Shape memory response and mechanical properties of warm deformed NiTi intermetallic alloy

The warm deformation method as an effective thermomechanical processing (TMP) route was applied on the equiatomic NiTi alloy to enhance its shape memory and mechanical properties. To this end, warm compression tests at different temperatures in the range of 100-500 degrees C under different strain rates of 0.003, 0.03 and 0.3 s(-1) were performed. The material flow behaviors at 100 degrees C revealed the occurrence of stress induced martensite formation; this caused the specimens to fail by fracture. However, the material flow behavior at 300 degrees C and 500 degrees C demonstrated a dynamic restoration phenomenon. The higher stability of austenite phase in the processed specimens at 300 C was indicated by microstructural and XRD analysis. In addition, the bimodal martensite packets were distinguished in the materials processed at 500 degrees C. The latter might well designate the occurrence of dynamic recrystallization phenomena. Furthermore these existence of austenite phase in 300 degrees C processed samples lead to an unfavorable shape memory response, while the fully martensitic structure of 500 degrees C processed samples materials exhibited a better shape memory recovery. The final mechanical properties of the alloy were improved through TMP, and applying the TMP at 500 degrees C increased the shape recovery strain of the alloy from approximately 1.5% (in as-hot rolled material) to 3.6%.