A BIO-COMPATIBLE TITANIUM ALLOY OPTIMISED FOR ADDITIVE MANUFACTURING

Abstract

A titanium-based alloy composition consisting in weight percent, of: between 15.0 and 35.0% niobium, between 0.0 and 7.5% molybdenum, between 0.0 and 20.0% tantalum, between 0 and 7.0% zirconium, between 0 and 6.0% tin, between 0.0 and 2.0% hafnium, between 0.0 and 0.5 % aluminium, between 0.0 and 0.5% vanadium, between 0.0 and 0.5% iron, between 0.0 and 0.5% chromium, between 0.0 and 0.5% cobalt, between 0.0 and 0.5% nickel, between 0.0 and 1.0% silicon, between 0.0 and 0.2% boron, between 0.0 and 0.5% calcium, between 0.0 and 0.5% carbon, between 0.0 and 0.5% manganese, between 0.0 and 0.5% gold, between 0.0 and 0.5% silver, between 0.0 and 0.5% oxygen, between 0.0 and 0.5% hydrogen, between 0.0 and 0.5% nitrogen, between 0.0 and 0.5% palladium, between 0.0 and 0.5% lanthanum, the balance being titanium and incidental impurities, wherein the composition satisfies the following equation: 0.0175Nb+0.0183Mo+0.03Ta+0.0116Zr+0.1Sn>1.0 where Nb, Mo, Ta, Zr and Sn represent the amounts of niobium, molybdenum, tantalum, zirconium and tin in wt % respectively.

Claims

1. A titanium-based alloy composition consisting in weight percent, of: 15.0 to 35.0% niobium, 0.0 to 7.5% molybdenum, 0.0 to 20.0% tantalum, 0 to 7.0% zirconium, 0 to 6.0% tin, 0.0 to 2.0% hafnium, 0.0 to 0.5% aluminium, between 0.0 to 0.5% vanadium, 0.0 to 0.5% iron, between 0.0 to 0.5% chromium, 0.0 to 0.5% cobalt, 0.0 to 0.5% nickel, 0.0 to 1.0% silicon, 0.0 to 0.2% boron, 0.0 to 0.5% calcium, 0.0 to 0.5% carbon, 0.0 to 0.5% manganese, 0.0 to 0.5% gold, 0.0 to 0.5% silver, 0.0 to 0.5% oxygen, 0.0 to 0.5% hydrogen, 0.0 to 0.5% nitrogen, 0.0 to 0.5% palladium, between 0.0 to 0.5% lanthanum, the balance being titanium and incidental impurities, wherein the composition satisfies the following equation:
0.0175Nb+0.0183Mo+0.03Ta +0.0116Zr +0.1Sn >1.0 where Nb, Mo, Ta, Zr and Sn represent the amounts of niobium, molybdenum, tantalum, zirconium and tin in wt % respectively.

2. The titanium-based alloy composition of claim 1, consisting of 7.0% or less molybdenum, preferably 6.0% or less molybdenum, more preferably 5.0 or less molybdenum.

3. The titanium-based alloy composition of claim 1, consisting of 1.5% or more molybdenum, preferably of 2.5% or more molybdenum.

4. The titanium-based alloy composition of claim 1, which satisfies the following equation 0.0375Nb+0.033Mo+0.0167Ta+0.05Zr+0.267(0.13Sn-0.516).sup.2>1.0, preferably >1.1 where Zr, Sn, Mo, Ta and Nb represent the amounts of zirconium, tin, molybdenum, tantalum and niobium in wt % respectively.

5. The titanium-based alloy composition of claim 1, consisting of 5.5% or less tin, preferably 5.0% or less tin, more preferably 4.75% or less tin.

6. The titanium-based alloy composition of claim 1, consisting of 2.0% or more tin, preferably 3.0% or more tin, more preferably 4.0% or more tin.

7. The titanium-based alloy composition of claim 1, consisting of 6.0% or less zirconium, preferably 5.5% or less zirconium, more preferably 4.5% or less zirconium, even more preferably 4.0% or less zirconium, most preferably 3.5% or less zirconium.

8. The titanium-based alloy composition of claim 1, consisting of 1.0% or more zirconium, preferably 1.5% or more zirconium, more preferably 2.0% or more zirconium

9. The titanium-based alloy composition of claim 1, which satisfies the flowing relationship
1.0Al wt. %+Sn wt. %+Zr wt. %2.5 in which Al, Sn and Zr represent the amounts of aluminium, tin and zirconium in wt % respectively.

10. The titanium-based alloy composition of claim 1, consisting of 17.5% or more niobium, preferably 20.0% or more niobium, more preferably 22.5% or more niobium.

11. (canceled)

12. The titanium-based alloy composition of claim 1, consisting of 17.5% or less tantalum, preferably 15.0% or less tantalum, more preferably 12.5% or less tantalum.

13. The titanium-based alloy composition of claim 1, consisting of 5.0% or more tantalum, preferably 7.5% or more tantalum, more preferably 10.0% or more tantalum, most preferably 12.5% or more tantalum.

14. The titanium-based alloy composition of claim 1, which satisfies the flowing equation
250883-150Fe wt. %-96Cr wt. %-49Mo wt. %-37 V wt. %-17Nb wt. %-12Ta wt. %-Zr wt. %-3Sn wt. %-+15Al wt. %, preferably
225883-150Fe wt. %-96Cr wt. %-49Mo wt. %-37 V wt. %17Nb wt. % -12Ta wt. %-7Zr wt. %-3Sn wt. %-+15Al wt. %, more preferably
200883-150Fe wt. %-96Cr wt. %-49Mo wt. %-37 V wt. %-17 Nb wt. % -12Ta wt. %-7Zr wt. %-3Sn wt. %+15Al wt. % in which Fe, Cr, Mo, V, Nb, Ta, Zr, Sn and Al represent the amounts of iron, chromium, molybdenum, vanadium, niobium, tantalum, zirconium, tin and aluminium in wt % respectively.

