GOLD AND TITANIUM BASED ALLOY FIELD OF THE INVENTION The invention relates to an alloy based on gold and titanium. This alloy, which also comprises a specific amount of grain refiner, can be used in horology or jewelry. BACKGROUND OF THE INVENTION Due to their properties, such as hardness or tensile strength, gold-titanium based alloys are commonly used in fields such as dentistry, ceramics, horology or jewelry. For instance, WO 2008/018109 discloses an alloy comprising 25-75 wt% of gold and titanium, as well as 0-40 wt% of nickel. Ti ₃ Au (in mole fraction) exhibits hardness values above most steels and approximately four times those of titanium. WO 2018/162745 discloses a material having a thin layer of Ti _1-x Au _x , in which the atomic fraction x is from 0.22 to 0.28, for instance 0.25 as in Ti ₃ Au. However, specific applications, for instance in horology or jewelry, require alloys having a greater gold fraction than in Ti ₃ Au (molar fraction). For instance, WO 2016/107755 discloses an alloy comprising at least 750 wt% (18 carats) of gold. This alloy is made of: - 41 to 49.5 at% Au - 45 to 55 at% Ti - 2.5 to 13 at% Nb, V, Pd, Pt, Fe - 0.1 to 2.5 at% at least one of Nb, V, Pd, Pt, Fe, Mo, Ta, W, Co, Ni, Ru, Rh, Ir, Cr, Mn, Cu, Zn, Ag, Al, B, Si, Ge, Sn, Sb, In. In addition to gold and titanium, this alloy comprises at least two alloying elements, as in Au _0.427 Ti _0.50 Fe _0.072 B _0.001 (in mole fraction) i.e., by weight, Au ₇₅₀ Ti ₂₁₃ Fe ₃₆ B ₁ , in which the total amount of iron and boron represents 37000 ppm by weight vs gold and titanium. Binary Ti1-xAux alloys have been reported (Svanidze, High hardness in the biocompatible intermetallic compound β-Ti ₃ Au, Sci. Adv., 2016, pages 1-6). These binary alloys are high hardness and biocompatible alloys. They are free of any grain refiner. When x (molar fraction) is between 0.15 and 0.4, the Ti _1-x Au _x alloys are optimized since they have low mass density and high hardness. Ott et al. have discussed the control of the grain size in gold or conventional alloys (AuAgCu) during the solidification of the molten alloy or during a recrystallization step (Grain Size of Gold and Gold Alloys, Gold. Bull., 1981, 14 (2), pages 69-74). Grain refiners, including titanium and cobalt, may be used. However, Ott et al. clearly specify that grain refinement depends on the concentration of the grain refiners and on the composition of the alloy. Furthermore, grain refiners used in dental gold casting alloys are not always suitable for gold alloys used in jewelry fabrication. Ott et al. do not discuss the shape memory effect of gold alloys. US 5,853,661 discloses biocompatible gold alloys comprising 91 to 99.4 wt% gold and 0.5 to 3 wt% (5000 ppm to 30000 ppm) titanium and/or tantalum, and tungsten as grain refiner. WO 2010/027329 discloses colored gold alloys comprising more than 10 wt% titanium. Examples include alloys comprising at least 0.5 wt% (5000 ppm) iron or cobalt. Using 50 to 1000 ppm of iridium, ruthenium, rhenium, molybdenum, tungsten or cobalt as grain refiner in conventional gold alloys (AuAgCu) has been disclosed (Fischer- Bühner, Metallurgy of Gold, pages 123-159). Fischer-Bühner et al. also teach that a small amount of titanium (no more than 1wt%) may be introduced in order to improve the hardness of 22-24 carat gold microalloys. They do not discuss the shape memory effect of gold alloys. Even though some of the above alloys may be used in jewelry or horology, there is still an interest in improving the properties of light colored gold alloys. Applicant has discovered that a small amount of grain refiner in gold-titanium alloys allows the formation of small grains, improves the hardness, and prevents the formation of cracks. It also affords an alloy having a small shape memory effect. SUMMARY OF THE INVENTION The invention relates to an alloy (two or three main elements) having improved properties due to the presence of a small amount of a specific grain refiner. This alloy is easy to shape by mechanical deformation since it exhibits a small shape memory effect, smaller than that of conventional gold-titanium alloys. This is quite surprising since, even though prior arts binary or ternary gold-titanium based alloys have satisfactory mechanical properties, they