In situ synchrotron X-ray microdiffraction analysis of thermomechanically induced phase transformations in Cu–Al–Ni shape-memory alloy

Abstract

Phase transformations in a single-crystal Cu–Al–Ni shape-memory alloy induced by thermomechanical effects were investigated in situ by high-resolution synchrotron X-ray microdiffraction. Contrary to the common belief, austenite texture maps revealed that austenite-to-martensite transformation occurred during heating of the partially transformed material under fixed specimen elongation. Twinned and detwinned types of martensite coexisted during this austenite-to-martensite phase transformation. Twinning and detwinning structures evolved to accommodate changes in stress and strain generated in the temperature-varying environment. Small amounts of austenite exhibiting distorted crystallographic orientation were detected in regions of stress-induced martensite during heating of the partially transformed material. The results of this investigation provide insight into intriguing stress rate-dependent phenomena intrinsic of shape-memory alloys and elucidate complex phase transformations due to thermal and mechanical stress effects.

Keywords:

• austenite
• elongation
• martensite
• phase transformation
• shape-memory alloy
• strain
• stress
• synchrotron radiation
• thermomechanical effects
• twinning
• X-ray diffraction

Acknowledgements

This work was performed at the Advanced Light Source and the Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the Office of Science, Office of Basic Energy Sciences, US Department of Energy, under Contract No. DE-AC02-05CH11231. The microdiffraction beamline 12.3.2 has been partially funded by the National Science Foundation under Grant No. 0416243. The authors acknowledge discussions with R. Ritchie, R. Gronsky, A. Pelton, M. Barney and S. Robertson, and technical assistance by N. Tamura and M. Kunz.