Thermomechanical response of an Ni–Ti–Cr shape-memory alloy at low and high strain rates

Abstract

The thermomechanical response of an Ni–Ti–Cr shape-memory alloy is investigated at various initial temperatures, over a wide range of strain rates, using an Instron hydraulic testing machine and one of the modified split-Hopkinson-bar systems at the Center of Excellence for Advanced Materials, University of California, San Diego. The transition stress for the stress-induced martensite formation is observed to be quite sensitive to the initial deformation temperature, but the yield stress of the resulting martensite is not. The linear transition stress–temperature relation with a slope of 8.5 MPa K⁻¹, obtained in a quasistatic loading regime, seems to remain valid for strain rates up to 500–700 s⁻¹. The transition stress and the yield stress of the stress-induced martensite show strain-rate sensitivity, increasing monotonically with increasing strain rate. There exists a certain critical strain rate at which the transition stress equals the yield stress of the material. This critical strain rate determines the material's deformation behaviour; the material deforms by the formation of stress-induced martensites and their subsequent yielding, when the strain rate is less than this critical value, and through dislocation-induced plastic slip of the parent austenite, when the strain rate exceeds the critical value. It appears that the critical strain rate increases slightly with decreasing initial temperature.

Acknowledgements

This work was supported by the Office of Naval Research (Multidisciplinary Research Program of the University Research Initiative) grant N000140210666 to the University of California, San Diego, USA. The authors wish to express their appreciation to Mr G.-Y. Ryu at the Research Institute of Industrial Science and Technology, Korea, for help with the TEM microstructural observations.