Macroscopic and Microstructural Aspects of the Transformation Behavior in a Polycrystalline NiTi Shape Memory Alloy

The mechanical and microstructural behavior of a polycrystalline Ni(49.9)Ti(50.1) (at.%) shape memory alloy was investigated as a function of temperature around the transformation regime. The bulk macroscopic responses, measured using ex situ tensile deformation and impulse excitation tests, were compared to the microstructural evolution captured using in situ neutron diffraction. The onset stress for inelastic deformation and dynamic Young's modulus were found to decrease with temperature, in the martensite regime, reaching a significant minimum at approximately 80 C followed by an increase in both properties, attributed to the martensite to austenite transformation. The initial decrease in material compliance during heating affected the ease with which martensite reorientation and detwinning could occur, ultimately impacting the stress for inelastic deformation prior to the start of the reverse transformation.