15. The titanium-based alloy composition of claim 1, which satisfies the flowing equation
75883-150Fe wt. %-96Cr wt. %-49Mo wt. %-37 V wt. %-17 Nb wt. %-12Ta wt. %-7Zr wt. %-3Sn wt. %+15Al wt. %.250 in which Fe, Cr, Mo, V, Nb, Ta, Zr, Sn and Al represent the amounts of iron, chromium, molybdenum, vanadium, niobium, tantalum, zirconium, tin and aluminium in wt % respectively.

16-19. (canceled)

20. The titanium-based alloy composition of claim 1, wherein the sum of wt % of each of aluminium, vanadium, iron, chromium, cobalt, nickel, manganese and boron is 1.0 wt. % or less, preferably 0.5 wt % or less.

21. The titanium-based alloy composition of claim 1, which satisfies the following equation 0.0178Nb+0.0143Mo+0.0243Ta+0.0285Sn<1.0 in which Mo, Nb, Ta, and Sn represent the amounts of molybdenum, niobium, tantalum and tin in wt % respectively.

22. The titanium-based alloy composition of claim 1, which satisfies the following equation
0.0298Nb+0.0272Mo+0.0246Ta+0.0376Zr+0.0259Sn>1.0, preferably
0.0298Nb+0.0272Mo+0.0246Ta+0.0376Zr+0.0259Sn>1.1 where Zr, Nb, Mo, Ta and Sn represent the amounts of zirconium niobium, molybdenum, tantalum and tin respectively in wt %.

23. The titanium-based alloy composition of claim 1, which satisfies the following equation 2.5>0.042Nb+0.06Mo+0.05Ta+0.03Zr+0.1Sn in which Mo, Nb, Ta, Zr and Sn represent the amounts of molybdenum, niobium, tantalum, zirconium and tin in wt % respectively.

24. The titanium-based alloy composition of claim 1, which satisfies the following equation
0.042Nb+0.06Mo+0.05Ta+0.03Zr+0.1Sn>1.0 in which Mo, Nb, Ta, Zr and Sn represent the amounts of molybdenum, niobium, tantalum, zirconium and tin in wt % respectively.

25. (canceled)

26. An implant or prosthetic device made of the titanium-based alloy of claim 1.

27-28. (canceled)

Description

[0030] The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which:

[0031] FIG. 1 illustrates the biocompatibility of various alloying elements;

[0032] FIG. 2 is a flow diagram illustrating the process by which the titanium-based alloy composition was determined;

[0033] FIG. 3 illustrates the contour plots for the crack susceptibility factor (CSF) for the TiNbMoTaZr system;

[0034] FIG. 4 illustrates the contour plots for the Scheil solidification (freezing) range for the TiNbMoTaZr system;

[0035] FIG. 5 illustrates the contour plots for the cost of the TiNbMoTaZr system;

[0036] FIG. 6 illustrates the contour plots for the martensitic start temperature of the TiNbMoTaZr system;

[0037] FIG. 7 illustrates the contour plots for the bonding order of the TiNbMoTaZr system;

[0038] FIG. 8 illustrates the contour plots for the growth restriction factor (GRF) of the TiNbMoTaZr system;

[0039] FIG. 9 illustrates the contour plots for the melting temperature of the TiNbMoTaZr system;

[0040] FIG. 10 illustrates qualitatively the effect of each element on the proposed merit indices;

[0041] FIG. 11 illustrates the constrains used to isolate an alloy space with merit indices comparable to or better than the current alloys;

[0042] FIG. 12 illustrates the effect of Sn and Zr on the contour plots for the solidification range of a few selected TiNbMoTa systems;

[0043] FIG. 13 illustrates the effect of Sn and Zr on the contour plots for the cracking susceptibility factor of a few selected TiNbMoTa systems;

[0044] FIG. 14 illustrates the effect of Sn and Zr on the contour plots for the growth restriction factor of a few selected TiNbMoTa systems;

[0045] FIG. 15 illustrates the effect of Sn and Zr on the contour plots for the melting temperature of a few selected TiNbMoTa systems;

[0046] FIG. 16 illustrates the effect of Sn and Zr on the contour plots for the martensite start temperature of a few selected TiNbMoTa systems;

[0047] FIG. 17 illustrates the effect of Sn and Zr on the contour plots for the raw material cost of a few selected TiNbMoTa systems;

[0048] FIG. 18 illustrates the effect of Sn and Zr on the contour plots for the bond order of a few selected TiNbMoTa systems;

[0049] FIG. 19 illustrates the effect of Sn and Zr on the contour plots for the aluminium equivalent of a few selected TiNbMoTa systems;

[0050] FIG. 20 illustrates the optimal alloy space as constrained by the threshold equations;

[0051] FIG. 21 illustrates the values of the threshold equations for each of the examples and baseline alloys;

[0052] FIG. 22 illustrates the scatter plots of the solidification range vs. growth restriction factor. Area highlighted in black indicates the optimal design space. Labelled data points correspond to literature alloys. Grey indicates the rest of the trial compositions;

[0053] FIG. 23 illustrates the scatter plots of the solidification range vs. cracking susceptibility factor. Area highlighted in black indicates the optimal design space. Labelled data points correspond to literature alloys. Grey indicates the rest of the trial compositions;

