Superelasticity and Shape Memory Effect in Copper Base Alloys

Among the copper base shape memory alloys the most popular are CuZnAl alloys which however show several features like: tendency to stabilisation, limitation of the effect to low temperatures, lattice defects multiplication after numerous cycling and low decomposition temperatures. This defines the directions of investigations of copper base alloys in order to avoid these negative features limiting their applications. In spite of a large number of investigations of several types of alloys like CuAlNi, CuAlMn, CuAlNiMnTi, CuAlAg, CuGa, CuZn and others, the shape memory properties of NiTi base alloys are considered to be superior and actually are used in a majority of applications in spite of a much higher components cost. In this presentation the investigations of superelastic deformation of mono- and polycrystalline CuAlMn alloys will be discussed. Structure changes during pseudoelastic deformation of CuAlMn single crystals were investigated using in situ optical and high voltage electron microscopy. The effects of ageing on the martensitic transformation temperatures in CuZnAl, CuAlMn and CuZnSn alloys and resulting shift of transformation temperatures are discussed based on the transmission electron microscopy studies. They indicate that changes of ordering within β phase are responsible for an increase, while precipitation of bainite or γ phase for a decrease in the martensitic transformation temperatures. Ribbons from CuAlNiMn alloy cast using the melt spinning method, crystallizing at a very high rate, show a large drop of M$\text{}_{s}$ temperatures. This drop of M$\text{}_{s}$ depends upon a wheel speed controlling the solidification rate. It is correlated with the decreasing grain size and its value is much larger for ribbons containing titanium due to its grain refinement capabilities. The two-way shape memory effect may appear after the training performed by a repetitive cooling under external stress and heating after unloading. It is demonstrated that in alloys transforming below 150°C saturation of the two-way shape memory effect occurs between 50 and 80 training cycles.