Molecular Dynamics Simulation On The One-way Shape Memory Effect Of NiTi Shape Memory Alloy Single Crystal

With the continuous development of nanotechnology and the progressive miniaturization of materials and devices,nanocrystalline NiTi alloys have been widely used in many fields such as electronics,machinery,aviation,energy,transportation,and medical treatment.The unique thermo-mechanical properties of NiTi shape memory alloy are determined by its martensite transformation and reorientation mechanism.Therefore,revealing the phase transition and reorientation microscopic mechanism on the atomic scale can not only provide theoretical guidance for the design and preparation of high-performance NiTi shape memory alloy,but also provide information for the establishment of constitutive model based on microscopic mechanism,which has important scientific and engineering significance.Although much progress has been made in molecular dynamics simulation of NiTi alloy stress/temperature-induced martensite transformation,the existing work rarely involves a process unique to the one-way shape memory effect,namely stress-induced martensite reorientation,and the dependence of the one-way shape memory effect on loading conditions is also not explored.In this paper,the one-way shape memory effect of nanocrystalline NiTi alloy was simulated by the molecular dynamics method based on the second nearest neighbor modified embedded atomic potential,and the crystal structure characteristics and microstructure evolution in the process of temperature-induced martensitic transformation and stress-induced martensite reorientation were revealed.The change of NiTi alloy martensitic transformation and martensite reorientation at atomic scale caused by different peak stresses and different loading and unloading rates was investigated.In addition,the influence of boundary conditions and size factors was also analyzed.The main work of this thesis is summarized as follows:(1)A molecular dynamics model of nano-single-crystal NiTi shape memory alloy was established,the shape memory effect of bulk single-crystal NiTi alloy under periodic boundary conditions was simulated,and the influence of peak stress and stress rate on the shape memory effect of NiTi alloy was analyzed.(2)Adopting a non-periodic boundary,the shape memory effect of the NiTi single crystal nano-pillar was simulated.The influence caused by free surface was further discussed.(3)Three NiTi single crystal nano-pillar models with different sizes were constructed by changing the length,width and height ratio of the original model.The effect of size on the temperature-induced phase transition and shape memory effect of single crystal nano-pillars under non-periodic boundary conditions were discussed.The research work in this paper is of great significance for further revealing the microscopic evolution mechanism of NiTi shape memory alloy.This study can clarify the atomic migration processes involved in the phase transition process(such as martensite nucleation and growth,crystal twinning behavior and the occurrence of unrecoverable strain),all of which have important practical guiding significance for the performance optimization,safety design and life evaluation of NiTi shape memory alloy products.