[0054] FIG. 24 illustrates the scatter plots of the cracking susceptibility vs. growth restriction factor. Area highlighted in black indicates the optimal design space. Labelled data points correspond to literature alloys. Grey indicates the rest of the trial compositions;

[0055] FIG. 25 illustrates the scatter plots of the growth restriction factor vs. other merit indices. Area highlighted in black indicates the optimal design space. Labelled data points correspond to literature alloys. Grey indicates the rest of the trial compositions;

[0056] FIG. 26 illustrates the scatter plots of the cracking susceptibility factor vs. other merit indices. Area highlighted in black indicates the optimal design space. Labelled data points correspond to literature alloys. Grey indicates the rest of the trial compositions;

[0057] Traditionally, titanium-based alloys have been designed through empiricism. Thus, their chemical compositions have been isolated using time consuming and expensive experimental development, involving small-scale processing of limited quantities of material and subsequent characterisation of their behaviour. The alloy composition adopted is then the one found to display the best, or most desirable, combination of properties. The large number of possible alloying elements indicates that these alloys are not entirely optimised and that improved alloys are likely to exist.

[0058] In titanium alloys, generally additions of aluminium (Al) are added as -stabiliser to improve the mechanical strength. However, aluminium has been associated with neurological related diseases. General additions of vanadium (V) are added as -stabiliser and to increase the mechanical strength without forming brittle intermetallic compounds. V makes a solid solution with the B phase. However, V is believed to have toxic and carcinogenic effects on the body. Additions of nickel (Ni), cobalt (Co), iron (Fe) and chromium (Cr) are also added as 8-stabiliser elements. However, all of these are considered to have low bio-compatibility and are associated with adverse responses to body functions. Therefore, the alloy contains a maximum amount of 0.5% of each of Al, V, Ni, Co, Mn, Fe and Cr preferably 0.2% or less of each of Al, V, Ni, Co, Mn, Fe and Cr and a total sum of these elements and boron of 1.0% or less, preferably 0.5% or less.

[0059] Calcium and carbon may be present at levels of up to 0.5% each and are not expected greatly to change the character of the alloy at this level. Oxygen, nitrogen and hydrogen may be present in the alloy at concentrations up to 0.5 wt % each without reducing the characteristics of the alloy significantly. Oxygen will significantly improve the strength of the alloy but high amounts may have a negative effect on any shape memory effects that the alloy may exhibit. The same is true of gold, silver and palladium, each of which may be present in an amount of up to 0.5 wt %. These may be added to increase bio-compatibility and to reduce infections. Lanthanum may be added in an amount up to 0.5 wt. % in order to further improve the microstructure during solidification (similarly to B).

[0060] Pure titanium, which is biocompatible has poor solidification characteristics (in particular crack susceptibility factor, CSF), discussed below. Therefore, alloying elements are added in the present invention to improve the additive manufacturability of the alloy. Because the alloy is intended for use in an article to be placed in the human or animal body including prosthetic devices, orthopaedic implants, in particular bone implants and/or artificial joints, it is desirable to achieve the desired properties of the titanium alloy using mainly or even exclusively so-called vital elements which have been shown to have high biocompatibility and osseo-integration. FIG. 1 is a graph showing the biocompatibility or otherwise of various possible alloying elements, obtained from Kuroda D, Niinomi M, Morinaga M, Kato Y, Yashiro T. Design and mechanical properties of new type titanium alloys for implant materials. Materials Science and Engineering: A. 1998.

[0061] In the alloy of the present invention, niobium (Nb) is used as -stabiliser and to lower the elastic modulus of the alloy. Tantalum (Ta) is also used as -stabiliser and to promote bone growth into the metal. Zirconium (Zr) is used as replacement of Ti further to reduce the elastic modulusin combination with Nb and Ta, Zr can perform as -stabiliser.

[0062] In the present invention, molybdenum (Mo) is used as a strong -stabiliser to improve the properties during manufacturing. Although Mo is not a so-called vital element, it has been shown that, when combined with Ti, it does not degrade the bio-compatibility of the alloy and that it has no adverse effects on the bodysee Nunome et. al. In vitro evaluation of biocompatibility of TiMoSnZr superelastic alloy, Journal of Biomaterials Applications, 2015.

[0063] Tin (Sn) is used to reduce the stiffness of the alloy and to add strength. Because Sn is an -stabiliser, the amount of Sn is limited to Sn7.0 wt. %, preferably 6.0% or less, more preferably 5.5% or less. In an embodiment at least 2.5% or more Sn is added to reduce stiffness, to increase strength and to obtain the most optimal crack susceptibility. More preferably there is 3.0% or more tin, yet further to reduce stiffness and increase strength. It is assumed that Sn can replace Zr in the present invention. A minimum of 4.0% tin is desirable for reduce stiffness and increased strength.

[0064] In an embodiment of the invention, the titanium alloy satisfies the flowing equation

1.0Al wt. %+Sn wt. %+Zr wt. %2.5

[0065] Tin and Zr may be interchangeable and when present in a preferable amount of 1.0% Al. eq. or more as illustrated by the above equation to increase the strength of the alloy and reduce the overall stiffness while improving on bio-compatibility. A preferable maximum amount of 2.5% Al. eq. ensures that the solidification range stays within acceptable limits while still obtaining the bio-compatibility benefits that Ta, Nb and Mo add while keeping a minimum amount of 52.5% of pure titanium in the final composition.

[0066] Hafnium (Hf) can optionally be used to increase the hardness of the alloy. However, Hf is a very expensive element and its used is restricted in the present invention to Hf 2.0 wt. %. Preferably Hf is present at a level of 1.0% or less to keep cost down. For applications where high hardness (to improve wear resistance) is important, Hf can be present in an amount of 0.1% or more preferably 0.5% or more.

