Effects of deformation on microstructure and mechanical properties of a Cu–Al–Ni shape memory alloy

In Cu-11.92 wt.%Al-3.78 wt.%Ni shape memory alloy, the influence of deformation and thermal treatments on the microstructure and mechanical properties under the compression test were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). Experiments show that the mechanical properties of the alloy can be enhanced by convenient heat treatments. The alloy exhibits good mechanical properties with high ultimate compression strength and ductility after annealing at high temperature. However, it exhibits brittle fracture and dramatic strain hardening, with linear stress-strain behavior after annealing at low temperature. The changes in the mechanical properties have been linked to the evolution of the degree of order, occurrence of precipitation, and variation of the grain size. From microstructural observations, it is seen that the β1′ (18R) and γ1′ (2H) martensite phases coexist at different fractions in the undeformed and deformed states. Deformation induces the changes between the β1′ and γ1′ martensites and deformation-induced martensites form at preferred orientations as mechanical twins. The β1′ martensite variants are twin-related with respect to the (1¯2¯8)18R mirror plane and a new orientation relationship for these twin variants is derived as (1¯2¯8)A∥ (1¯2¯8)C: [4¯61] A∥ [4¯61]C. Additionally, an increase in the amount of deformation causes martensite reorientation, de-twinning, and dislocation generation; also, the martensite plates are seen to have rearranged in the same orientation to be parallel with each other.