| Nickel-titani?m shape memory alloy has been widely studied due to its excellent superelasticity and shape memory effect.These mechanical properties are derived from stress-and temperature-dependent martensitic phase transition behavior in the nickel-titani?m alloy.However,texture induced anisotropy of martensitic phase transformation still remains unexplored,and this thesis presents an in-depth study on this aspect.The as-received rolled NiTi alloy plate is composed of single-phase austenite,and has a < 110 > || ND and < 111 > || ND(normal direction)fiber texture.NiTi alloy samples are subjected to cyclic tensile loading along rolling direction(RD)and transverse direction(TD)with a mini material testing machine.During loading,X-ray diffraction is used to measure in situ the texture components and martensite content in NiTi alloy.The samples before and after loading are characterized by electron backscatter diffraction(EBSD)technique to obtain the residual martensite,grain orientation,grain size and so on.During loading,stress causes phase transformation from austenite to martensite in NiTi alloy.During unloading,reverse phase transformation from martensite to austenite occurs.Based on the crystallographic theory of texture evolution and trace analysis of the habit planes obtained from the EBSD data,the types of activated martensitic variants can be determined.The experimental results show that stress-strain curves of RD and TD samples have larger and smaller phase transition platforms for single cyclic loading,respectively.For RD sample,six martensite variants are activated and {111} < 112 > texture components are newly formed.For TD sample,only three martensitic variants are activated and strength of the initial {110} < 110 > texture component increases.A clear Ludes band can be observed on the surface of RD and TD samples.The Ludes bands firstly form in matrix grains with favorable orientation for martensitic transformation,and then in grains with unfavorable orientation for martensitic transformation.With increasing cycle n?mber,the transformation plateau on the stress-strain curve decreases to zero.Due to the disappearance of martensitic transformation plateau,martensitic transformation nucleation can not be judged from the stress-strain curve,but can be detected by XRD pattern,and martensitic transformation stress is less than the stress at martensitic transformation platform in first cycle.EBSD results show that the n?mber of residual martensite and twin increases with increasing cycle n?mber,resulting in a large amount of transformation twins and grain refinement.At the same time,deformation leads to orientation rotation in matrix grain and the increase in the proportion of grains with soft orientation,promoting the martensitic transformation.According to the Schmid factor analysis of the residual martensite trace in EBSD map,activated martensitic variants has larger Schmid factor than most other martensitic variants.In addition,more grain boundaries and residual martensites provide more nucleation sites for subsequent martensitic transformation,leading to the disappearance of macroscopic Ludes band and the reduction of martensitic transformation stress.The increase of martensite n?mber and grain refinement lead to strain hardening,resulting that stress increases continuously with increasing strain and phase transformation platform disappears.The hyperelastic behavior and martensitic transformation of NiTi alloy depend not only on the texture of the sample and the n?mber of loading cycles,but also on the loading strain.When cyclic loading strain is small,NiTi alloy has excellent hyperelasticity.The macro deformation is mainly mediated by martensite phase transformation,and negligible amount of residual martensite can be found in the sample after stress unloading.Large cyclic loading strain can disable hyperelasticity in NiTi alloy,part of the macro deformation is mediated by dislocation slip,and n?merous residual martensites can be found in the sample after stress unloading. |
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