Temperature and microstructure dependent tensile behavior of coarse grained superelastic NiTi

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 lm. Effects of temperature in the range 30-130 degrees 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 ana-lyzed post-mortem using Electron Backscatter Diffraction and x-ray diffraction. Except the least recov-ered microstructure, the samples did not follow linear temperature trends according to Clausius-Clapeyron. With increasing temperature, the plateau stress of localized martensitic transformation stabi-lized 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 recrystal-lization. Large ductility was positively correlated with occurrence of austenite deformation twins in deformed microstructure.(c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

This work analyzed the effects of temperature and microstructure on the tensile behavior of superelastic NiTi wires. Cold-drawn NiTi wires with different final heat treatments were subjected to temperature gradients, and the deformation processes and microstructure changes were characterized. The results showed that the stress of localized martensitic transformation stabilized with increasing temperature, while the volume fraction of martensite decreased. Ductility exhibited a non-monotonic evolution with temperature, with higher ductility observed in samples with more microstructure recovery or recrystallization.