Abstract
Shape memory effect is a phenomenon in which a specimen, apparently plastically deformed at some low temperature, reverts to its undeformed original shape on heating to a somewhat higher temperature. A common property of all the systems that show this effect is that they all exhibit a thermo-elastic martensitic transformation, and the shape memory effect is observed when the alloy is deformed at a partly or wholly martensitic condition and then heated to the matrix phase. The matrix is usually highly ordered and the martensite invariably inherits the order of the matrix phase. In the present investigations, an attempt has been made to study the effect of order on the martensitic transformation characteristics in the system Cu-Zn-Mn, showing the shape memory effect. Investigations have been directed towards determining the effect of manganese content and the effect of low temperature ageing and quenching on ordering, and thereby on martensitic transformations. Resistance measurement with temperature and tensile testing methods have been used to show that manganese up to 10 at % acts almost like zinc in Cu-Zn-Mn alloys. Low temperature ageing and quenching of the matrix phase result in rises in Ms temperature which are proportional to the manganese content and the temperature of ageing. The entropy change accompanying the martensitic transformation is independent of heat treatment and is almost the same as that found in Cu-Zn alloys, suggesting that the martensite inherits the order of the parent phase. The matrix phase undergoes secondary ordering transition, B2 → DO3/L2, in addition to the primary ordering transition A2 → B2 at high temperatures. Because of rapid diffusion, ageing above the B2 → DO3/L21. transition temperature does not produce markedly different structures than ageing at lower temperatures. The effects on Mg due to ageing and quenching are essentially attributed to disordering of the parent phase. Starting with calculations based on an improved BWG model to take into account the total ordering behaviour in the 3-phase, it is demonstrated that an explicit calculation of the temperature can be made by combining ordering data for both the parent- and product phases with a regular solution model and phase stabilities. It is also shown that such a thermodynamic analysis performed on binary Cu-Zn and ternary Cu-Zn-Mn alloys satisfactorily accounts for both the retrograde solidus that are so characteristic of these alloys, and the variation of Ms with composition. The rises in Ms with low temperature ageing and quenching are shown to be due to the ordering energy being higher in the ?-phase than in the a-phase. It is suggested that in the system Cu-Zn-Al, where Ms is depressed on disordering, the reverse is the case.