Strain effects on the macroscopic behaviour and martensite morphology in shape-memory CuZnAl alloys

Shape-memory alloys exhibit super-elasticity when they are deformed at a te mperature less than the transformation temperature. The specimen remains in the deformed shape on releasing the strain at this temperature but recover s the undeformed original shape on heating to temperatures over the reverse transformation temperature. As a result of their ability to generate spont aneous shape changes on thermal cycling, these alloys can convert heat dire ctly into mechanical work due to the substantial recovery stresses generate d during the martensite-to-parent transformation. These alloys are very sen sitive to thermal effects and are influenced strongly by both quenching and post-quench heat-treatments. The effects of external stress on the charact er of the macroscopic shape in two shape-memory CuZnAl alloys are investiga ted. The effects of post-quench heat-treatment on martensite morphology wer e observed by means of optical microscopy. Thermal hysteresis occurring dur ing the heating and cooling processes is attributed to internal friction. F urthermore, parallel-sided martensite plates which can be grown almost in o ne dimension have been observed in the post-quench hear-treated specimens. It is suggested that the equilibrium phase (alpha-fcc) formed in the aged s pecimens causes the loss of shape memory. (C) 2000 Elsevier Science S.A. Al l rights reserved.