Crossover from Mechanical Stabilization of Martensite to Spanning Avalanche-Type Reverse Transformation of Deformed Cu-Al-Ni Shape Memory Crystals

A new interpretation of factors controlling the kinetics of reverse transformation of martensites stabilized by prestrain is suggested. We investigate the effect of amount of prestrain in the β1´martensite and in the β-phase on temperature and kinetics of the reverse martensitic transformation in Cu-Al-Ni shape memory alloy single crystals. Calorimetry and video recording of the jumping samples, provoked by the burst strain recovery during temperature-induced reverse transformation, are used in experiments. For crystals deformed both in the β´1 martensite and in the β-phase, we report the existence of a sharp transition (crossover) from the mechanical stabilization of martensite to the burst reverse martensitic transformation which proceeds as a single avalanche. The crossover occurs for prestrain values close to the maximum transformation strain. The specific energy of the jump during burst strain recovery depends on the maximum stress applied during prestraining the sample. The crossover from a broad transformation range to an infinite or spanning avalanche is predicted by 3D random field Ising model for disordered systems undergoing the first order phase transition. Based on this model, we explain the crossover in deformed Cu-Al-Ni crystals by a rapid decrease of structural disorder towards the critical one at the final stage of the formation of detwinned γ´1martensite. A simple solution is obtained for the critical transformation rate which produces jumping of the sample during burst reverse transformation after prestrain.