride, or nitride particles; wherein the Ti-6A1-4V particles and the Ti particles each have at least a partial coating of the oxide, carbide, boride, or nitride particles.

12. The composition of any one of forms 7 to 11, which comprises: from about 20 wt.% to about 80 wt.% Ti particles; from about 0.05 wt.% to about 5 wt.% oxide, carbide, boride, or nitride particles; and from about 20 wt.% to about 80 wt.% Ti-6A1-4V particles.

13. The composition of any one of forms 1 to 12, which is in the form of a powder.

14. The composition of any one of forms 1 to 13, wherein the Ti particles and Ti-6A1-4V particles have an average diameter of from about 10 pm to about 100 pm.

15. The composition of any one of forms 1 to 14, wherein the weight ratio of Ti particles to Ti-6A1-4V particles is from about 1:5 to about 5:1.

16. The composition of any one of forms 1 to 15, which comprises: from about 25 wt.% to about 75 wt.% Ti particles; from about 0.25 wt.% to about 0.5 wt.% iron (III) oxide particles; and from about 25 wt.% to about 75 wt.% Ti-6A1-4V particles.

17. The composition of any one of forms 1 to 16, wherein the Ti particles comprise at least about 99% by weight of Ti.

18. The composition of any one of forms 1 to 17, wherein the Ti-6A1-4V particles consist essentially of: from about 5.5 to about 6.75 wt.% Al; from about 3.5 to about 4.5 wt.% V; from about 0 to about 1 wt.% in total of one or more elements which are not V, Al, or Ti; and the balance Ti.

19. The composition of any one of forms 1 to 18, which is a feedstock for a 3D printer.

20. A method for preparing a composition for additive manufacturing, the method comprising the steps of: forming a mixture of charged particles of Ti-6A1-4V and Ti; and contacting the mixture with oppositely charged particles to form at least a partial coating of the oppositely charged particles on the particles of Ti-6A1-4V and Ti, thereby producing said composition; wherein the oppositely charged particles comprise a beta- stabilising element.

21. The method of form 20, wherein the oppositely charge particles are oxide, carbide, boride, or nitride particles.

22. The method of form 21, wherein the Ti-6A1-4V and Ti particles are negatively charged, and the oxide, carbide, boride, or nitride particles are positively charged.

23. The method of any one of forms 20 to 22, wherein the mixing step is conducted in a liquid, which is subsequently substantially removed to produce the composition in a powder form.

24. A method for additive manufacturing using the composition of any one of forms 1 to 19.

25. The method of form 24, which does not comprise a post-formation heating step.

26. Use of a composition of any one of forms 1 to 19 for additive manufacturing.

27. A 3D printed part made from the composition of any one of forms 1 to 19, or prepared according to the method of any one of forms 20 to 23.

28. The 3D printed part of form 27, which has substantially isotropic mechanical properties.

29. The 3D printed part of form 27 or 28, wherein the mechanical properties are tensile strength and/or ductility.

30. The 3D printed part of any one of forms 27 to 29, wherein the surface mechanical properties of the part are substantially the same as, or are identical to, the bulk mechanical properties of the part.

31. An alloy which consists or consists essentially of: from about 1.1 to about 5.4 wt.% Al; from about 0.7 to about 3.6 wt.% V; from about 0.005 to about 4.8 wt.% Ml; from about 0.01 to about 4.8 wt.% M2; from about 0 to about 1 % impurities; and the balance Ti; wherein Ml is one or more element selected from the group consisting of H, B, C, N, and O; and M2 is one or more beta-stabilising element.

32. The alloy of form 31, which consists or consists essentially of: from about 1.4 to about 5 wt.% Al; from about 0.9 to about 3.5 wt.% V; from about 0.005 to about 0.5 wt.% O; from about 0.1 to about 0.6 wt.% Fe; from about 0 to about 1 wt.% impurities; and the balance Ti.