Study On Plastic Deformation Mechanism Of Martensite NiTi Shape Memory Alloy Under Local Canning Compression

Nickel-titanium shape memory alloys(NiTi SMA) are widely used in medicine,aviation and aerospace fields because of their excellent comprehensive mechanic performance, such as shape memory effect, superelasticity, corrosion resistance and biocompatibility. Plastic working has significant influence on shape memory effect as well as superelasticity of NiTi SMA. Severe plastic deformation under a certain temperature is able to form the amorphous NiTi SMA, nanocrystalline phase subsequently obtained by means of heat treatment.Nanocrystalline NiTi SMA has more excellent mechanical properties, superelasticity,corrosion and biocompatibility compared with the coarse grain NiTi SMA. Therefore, to carry out the NiTi SMA severe plastic deformation technology possess the important scientific significance and project application value.In the paper,The equiatomic NiTi SMA(Ni5OTi50) and Ni50-x/xTi50-x/2Nbx series alloy(x=2,4, 6) as research object, its microstructure is characterized. The analysis shows that the Ni50Ti50 SMA is martensitic structure which quenching to liquid nitrogen after heat the temperature to 850? and keep for 2 hour. It contains a large number of martensite type I twin with some secondary twin happened. The Ni50-x/2Ti50-x/2Nbx alloy quenching results show that the content of Ni50-x/2Ti50-x/2Nbx alloy phase structure varies from Nb content, which mainly including B2 austenite phase, B19' martensite phase, NiTi2phase and ?-Nb phase.With solution treatment Ni50Ti50 SMA as the research object, established the martensite NiTi SMA macro finite element model based on the local canning compression experiment.Rigid-viscoplastic finite element method is used to simulate the local canning compression of NiTi SMA. Information about the deformed sample, such as, distribution of equivalent strain and stress, velocity field and load-stroke curve has been got,which lays an important foundation for metal flow mechanism based on rules of NiTi SMA subject to the local canning compression.The compression experiment of solution treatment NiTi SMA under local canning is carried out. Experiment results show that Nanocrystalline and amorphous phase are observed at 50% deformation of NiTi SMA, and there is uneven distribution of deformation.amorphous tendency of the alloys increases with the increasing deformation of NiTi SMA. In addition, deformation twins formed in SMA alloy which belong to the compound twin.In order to form the amorphous structure the locally canned solution treatment NiTi SMA is compressed to the reduction in height by 75%. Then the amorphous NiTi SMA are annealed at 300? 450? and 600? for 2 h, respectively. The true stress-strain curves are obtained by compression experiments of the crystallization NiTi SMA. It can be found that the NiTi sample crystallization at 300? exhibites an extremely high elastic limite, but the plasticity is poor. The NiTi samples crystallization at 450? exhibite a high elastic limite and high plasticity. The NiTi samples crystallization at 600? exhibite lower yield strength but appropriate plasticity.