also exhibit a high shape memory effect, which results in alloys that are difficult to shape by mechanical deformation (stamping, cold rolling…). The invention also overcomes this technical problem due to specific Au/Ti weight ratio and to the presence of a grain refiner in a narrow weight range. More specifically, the invention relates to an alloy of formula Au _x Ti _y R _z M _j , wherein: x = 375 to 917 parts by weight, y = 83 to 625 parts by weight, x + y + j = 1000 parts by weight, z = 30 to 200 ppm by weight of x + y + j, 0 ≤ j < y, with 0 ≤ j < 125 when x = 750 to 917, wherein R is a grain refiner selected from the group consisting of iridium, bore, vanadium, iron, cobalt, barium, yttrium, zirconium, and mixtures thereof, and wherein M is selected from the group consisting of silver, aluminum and mixtures thereof. When j = 0, the alloy is of formula Au _x Ti _y R _z , wherein: x = 375 to 917 parts by weight, y = 83 to 625 parts by weight, x + y = 1000 parts by weight, z = 30 to 200 ppm by weight of x + y, and wherein R is a grain refiner selected from the group consisting of iridium, bore, vanadium, iron, cobalt, barium, yttrium, zirconium, and mixtures thereof. This alloy is therefore free of palladium. This alloys consists in gold, titanium, optionally silver and/or aluminum, and one or more grain refiner selected from a specific list of elements. The amount of grain refiner is 30 to 200 ppm by weight with respect to the total amount of gold, titanium and M element (Ag and/or Al). When z is less than 30 ppm by weight of x + y + j, the amount of refiner is too small to afford any refining effect to the alloy. On the other hand, when z is more than 200 ppm by weight of x + y + j, aggregates form within the alloy. Aggregates can weaken the alloy, lead to the formation of cracks and even aesthetic defects. The alloying elements silver and aluminum allow lowering the temperature of transformation of the alloy. They facilitate manufacturing processes of the alloy, for instance shaping, forging, casting. Their presence does not significantly change the density of the alloy, which remains low as compared to gold based alloys containing palladium. All ranges include the end-points. For instance, the “375 to 917” and “between 375 and 917” ranges include the 375 and 917 values. According to a preferred embodiment, the alloy comprises a single grain refiner, preferably iridium (R = iridium). The alloy can therefore consists of three elements: gold, titanium and one grain refiner, for instance gold, titanium and iridium. In the alloy of formula Au _x Ti _y R _z M _j , the amount z of grain refiner ranges from 30 to 200 ppm by weight when x + y + j is equal to 1000 parts by weight. According to a preferred embodiment, z ranges from 40 to 100 ppm, more preferably from 40 to 60 ppm, even more preferably z is equal to 50 ppm. In the alloy of formula Au _x Ti _y R _z M _j , the amount x of gold ranges from 375 to 917 parts by weight, for instance from 580 to 917 or from 750 to 917 parts by weight. It preferably ranges from 580 to 760 parts by weight. In the alloy of formula Au _x Ti _y R _z M _j , the amount y of titanium ranges from 83 to 625 parts by weight. It preferably ranges from 240 to 420 parts by weight. For instance, in parts by weight, x can range from 750 to 760, y from more than 125 to 250 or less and j from 0 to less than 125, with 240 ≤ y + j ≤ 250 and x + y + j = 1000. According to a specific embodiment, the alloy has any one of the following formula (in weight fraction): Au ₅₈₃ Ti ₄₁₇ R _z , Au ₇₅₃ Ti ₂₄₇ R _z or Au ₈₀₄ Ti ₁₉₆ R _z , wherein the amount z of grain refiner ranges from 30 to 200 ppm by weight of the total amount of gold and titanium. According to a particular embodiment of the invention, the alloy has a molar ratio gold/titanium of between 25.39/74.61 (14 carats) to 54.86/45.14 (20 carats), more preferably from 42.17/57.83 (18 carats, based on the weight of titanium and gold) to 54.86/45.14 (20 carats). When x varies from 375 to 917, the amount of gold approximately ranges from 9 to 22 carats as compared to the total amount of gold and titanium. The alloy may comprise impurities. In general, impurities may result from the metals used to form the alloy. Advantageously, these possible impurities amount