Using DSC for the detection of diffusion-controlled phenomena in Cu-based shape memory alloys

Abstract

The thermally induced reversible diffusionless martensitic transformation, that governs the mechanism of shape memory alloys (SMAs), can be conveniently illustrated, on differential scanning calorimetry (DSC) curves, by a close variation loop of heat flow with temperature. Cooling-induced direct (forward) martensitic formation causes an exothermic maximum, while martensite reversion, on heating, is associated with an endothermic peak. The occurrence of diffusion-controlled phenomena is emphasized by discontinuities of heat flow variations, such as those observed in the case of Cu–Zn–Al SMAs, experiencing thermoelastic martensitic transformation. When heating such a martensitic Cu–Zn–Al SMA, four solid-state transitions were observed on DSC curves, under the form of: (1) an endothermic minimum ascribed to martensite reversion to parent phase; (2) an endothermic minimum associated with transitory formation of bainite; (3) an exothermic maximum corresponding to the precipitation of equilibrium α-phase and (4) an endothermic peak representing the order–disorder transition of parent phase. The first transition is diffusionless, while the other three are diffusion-controlled. The DSC equipment was used to investigate the effects of: (1) heat treatment temperature, (2) number of cycles of thermomechanical treatment and (3) thermal cycling within the temperature range where diffusion-controlled solid-state transitions occur in a Cu–Zn–Al SMA, the results being corroborated with that obtained by optical (OM) and transmission (TEM) electron microscopy.