Microstructure And Properties Of A New Metastable β Type Ti-Nb-Ta-Zr-O Alloy

In 2003, a group of metastableβ-type titanium alloys basically expressed as Ti3(Ta+Nb+V) + (Zr,Hf) + O were developed by Saito et. al.. These alloys were called as multifunctional alloys due to their many"super"properties, such as super elasticity, super plasticity, and Invar and Elinvar properties. Particularly, these are the alloys that combine the ultralow elastic modulus and ultrahigh strength, which breaks the norm that a metal can not have both simultaneously. Therefore, they can be expected to be very attractive materials in dental and orthopedic implants. In order to exhibit these properties, according to Saito et. al.,"each alloy system requires substantial cold working". The unique properties of these alloys are attributed to a dislocation-free plastic deformation mechanism because no dislocation or twin crystal is observed after hard cold-working.In this study, a multifunctional alloy with a typical chemical composition of Ti-23Nb-0.7Ta-2Zr-O (at.%, TNTZO) was prepared by cold crucible levitation melting (CCLM) technique. Then the microstructure of the TNTZO alloy after cold working and the microstructural evolution during recrystallization were investigated and the deformation mechanism was analyzed. Additionally, the mechanical, physical, and corrosion properties of this alloy were also studied.The results revealed that the TNTZO alloy is still composed of singleβphase and no transformation such as stress-inducedα" martensite andωphase occur after deformation. After severe cold swaging, the microstructure of the TNTZO alloy changed into characteristic"marble-like"structure because grains undergo plane-strain elongation. <100> and <111> compressed texture and <110> fiber texture, which are typical texture components of a bcc metal, are detected in the cold worked TNTZO alloy. Additionally, TEM results show that high density dislocations occur in the deformed TNTZO alloy. Therefore, the TNTZO alloy deforms by the traditional dislocation glide on slip systems, rather than by the dislocation-free mechanism.The relationship of the recrystallized fraction versus annealing time for the TNTZO alloy can be described by a typically sigmoid curve, which indicates that the recrystallization process is a typical process of nucleation and growth of new grains. The activation energy of recrystallization was calculated to be 180 kJ/mol for the 90% cold swaged TNTZO alloy. TEM results show that recovery of the TNTZO alloy proceeds by polygonization mechanism, and recrystallization is achieved by the nucleation and growth of new grains at the expense of the deformed structure. Therefore, the number of low angle grain boundaries decreases, however, the number of the coincidence site lattice (CSL) and random boundaries increases with prolonging annealing time. Major texture components of the cold worked TNTZO alloy become diffused after complete recrystallization, thus no recrystallization texture forms. The TNTZO alloy has the same recovery and recrystallization mechanisms as the ordinary bcc metals, which also indicates that the TNTZO alloy deforms by the traditional dislocation glide on slip systems.After cold working, Young's modulus of the TNTZO alloy decrease because of the occurrence of deformation texture. Additionally, the cold-worked TNTZO alloy exhibits super elastic property. After annealing at 820℃for 5 min, Young's modulus of the TNTZO alloy recovered to the level before cold working due to the recrystallization of the alloy. The thermal expansion coefficient of the cold swaged TNTZO alloy is extremely low, not exceeding 5×10-6 K-1 from room temperature to 400℃, however, it dramatically increases after annealing, is approximately 9×10-6 K-1. The endothermic peaks in DSC curves reveal that phase transformation occurs in the TNTZO alloy from about 400℃to 480℃.The passive film of the TNTZO alloy is mainly composed of the oxides of Ti, Nb, Ta and Zr, and the film of the Ti-6Al-4V alloy consists of the oxides of Ti, Al and V. The passive TiO2 film modified by the oxides of Nb, Ta and Zr possesses higher stability and protective quality than the TiO2 film modified by the oxides of Al and V. Therefore the TNTZO alloy exhibits a better corrosion property than Ti-6Al-4V alloy in Ringer's solution. Due to the existence of the strain and deformation texture, the effect of deformation on corrosion property of the TNTZO alloy is not monotonic. Small deformation causes the deterioration of the corrosion resistance, at high deformation level, however, the TNTZO alloy shows similar corrosion property to the as-soluted one.