Improvement of Hardness and Microstructures by Ageing in Shape Memory CuAlNi Alloys

Abstract
The shape memory CuAlNi alloys, having a composition near that of Cu₃Al, exhibit disorder-order transition from the disordered bcc β-phase to the ordered β₁-phase (DO₃ type) at high temperatures. Although the β₁-phase is a non-equilibrium phase, it nonetheless remains stable at room temperature upon quenching from the high temperature β-phase. At the lower temperatures, the β₁-phase itself undergoes martensitic transformation and displays layered structure called M18R. The influence of ageing on two CuAlNi β-phase alloys was studied by measuring the time dependence of hardness, lattice parameters and by optical microscopy observation at 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.