How and When the Second Austenite Variant Could Affect the Superelasticity in Cu–Al–Be Single Crystals

Abstract

The superelasticity in shape memory alloys is a useful property for many applications. It is due to a martensitic transformation induced by applied stress and can reach up to about 10% recoverable strain. Cu-based alloys transforming to 18R martensite are very promising systems. Due to the crystal symmetry of the 18R martensite, it may occur that the sample does not return completely to the original austenite but to a second austenite variant that gives another shape to the sample. Both variants are related by a symmetry operation of the martensite point group. An example of thisphenomenon in a Cu–Al–Be single crystal is presented in this work. The 18R martensite was induced using a tensile straining holder in a transmission electron microscope. The presence of the second austenite was found after unloading. Electron diffraction confirmed their crystallographicrelationship, in agreement with predictions from Wechsler–Lieberman–Read theory. The conditions for the second austenite variant to form were established. The appearance of the second austenite variant is shown to alter the local strain compatibility and is expected to negatively affect superelasticity during stress-induced cycling.