Microstructure and martensitic transformation behavior of CuAlNb high temperature shape memory alloy

Microstructure and martensitic transformation behavior of the Cu₇₂Al_26.5Nb_1.5 high temperature shape memory alloy have been studied. The experimental results show that the microstructure of the as-melted Cu₇₂Al_26.5Nb_1.5 alloy consists of four phases: matrix, γ₂ phase, Nb(Cu, Al) phase and Nb(Cu, Al)₂ phase. With the increase of cooling rate, the amount of the γ₂ phase decreases, while the shape and amount of the Nb(Cu, Al) phase and Nb(Cu, Al)₂ phase have no obvious change. β₁' ↔ β₁ martensitic transformation/reverse transformation occurs during cooling/heating. The parent phase has a D0₃-ordered BCC structure while the martensite has a M18R monoclinic structure. The martensite of the experimental alloy mainly exhibits self-accommodation morphology, and the substructure is mainly (1210) twin. The alloy aged in the martensite shows the pronounced martensite stabilization, i.e. reverse transformation temperatures increase remarkably with the increase of aging time. When aging in the parent phase, martensitic transformation temperatures drop at the first stage, then increase with the increase of aging time. With further aging, the transformation temperature shows stable value. The reverse transformation temperature increases at the early stage of aging and keeps constant with the further increase of aging time.