to a total of less than 1000 ppm, more preferably, less than 500 ppm, more preferably less than 250 ppm, by weight of the alloy (x + y + j + z). The alloy may comprise a total amount of impurities of less than 100 ppm. Accordingly, the alloy may comprise more impurities than grain refiner. The benefit resulting from the presence of a grain refiner is greater than any negative effect that might result from the presence of impurities, even when the amount of impurities exceeds that of grain refiner. Impurities do not affect the poor (or lack of) shape memory resulting from the presence of 30-200 ppm of grain refiner. In particular, impurities can include any one or more of carbon, oxygen and nitrogen. In general, the amount of oxygen may be greater than that of nitrogen, which may be greater than that of carbon. The total amount of one or more of carbon, oxygen and nitrogen is preferably less than 1000 ppm, more preferably, less than 500 ppm, more preferably less than 300 ppm even more preferably less than 250 ppm, by weight of the alloy (x + y + j + z). However, in some cases, the total amount of one or more of oxygen and nitrogen may amount to more than 1000 ppm, preferably less than 10000 ppm, more preferably less than 7500 ppm, even more preferably less than 5000 ppm. This large amount of oxygen and/or nitrogen may result from the process for preparing the alloy, in particular if the melted/solubilized Au, Ti, M and R elements are more or less slightly exposed to air. Experimental conditions may therefore impact the amount of impurities such as oxygen and nitrogen; however, in general, metal or metalloid impurities result from the Au, Ti, R and M materials used to prepare the alloy. The total amount of impurities other than carbon, nitrogen and oxygen is preferably less than 500 ppm, more preferably less than 250 ppm, even more preferably less than 230 ppm, by weight of the alloy (x + y + j + z). These other impurities may include but not limited to hydrogen, sulfur; silicon; phosphorous; selenium; halogens; metals other than gold, titanium, silver, aluminum and the grain refiner R; metalloids other than the grain refiner R. According to a preferred embodiment, non-metallic and non-metalloid impurities represent less than less than 1000 ppm, more preferably, less than 500 ppm, more preferably less than 300 ppm even more preferably less than 250 ppm) while metallic impurities (metals other than gold, titanium, silver, aluminum and the grain refiner R; metalloids other than the grain refiner R) preferably represent less than 500 ppm, more preferably less than 100 ppm, even more preferably less than 50 ppm, by weight of the alloy (x + y + j + z). In addition to a smaller grain size of the alloy, the grain refiner also contributes to the control of the precipitation step. A smaller grain size reduces the macroscopic deformation of the alloy. As a result, the grain refiner also improves the ductility of the alloy as its ability to elongate increases as compared to alloys that comprise less than 30 ppm or more than 200 ppm of grain refiner. The Au _x Ti _y R _z M _j alloy has a grain size that preferably ranges from 10 to 300 µm, more preferably from 20 to 200 µm. The Au _x Ti _y R _z M _j alloy preferably comprises between 10 and 70 % by volume of Ti ₃ Au (molar fraction) precipitates (Ti ₃ Au or Ti _42.17 Au _57.83 by weight), more preferably between 20 and 60 %. Formula Ti ₃ Au corresponds to an alloy consisting of gold and titanium, wherein the molar ratio gold/titanium is 1/3. All other formulae, for instance Au _x Ti _y R _z M _j or Au _x Ti _y R _z , are in weight. Ti ₃ Au precipitates actually improves the hardness of the alloy. These Ti ₃ Au precipitates are dispersed within the Au _x Ti _y R _z M _j matrix. They are located at the interface between the grains i.e. at the grain boundary and/or within the matrix. The alloy has a hardness that preferably ranges from 220 to 650 Hv, more preferably from 400 to 650 Hv, for instance from 450 to 500 Hv. Due to its hardness properties, the alloy resists to scratches. In addition, the grain refiner, which prevents crack initiation, improves the aesthetic properties of the alloy. The present invention also relates to a process for preparing the alloy of formula Au _x Ti _y R _z M _j . This process comprises the following steps: 1/ casting a material: melting x parts by weight of gold, y parts by weight of titanium and z ppm of a grain refiner selected from the group consisting of iridium, bore, vanadium, iron, cobalt, barium, yttrium, zirconium, and mixtures thereof, wherein x = 375 to 917 parts by weight and y = 83 to 625 parts by weight, wherein x + y + j = 1000 and z = 30 to 200 ppm by weight of x + y + j, 0 ≤ j < y, with 0 ≤ j < 125 when x = 750 to 917, and wherein M is selected from the group consisting of silver, aluminum and mixtures thereof, 2/ homogenizing the resulting material of formula Au _x Ti _y R _z M _j , 3/ optionally, deformation of the material from step 2/, 4/ optionally, precipitation by thermal treatment of the material from step 2/ or 3/ and obtaining an alloy of formula Au _x Ti _y R _z M _j . The casting step consists in melting/solubilizing the Au, Ti, M and R elements at a temperature TCAST that may be greater than that of the respective melting points of these elements (or mixtures thereof) and below that of their (or mixtures thereof) respective boiling points. For instance, casting gold, titanium and iridium may be carried out at a temperature of more than 2466°C (melting point of iridium) and less than 2856°C (boiling point of gold). The casting temperature TCAST may be below the melting point of Ti or R. It is preferably above the melting point of gold. Melted gold can then solubilize titanium and/or the grain refiner R and/or M. The casting step 1/ may include making a pre-alloy. Making a pre-alloy allows a lower casting temperature. The pre-alloying step is preferably carried out in an arc furnace. When the process includes a pre-alloying step, the casting step may be carried out The casting conditions (casting temperature and time) can be adjusted. In general, pre-alloying or casting (without pre-alloying) the alloy, in particular in an arc furnace, is carried out at a temperature that preferably ranges from 2470 to 2850°C, more preferably from 2480 to 2700 °C. In that case, the different elements of the alloy are preferably maintained at this temperature for 5 seconds to 30 minutes, for instance for 5 seconds to 20 minutes. For instance, it can last from 5 seconds to 5 minutes, more preferably for 10 seconds to 2 minutes. In general, casting the alloy, following a pre-alloying step, is carried out at a temperature of 1100 to 1500°C, more preferably from 1250 to 1450°C, in particular in an induction furnace or a cold crucible induction melting. In that case, the different elements of the alloy are preferably maintained at this temperature for 5min to 60min, for instance for 10min to 30min. Casting the different elements of the alloy can consist in casting the appropriate amounts of elements by any means, for instance any one of: arc furnace, induction furnace, cold crucible induction melting… In general, the casting step 1/ is followed by a cooling stage, for instance by air quenching or water quenching, preferably by water quenching. The grain refiner may also act as seeding material for the Au _x Ti _y R _z M _j alloy as the material resulting from the casting step has an improved microstructure as compared to alloys consisting of gold and titanium. While thermal treatments increase the grain size, the grain refiner counterbalances this effect by slowing down the grain’s growth. According to a preferred embodiment, the homogenizing step 2/ is carried out at a temperature, T _HOM , of from 1200 to 1400°C, more preferably between 1250 and 1350°C. The homogenizing step 2/ is preferably carried out for 2 hours to 12 hours, more preferably for 5 hours to 10 hours, for instance for 5 hours to 8 hours. For instance, homogenizing step 2/ may be carried out between 1250 and 1350°C, for 5 hours to 10 hours. The process preferably includes a cooling stage between steps 2/ and 3/. The homogenized material is preferably rapidly cooled, by air quenching or water quenching, more preferably by water quenching. Deforming step 3/ is preferably carried out by cold compression or by rolling compression or by hot forging, more preferably by cold compression. The deformation is preferably comprised between 20 and 80%, more preferably between 30 and 70%. The deforming