Thermal Cycling Effect in Cu–Zn–Al Shape Memory Alloys with B2 and D03 Type Ordered Structures in Parent Phase

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Abstract
Effect of thermal cycling on the martensitic transformation in two kinds of Cu–Zn–Al shape memory alloys, whose parent phases are ordered into D03 and B2 types, has been examined by electrical resistivity vs. temperature measurement, optical and transmission electron microscopy and X-ray diffraction. The thermal cycling was carried out between 77 K below Mf temperature and 288 K above Af temperature up to 104 times. With increasing thermal cycle, Ms temperature of the B2 type alloy (Cu-29.1Zn-6.7Al (at.%)) increases, while that of the D03 type alloy (Cu-12.5Zn-19.4Al (at.%)) decreases. The change in Ms temperature is about 10 and 15 K in the B2 and D03 type alloys, respectively, after 103 thermal cycles. The martensitic structure in the B2 type alloy, consisting of acicular β′2 martensite, is reproducible after 10 cycles or so, and in addition, residual martensites at room temperature become noticeable roughly after 102 cycles, as reported previously. On the other hand, the martensitic structure in the D03 alloy, initially consisting of acicular β′1 and spear-like γ′1 martensites, becomes finer with increasing thermal cycle, and the microstructural reproducibility is poor even after 103 cycles, as in a Cu–Al–Ni alloy previously studied. Numerous dislocations are accumulated in the parent phases of both the thermal cycled alloys. The density of the dislocations appears to be higher in the B2 alloy than in the D03 alloy. Disordering takes place in the parent phases of both the alloys with thermal cycling, as in the Cu–Al–Ni alloy. The observed change in Ms temperature in each alloy is considered to be caused by the disordering. Characteristics of the disordering suggest that dislocations with the Burgers vector of 1/4·⟨111⟩ and 1/2·⟨111⟩ are generated in the D03 and B2 parent lattices, respectively, by thermal cycling. The feature of disordering is different from that in the Cu–Al–Ni alloy previously examined, and is inconsistent with the result of a previous analysis made on B2 type Cu–Zn–Al alloys. Mechanisms of the disordering by thermal cycling in both the alloys are discussed.