A Two-Stage Martensite Transformation in a Cu-13.99 mass% Al-3.5 mass% Ni Alloy

Abstract

The reverse martensite transformation of an as-quenched Cu–Al–Ni shape memory alloy presents two separate stages which are studied by means of calorimetry. The first stage, corresponding to the lower temperatures, presents a thermal hysteresis of about 10 K, a continuous behaviour of the energy dissipation and a very low acoustic emission. The second stage, at higher temperatures, presents a thermal hysteresis of about 40 K, a rather jerky character in the energy dissipation and a higher level of acoustic emission. The first stage progressively disappears and the transformation temperatures increase when the alloy is aged at 473 K or 573 K but the difference in temperatures between the two stages, of about 25 degrees, remains constant. An analysis by SEM of the microstructure during the transformation, in accordance with the above-mentioned features, shows that the β1′-and γ1′-phases coexist in the martensite. The first stage is a β1′→β1 transformation, while the second stage is a γ1′→β1 transformation. The occurrence of the β1′ martensite is assisted by internal strain energy created during the direct transformation. The relative amount of γ1′-martensite increases by ageing at 473 K or 573 K.