step is preferably carried out at a temperature TDEF of between 0 and 50°C, more preferably between 20 and 30°C. Precipitation step 4/ is preferably carried out at a temperature TPRE of from 400 to 1000°C, more preferably between 500°C and 700°C, for instance between 500°C and 600°C. When the process includes deformation step 3/, precipitation step 4/ is preferably carried out for 10 minutes to 300 minutes, more preferably for 30 minutes to 180 minutes, for instance for 30 minutes to 120 minutes. When the process does not include deformation step 3/, precipitation step 4/ is preferably carried out for 1 hour to 15 hours, more preferably for 2 hours to 12 hours. For instance, after deformation step 3/, precipitation step 4/ may be carried out between 500°C and 600°C, for 30 minutes to 2 hours. At the end of the precipitation step 4/, the alloy is cooled to room temperature (20 to 25°C), preferably by air quenching or water quenching, more preferably by water quenching. The precipitation step 4/ promotes the formation of a Au _x Ti _y R _z M _j matrix comprising Ti ₃ Au precipitates, resulting in an improved hardness. Applicant has noticed that step 2/ and/or step 4/ may increase the amount of oxygen impurity, which is preferably less than 1000 ppm. At least one of steps 1/ to 4/ may be followed by a cooling stage to room temperature. Steps 1/ to 4/ are preferably followed by a cooling stage to room temperature, preferably by water quenching. In general, T _CAST is greater than T _HOM , which is greater than T _DEF . On the other hand, T _PRE is generally greater than T _DEF but smaller than T _HOM . The grain refiner affords smaller grain size. On the other hand, the deformation step and the precipitation step improve the hardness of the alloy. The present invention also relates to an item comprising or consisting of the alloy of formula Au _x Ti _y R _z M _j . Accordingly, this alloy can be used in order to manufacture luxury goods. For instance, it may be a watch component comprising (or consisting of) the alloy. It may be a jewel comprising (or consisting of) the alloy. The item comprising or consisting of the alloy of formula Au _x Ti _y R _z M _j can be a jewel, a leather good, or a clothing accessory. It may also be a watch, a writing accessory, or a decorative item. For instance, it can be any of the followings: ring, ear ring, necklace, bracelet, pendant, watch or watch movement component (case, bezel, case back, crown, other case small parts, balance wheel, gear wheel, axis, screw…), buckle (belt, purse…), tie bar, cuff links, money clip, hair pin, pen, paper knife… The alloy according to the invention can therefore be used in a field selected from the group consisting of: jewelry, horology, clothing, writing accessories, leather goods, and ornaments. The invention and its advantages will become more apparent to one skilled in the art from the following figures and examples. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 show the grain size of Au _x Ti _y Ir _z alloys (x = 750, y = 250, z = 30-200 ppm). Figure 3 shows Ti3Au precipitates in Au ₇₅₀ Ti ₂₅₀ Ir ₅₀ ppm (weight fraction). Figures 4 shows Au _x Ti _y Ir _z alloys (x = 750, y = 250, z = 30-200 ppm). Figures 5 and 6 show Ti ₃ Au (molar fraction) precipitates in Au ₇₅₀ Ti ₂₅₀ Ir _50ppm (weight fraction). Figure 7 shows the recovered deformation of gold-titanium alloys (Au ₇₅₀ Ti ₂₅₀ and Au ₇₅₀ Ti ₂₅₀ Ir ₅₀ ppm) after a cold deformation of 0-15%. Figure 8 shows the grain size evolution of gold-titanium alloys (Au ₇₅₀ Ti ₂₅₀ and Au ₇₅₀ Ti ₂₅₀ Ir ₅₀ ppm). EXAMPLES Alloys of formula Au _x Ti _y R _z (x = 750 parts by weight, y = 250 parts by weight and z = 0 to 200 ppm by weight vs x+y) have been prepared following different experimental conditions, as outlined in Table 1. Samples “INV” refer to alloys according to the invention while samples “REF” are reference examples. The process for preparing the different alloys comprises the following steps: 1/ Casting a material (samples 1-30): melting the appropriate amounts of gold, titanium and, eventually, iridium elements, for instance in an arc furnace, preferably between 2466°C and 2856°C. 2/ Homogenizing the resulting material (samples 6-30). This thermal treatment may be followed by a quenching step, preferably in water. 3/ Optionally, deformation by cold compression (samples 11-12 and 22-30). 4/ Optionally, precipitation by thermal treatment (samples 13-30). This thermal treatment may be followed by a quenching step (preferably in water).

