On the nature of the thermoelastic martensitic phase transformation in Au-47.5 at.% Cd determined by acoustic emission

Abstract

The acoustic emission activity accompanying thermoelastic martensitic phase change in Au-47.5 at.% Cd alloy during the forward and reverse transformations was found to be asymmetric in energy and pulse-width distribution. The amount of acoustic energy recorded at the transducer was higher for the more complex martensitic microstructure. When the alloy was allowed to perform the transformation shear strain without constraints, as in the case of single-crystal single-interface experiments, no acoustic activity was detected. Conversely, when the movement of the single-interface was impeded and could proceed only by relaxation of local stresses, acoustic emission was recorded. The acoustic emission technique enables evaluation of the nonchemical contribution to the free energy change of the phase transformation. The acoustic energy emitted during the forward transformation, on cooling, was related to the frictional resistance opposing the movement of phase interfaces. A similar frictional resistance evolves during the reverse transformation, on heating. However, the elastic stored energy related to the change of shape upon formation of the martensite, is released during the reverse transformation. The extent of this contribution in acoustic activity depends on the morphologies of the transforming phases.