As early as the 1980s, foreign researchers had already conducted studies on Ti-Zr alloys. Mehjabeen and others from Japan investigated the mechanical properties of Ti-Zr alloys, revealing that the strength and hardness were 2 to 3 times those of pure Ti and pure Zr. The Ti-50at.%Zr alloy had the highest strength and hardness, as well as the smallest grain structure. Sista and others studied the biological properties of Ti-Zr alloys containing 50% Ti, and found that compared with pure Ti and Ti-Nb alloys, the surface of Ti-Zr alloys was more conducive to cell adhesion and growth. Vicente and others added 0.02% to 0.04% oxygen to Ti-Zr alloys and discovered that the oxygen content had little effect on the microstructure and biocompatibility, but significantly increased the hardness and elastic modulus of the alloy. Ho and others from Taiwan studied the effects of Zr content on the microstructure and mechanical properties of Ti-(10-40wt.%)Zr alloys and the changes in microstructure and properties during subsequent heat treatment. The results showed that the strength, hardness and elastic modulus of the alloy were significantly related to the increase in Zr content. After different cooling rates following heat treatment, the alloys formed α+ω phase, β+α+ω phase, and β+α phase, etc. They also found that adding other elements such as Nb, Mo, Cr, and Fe to Ti-10Zr-X could significantly improve the mechanical properties and wear resistance of the alloy, making it an ideal dental restoration material. Research on the microstructure transformation rules of Ti-(10-70wt.%)Zr binary alloys and the effects of heat treatment on microstructure transformation and surface bioactivity indicated that when the Zr content was less than 20%, the alloy was a single α phase; when the Zr content was between 20% and 60%, the alloy was composed of α and β phases; and when the Zr content was greater than 60%, the alloy was a single β phase. The α phase had a needle-like structure, while the β phase had an equiaxed structure. The hardness of the alloy increased first and then stabilized as the Zr content increased, with the maximum hardness value of 330 (HV3) in the Ti-50wt.%Zr alloy.
The relationship between the martensitic structure and Ta content in Ti-Ta alloys is as follows: when Ta < 8.7 at.%, the alloy only has the α' phase at room temperature; when 8.7 at.% < Ta < 32 at.%, the alloy only has the α" phase at room temperature; when Ta > 32 at.%, the alloy only has the β phase at room temperature. Buenconsejo et al. found that due to the β phase stability of Ta element, the phase transformation stability of Ti-Ta alloys is higher than that of Ti-Nb and Ti-Mo alloys, and thus there is no ω phase precipitation during quenching. Meanwhile, the shape memory properties of Ti-(30-40 at.%)Ta alloys were studied. For every 1 at.% increase in Ta content, the martensitic start transformation temperature Ms decreases by 30 K. The increase in Ta content can inhibit the precipitation of ω phase during aging. During the thermal cycling process from 173 to 513 K, Ti-32Ta (Ms = 440 K) has a stable high-temperature shape memory effect.
The relationship between the microstructure, mechanical properties and Ta content in Ti-Ta alloys was studied. It was found that the quenched microstructure of Ti-Ta alloys is highly related to Ta content. When Ta < 20 wt.%, the quenched microstructure is a lamellar α' structure; when 30 wt.% < Ta < 50 wt.%, the quenched microstructure is a needle-like α" phase; when Ta = 60 wt.%, the β + α" phase appears; when Ta > 60 wt.%, a single β phase appears. In Ti-30%Ta and Ti-70%Ta alloys, the best match of low elastic modulus and high strength is achieved, which is very suitable for biomedical materials. Zheng et al. added Zr element to Ti-Ta alloys to inhibit the precipitation of ω phase during thermal cycling and improve the stability of phase transformation temperature. In Ti-15Ta-15Zr alloy, the phase transformation temperature decreased by less than 5 K in the first five thermal cycling processes and remained unchanged thereafter, showing excellent thermal cycling stability. Therefore, the addition of Zr element increases the critical slip stress of Ti-Ta alloys and improves the shape memory performance.