Formation of stress-induced martensite in the presence of γ-phase, in a Cu–Al–Ni–Mn–Fe shape memory alloy

The structure of a Cu–13.67Al–4.28Ni–0.54Mn–0.14Fe (wt.%) shape memory alloy was analyzed in the initial condition, during a heating–cooling cycle, and during a room temperature loading–unloading mechanical cycle. A fully reversible martensitic transformation was revealed which was associated with a shape recovery degree reaching a saturation value at 97.7% for a relative permanent deflection of 90 %. By X-ray diffraction, the occurrence of a reduced amount of β ′ 1 stress-induced martensite (SIM) accompanied by the precipitation of the hard γ2-phase was noticed both during the heating of a martensitic specimen and during the air-cooling of the alloy in the austenitic state. It was assumed that a possible cause for the formation of β ′ 1 SIM during heating or cooling could be the strains caused by the volume increase of hard γ2-phase that deformed the softer martensitic or the austenitic matrix. The irreversible formation of β ′ 1 SIM at 100 MPa was revealed by X-ray diffraction in elongated condition and confirmed by OM at room temperature, after unloading.