Recovery Mechanisms in a Compressed Ni-Ti Superelastic Alloy

Abstract

The present work studied the influence of annealing on a Ni-Ti alloy submitted to uniaxial compression. The stress-strain curve under compression showed the absence of the Stress-Induced-Martensite (SIM) plateau and 42% of remaining austenitic phase (B2). identified by the Electron Backscattered Diffraction (EBSD) technique on a Scanning Electronic Microscope (SEM). Annealing at 150 ºC increased the volume fraction of austenite to 70%. Annealing at 200 ºC reduced the volume fraction of austenite to 33% and increased the classical Vickers hardness and the dynamic elastoplastic hardness. This increase suggests the presence of a phase with higher indentation resistance, which can be the R phase reported in the literature. Annealing at 250 ºC decreased the classical Vickers hardness and the dynamic elastoplastic hardness, probably due to recovery. Annealing at 300 ºC advanced the recovery process and promoted some recrystallization, as suggested by the large standard deviation observed in the hardness results.

Keywords

• Nitinol
• compression
• recovery
• ultramicrohardness
• EBSD