Stress induced martensitic transformations and phases stability in Cu–Al–Be shape-memory single crystals

The stress induced martensitic transformations between different metastable phases in Cu–Al–Be shape-memory alloy single crystals were assessed experimentally. In particular, the successive stress induced transformations from the high temperature austenitic DO3 structure (β1) to the 18R and to the 6R martensites were considered. Several distinct features which characterize the stress induced transformations in Cu–Al–Be and distinguish this system from other Cu-based shape-memory alloys are reported in this work. It was found that the 6R phase forms from a distorted 18R and that this stress induced transformation is highly reversible with no plastic deformation accompanying the process. The stress induced transformation from the distorted 18R to the 6R structure exhibits wide hysteresis, comparable with values observed in the NiTi system. A slightly negative temperature dependence of the critical stress to induce the 6R martensite was determined. Finally, a stress–temperature phase transformation diagram involving the metastable β1, 18R and 6R phases is proposed.