KINETICS OF MATERIALS WITH WIGGLY ENERGIES - THEORY AND APPLICATION TO THE EVOLUTION OF TWINNING MICROSTRUCTURES IN A CU-AL-NI SHAPE-MEMORYALLOY

We analyse the kinetics of the transition between two variants of mart ensite during biaxial dead loading. The volume fraction of one martens ite variant versus the applied loads exhibits an unusual hysteresis, c haracterized by a sensitive dependence on the amplitude of the loads a nd a dissipationless response at small amplitude. Observation of the m icroscopic volume fraction at the level of a few bands of martensite r eveals that the main mechanism by which one variant grows at the expen se of another is a tip-splitting event; the tips of martensite needles present in the specimen suddenly split. This leads us to adopt a form of the energy in which many little wiggles are superposed on a slowly varying function that accounts for the loading device, elastic and in terfacial energies. We analyse the resulting microscopic kinetic law b y deriving from it a macroscopic kinetic equation that governs the ave rage response. This law inherits the phenomenon of 'getting stuck in l ocal minima of the energy'. It leads to good qualitative (and fair qua ntitative) agreement with observation over a very wide range of differ ent kinds of experiment.