Improvement of hardness and microstructures by ageing in shape memory CuAlNi alloys

The shape memory CuAlNi alloys, having a composition near that of Cu,AI, exhibit disorder-order transition from the disordered bce beta-phase to the ordered beta(1)-phase (DO3 type) at high temperatures. Although the beta(1)-phase is a non-equilibrium phase, it nonetheless remains stable at room temperature upon quenching from the high temperature beta-phase. At the lower temperatures, the beta(1)-phase itself undergoes martensitic transformation and displays layered structure called M18R. The influence of ageing on two CuAlNi beta-phase alloys was studied by measuring the time dependence of hardness, lattice parameters and by optical microscopy observation al room temperature. Hardness shows a trend to decrease with the ageing duration due to the formation of precipitation and grain size is also affected by ageing.