Molecular Dynamics Simulation Of Shape Memory Effect And Superelastic Mechanical Properties Of Porous NiTi Alloys

NiTi shape memory alloy has been widely used in aerospace,civil engineering,medical and health fields due to its shape memory effect,superelasticity,low density,high porosity and biocompatibility properties.The temperature-induced and strain-induced phase transformation of porous NiTi alloy is closely related to porosity.It is of great theoretical significance to study the effect of porosity on the mechanical behavior and microstructure evolution of porous NiTi shape memory alloy phase transformation.In this paper,a porous NiTi alloy model and four porous NiTi alloy models with different porosity（porosity is 1.5%,5.0%,11.7%and 22.6%respectively）were established.Based on the second nearest neighbor modified embedded 2NN-MEAM potential function,Nose Hoover temperature controlled relaxation method,NPT ensemble and periodic boundary,the shape memory effect and superelasticity of porous NiTi alloys were simulated by molecular dynamics method,and the porosity The influence of loading and unloading rates and other factors on the mechanical behavior and microstructure evolution of porous NiTi alloys under temperature and strain induced phase transformation.The main content of the paper includes:（1）Temperature induced phase transition:A temperature induced phase transition simulation was conducted on the established NiTi alloy porous models with no pores and four different porosities,with a temperature range of 5K-500K and a temperature rise and fall rate of 10K/ps.By analyzing the temperature induced martensite transformation and austenite transformation process of porous and non porous NiTi alloys,the critical transformation temperature of porous NiTi shape memory alloys was obtained,and the rule of the critical transformation temperature changing with porosity was given.（2）Shape memory effect:cooling-loading-unloading–warming shape memory effect simulation was carried out on the established porous models of NiTi alloy with no pores and four different porosity.At a temperature of 5K lower than the end temperature of martensitic transformation,five NiTi alloy models were simulated along the direction[001]with single compression-tensile loading and unloading and 5k-500K heating at 2×10^-3/ps and 4×10^-3/ps strain loading and unloading rates.The shape memory effect of porous NiTi alloys with porosity and loading and unloading rate was discussed by analyzing the mechanical behavior and microscopic process evolution during loading and unloading and temperature simulation.（3）Hyperelastic:The superelastic effect of loading-unloading was simulated for the porous models with no pores and four different porosity of NiTi alloy.The simulated loading and unloading temperatures were T=400K,450K and 500K（the temperature was higher than the end temperature of austenitic phase transformation）,and the loading and unloading rates were1×10^-3/ps,2×10^-3/ps and 4×10^-3/ps,respectively.The effects of porosity,simulated temperature and loading and unloading rate on the mechanical behavior and microstructure evolution of porous NiTi shape memory alloy were analyzed.The results of this study are helpful to further understand the phase transition behavior of porous NiTi shape memory alloy at the nanoscale,and provide some references for the optimization design of porous NiTi alloy in industrial applications.