Study On Super-elasticity Degeneration Mechanism Of NiTi Shape Memory Alloy By Molecular Dynamics Simulations

Nearly equiatomic NiTi shape memory alloys(SMAs)are one of the most promising smart materials due to their excellent mechanical properties,and are widely used in aerospace,automotive,biomedical and daily life.The devices made from super-elastic NiTi SMAs are frequently subjected to a cyclic loading in actual service.During cyclic deformation,super-elastic NiTi SMA would exhibit functional degeneration,that is,super-elasticity degeneration.Cyclic deformation and related structure damage evolution are very important in evaluating the service capability and reliability of NiTi SMA devices.At atomic scale,the super-elastic degeneration of NiTi SMA is manifested by atomic rearrangement,the change of crystal structure,and the slipping,dislocation,twinning and the accumulation of defect caused by the changes in the spatial position of atoms and crystals.So,the study of cyclic deformation behavior of super-elastic NiTi SMA from the atomic scale is helpful to understand the cyclic deformation characteristics of super-elastic NiTi SMA and the deformation mechanisms of super-elastic NiTi SMA at nanometer scale.The deformation mechanims of NiTi SMA at atomic scale were widely investigated by molecular dynamics simulation in recent years.However,the related research is mainly focused on the super-elasticity of NiTi SMA in a single loading-unloading condition,rather than the cyclic phase transformation,plastic deformation and its interaction.In order to investigate the core problems of cyclic transformation,plastic deformation and its interaction and thermo-mechanical coupled characteristics involved in the super-elastic NiTi SMA subjected to a cyclic deformation,the following molecular dynamics simulations were carried out:(1)By the molecular dynamics simulation to the thermo-mechanical coupled response of the single crystal bulk NiTi SMA under the compression/unloading and an adiabatic condition,the temperature change and the nucleation and growth of martensite transition were discussed.The simulated results of molecular dynamics exhibited that the single crystal bulk NiTi SMA showed a significant temperature change during the martensite transition and its reverse under an adiabatic condition;moreover,a localized instability occurred apparently during martensite transition,which was closely related to the nucleation and growth rates of martensite phase;no instability was observed in the simulated stress-strain curves if the model size was relatively larger.(2)The super-elasticity of equiatomic NiTi SMA nano-pillar subjected to a uniaxial cyclic compression and unloading was investigated by molecular dynamics simulation.By analyzing the local atomic structure,the nucleation sites and propagation paths of martensite transition and its reverse,the degeneration of super-elasticity and the interaction between martensitic transition and defects at nano-scale during cyclic compressive deformation were discussed.It was indicated that the dislocation and twinning were responsible for the changes in the atomic structure and mechanical response,the accumulated irreversible strain and the degeneration of super-elasticity.(3)By establishing some atomistic simulation cells with the same size but different numbers of grains,molecular dynamics simulations were performed to investigate the super-elasticity of nanocrystalline NiTi SMA subjected to a cyclic tension-unloading and its dependence on the grain size.The effect of grain boundaries on the martensite transformation stress as well as the nucleation and growth of martensite phase was addressed.The degeneration of super-elasticity and the initiation and growth of defects in the nanocrystalline NiTi SMA during the cyclic tension-unloading were discussed.The results showed that the super-elasticity degeneration occurred during the cyclic deformation of nanocrystalline NiTi SMA,and the residual strain accumulated progressively with the increasing number of cycles,which became more significant with the decrease of grain size.The grain boundaries could enhance the martensite transformation stress of nanocrystalline NiTi SMA and suppress the instability occurred during the martensite transformation.(4)The transformation ratchetting behavior(i.e.the cyclic accumulation of peak/valley strain)of super-elastic nanocrystalline NiTi SMA subjected to a cyclic loading and its deformation mechanism at atomic scale were studied by molecular dynamics simulations.The effects of ambient temperature,applied peak stress and stress rate on the transformation ratchetting behavior of nanocrystalline NiTi SMA were discussed under an isothermal condition;in addition,the effect of temperature rise caused by the transformation latent heat and inelastic dissipation on the transformation ratchetting behavior of nanocrystalline NiTi SMA was further investigated under an adiabatic condition by setting different stress rates.The molecular dynamics simulations showed that the transformation ratchetting of nanocrystalline NiTi SMA occurred at atomic level and depended on different thermal boundary conditions.Under an isothermal condition,the transformation ratchetting was caused by both the plastic deformation occurred at grain boundaries and in the disordered structures within the grains and the accumulation of residual B19? martensite phase;but,it was determined only by the plastic deformation at grain boundaries and in the disordered structures within the grains under an adiabatic condition.