Acoustic emission of Ni–Mn–Ga magnetic shape memory alloy in different straining modes

We investigated a Ni49.7Mn29.1Ga21.2 single crystalline sample exhibiting 5.7% magnetic-field-induced strain (magnetic shape memory effect) by means of acoustic emission (AE) experiments. The AE was monitored during thermally- and stress-induced martensitic transformations and during compressive stress-induced martensite reorientation. Partial cycles were measured for the stress-induced transformation. All processes are acoustically active. The highest AE activity was detected during stress-induced transformation from martensite to austenite upon unloading. Observed difference between AE activity during stress-induced forward and reverse transformations might be attributed to different dynamics of transformation processes. Austenite remains acoustically inactive during loading and unloading as well as during thermal cycling. Martensite exhibits acoustic activity during stress-induced reorientation, during loading and unloading, and lower activity during thermal cycling. This acoustic activity can be tentatively ascribed to twinning processes.