Isothermal ageing at low temperatures of a smart material: a CuZnAlCo shape memory alloy

Abstract

Shape memory properties are found in alloys having a martensitic or [beta] structure. These structures are unstable at high and also at low aging temperatures transforming into the eutectoid phases by diffusion controlled processes. The presence of cobalt in heat-treated aluminum bronze was found to cause changes in the proportions of the equilibrium phases, increasing the [alpha] content and decreasing the fraction of the eutectoid [alpha] and [gamma]. On aging the cobalt containing alloy, thee were no structural changes until the temperature was below 350 C. At higher temperatures the martensite decomposed by two different modes. In the first mode, [alpha] phase precipitates at the grain boundaries or around CoAl(Cu) particles and the [gamma] phase is formed afterwards. The second mode of decomposition took place by a transitional [gamma][double prime] precipitation, which takes Al from the matrix, so allowing it to transform to the [gamma] phase. Little work has been carried out on systems such as Cu-Zn-Al-X, where X is a grain refining element. Authors have extensively studied the improvements of the properties of a Cu-Zn-Al-Co shape memory alloy. Calorimetric methods have also been used by the present authors to study the decomposition phenomena of this alloy. The present workmore » is a continuation of that study to determine the hardness and structural changes brought about by the addition of Co.« less