Irreversible Processes During Martensitic Transformation in Zr-Based Shape Memory Alloys

The purpose of this paper is to explain the thermodynamical and shape memory behaviour in comparison with structural parameters for Zr-based intermetallics - a new class of potential shape memory alloys (SMA) for high-temperature applications [1]. Electrical resistivity, structural, shape memory and calorimetric measurements were carried out for the Zr2CuNi - Zr2CuCo quasibinary cross-section. It was shown that the martensitic transformation (MT) of the high -temperature B2 phase resulted in the simultaneous formation of the two martensitic phases belonging to the P21/m (B19' type) and Cm space groups in the case of Zr2CuNi similar to ZrCu [2]. Co for Ni substitution causes the changes in the martensite volume fractions up to formation of only B19' type martensite in Zr2CuCo compound without significant changes in lattice parameters for both martensites. Such substitution also decreases generally the transformation heats and energy dissipated during the full cycle of MT. The non-thermoelastic behaviour that was observed in [l] for Zr2CuNi changes to a thermoelastic one in the case of MT in Zr2CuCo. Shape memory effect (SME) is nearly complete for alloys with high Ni content (not less than 85% of shape recovery ratio (Ksme)). It becomes complete at Co additions. The effect of the interaction between two martensites on the non-thermoelastic MT behaviour and SME in Zr-based intermetallics is discussed in the present paper