Low Cycle Fatigue Behavior Of Novel ? Titanium-based Shape Memory Alloys

New ? Ti based shape memory alloys as implant material has great potential applications in biomedical field, which has non-toxicity to human body, excellent superelasticity and good biocompatibility. Depth study low-cycle fatigue properties and microstructure of ? titanium shape memory alloy effect of its fatigue properties,and further discussion and analysis ? shape memory alloy the process of martensitic transformation under alternating loads, crack propagation, martensitic phase transformation and the interaction mechanism between martensite phase transformation, which are of significance to promoting practical of alloys. In this thesis, based on the excellent capacity of elastic energy Ti-7.5Nb-4Mo-2Sn memory alloy as the research object, we situ observe the deformation of ? titanium shape memory alloy was observed in situ by using digital speckle technique, and research its deformation characteristics. Fatigue life of the alloy were evaluated under the cyclic load, and studied the microstructure, elastic modulus and the super elasting change rule in the process of fatigue deformation. Finally, the crack propagation process and the evolution strain field at the crack tip wavre obserred in situ by using DIC. The influence mechanism of crack propagation ? and ? phase boundary was investigated.The main results are summarized as follows.(1) The ??? phase transition point of Ti-7.5Nb-4Mo-2Sn alloy between 700 ?and 800 ?.700 ? and 800 ? alloy exhibits superelasticity at room temperature.The alloy of 700 ? annealed at 700 ? indicates excellent superelasticity because of its tiny structure and strengthening the role of ? phase. Martensite phase transformation originated from the surface of sample, and then to expand internal sample. The process of expand is tend to the martensitic phase transformation in the grain which expand is uneven.(2) The alloy annealed at 700 ? then that of the alloy annealed at 800 ?exhibits higher fatigue life. Shape Memory alloy releases heat result in the alloys temperature rise at repeated deformation martensite transformation process,promoting ??? and ??? phase transition process. In the process of fatigue deformation, alloy show softening after hardening phenomenon. Alloy softening is caused by twin consolidation, subsequently Alloy hardening is due to the increase of? and ? two-phase content increasing with an increase in the number of fatigue.(3) Under the same loading rate, the alloy annealed at 700? exhibits relalivelyhigh crack propagation rate and the deflection angle compared with the alloy of annealed at 800? annealing treatment. It is associated with that of alloy annealed at700 ? annealing treatment contains a lot of ? phase, resulting in more ?/? phase boundaries which is faron to generation of microcracks or makes the microcracks crack deflection. DIC observations reveal that the stress concertration occurs at the tip of the cracks, result in the Ti-7.5Nb-4Mo-2Sn in homengenous deformation, the martensite transformation occurs at the place with big deformation, which block the crack propogation.