Investigation On Microstructural Evolution And Mechanical Behavior Of NiTi Shape Memory Alloys Subjected To Thermomechanical Processing

Because of their excellent superelasticity and shape memory effect,Ni Ti shape memory alloys（SMAs）have been widely applied in engineering and medical domains.Due to grain refinement,nanostructured Ni Ti SMAs possess enhanced functional properties compared with conventional coarse-grained one.Thermomechanical processing involving severe plastic deformation（SPD）and subsequent annealing can be effectively used for producing nanostructured Ni Ti SMAs.In the present study,Ni_50.7Ti_49.3SMA with B2 cubic austenite structure and Ni₅₀Ti₅₀SMA with B19′monoclinic martensite structure experience SPD by means of local canning compression at room temperature,and subsequently they are subjected to annealing at 450℃for various times.It lays the theoretical foundation for manufacturing high-performance nanocrystalline Ni Ti SMAs to investigate the microstructure evolution and mechanical behaviors of Ni_50.7Ti_49.3SMAs and Ni₅₀Ti₅₀SMAs during thermomechanical processing by combining compressive mechanical property experiment technology,scanning electron microscopy（SEM）technology,electron backscattered diffraction（EBSD）technology and transmission electron microscopy（TEM）technology,crystal plasticity finite element simulation technology and discrete dislocation dynamic simulation technology.The main research results of the present work are as follows.Local amorphization and nanocrystallization can take place when Ni₅₀Ti₅₀SMA is subjected to SPD based on local canning compression at room temperature.The subsequent annealing at 450℃leads to the nanocrystallization of amorphous phase processed by SPD.The annealed Ni₅₀Ti₅₀SMA presents much lower plasticity but higher fracture strength compared with the solution-treated one.The plasticity of the annealed alloy increases with increasing annealing time.Moreover,the microstructure difference induced by thermomechanical processing also leads to various fracture characteristics,where the solution-treated sample presents ductile fracture while the samples processed by 50%local canning compression and thermomechanical processing exhibit brittle fracture.The reasons for different fracture characteristics of the annealed sample and the solution-treated one are attributed to the following two aspects.On one hand,the decreases of both plastic and reorientation-detwinning zones weaken their shielding effect of preventing the crack growth.On the other hand,the difference of crack-path configuration leads to larger proportion of fracture modeⅠin the annealed alloy.Similarly,local amorphization and nanocrystallization can be also implemented when Ni_50.7i T_49.3SMA is subjected to SPD based on local canning compression.Nanocrystallization can be realized in the Ni_50.7Ti_49.3SMAs subjected to SPD and subsequent annealing at 450℃.Meanwhile,Ni_50.7Ti_49.3SMA processes higher grain growth rate compared with Ni₅₀Ti₅₀SMA.In addition,{111}and{111}type I martensite twins are found in the annealed Ni_50.7Ti_49.3SMAs where B2 austenite phase is dominant in the matrix,which is attributed to the fact that local inhomogeneous chemical composition in the microstructure of the Ni_50.7Ti_49.3samples is caused by thermomechanical processing,and the appearance of Ni-poor region leads to the increase of martensite transformation start temperature in the local area.The annealed Ni_50.7Ti_49.3SMA possesses extremely high yield stress and poor plasticity compared with the solution-treated one.Furthermore,stress-induced martensite transformation of Ni_50.7Ti_49.3SMA is related closely to grain size and it is gradually inhibited with the decreasing nanocrystalline grain size.Crystal plasticity finite element simulation is employed for revealing grain size effect of nanocrystalline Ni_50.7Ti_49.3SMA under uniaxial compression at 400℃.Crystal plasticity finite element models of nanocrystalline Ni_50.7Ti_49.3SMA with different grain sizes are established based on experimental results.It should be pointed out that statistically stored dislocation（SSD）and geometrically necessary dislocation（GND）densities are incorporated into crystal plasticity constitutive model.Crystal plasticity finite element simulation results agree well with compressive experiment results of nanocrystalline Ni_50.7Ti_49.3SMA at 400℃.The results indicate that the variation trend of yield stress with average grain size satisfies Hall-Petch effect.Furthermore,it is found that flow stress,SSD density and GND density of nanocrystalline Ni_50.7Ti_49.3SMAs decrease with the increase of average grain size.The effects of different grain sizes and different loading directions on the evolution of the yield stress and dislocation configuration of single grain of nanocrystalline polycrystalline Ni Ti SMA are investigated based on discrete dislocation dynamic simulation.It is found that the yield stress of single grain of Ni Ti SMA decreases significantly and the dislocation density decreases as the grain size increases.The simulation results show that single grain of Ni Ti SMA exhibits strong strain hardening due to the rapid multiplication of dislocation for any grain size and any loading direction.Furthermore,different loading directions result in different rates of dislocation multiplication.Therefore,the strain hardening of single grain of Ni Ti SMA has a strong orientation dependence.