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

Abstract

Compression tests have been carried out for a Cu-Al-Ni single crystal at temperatures well above the martensitic transformation, near the transformation or below it (in martensitic state). The composition was selected in order to obtain either β-β′ or β-γ′ thermal martensitic transformations, after suitable thermal treatments. The characteristics of the martensitic transformation and structural changes after the compression tests have been studied by means of calorimetry (DSC) and TEM. The obtained results show that when a compressive stress is applied on quenched samples (TTA treatment, β-β′ thermal transformation) a β-(β′)-γ′ transformation or a β′-γ′ one are stress-induced, depending whether the initial state is the parent or the martensitic phase. For aged samples (TTB treatment, β-γ′ thermal transformation) the application of stress brings about the β-γ′ transformation or γ′ re-orientation, depending on the initial state. In all the cases a notable martensite stabilisation is observed only when the stress–strain loop is not closed, that means when a permanent strain remains in the material after unloading. A direct relationship between the applied deformation when stressing the sample and the degree of stabilisation has been obtained for different strain values (between 5% and 12%) and for each set of samples (TTA and TTB). At the same time, the evolution of the characteristics of the martensitic phases with the degree of deformation has been studied. The stress induced stabilisation mechanism is related to the presence of non-twinned γ′ martensite which makes difficult the retransformation to the parent phase.