Sarah M. Kamel

Acoustic emission, AE, belonging to rubber-like deformation in martensitic state after stress induced martensite stabilization (SIM) of Ni51Fe18Ga27Co4 single crystals, in compression were investigated. AE activity in the plateau regions of the stress-strain loop is due to massive reorientation from variant, 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-φ as well as 2-φ, respectively with φ≅0.7. Normalized universal temporal shapes of avalanches (i.e. the UA versus tA1-φ plots, where U is the detected voltage) for 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 intrinsic absorption of AE signals and/or to overlap of sub-avalanches.

In some shape-memory single crystals the stress–strain (σ~ε) curves, belonging to stress induced martensitic transformations from austenite to martensite at fixed temperature, instead of being the usual slightly increasing function or horizontal, have an overall negative slope with sudden stress drops in it. We discuss this phenomenon by using a local equilibrium thermodynamic approach and analysing the sign of the second derivative of the difference of the Gibbs free energy. We show that, considering also the possible nucleation and growth of two martensite structural modifications/variants, the stress–strain loops can be unstable. This means that the overall slope of the uploading branch of the stress–strain curve can be negative for smooth transformation if the second martensite, which is more stable with larger transformation strain, is the final product. We also show that local stress-drops on the stress–strain curve can appear if the nucleation of the second martensite is difficult, and the presence of such local stress-drops alone can also result in an overall negative slope of the stress–strain curves. It is illustrated that the increase of the temperature of the thermal recovery during burst-like transition is a measure of the change of the nucleation energy: the more stable martensite has larger nucleation energy.