Investigation Of Plastic Deformation Mechanism And Local Amorphization Mechamism Of NiTi Shape Memory Alloy Based On Canning Compression

Because of their excellent shape memory effect and outstanding superelasticity,NiTi shape memory alloys are exploited for an ever-increasing diversity of applications in the field of materials science and engineering.In general,shape memory effect and superelasticity of NiTi shape memory alloys are attributed to a crystallographically reversible martensite phase transformation between a high-symmetry B2 parent austenite phase with cubic structure and a low-symmetry B19? martensite phase with monoclinic structure.As we know,plastic deformation is the main approach to manufacturing NiTi shape memory alloy products,such as NiTi wire,bar,tube,strip and sheet.Moreover,plastic deformation plays an important role in the application of NiTi shape memory alloy in the field of engineering.It is generally accepted that plastic deformation has a significant influence on the microstructure evolution of NiTi shape memory alloys,which further affects the shape memory effect,superelasticity and mechanical properties of NiTi shape memory alloys.However,depending on temperatures,NiTi shape memory alloys belong to either B2 austenite or B19? martensite,which means that plastic deformation mechanisms of NiTi shape memory alloys are also temperature-dependent and thus exhibit a predominant distinction in the case of various temperatures.Therefore,it shall be able to lay the scientific foundation for revealing the plastic deformation mechanisms of NiTi shape memory alloys in the case of various temperatures.In the present study,as a novel plastic deformation technique,canning compression is proposed.During canning compression,NiTi shape memory alloy samples are in a three-dimension compressive stress state,which contributes to suppressing the initiation and propagation of micro-crack in NiTi shape memory alloys,improving the capability of plastic deformation of NiTi shape memory alloys and even realizing the amorphization of NiTi shape memory alloys at room temperature and lower temperature.In the present study,the NiTi shape memory alloy with a nominal composition of Ni50.9at%Ti49.1at% is used.By combining canning compression experiment,crystal plasticity finite element method(CPFEM),electron back-scattered diffraction(EBSD)technique,transmission electron microscopy(TEM)technique and high resolution transmission electron microscopy(HRTEM)technique,from the macroscale,mesoscale,microscale and atomicscale perspectives,it shall be able to lay the theoretical foundation for manufacturing high-performance NiTi shape memory alloy to reveal the plastic deformation mechanism as well as the amorphization mechanism of NiTi shape memory alloy based on canning compression.The main achievements of the present study are as follows.By means of characterizing the microstructure evolution of NiTi shape memory alloy in the case of plastic deformation based on canning compression at various temperatures ranging from-150 to 800?,it is found that NiTi shape memory alloy possesses different plastic deformation mechanisms in the case of various temperatures,such as dislocation slip,deformation twinning,stress-induced martensite transformation and martensite reorientation.In addition,the amorphization of NiTi shape memory alloy is closely related to deformation degree and deformation temperature,where the degree of amorphization of NiTi shape memory alloy increases with the decrease of deformation temperature,and increases with increasing the deformation degree.In other words,there exist the critical deformation degree and the critical deformation temperature in terms of amorphization of NiTi shape memory alloy.Moreover,dislocation density plays a significant role in the amorphization of NiTi shape memory alloy in the case of plastic deformation based on canning compression.The amorphization of NiTi shape memory alloy can be regarded as the product of high dislocation density.Based on the statistically stored dislocation(SSD)and geometrically necessary dislocation(GND),a critical dislocation density model for the occurrence of amorphous phase in the case of plastic deformation based on canning compression is proposed in the present study.In the case of-150?,the plastic deformation mechanisms of martensitic NiTi shape memory alloy are involved in martensite reorientation and dislocation slip.The amorphization of NiTi shape memory alloy can occur under a certain deformation degree.Furthermore,it is found that a relatively large plastic strain contributes to realizing the mechanical stabilization of temperature-induced B19? martensite.In the case of room temperature,the plastic deformation mechanisms of austenitic NiTi shape memory alloy comprise of stress-induced martensite transformation,deformation twinning and dislocation slip.The amorphization of NiTi shape memory alloy can also occur under a certain deformation degree.Moreover,it is found that a relatively large plastic strain contributes to realizing the mechanical stabilization of stress-induced B19? martensite.In the case of 300?,austenitic NiTi shape memory alloy subjected to canning compression contributes to forming(114)deformation twin.In the case of 400? and higher temperature,the plastic deformation mechanisms of austenitic Ni Ti shape memory alloy subjected to canning compression are based on dislocation slip.Based on the classic crystal plasticity theory and the corresponding numeralization algorithm,in the present study,different polycrystal models are constructed and the corresponding crystal plasticity material parameters are identified by using the “trial and error” method.Crystal plasticity finite element simulations based on the constructed polycrystal models are conducted in order to investigate plastic deformation of NiTi shape memory alloy during uniaxial compression at 400?.Simulation results indicate that various loading directions have an influence on the mechanical response of material points within individual grains.In addition,the inhomogeneous microstructure evolution of NiTi shape memory alloy subject to uniaxial compression is well predicted on the basis of GND density.Furthermore,the contributions of various slip modes to plastic strain are investigated.As a consequence,the contribution of {110}<100> slip mode to plastic strain is primary in plastic deformation of NiTi shape memory alloy,then the contribution of {110}<111> slip mode is secondary and the contribution of {010}<100> slip mode is the least.In the case of large plastic strain,{110}<100> and {110}<111> slip modes facilitate the formation of ?(<111>)fiber texture,while the {010}<100> slip mode contributes to the formation of(001)[0 10] texture component.All the conducted simulations provide the parameter basis for the subsequent crystal plasticity finite element(CPFE)simulation of NiTi shape memory alloy subjected to canning compression.Based on the mechanism-based strain gradient crystal plasticity(MBSGCP)theory and the corresponding numeralization algorithm,in the present study,polycrystal model with optimal grain number and element number is constructed and the corresponding crystal plasticity material parameters are identified by using the “trial and error” method.Crystal plasticity finite element simulation based on the constructed polycrystal model is used for investigating plastic deformation of NiTi shape memory alloy subjected to canning compression at 400?.As a result,the texture evolution is predicted and the evolution of SSD density and GND density is also illustrated.It is found that ?(<111>)fiber texture is also formed during canning compression and the simulated results are in good accordance with the experimental ones.Moreover,during canning compression,the SSD and GND are aggregated near the grain boundaries.The aggregation of SSD results from the fact that the slip of SSD is blocked by GND near the grain boundaries,while the aggregation of GND stems from the generated strain gradient near the grain boundaries and it contributes to keeping the lattice continuity during plastic deformation.With the progression of plastic deformation,SSD density and GND density show the opposite variation rules.