The stress-strain-temperature thermomechanical responses of NiTi shape memory alloys due to B2-B19′ martensitic transformation (MT) should ideally be phase and strain reversible in closed-loop thermomechanical load cycles, where the austenite and martensite phases do not undergo plastic deformation. However, this ideal behavior is only observed when MT occurs under zero or very low externally applied stresses. When MT occurs under higher externally applied stresses, it generates small plastic strains. These strains accumulate whenever MT proceeds under external stress, leading to the accumulation of residual plastic strains, internal stress, and lattice defects during cyclic thermomechanical loads. This accumulation results in the instability of cyclic thermomechanical responses, a phenomenon known as “functional fatigue.”

Compared to conventional engineering materials, NiTi shape memory alloys deform via a wide range of deformation mechanisms owing to the B2⇔B19’ martensitic transformation, including twinning in martensite and plastic deformation by dislocation slip. A detailed understanding of the functional properties of NiTi requires comprehensive knowledge of all deformation processes possibly activated in thermomechanical loads. A stress-temperature diagram (Fig. 1c), constructed from the results of isothermal (Fig. 1a) and isostress (Fig. 1b) tensile tests on superelastic NiTi wire (Fig. 1a,b), provides basic information on the critical stress and temperature conditions at which individual deformation/transformation processes are activated in thermomechanical loads. The σ-T diagram is a very useful tool in NiTi research since it defines stress and temperature conditions under which martensitic transformation occurs and plastic deformation is avoided. Problems arise when multiple deformation processes are activated simultaneously, and one cannot be sure which deformation mechanism is activated. In such cases, in-situ experimental methods (e.g., in- situ electric resistivity, in-situ ultrasonic methods, in-situ x-ray diffraction) are beneficially employed. In this work, we report on the application of in-situ Dynamic Mechanical Analysis (DMA) to detect and distinguish the activation of various deformation/transformation processes during the tensile thermomechanical loading of nanocrystalline NiTi wires, particularly upon isostress heating under a wide range of tensile stresses up to fracture (Fig. 1b,d).