[0067] A modelling-based approach used for the isolation of new grades of titanium-based bio-compatible alloys is described here, termed the Alloys-By-Design (ABD) method. This approach utilises a framework of computational materials models to estimate design relevant properties across a very broad compositional space. FIG. 2 illustrates the various steps of the method. In principle, this alloy design tool allows the so-called inverse problem to be solved; identifying optimum alloy compositions that best satisfy a specified set of design constraints.

[0068] The first step in the design process is the definition of an elemental list along with the associated upper and lower compositional limits. The compositional limits for each of the elemental additions considered in this inventionreferred to as the alloy design spaceare detailed in Table 2.

TABLE-US-00002 TABLE 2 Alloys design space in wt. % searched using the Alloy-by-Design method. Nb Mo Ta Zr Sn Min. 0.0 0.0 0.0 0.0 0.0 Max. 40.0 15.0 30.0 20.0 12.5

[0069] The second step relies upon thermodynamic calculations used to calculate the phase diagram and thermodynamic properties for a specific alloy composition. Often this is referred to as the CALPHAD method (CALculate PHAse Diagram). These calculations are conducted for those temperatures where an optimal phase architecture of the new alloy is found.

[0070] A third stage involves isolating alloy compositions which have the desired microstructural architecture for additive manufacturing and for bio-medical purposes. In the case of titanium alloys, the formability via additive manufacturing is related directly to the weldability of the alloy. In titanium alloys, the weldability can be correlated in a first instance to the microstructural architecture in terms of and phase proportions. For near- alloys, the weldability is good, these alloys are usually welded in annealed condition. For the / alloys, weldability is dependent on the amount of phase present. The most strongly beta stabilised alloys are usually embrittled during weldinghigh beta content alloys are rarely welded. The exception is Ti6Al4V which has good weldability and can have good mechanical properties after heat-treatment. Metastable alloys have good weldability and retain good mechanical properties after welding even without the need of post-heat treatment. In the bio-medical field, there is a need to design alloys that have low elastic modulus (to be close to that of the bone) and to use alloyants that improve bio-compatibility and osseo-integration. Metastable alloys (BCC) have been shown to decrease substantially the elastic modulus when compared to and alloys (HCP+BCC). Moreover, the use of Nb, Zr, and Ta (all of them stabilisers) is known to improve the bio-compatibility and osseo-integration of titanium alloys. Thus, it is desirable for the purposes here exposed to design a metastable alloy employing the so-called vital elements.

[0071] Thus, the model isolates all compositions in the design space which are the most bio-compatible, which tend to form stable microstructures for optimal additive manufacturability, and which have a low elastic modulus for good osseo-integration and reduced stress-shielding. Moreover, the important factors such as melting temperature and amount of alloyantswhich are important to obtain a uniform and chemically homogeneous powder particleare also weighted into the design process.

[0072] In the fourth stage, merit indices are estimated for the remaining isolated alloy compositions in the dataset. Examples of these include: martensitic transformation merit index (which describes the temperature at which the transformation starts), elastic modulus and bone-growth compatibility (which are related to the bond order of the composition), manufacturability (which is related to the freezing range and the susceptibility of the alloy to crack which is a function of the transient solidification behaviour and the phase proportions of the alloy), and powder processability (which is related to the melting temperature and the amount of titanium in the alloy).

[0073] The first merit index is the bone-compatibility merit index, which highlights the speed at which cells grow into the metal interface. Okazaki and Tetsuya. (1998), Corrosion resistance, mechanical properties: corrosion fatigue strength and cytocompatibility of new Ti alloys without Al and V. Biomaterials, has shown that the bone-growth correlates with the bonding order of the alloy. Thus, the merit index is calculated using a rule of mixtures following

Bo=.sub.ix.sub.iBo.sub.i

[0074] where x.sub.i is the concentration of element i and Bo.sub.i the ith bonding order value of the element. Moreover, this merit index also measures the critical current density for passivation (which gives a measurement of the cytotoxicity) and an approximation to the elastic modulus of the alloy. Increasing the bonding order helps decreasing both the cytotoxicity and the elastic stiffnesssee Brown S A, Lemons J E (1996) Medical Applications of Titanium and Its Alloys: The Material and Biological Issues.

TABLE-US-00003 TABLE 3 Bonding order of each element. Ti V Fe Ni Zr Nb Mo Hf Ta Al Si Sn 2.79 2.8 2.65 2.41 3.08 3.09 3.06 3.11 3.14 2.42 2.56 2.28

[0075] The second merit index relates to the susceptibility of the alloy to form the martensitic phase. This merit index uses the martensite start temperature model as disclosed in Suresh Neelakantana, Prediction of the martensite start temperature for titanium alloys as a function of composition, 60 Scripta Materialia 611 (2009). The martensitic start temperature ( C.) is calculated as a function of the alloy composition following

M.sub.S=883-150Fe wt. %-96Cr wt. %-49Mo wt. %-37 V wt.-% 17Nb wt. %-12Ta wt. %-7Zr wt. %-3Sn wt. %+15Al wt. %.

[0076] If the martensitic start temperature is below ambient temperature, martensitic phase will not be present at the microstructure level. Martensitic phase is hard and brittle, and it could affect the crack susceptibility of the alloy if the thermal strains during the manufacturing process are high enough to crack this brittle phase. A preferable upper limit of the martensitic start temperature is defined so that the phase transformation does not compromise the manufacturing process (250 C.). A reduction in the likelihood of martnesite formation is achieved by reducing the martensite start temperature even further. Therefore, a martensite start temperature of 225 C. or less is preferred and the most desirable alloys have a martensite start temperature of 200 C. or less. A lower limit is also defined, a martensitic transformation above the service temperature can be used to add strength, shape memory effect, and to further lower the stiffness of the alloy (75 C.). The optimal martensitic start temperature is regarded as being between 75 to 200 C.

