Inelasticity-driven and path-dependent shifts in the transformation temperatures of high-temperature shape memory alloys

Transformation temperatures of high-temperature shape memory alloys (HTSMAs) can be modified by various thermo-mechanical processes, including those involving the introduction of inelastic deformation. This work focuses on the influence of the thermo-mechanical path used to introduce inelastic deformation on the transformation temperatures. Inelastic deformation is created following three distinct thermomechanical paths: isothermal, out-of-phase (involving a decrease of temperature while load is increased) and a combination of isothermal and out-of-phase. Each path can trigger different inelastic mechanisms (detwinning, TRIP and/or plastic yielding) resulting in different shifts of transformation temperatures. Changes in transformation temperatures are assessed using Differential Scanning Calorimetry (DSC) after unloading. Several DSC cycles are performed to evaluate the new transformation temperatures of the deformed material as well as any temporary effect recovered upon thermal cycling. Transformation temperatures show significant path dependency during the first DSC cycle. This path dependency is attributed to whether or not the path allows detwinning/reorientation to occur. However, the path-dependency of the transformation temperature shift disappears after the first DSC cycle. Only the absolute value of irrecoverable deformation affects the transformation temperatures after the initial reverse transformation. This feature is attributed to the recovery of detwinned/reoriented structures generated by some thermomechanical paths.