Study Of Microscopic Deformation Behavior Of Martensitic NiTi(Nb) Shape Memory Alloys

Martensitic B19’-NiTi shape memory alloys undergo martensite variant reorientation during the deformation process,which is accompanied by great changes in the texture and internal stress state,and then affects the mechanical behavior and shape memory effect of the NiTi alloys.The texture evolution is affected by the initial textures and deformation mode,and the load transfer behavior between martensite variants seems to be related to the texture.However,the texture evolution and load transfer behavior of the martensitic NiTi alloy wire during tensile deformation process have not been systematically studied.In addition,for second phase reinforced martensitic NiTi alloy composite wires,the influence of nanowires and nanoparticles on the martensite variant reorientation of the NiTi matrix,as well as the interaction between the nano second phases and NiTi matrix,are still unclear.This information is of great significance to understand the mechanical behavior of martensitic NiTi（based）shape memory alloy wires and thus guiding the design,processing and application of these alloys.This paper mainly studies the micro-deformation behavior of binary NiTi wire during the martensite variant reorientation process,such as texture evolution and load transfer among martensite variants,the micro-deformation behavior of NiTi matrix and Nb nanowires（nanoparticles）in the Nb nanowires（nanoparticles）-martensitic NiTi alloy composite wires,and the interaction between NiTi matrix and Nb nanowires（nanoparticles）in the composite wires.The main research contents and results of the thesis are listed as follows:（1）The binary B19’-NiTi wire has three fiber textures,which are（120）_B19’,（102）_B19’and（（?）02）_B19’,and the strength distribution of these three textures is uniform.In the composite wires,the restraint and coordination effect of Nb nanowires and Nb nanoparticles results in the dispersion of the intensity distribution of the NiTi textures,and the strength of（102）_B19’texture is stronger than those of the other two textures.In addition,no deformation twins were produced in NiTi matrix during deformation process due to the strong constraint of Nb nanowires,while the weaker constraint of the Nb nanoparticles resulted in dislocation slip of NiTi matrix under large deformation strain,and（20（?））deformation twins were generated,accompanied by the emergence of new（101）_B19’fiber texture.（2）In the process of NiTi martensite variant reorientation,the lattice strains of favorably oriented variants increase while those of unfavorably oriented variants decrease,in other word,the load of unfavorably oriented variants is transferred to favorably oriented variants.This is because the inter-variant coupling and inter-granular constraints in the self-accommodating B19’martensite make the variants nearly identical in elastic strain but greatly different in load bearing during elastic deformation,but these constraints are weakened and then load transfer occurs when variant reorientation occurs.Moreover,some unfavored variants produce negative lattice strains along the tensile direction,which is attributed to the strong initial texture of NiTi wire and the requirement for compatibility of the shape strains among the neighboring grains and variants to maintain matrix continuity.The dominant stress state on the unfavored variants is dictated by the surrounding environment instead of the externally applied stress.In addition,Nb nanowires and Nb nanoparticles have different degrees of influence on the load transfer behavior of martensite variants in NiTi matrix,and the influence of Nb nanowires is significantly greater than that of Nb nanoparticles.（3）The Nb nanowires in the composite wire achieve an ultra-large elastic strain of4.7%during tensile deformation,which is attributed to the"lattice strain matching"effect between the martensite reorientation lattice shear of NiTi matrix and the elastic strain of Nb nanowires.However,the synchrotron X-ray diffraction results show that the lattice strain of Nb nanoparticles is only 1.78%,which is attributed to the stress relaxation behavior of Nb nanoparticles during the deformation process.This strain result is the statistical average of lattice strains of many Nb nanoparticles after stress relaxation,and cannot represent the true lattice strain of Nb nanoparticles.TEM results show that the residual lattice strain of a single Nb nanoparticle in the pre-deformed wire reaches 2.78%,which indirectly proves that Nb nanoparticles can achieve large elastic strain in B19’-NiTi matrix.In addition,due to the different reorientation strain of differently oriented martensite variants in NiTi matrix,heterogeneous strain distribution exists in both Nb nanowires/nanoparticles.