[0077] The third merit index is the freezing range. The freezing range is effectively the temperature range of the two-phase liquid+ phase region.

[0078] This merit index is calculated using the Scheil thermodynamic calculation in ThermoCalc and provides the transient solidification route of the alloy. An expanded solidification range has the potential to increase hot tearing and remelting of underneath layers during laser manufacturing. Thus, it is desirable to monitor the temperature range at which the transition from liquid to solid occurs. Although not the most critical factor, it is desirable to minimise the solidification range, at least to a value comparable to those alloys in the literature.

[0079] The fourth merit index is the crack susceptibility factor (CSF), it is calculated as a function of the time needed to solidify the last 10% of the liquid versus the time necessary to go from a 40% solid fraction to 90% solid fraction. The solidification is assumed to occur at a constant rate of decrease in temperature in order to facilitate the estimation of the solidification time. It is assumed that the risk of hot tearing is stronger during the last instants of the solidification, thus the goal is to minimise the time of that part of the solidification process. The CSF is calculated following

[00001]$\mathrm{CSF}=\frac{\mathrm{time}\left(f_{\mathrm{solid}}=0.9\right)-\mathrm{time}\left(f_{\mathrm{so1id}}=1.0\right)}{\mathrm{time}\left(f_{\mathrm{solid}}=0.4\right)-\mathrm{time}\left(f_{\mathrm{solid}}=0.9\right)}$

[0080] where time(f.sub.solid) is the time required to reach a particular volume fraction of solid.

[0081] The fifth merit index is the growth restriction factor (GRF). The methodology to derive it is described in T. E. Quested, A. T. Dinsdate Et A. L. Greer, Thermodynamic Modeling of Growth-Restriction Effects in Aluminum Alloys, 53 Acta Materiatia 1323 (2005). Generally, it is accepted that an alloy with a large growth restriction factor tends to reject solute atoms during solidification which translate into finer grain size microstructures which are beneficial for additive manufacturing. The GRF is calculated as the derivative of the fraction of solid with respect to undercooling following

[00002]$\mathrm{GRF}={\left(\frac{\left(.Math.T_S\right)}{f_s}\right)}_{f_s.fwdarw.0}$

[0082] where f.sub.s is the fraction of solid and T.sub.S is the solutal undercooling. This can be approximated using the Scheil analysis by calculating the slope of a linear regression fit to the temperature profile as a function of the solid fraction for solid fractions below 0.1. Further details can be found in the aforementioned scientific reference.

[0083] The sixth merit index is the powder processability. In order to facilitate the powder production process and obtain optimal elemental homogenisation, the melting temperature and the amount of alloyants must be minimised. Thus, for the sixth index, the temperature of the melting point is calculated, this is desirably below 1900 C. The sixth merit index also limits the amount of alloyants. Preferably pure titanium is at least 50.0 wt. % of the final composition. It is thought that it is desirable to maintain titanium in at least an amount of 52.5 wt. %, preferably at least 55.0 wt. % of the final composition to ease powder processing. If too little titanium is present it is believed that it may make processing of the powder (produced by atomisation of an ingot) difficult. That is, when the ingot of the correct composition is atomised, if too little titanium is present this may cause segregation whose occurrence is also related to the melting point. The atomised powder is then fed into a selective laser melting (SLM) machine for additive manufacture.

[0084] The ABD method described above was used to isolate the inventive alloy composition. The design intent for this alloy was to isolate the composition of a new titanium alloy which exhibits a combination of stiffness, strength, manufacturability, processability, cytotoxity and osseo-integration which is comparable or better than equivalent grades of alloy. The cost of the alloy has also been considered in the design of the new alloy. The cost is based on the amount of each element contained in the alloy at 2018 prices.

[0085] The material propertiesdetermined using the ABD methodfor the literature titanium alloys are listed in Table 4. The design of the new alloy was considered in relation to the predicted properties listed for these alloys. The method was used to propose nine optimised alloy compositions which target different properties: ranging from highest GRF to lowest GRF and lowest CSC to highest CSC respectively. The calculated material properties for the optimised alloys with nominal compositions according to Table 5 and in accordance with the present invention are also given.

