Plastic Deformation And Anisotropy Of Nanocrystalline NiTi Shape Memory Alloy During Phase Transformation

NiTi shape memory alloy has excellent shape memory effect,super elasticity and biocompatibility,and is currently the most potential shape memory material.The nanocrystalline NiTi has better performance,but the stress-induced martensite transformation of nanocrystalline NiTi alloy has the problem of poor cycle stability,and the mechanism is still unclear.Therefore,it is important to study the microscopic deformation mechanism of stress-induced martensitic phase transformation of nanocrystalline NiTi shape memory alloy.In view of the above problems,this paper adopts heat treatment methods such as cold drawing and annealing to prepare Ni_51.2Ti_48.8 shape memory alloy wire with different grain sizes;and through solid solution and aging precipitation of Ni₄Ti₃ second phase to improve the stability of NiTi alloy and extend NiTi service life of the alloy and the expansion of the alloy application range.The cyclic tensile properties of nanocrystalline NiTi alloys were studied,and the deformation mechanism of nanocrystalline NiTi during phase transformation was analyzed.It was found that the inflection point of the stress-induced phase transformation in the tensile stress-strain curve of the nanocrystalline sample has shifted to the left,while the ultrafine-grained NiTi alloy has no obvious change.In addition,the nanocrystalline samples showed a significant migration of<7 7 11>direction texture towards the<2 2 3>direction after stretching,while the ultra-fine-grained NiTi alloy has no obvious texture migration;The orientation distribution function of nanocrystalline?2=45°shows strong texture in the direction of<1 1 0>the structure was decomposed into weak textures in the directions of<3 2 0>and<2 3 0>.The above results prove that the plastic deformation mechanism in the stress-induced phase transformation of nanocrystalline NiTi was mainly the grain rotation and the reorientation of the martensite variants.The effect of aging on the phase transformation of nanocrystalline NiTi alloy was studied.It was found that the grains grow up with the increase of annealing/aging temperature,and Ni₄Ti₃ precipitated particles are precipitated at the intersection of grain boundaries in the sample after annealing.As the annealing temperature(350??500?)increases,the grain size increases,and the phase transformation initiation temperature R_sof the nanocrystalline NiTi alloy decreases,indicating that the grain size has a major effect on the phase transformation point of the nanocrystalline NiTi alloy.As the increase of aging temperature(250??300?),the initial phase transition temperature R_s of nanocrystalline NiTi alloy increases.The main reason is that the precipitated phase Ni₄Ti₃is more likely to increase and grow in the large grain size,which leads to the decrease of the content of Ni in the matrix and the increase of R_s;That is,after the grain size grows,the influence of the Ni content on the phase transition temperature will play a major role.The test results of cyclic stretching show that the stress plateau of the sample after aging is smaller than that of the sample before aging.It can be seen that low-temperature aging can significantly improve the functional stability of nanocrystals.The micromechanical properties of nanocrystalline NiTi alloys were studied.It was found that the mechanical properties of the R phase and the B19?phase of the NiTi alloy with a grain size of less than 60nm are anisotropic,and the micromechanical properties of the R phase are relatively weakly dependent on the crystal orientation,while the B19?phase has serious crystallography orientation dependence.Combined with the analysis of the martensite pole figure and the inverse pole figure,the single-variant martensite in the nanocrystalline NiTi alloy appears in a specific<1 2 0>orientation arrangement during the stress-induced martensite phase has a great influence on oriented martensite texture,making the anisotropy of martensite mechanical properties more obvious.