Precipitation Effects on the Martensitic Transformation in a Cu-Al-Ni Shape Memory Alloy

This paper describes the effects of precipitation of alpha-phase on a Cu-Al-Ni shape memory alloy (SMA) with chemical composition bordering on beta region. By differential scanning calorimetry, a series of reproducible heat flow fluctuations was determined on heating a hot-rolled martensitic Cu-Al-Ni SMA, which was associated with the precipitation of alpha-phase. Two heat treatments were given to the SMA so as to "freeze" its states before and after the thermal range for precipitation, respectively. The corresponding microstructures of the two heat-treated states were observed by optical and scanning electron microscopy and were compared with the initial martensitic state. Energy dispersive spectroscopy experiments were carried out to determine the chemical compositions of the different phases formed in heat-treated specimens. The initial as well as the heat-treated specimens with a lamellar shape were further comparatively investigated by dynamic mechanical analysis and two-way shape memory effect (TWSME) tests comprising heating-cooling cycles under a bending load. Temperature scans were applied to the three types of specimens (initial and heat-treated states), so as to bring out the effects of heat treatment. The storage modulus increased, corresponding to the reversion of thermoelastic martensite and disappeared with the formation of precipitates. These features are finally discussed in association with TWSME under bending.