Acoustic Emission During Rubber-like Deformation in Ni₅₁Fe₁₈Ga₂₇Co₄ Single Crystalline Shape Memory Alloys

Acoustic emission, AE, belonging to a rubber-like deformation in a martensitic state after the stabilization aging of the stress-induced martensite (SIM aging) of Ni₅₁Fe₁₈Ga₂₇Co₄ single crystals in compression, were investigated. AE activity in the plateau regions of the stress–strain loop is due to a massive reorientation from the variants produced by SIM aging to the variants preferred by the compressive stress (perpendicular to the stress used in SIM aging) and vice versa. For unloading, the large AE activity just at the knee point of the stress–stain curve is attributed to the difficulty of the re-nucleation of the SIM aging-stabilized martensite variant. The amplitude, peak energy, and area of signals can be described by power-like distributions and the characteristic exponents are in good agreement with data obtained in other alloys. Power law cross-correlations between the energy, E, and amplitude, A, as well as between the area, S, and the amplitude, A, were also analyzed. It was found that the exponents are given by $3-\phi$ as well as $2-\phi$ , respectively, with $\phi \cong \mathrm{0.7.}$ Normalized universal temporal shapes of avalanches (i.e., the ${\displaystyle \frac{U}{A}}$ versus ${\displaystyle \frac{t}{A^{1-\phi }}}$ plots, where U is the detected voltage) for a fixed area scale very well together. The tail of the normalized temporal shape decays more slowly than the theoretical prediction, which can be attributed to an intrinsic absorption of AE signals and/or to the overlap of sub-avalanches.