Temperature and microstructure dependent tensile behavior of coarse grained superelastic NiTi

Abstract

In this work, effects of temperature and microstructure on tensile behavior of superelastic NiTi wires were analyzed. We analyzed cold-drawn NiTi wires of 1.78 mm that received four different final heat treatments using short direct electrical current pulses. Final coarse-grained microstructures ranged from recovered up to fully recrystallized microstructure with grain size of 25 µm. Effects of temperature in the range 30–130 °C were analyzed using monotonous tensile tests where a temperature gradient was induced along the sample axis. Deformation processes related to martensitic transformation and plastic deformation were characterized by temperature resolved stress–strain responses, deformation work, released heat, volume fraction of martensite. Deformation-induced microstructure changes were analyzed post-mortem using Electron Backscatter Diffraction and x-ray diffraction. Except the least recovered microstructure, the samples did not follow linear temperature trends according to Clausius-Clapeyron. With increasing temperature, the plateau stress of localized martensitic transformation stabilized at a constant stress while volume fraction of martensite decreased down to zero. Localized strains induced by martensitic transformation and ductility evolved non-monotonously with temperature, reaching maxima at temperatures that decreased with increasing microstructure recovery or recrystallization. Large ductility was positively correlated with occurrence of austenite deformation twins in deformed microstructure.

Keywords

• Martensitic transformation
• Shape memory alloys
• Superelasticity
• Deformation twinning
• NiTi
• Microstructure