TABLE-US-00004 TABLE 4 Calculated phase fractions and merit indices made with the Alloys-by-Design software. Results for four commonly used biomedical Ti alloys as listed in Table 1 and the nominal composition of the new alloys listed in Table 5. Freezing CSF GRF Bo Ms Vital Weldability Melting Al eq. Ti Cost ID ( C.) () () () ( C.) () () () (wt. %) () ($/ton) Ti6Al4V 1 27 1.31 5.5 2.75 825 No + 1703 6.0 90 5529 Ti6Al7Nb 2 26 1.06 7.3 2.76 854 No + 1717 6.0 87 7170 Ti13Nb13Zr 3 62 1.11 20.0 2.84 571 Yes + 1662 2.2 74 28447 Ti12Mo6Zr2Fe (TMZF) 4 580 3.06 53.2 2.82 47 No Metastable 1680 1.0 80 14136 Ti15Mo5Zr3Al 5 78 0.90 30.4 2.80 158 No Metastable 1735 3.8 77 12860 Ti12Mo3Nb 6 49 0.51 28.8 2.81 244 Yes Metastable 1726 0.0 85 6621 Ti12Mo5Ta 7 64 0.48 39.0 2.81 235 Yes Metastable 1741 0.0 83 12830 Ti16Nb10Hf (Tiadyne 1610) 8 24 0.52 14.8 2.83 541 Yes (martensitic) 1699 0.0 74 110192 Ti35Nb7Zr5Ta (TNZT) 9 210 0.48 122.5 2.88 179 Yes Metastable 1834 1.2 53 35119 Ti29Nb13Ta4.6Zr (TNTZ) 10 206 0.45 124.4 2.88 202 Yes Metastable 1850 0.8 53 41128 Ti28Nb13Zr0.5Fe (TNZF) 11 369 2.32 93.7 2.87 241 No Metastable 1733 2.2 59 33928 Ti24Nb4Zr7.9Sn (Ti2448) 12 111 0.76 109.9 2.82 423 Yes (martensitic) 1740 3.3 64 20696 Ti30Ta 13 115 0.47 63.3 2.83 523 Yes (martensitic) 1792 0.0 70 48660 Ti50Ta 14 226 0.55 109.9 2.86 283 Yes (martensitic) 1912 0.0 50 77900 Ti18Nb10Ta2Mo 15 98 0.36 73.0 2.84 359 Yes Metastable 1777 0.0 70 26146 Ti29Nb13Ta4Mo 16 211 0.31 142.8 2.88 38 Yes Metastable 1895 0.0 54 34689 Ti16Nb13Ta7Mo 17 146 0.34 111.2 2.85 112 Yes Metastable 1827 0.0 64 30098 Ti16Nb13Ta4Mo 18 121 0.36 90.3 2.84 259 Yes Metastable 1801 0.0 67 29918 Example 1 1 234 0.33 231 2.85 136 Yes Metastable 1884 1.8 53 37541 Example 2 2 212 0.33 188 2.85 239 Yes Metastable 1859 1.8 54 42037 Example 3 3 230 0.32 218 2.85 91 Yes Metastable 1875 1.8 54 34251 Example 4 4 236 0.30 219 2.86 87 Yes Metastable 1882 1.8 53 33568 Example 5 5 245 0.31 221 2.86 77 Yes Metastable 1884 1.9 53 31610 Example 6 6 230 0.29 210 2.86 111 Yes Metastable 1875 1.8 54 29315 Example 7 7 238 0.28 208 2.86 105 Yes Metastable 1882 1.8 53 28677 Example 8 8 247 0.27 204 2.87 99 Yes Metastable 1888 1.8 52 28039 Example 9 9 248 0.25 196 2.87 96 Yes Metastable 1894 1.7 51 27311

[0086] An important characteristic in order to improve the additive manufacturing properties of titanium alloys is a low crack susceptibility factor (CSF), namely the fourth merit index. FIGS. 3 and 13 illustrate the effects of varying amounts of molybdenum, niobium, tantalum, zirconium and tin on the crack susceptibility factor. The effects of all elements is positive, but the effect of niobium and tin are strongest.

[0087] Another important characteristic in order to improve the additive manufacturing properties of titanium alloys is a high growth restriction factor (GRF). FIGS. 8 and 14 illustrate the effects of varying amounts of molybdenum, niobium, tantalum, zirconium and tin on the growth restriction factor. The effects of all elements is positive, but the effect of niobium, molybdenum and tin is stronger (tin being the strongest of these).

[0088] As can be seen, a minimum crack susceptibility factor is achieved with niobium of around 20.0% or more for lower amounts of molybdenum and higher amounts of tantalum or zirconium. Conversely, as can be seen from the contour maps of FIG. 4, increased amounts of niobium can deleteriously lead to a greater solidification (freezing) range. This, along with cost (FIG. 5), a deleterious effect of decreasing the martensitic start temperature below the desired range (FIG. 6) and the desire to keep a minimum amount of titanium as high as possible means that the best balance of properties is achieved when the alloy contains 35.0% or less niobium. An upper limit of 35.0% Nb is set to keep the melting temperature within limits and to allow the addition of other important alloyants which have other positive effects. However, a minimum amount of 15.0% niobium is necessary to achieve a sufficiently low CSF value.

[0089] Desirably niobium is added in an amount of at least 17.5 wt. % so as to achieve an even lower CSF and therefore excellent additive manufacturing properties. In a preferred embodiment, niobium is added in an amount of at least 20.0% or even 22.5% yet further to reduce CSF. In an embodiment niobium is limited to 32.5% or less to reduce expense and ease manufacturability of the powder and increase strength and reduce solidification range whilst still maintaining an acceptable level of CSF. In a further preferred embodiment niobium is limited to 30.0 wt. % or less or even 27.5 wt. % or less further reducing the cost of the alloy whilst increasing its strength and ease of manufacture of the starting powder.

[0090] Example 2 has a significantly higher martensite start temperature than the other examples. However example 2 uses low niobium and molybdenum levels whilst still achieving an acceptable growth restriction factor. The advantage of such an alloy is that the high levels of Ta and Zr may result in good bio-compatible effects. However, the making of the ingot is the most difficult (Ta very high) and the GRF is not as high as the other examples.

