Phase stability of CuAlMn shape memory alloys

Thermoelastic martensitic transformations in single crystals of two CuAlMn shape memory alloys were investigated using tension/compression stress–strain tests, thermal cycling tests, stress recovery tests and calorimetric measurements. Pseudoelastic behaviour was observed in the as-quenched samples stressed above the Af temperature. Near the Ms temperature the stress–strain response changed and became pseudoplastic. A pronounced dependence of the thermoplastic behaviour on the transformation history was found. The samples once subjected to tension/compression cycling at a temperature near or below Ms remained pseudoplastic during subsequent stress–strain experiments at higher temperatures, and a large increase of the Af temperature was observed. This stabilisation of martensite is similar to that observed on CuAlNi, and can be ascribed to the detwinning of the martensitic phase and formation of a single variant of the γ1′ martensite.