Stabilisation of martensite by applying compressive stress in Cu-Al-Ni single crystals

Compression tests have been carried out for a Cu-AI-Ni single crystal at te mperatures well above the martensitic transformation. near the transformati on or below it (in martensitic state). The composition was selected in orde r to obtain either beta-beta' or beta-gamma' thermal martensitic transforma tions, after suitable thermal treatments. The characteristics of the marten sitic transformation and structural changes after the compression tests hav e been studied by means of calorimetry (DSC) and TEM. The obtained results show that when a compressive stress is applied on quenched samples (TTA tre atment, beta-beta' thermal transformation) a beta beta-(beta')-gamma' trans formation or a beta'-gamma' one are stress-induced, depending whether the i nitial state is the parent or the martensitic phase. For aged samples (TTB treatment, beta-gamma' thermal transformation) the application of stress br ings about the beta-gamma' transformation or gamma' re-orientation, dependi ng on the initial state. In all the cases a notable martensite stabilisatio n is observed only when the stress-strain loop is not closed. that means wh en a permanent strain remains in the material after unloading. A direct rel ationship between the applied deformation when stressing the sample and the degree of stabilisation has been obtained for different strain values (bet ween 5% and 12%) and for each set of samples (TTA and TTB). At the same tim e, the evolution of the characteristics of the martensitic phases with the degree of deformation has been studied. The stress induced stabilisation me chanism is related to the presence of non-twinned gamma' martensite which m akes difficult the retransformation to the parent phase. (C) 2001 Acta Mate rialia Inc. Published by Elsevier Science Ltd. All rights reserved.