Low Cycle Fatigue Behavior Of Biomedical Near ? Titanium Alloy TLM-Hf

At present,near-? titanium alloys have become a hot spot in the research of biomedical metal materials due to it has low elastic modulus and wide adjustable structure and properties.In the medical titanium alloy Ti-25Nb-3Mo-3Zr-2Sn(hereinafter referred to as TLM)developed by the Northwest Institute of Nonferrous Metals,industrial grade pure zirconium(impurity element mainly Hf)is used instead of nuclear grade pure zirconium,and trace amounts Hf are added,obtaining Ti-25Nb-3Mo-3Zr-2Sn-0.25Hf(hereinafter referred to as TLM-Hf),studying the influence of trace Hf elements on the microstructure and properties of TLM alloys,and judging the feasibility of using industrial grade pure zirconium instead of nuclear grade pure zirconium to reduce costs.Four typical heat treatment processes were set:solution treatment:750?/30min water cooling;aging treatment:750?/30min water cooling,380?/4h air cooling;750?/30min water cooling,480?/4h air cooling;750?/30min water-cooled,510?/4h air-cooled.It was found that TLM-Hf micro-structure and mechanical property were not much different from TLM,indicating that it is feasible to reduce the cost in terms of structure and mechanical properties by using industrial grade sponge zirconium instead of nuclear grade sponge zirconium in TLM alloy.Secondly,the low-cycle fatigue test was carried out by using TLM-Hf alloy with good mechanical property-solid-state and high-temperature aging.The half strain amplitude ??t/2 of the solid solution state TLM-Hf alloy is 1.0%,1.3%,1.6%,2.0%,and the aging TLM-Hf alloy is performed.??t/2 is a cyclic loading of 0.6%,0.8%,1.0%,and 1.3%.The elastic modulus of the alloy during the cycle is obtained by processing the stress-strain hysteresis curve.It is found that the elastic modulus of the solid solution TLM-Hf alloy increases more faster with the increase of the strain amplitude.It was found that this is related to the precipitation of ?" induced by meta-stable ? alloy.The elastic modulus of the aged TLM-Hf alloy is relatively stable in cyclic loading.By the fatigue test,we find that the alloys of the two tenses dominate the elastic strain amplitude at the corresponding half strain amplitude.The stress-strain behavior of the solid solution TLM-Hf alloy at the lower strain amplitude is more complicated,which is related to the start of the slippage dislocation,the interaction of dislocations and the quenching.At higher strain amplitudes,the alloy exhibits cyclic hardening,which is related to the deformation-induced precipitation phase and the occurrence of twins in the solid solution TLM-Hf alloy under high strain amplitude loading.The aged TLM-Hf alloy is more stable under different strain amplitude cyclic loading.The relational expressions of the semi-stress amplitude ??t/2 and the semi-plastic strain amplitude ??p/2 of the solid solution and the aged state of the TLM-Hf alloy and the fatigue life expression are obtained.The analysis of fatigue fracture of alloy shows that the two TLM-Hf alloys show a tendency of multi-source fracture with the increase of strain amplitude.The secondary cracks become more and deeper,and the fatigue bands become less widened.Through the observation of the micro-structure,it is found that the mechanism of plastic deformation is mainly slip and twinning for the solid solution TLM-Hf alloy under high strain amplitude circulation,and the dislocation cells and twin crystals in the crystal are favorable for the nucleation of the crack.The aged TLM-Hf alloy exhibits a large number of dislocations around the acicular a phase,causing stress concentration,resulting in crack nucleation.