The maximum recovery strain (εr) of Ti-Ni alloy can reach 8.0%, showing excellent shape memory effect and superelasticity, and is widely used as bone plates, vascular scaffolds and orthodontic frames. However, when Ti-Ni alloy is implanted into the human body, it can release Ni+ which is sensitizing and carcinogenic, leading to serious health problems. β titanium alloy has good biocompatibility, corrosion resistance and low elastic modulus, and can get better strength and plasticity match after reasonable heat treatment, it is a kind of metal material that can be used for hard tissue replacement. At the same time, reversible thermoelastic martensitic transformation exists in some β titanium alloys, showing certain superelastic and shape memory effects, which further expands its application in the biomedical field. The development of β-titanium alloy which is composed of non-toxic elements and has high elasticity has become a research hotspot of medical titanium alloy in recent years.
At present, many β-titanium alloys with superelasticity and shape memory effects at room temperature have been developed, such as Ti-Mo, Ti-Ta, Ti-Zr and Ti-Nb alloys. However, the superelastic recovery of these alloys is small, such as the maximum εr of Ti-(26, 27)Nb (26 and 27 are atomic fractions, if not specially marked, the titanium alloy components involved in this paper are atomic fractions) is only 3.0%, much lower than Ti-Ni alloy. How to further improve the superelasticity of β titanium alloy is an urgent problem to be solved. In this paper, the factors affecting the superelasticity of β titanium alloy are analyzed, and the methods for improving the superelasticity are summarized systematically.
Superelasticity 1.1 Reversible stress-induced martensitic transformation of 1β titanium alloys
The superelasticity of β titanium alloys is usually caused by reversible stress-induced martensitic transformation, that is, the β phase of the body-centered cubic lattice structure is transformed into the α" phase of the rhombic lattice structure when the strain is loaded. During unloading, the α" phase changes into β phase and the strain recovers. In the superelastic β titanium alloy, the β phase of the body-centered cubic structure is called "austenite" and the α phase of the rhombic structure is called "martensite". The beginning temperature of martensitic phase transition, the end temperature of martensitic phase transition, the beginning temperature of austenite phase transition and the end temperature of austenite phase transition are expressed by Ms, Mf, As and Af, and Af is usually several kelvin to tens of Kelvin higher than Ms. The loading and unloading process of β titanium alloy with stress-induced martensitic transformation is shown in Figure 1. First occurs an elastic deformation of the β phase, which transforms into the α" phase in the form of shear when the load reaches the critical stress (σSIM) required to induce the martensitic phase transition. As the load increases, the martensitic phase transition (β→α") continues until the stress required for the end (or end) of the martensitic phase transition is reached, and then the elastic deformation of the α" phase occurs. When the load further increases beyond the critical stress required for β phase slip (σCSS), the plastic deformation of β phase occurs. During unloading, in addition to the elastic recovery of α" phase and β phase, α"→β phase transition also causes strain recovery. The superelastic or shape memory effect of the alloy depends on the relationship between the phase transition temperature and the test temperature. When Af is slightly lower than the test temperature, the α phase induced by stress during loading undergoes α →β phase transition during unloading, and the strain corresponding to the stress-induced phase transition can completely recover, and the alloy exhibits superelasticity. When the test temperature is between As and Af, a part of α phase is transformed into β phase during unloading, and the strain corresponding to the stress-induced phase transition is recovered, and the alloy exhibits certain superelasticity. If the alloy is further heated above Af, the remaining α" phase is transformed into β phase, the phase transition strain is completely recovered, and the alloy exhibits certain shape memory effect. When the test temperature is lower than As, the stress-induced martensitic transformation strain does not automatically recover at the test temperature, and the alloy does not have superelasticity. However, when the alloy is heated above Af, the phase change strain is completely restored, and the alloy exhibits shape memory effect.