Identification of 6R martensite in aged Cu-Al-Ni after pseudoelastic mechanical cycles

Abstract

Martensitic transformations in Cu-Al-Ni shape memory alloys can be induced by temperature or mechanical stress. Several martensitic phases have been reported in Cu-Al-Ni single crystals, depending on chemical composition, crystallographic orientation, temperature and applied mechanical stress. In tensile tests, the morphology of stress vs. strain curves is directly related to the martensitic transitions taking place in the sample, which provides evidence as to which phases are formed under different conditions. In this work, we studied aged Cu-Al-Ni single crystals and found unusual morphology in some stress vs. strain curves. These samples were studied with electron microscopy and we confirmed the presence of 6R martensite due to the extra spots in the selected area diffraction patterns. The formation of 6R martensite from the β phase in Cu-Al-Ni during tensile tests is mentioned in the literature, but this is the first time that twinned 6R retained martensite is confirmed experimentally in samples that also present γ precipitation due to ageing thermal treatments.