Compressive stresses and stabilisation in Cu-Al-Ni single crystals

Abstract

Cu-Al-Ni single crystals which present β-β' (quenched samples) or β-γ' (aged at 570K samples) thermally induced martensitic transformations, were submitted to compressive stresses at different temperatures. The test temperatures were chosen in order to have martensite-martensite stress induced transformations. The characteristics of the martensitic transformation and structural changes before and after the compression tests have been studied by means of calorimetry (DSC) and TEM. The obtained results show that when applying compressive stresses in both ensembles of samples, a non-twinned γ' martensite appears. In the quenched samples it is formed through the β' phase already present at room temperature. In the aged samples the non twinned γ' phase is stress induced from the γ' thermally formed (internally twinned). The amount of non twinned γ' phase increases with the applied deformation, for low strain values it coexists with the β' or y' (twinned) phases and is the only present phase for higher applied strain values. In all the cases a notable martensite stabilisation is observed in the first thermal induced transformation after the mechanical test only when the stress-strain loops are not closed, that means when a permanent strain remains in the material after unloading. The degree of mechanical stabilisation is more important as higher is the applied deformation and as much amount of γ' phase mechanically formed (non twinned) is still present in the material and it is slightly dependent on the test temperature if lower than Af. The stress induced stabilisation mechanism is related to the presence of this non twinned martensite and its difficult retransformation to the parent phase.