Defect pinning of interface motion in thermoelastic structural transitions of Cu-Al-Ni shape-memory alloy

The high mobility of austenite-martensite interfaces is a characteristic of a thermoelastic martensitic transformation. Internal friction and elastic constants are very suitable probes to analyze this mobility. In this work, resonant ultrasound spectroscopy, differential scanning calorimetry, and neutron powder diffraction have been employed to analyze the role of defects in a first-order transformation. An anomalous behavior associated with the martensitic transformation in a Cu-Al-Ni shape-memory alloy has been observed; the internal friction peak measured during cooling completely disappears on heating. The elastic constants also show different behavior on heating and cooling. The different mobility of defects in the two phases, and the simultaneous occurrence of both the defect recovery processes and the martensitic transformation in the same temperature range, are the origin of the observed behavior. These effects show an exceptional influence of defects on thermoelastic equilibrium during a first-order structural transition. The proposed mechanism is general and may apply to other transitions than the one reported in this paper.