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

Abstract
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 (K_sme)). 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.