Study On Crystallization Mechanism Of NiTi Shape Memory Alloy Under Cold Plastic Deformation And Annealing

NiTi shape memory alloy(NiTi SMA)is widely used in biomedical and aerospace fields due to its shape-memory effect as well as superelasticity.However,with the rapid development of science and technology,NiTi SMA must meet the higher and higher requirements in terms of functions and properties.The involved studies have shown that nanocrystaline NiTi SMA possesses excellent mechanical properties,superelasticity,corrosion resistance and biocompatibility compared with coarse-grained one.Therefore,preparation of nanocrystaline NiTi SMA has potentially broad application perspective.It is well known that severe plastic deformation(SPD)is able to lead to large plastic strain of metallic materials and thus refine the grains by means of changing microstructures of bulk metal materials based on high strain,which contributes to enhancing or improving the mechanical properties of metallic materials.In the present study,by combining SPD technique based on local canning compression,crystallization technique by heat treatment,electron back scattering diffraction(EBSD),transmission electron microscopy(TEM),differential scanning calorimetry(DSC)and compressive mechanical tests,it lays scientific and theoretical foundations for manufacturing high-performance nanocrystalline NiTi SMA to investigate amorphization mechanism of equiatomic NiTi SMA subjected to SPD based on local canning compression,understand the mechanical properties and phase transformation behavior of NiTi SMA after crystallization,and finally reveal crystallization mechanism of amorphous NiTi SMA during heat treatment.In the present work,the main results are as follows.As-received hot-rolled equiatomic NiTi SMA was subjected to solution treatment,where it was held at 850? for 2h and subsequently was quenched into liquid nitrogen,in order to lead to complete martensite phase transformation.According to XRD analysis and TEM observation,it can be found that complete martensite phase transformation does take place in the NiTi SMA subjected to solution treatment.In addition,the solution-treated NiTi SMA contains plenty of martensite twins,which belong to type?twins,where secondary twins occur simultaneously.The solution-treated NiTi SMA was subjected to local canning compression.When NiTi SMA experiences local canning compression by 25%,the deformation characteristic ofmartensite laths can be observed and detwinning can take place.In addition,there exists a small amount of nanocrystalline phase and amorphous phase.When NiTi SMA experiences local canning compression by 50%,it can be observed that nanocrystalline phase and amorphous phase coexist.In the same manner,the feature of detwinning can be found and there exist deformation twinning.When NiTi SMA undergoes local canning compression by75%,it is subjected to severe amorphization and simultaneously the characteristic of detwinning can be captured.When martensitic NiTi SMA is subjected to amorphization during severe plastic deformation based on local canning compression,the mechanism for amorphization deals with martensite reorientation,deformation twinning,detwinning and dislocation slip.Equiatomic NiTi SMA,which was subjected to severe plastic deformation based on local canning compression by 75%,is crystallized for 2 h at 300,450 and 600?,respectively.TEM observation results indicate that the size of grain increases with the increase in the crystallization temperature during heat treatment.In the case of crystallization at 300?,nanocrystalline phase is dominant in NiTi SMA,where there exists local amorphous zone.In the case of crystallization at 450?,almost complete nanocrystalline phase can be obtained.In the case of crystallization at 600?,the size of grain is enhanced substantially,but there still exists a small amount of nanocrstalline phase,where twins can be observed.As for crystallized Ni Ti SMA,the compression experiment is implemented in order to the corresponding stress-strain curves.The experimental results demonstrate that in the case of crystallization at 300?,NiTi SMA exhibits extremely high elastic limit,but its plasticity is poorer.In the case of crystallization at 450?,NiTi SMA exhibits very high yield strength,but its plasticity is also better.In the case of crystallization at 600?,NiTi SMA exhibits relatively lower yield strength,but its yield strength is still higher than that of as-received NiTi SMA and its plasticity is also better.In the case of crystallization at 300 and 450?,NiTi SMA exhibits one-stage phase transformation during heating and cooling.In other words,phase transformation from anstenite to martensite can take place on cooling,whereas phase transformation from martensite to anstenite can occur on heating.In the case of crystallization at 600?,NiTi SMA exhibits one-stage phase transformation from martensite to anstenite during heating,whereas exhibits two-stage phase transformation during cooling,namely one-stage phasetransformation from anstenite to R phase and two-stage phase transformation from R phase to martensite.In addition,in the case of crystallization at 300?,NiTi SMA exhibits very small phase transformation hysteresis.