In-situ Diffraction Studies On The Phase Transformation And Micro-mechanical Behavior Of NiTiNb Shape Memory Alloys

An accurate understanding and control of functional mechanism is one of the key technologies for the development and performance optimization of Functional Materials and its components. Wide hysteresis NiTiNb shape memory alloys show a martensitic transformation behavior induced by stress or temperature changes. Microscopic stress field will emerge as the martensite variants take preferred orientations. The interaction effect of macro-and micro-stress field has a great influence on the performance of related components. In this work, the evolution of macro-and micro-mechanical behavior of Ni47Ti44Nb9shape memory alloys is investigated through SEM, TEM and in situ high energy X-ray synchrotron diffraction. The conclusions are shown as follows:The microstructure of NiTiNb shape memory alloys during tensile test is observed by TEM. The Nb particles are embedded in NiTi matrix and twin martensite variants induced by applied stress have various morphologies, including twin variants which arrange alternatively, thick variants which occur after the self-accommodation and reorientation of various variants. The single large lath martensite contains a high density of dislocation and distribute across one or several grains sometimes. Some martensite variants even intersect each other. The ellipsoidal Nb particles are stretched along the loading direction and intersect with NiTi matrix to cause larger distortion and the bending of interfaces between martensite variants.The in situ tensile test of NiTiNb shape memory alloys is investigated through high energy X-ray synchrotron radiation at room temperature. The tensile test is divided into three stages, namely the stage of elastic deformation, the stage of martensitic stress-induced transformation and the stage of plastic deformation.The stage of elastic deformation:The macroscopic strain of austenite and Niobium increase linearly with applied loadings, which the grains are stretched on loading direction and compressed on transverse direction. As for the microscopic strain, lattice strain of crystal planes of austenite increase linearly with applied stress within the range of0-280MPa and present the anisotropy at280MPa on loading direction, which the trend of stress-strain curve of (211) crystal plane of austenite is lower than that of (110),(220) crystal planes of austenite; the evolution mechanism of micro-mechanical behavior on transverse direction is similar to the evolution on loading direction, except that the anisotropy is presented at220MPa.The stage of martensitic stress-induced transformation:The macroscopic strain remains about the same with applied loadings. As for the microscopic strain, the lattice strain of each crystal plane changes drastically with the remarkable decrease of austenitic peaks and the significant increase of martensitic peaks due to the stress-induced transformation from austenite to martensite. The stress-induced martensite are found to be highly anisotropic while the peaks of (010),(020) crystal planes of martensite only exit in loading direction and the peaks of (101),(111),(132) crystal planes of martensite only exit in transverse direction.The stage of plastic deformation:A small amount of austenite transform to martensite due to the stress-induced transformation. The martensite variants distributed along the (132) crystal planes are not in a preferable direction so as to rotate to the preferred orientation since the stress is496MPa.The evolution mechanism of micro-mechanical behavior of NiTiNb shape memory alloys during tensile cycle1to cycle5is studied by in situ synchrotron X-ray diffraction technique at203K. It is shown that NiTiNb shape memory alloys do have two evolution mechanisms. The evolution mechanism I is stress-induced transformation from austenite to martensite when the stress increases to the critical stress of martensitic transformation and reverse transformation from martensite to austenite during unloading. The evolution mechanism Ⅱ is that the martensite variants preferentially merge and re-orientate due to plastic deformation of martensite when the stress increases to the yield stress of martensite and the residual strain is irreversible after unloading. Cycle1and cycle2are based on the evolution mechanism Ⅰ; cycle3is mainly based on the evolution mechanism Ⅰ, supplemented by the evolution mechanism Ⅱ; cycle4is mainly based on the evolution mechanism Ⅱ, supplemented by the evolution mechanism Ⅰ; cycle5is based on the evolution mechanism Ⅱ. The micro-mechanical evolution mechanisms of various crystal planes of martensite in each cycle also show a significant anisotropy.