[0091] The lowest cracking susceptibility factor is generally achieved for an amount of tin of around 4.5-5.0% (see FIG. 13). For amounts of tin higher than 5.0% the CSF begins to deteriorate. At Tin approximately 6.5%, the value of cracking susceptibility tends to reach the restriction limit. Moreover, large amounts of tin tend to lower the bond order of the alloyFIG. 18 shows that for the preferred alloy space, amounts of tin approximately larger than 7.0% tend to produce a value of bond order which is too low. Thus, an upper limit for Tin is imposed at Sn=6.0%. Preferably, tin is present in amounts lower than 5.5% in order to be closer to the optimal CSF value, more preferably lower than 5.0%. In a preferred embodiment, the amount of tin is 4.5 wt. % or less in order to avoid a significant increase of the start martensitic temperature and the amount of a-phase present. Moreover, addition of tin can lower the melting temperature of the alloy - this is beneficial. In a preferred embodiment, the amount of tin is 2.0 wt. % or more to add strength and to lower the stiffness but more importantly to increase growth restriction (FIG. 14) and to decrease cracking susceptibility. Preferably tin is present in amount of 3.0% or larger. More preferably in an amount of 4.0% or larger.

[0092] As can be seen from the contour plots of the bond order in FIG. 7, tantalum has a strong influence on advantageously increasing the bond order. This promotes cell growth and indicates decreased toxicity. On the other hand, tantalum is deleterious in terms of cost of the alloy (FIG. 5) and in terms of reduced manufacturability due to increasing of the solidification range (FIG. 4) and in terms of reduced strength due to lowering of the martensitic start temperature (FIG. 6), although this deleterious effect on the martensitic start temperature is not so strong. Tantalum also increases the melting temperature of the alloy (FIG. 9). An upper limit of 20.0% Ta is set in order to maintain a sufficiently low melting point. Desirably the maximum amount of tantalum is 17.5% or less (for instance preferably 15.0% or less or 12.5% or less tantalum) as this can help in the manufacturability of the atomised alloy (because this allows a higher amount of titanium so that the possibility of segregation during atomisation is reduced) and reduces the raw cost of the alloy significantly.

[0093] To achieve a bond order which is comparable with the best performing existing alloys shown in Table 3, the amount of tantalum is preferably 5.0% or more, more preferably 7.5% or more. A bonding order even greater can be achieved by increasing the minimum amount of tantalum to 10.0 wt. % or more or even 12.5% or more. However, tantalum may be absent, depending upon the combination of properties desired.

[0094] Molybdenum has a strong effect on reducing the martensitic start temperature and thus -stabilising the alloy (FIG. 6) while keeping the content of pure titanium high. Molybdenum is also important in increasing the growth restriction factor (FIG. 8) and to some extent reducing the cracking susceptibility factor (FIG. 3). However, molybdenum has the effect of increasing the overall solidification range (FIG. 4). Nonetheless, molybdenum is a known grain refiner and a strong stabiliser, thus, it allows an alloy with a high titanium content to be produced (thereby increasing the ease of powder processing and its chemical homogeneity). Along with niobium and tantalum, molybdenum undesirably increases the melting temperature (FIG. 9). Therefore, it has been found that a suitable level of molybdenum is 7.5 wt. % or less. Because of its beneficial effect on

[0095] GRF, molybdenum is preferably present in an amount of at least 1.5 wt. % or more. Desirably molybdenum is present in amount of 7.0 wt. % or less to reduce its effect on the increase on solidification range and to assure that the martensite start temperature range remains within the desired range. Levels of molybdenum of 6.0% or less or even 5.0% or less are even more desirable for the same reason. On the other hand, an amount of molybdenum of 2.5 wt % or more, preferably 3.5 wt. % or more is advantageous as this ensures the GRF remaining high without needing further to increase the amount of niobium.

[0096] The contour plots of FIGS. 3-6 show that the effect of zirconium is not as strong as the other alloying elements except in a deleterious way relating to the cost (FIG. 5). Zirconium has little effect on the melting temperature (FIG. 9) or on the martensitic start temperature (FIG. 6) and its effect on the CSF (FIG. 3) seems less strong than Nb, Mo and Ta. However, Zr is known to substitute for Ti while adding bio-compatibility. Moreover, Zr is known to lower the stiffness (it substantially increases bonding ordersee FIG. 7) and adding strength by solid-solutioning and grain refinement (Zr increases GRFsee FIG. 8). However, large amounts of Zr are shown to have a negative effect on the cracking susceptibility (see FIG. 3). With the above in mind, small amounts of zirconium may be added, particularly because of its known biocompatibility (see FIG. 1) and its strengthening effect. However, due to the desire of keeping a low cracking susceptibility factor, the suitable maximum amount of zirconium is preferably 6.0wt % or less, even 5.5% or less. However, up to 7.0% or less is permitted, particularly where a strong alloy is desired at the expense of additive manufacturability. Limiting the amount of Zr also ensures a high Ti content and a low solidification range (FIG. 4). Moreover, Zr is restricted to small amounts because its effect on the CSF is not as beneficial as Nb, Ta and Mo. In a preferred embodiment the alloy consists of 4.5% or less zirconium, preferably 4.0% or less or even 3.5% or less zirconium to ensure a low cracking susceptibility. In a preferred embodiment, a minimum amount of 1.0% or more is desirable in order to increase the bond order, to increase the growth restriction factor and to add strength. More preferably 1.5% or more or 2.0% or more further to increase the bond order, to further increase the growth restriction factor and to add strength.

[0097] FIG. 20 is a summary of the preferable requirements of the alloy together with a series of linearised restrictions which were derived to isolate the optimal alloy space.

[0098] FIG. 20 has plots for compositions falling within the preferred range and is simply used to illustrate further restrictions on the alloy which result in the best combination of the first-sixth merit indices.

