Resonant ultrasound spectroscopy for characterization of lattice instability of shape memory alloys

Abstract

Shape memory alloys (SMAs) are materials able to undergo reversible displacive transitions between different crystal lattices. This ability is reflected by the elastic constants of the individual phases, as certain combinations of these constants exhibit anomalous softening when the crystal approaches the transformation temperature or when the lattice is destabilized by the presence of mobile interfaces. The lecture will summarize the advantages of using resonant ultrasound spectroscopy (RUS), and in particular its contact-less laser-based modification, for detecting and evaluating these elastic anomalies. Three recently studied cases will be discussed: i) single crystals of ferromagnetic shape memory alloys in which the instability-induced softening couples with magnetoelasticity; ii) fine regular laminates in tetragonal martensitic lattices in which the soft combinations of the elastic constants have to be calculated from the effective elastic response of the microstructure; iii) modulated crystals with highly mobile interfaces that act as strongly-nonlinear mechanical oscillators and where the properties of the interfaces can be achieved by analyzing the non-linearity.