Underlying mechanism of dual hysteresis in NiMnGa single crystals

NiMnGa single crystals are compressed in the [0 0 1] orientation at room temperature. The stress–strain response exhibits multiple stages as the deformation proceeds. Initially, the response exhibits a stress drop in the stress–strain curve preceding a plateau stress. By contrast, the second transition, which occurs at five times the initial critical transformation stress, produces a work hardening like response. Complete pseudoelastic (PE) recovery ensues upon unloading with the reverse transformation exhibiting the same two-stages. Furthermore, for the dual hysteresis that results, the second hysteresis is nearly four times wider than the first. The underlying transformation path for each stage is ascertained from local strain analysis utilizing variable magnification in situ digital image correlation (DIC). We distinguish three different morphological transitions; band formation, phase front propagation, and heterogeneous growth. The morphologies can be attributed to austenite undergoing the successive transitions 10M → 14M (modulated to modulated) and 10M → L10 (modulated to non-modulated). Differential critical stress and hysteresis levels are rationalized based on the initial modulated-to-modulated and successive modulated-to-non-modulated transition. The strain–temperature response is reported, as well, and exhibits a tiny thermal hysteresis (5 °C), which is attributed to the modulated-to-modulated conversion.