[0099] Regression analysis was performed on the results of the calculations of the merit indices to determine the following seven relationships which are valid within the elemental range of the alloy (Nb 15-35, Mo 0-7.5, Ta 0-20.0, Zr 0-7.0 and Sn 0-6.0). As a main requirement for the alloy a growth restriction factor (GRF) of at least 150 was chosen. This is greater than the GRF achieved by any of the prior art alloys in table 4. As described above in relation to the fifth merit index, a higher growth restriction factor is beneficial for additive manufacturing. From FIG. 8 it becomes clear that a region of the alloy space investigated results in alloys with a growth restriction factor greater than those of the prior art (as shown in Table 4). Analysis shows that within the elemental range of the alloy space, a growth restriction factor of greater than 150 can be achieved if the following equation (equation 2) is satisfied:

0.0175Nb+0.0183Mo+0.03Ta+0.0116Zr+0.1Sn>1.0, preferably >1.1, more preferably >1.2

[0100] where Nb, Mo, Ta, Zr and Sn represent the amounts of niobium, molybdenum, tantalum, zirconium and tin in weight percent respectively. Preferably the growth restriction factor is even greater meaning that the sum on the left hand side of equation 2 is greater than 1.1, even more preferably greater than 1.2, as shown above. As shown in Table 5, all the example alloys have a growth restriction factor of greater than 180 and the above sum is at least 1.28. The best performing alloys in this respect have a growth restriction factor of greater than 200 and this is also preferred as the greater the growth restriction factor the greater the additive manufacturability. This equation is marked as equation 2 in FIG. 20.

[0101] Line 1 in FIG. 20 represents the following equation

1.0Al wt. %+Sn wt. %+Zr wt. %2.5

[0102] Meeting this criterion ensures that the strength benefits of having enough aluminium equivalent elements is metbecause aluminium itself is restricted due to low bio-compatibility. Because of the low levels of allowed aluminium, aluminium can be excluded from this equation.

[0103] The fourth merit index (crack susceptibility factor) is represented by two equations marked as 3a and 3b in FIG. 20. As can be seen in FIG. 3, in the design space considered a minimum crack susceptibility factor is present in the alloy design space and the relationship is not linear. Therefore two linear equations are used to model that desired space. These equations are 0.0375Nb+0.033Mo+0.0167Ta+0.05Zr+0.267(0.13Sn -0.516).sup.2>1.0 which is shown as 3a in FIG. 20 and Zr>4 which is shown as equation 3b in FIG. 20 These equations are generated with the requirement that the crack susceptibility factor is 0.33 or less which can be seen to be lower than the crack susceptibility factor of most of the prior art alloys shown in Table 5. Preferably, the crack susceptibility value is lower than 0.3 or even 0.28. For those cases, one wants to achieve values of Eq. 3a approximately higher than 1.17 but values of equation 3a of higher than 1.1 also have very low crack susceptibility factor and are only slightly less preferred. As shown in Table 5, the example alloys 6 to 9 have a cracking susceptibility lower than 0.3thus better than all the prior art alloys. When equation 3a is used, these example alloys have a value greater than 1.17this is preferred as the greater the value of equation 3a, the better the resistance to cracking during the additive manufacturing process.

[0104] The sixth merit index is modelled by the following equation (equation 4) 0.0178Nb+0.0143Mo+0.0243Ta+0.0285Sn<1.0 which is shown as line 4 in FIG. 20. Alloys meeting this equation have a melting point of below 1900 C.

[0105] The first merit index is desirably an alloy with a bonding order of greater than 2.85. This requirement is met by an alloy satisfying the following equation (equation 5) 0.0298Nb+0.0272Mo +0.0246Ta+0.0376Zr+0.0259Sn>1.0 where Zr, Nb, Mo, Ta and Sn represent the amounts of zirconium, niobium, molybdenum, tantalum and tin respectively in wt %. Such an alloy has a bonding order comparable to that of prior art alloys as shown in Table 4. This equation is shown as line 5 in FIG. 20. These alloys have high enough bond order for improved bio-compatibility and low elastic modulus. If the sum of equation 5 is greater than 1.1, the bond order is even higher which is desirable.

[0106] The second merit index, namely the susceptibility to martensitic phase is already described above with respect to the equation for Ms. The same line is plotted in FIG. 20 for the components illustrated in FIG. 20. Lines 6 and 7 marking the maximum and minimum desired martensitic start temperature (250 C. and 75 C.) are plotted in FIG. 20 and the equation is as follows:

75>883150Fe49Mo17Nb12Ta7Zr3Sn<250.

[0107] The desirable maximum allowable martensite start levels of 225 C. and 220 C. are not plotted in FIG. 20, but from FIG. 16 the effect of such preferred restrictions on allowable amounts of alloyants can be seen.

[0108] The third merit index, namely the freezing range, is also plotted with a desirable range of less than 250 C. is plotted as line 8 in FIG. 20 (the upper end of the range; all the alloys in FIG. 20 have a freezing range above the lower end of the range. This follows the following equation (equation 8):

2.5>0.042Nb+0.06Mo+0.05Ta+0.03Zr+0.1Sn>1.0.

[0109] This equation 8 isolates those alloys which have an optimal solidification rangehigh enough to allow optimal layer adhesion during additive manufacturing and low enough so that the risk of hot tearing is kept low.

[0110] FIGS. 22-26 illustrate the range of merit indices of alloys in the range in which the model was run (Table 2) with the properties of existing alloys numbered and plotted on the same graphs as well as areas of the inventive range plotted (as restricted by FIG. 11). As can be seen, the alloys of the present invention have improved properties over the otherwise most suitable existing alloys.

[0111] The amounts of silicon and boron are not arrived at from thermodynamic calculations but instead are added from knowledge that they will increase strength and creep resistance and enhance ductility of the alloy. As such, they are allowable up to 0.2 wt % boron and 1.0 wt % silicon.