Stabilisation and Irreversibility of Martensite in Copper Base Shape Memory Alloys

Abstract

Application of martensitic shape memory alloys requires reproducible transformation temperatures independently of the time of low temperature ageing, in order to ensure the shape recovery in the same temperature interval. Martensite in copper base alloys is a metastable phase, since it decomposes into equilibrium phases below the temperature of the start of reversible martensitic transformation - A_s. The tempered martensite cannot be retransformed to the parent β as shown in CuZnSi alloys¹. The thermal stability of martensite is rather poor in CuZnSi alloys and in CuZnSn alloys decomposing rapidly at 200°C^1,2, while other alloys still retransform after prolonged ageing at 300°C as demonstrated by Dutkiewicz³ for CuAIMn and by Kobus⁴ in CuAlNiMnTi alloys. Martensite in copper base alloys possess in most cases an orthorhombic disordered 2H, 3R or 9R or ordered 4H, 6R or 18R structures⁵. These structures result from a periodic arrangement of stacking faults and usually possess a high density of random stacking faults. It is caused by a low stacking fault energy (SFE) in these alloys below 30 erg/cm². In fact alloying additions used in shape memory alloys were reported to lower the SFE in α phase copper base alloys as summarised by Gallagher⁶. When alloys are kept in the martensitic state, they tend to raise the retransformation temperature to parent β as pointed out by Hansen⁷ and Scarsbrook⁸. This phenomenon is known as stabilisation of martensite. Several mechanisms have been proposed for the stabilisation of martensite: (i) vacancy pinning of interfaces^7,9,10 preventing martensite/matrix interfaces to move; higher temperature is needed to overcome its resistance.