Table 1: Experimental conditions for the preparation of Au _x Ti _y R _z alloys. Y: Yes N: No ^(wq) : water quenching Hom: homogenization at 1310°C for 7 hours Def: deformation by cold compression or by cold rolling (sample 25 only) PP: thermal treatment for precipitation Table 1 shows that, depending on the experimental conditions, the alloy according to the invention exhibits a hardness that can range from 227 to 609 Hv. The alloy hardness is improved when the process successively involves a homogenization step, a deformation step and a precipitation step. These steps are preferably following by a quenching step, for instance water quenching. Figure 8 shows the grain size evolution of a sample after a homogenization step (1310°C for 7 hours). The presence of 50 ppm of a grain refiner (Ir) in Au ₇₅₀ Ti ₂₅₀ affords smaller grains (less than 1 mm vs 2-3 mm). The technical effect resulting from the presence of smaller grains is the reduction of macroscopic deformation that leads to crack initiation. In other words, the alloy according to the invention exhibits a more homogeneous deformation through the bulk, less segregation, and a better aesthetics since grains cannot be seen at a macroscopic scale. However, figure 1 shows that the amount of grain refiner should be limited to 200 ppm in order to keep the smaller grain size benefit. Additionally, since the grain refiner prevents crack initiation, it also improves the aesthetic properties of the alloy, especially after a polishing step. It improves the deformation of the alloy without generating cracks. The homogenization step (samples 6-10) may be followed by a quenching step, preferably in water. The deformation step, preferably by cold compression, improves the hardness of the alloy (samples 11 and 12 vs samples 6 and 8). The precipitation step is a thermal treatment, preferably at a temperature of at least 400°C and for 1 hour or more. This thermal treatment may be followed by a quenching step, preferably in water. It promotes the precipitation of Ti ₃ Au. Ti ₃ Au precipitates improves the hardness of the alloy. As compared to Au ₇₅₀ Ti ₂₅₀ , the presence of 30-200 ppm of grain refiner promotes the Ti ₃ Au precipitation kinetics. In general, the deformation step (in the presence of a grain refiner or not) contribute to this phenomenon as well. In the presence of a grain refiner (30-200 ppm), the above precipitation step 4/ promotes a mixed structure of Au-Ti and Ti ₃ Au. More specifically, the resulting alloy comprises Ti ₃ Au precipitates within a gold-titanium, or gold-titanium-grain refiner, matrix (figures 3-6). Figure 7 shows the recovered deformation of different alloys (Au ₇₅₀ Ti ₂₅₀ and Au ₇₅₀ Ti ₂₅₀ Ir ₅₀ ppm) after an incremental cold deformation of 0-15% and a mild heating step between 450°C and 700°C. Prior to this test, steps “1/ + 2/” or steps “1/ + 2/ + 4/” have been applied to these samples. Step 2/ is a homogenizing step carried out at 1310°C for 7 hours while step 4/ is a precipitation step carried out at 750°C for 10 hours. Above 10% deformation, the alloy according to the invention exhibits a recovered deformation of 2% or less vs 3% or more for Au ₇₅₀ Ti ₂₅₀ . This is a significant improvement of roughly 33%. The recovered deformation relates to the difference between the size of a sample, following an incremental deformation stage (0-15%), and its final size after a mild heating step and the subsequent cooling stage. This incremental deformation is not the deformation of step 3/. The mild heating step following the incremental deformation is not a precipitation step 4/. This mild heating step allows change from martensitic phase to austenitic phase. If upon cooling, the alloy recovers its initial size, it also loses the benefits of the mild heating step and means a return to its martensitic phase. Accordingly, it is highly beneficial to obtain a recovered deformation close to 0%, which results in an alloy that is easier to shape by mechanical deformation (stamping, cold rolling…) due to a reduced shape memory.