Thermal Cycling Effect in Cu&ndash;Zn&ndash;Al Shape Memory Alloys with B2 and D0<SUB>3</SUB> Type Ordered Structures in Parent Phase

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 reproducible after 10 cycles or so, and in addition, residual martensites at room temperature become noticeable roughly after 102 cylces, as reported previously. On the other hand, the martensitic structure creasing thermal cycle, and the microstructural reproducibility is poor even after 103 cycles, as in a CuAl-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 CuAl-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.

Calorimetric study of the influence of thermal cycling on the martensitic transformation of Cu-Zn-Al alloys

“…The temperatures marked by an asterisk are associated with the transformation of an amount of material already detectable by other techniques, and we can thus compare our results to those obtained using other less sensitive techniques, such as electrical resistivity measurements. Tadaki et a1 [4], studying samples quenched at 263 K, have reported that for DO3 ordered Cu-Zn-A1 (as is the case for our samples) the M , temperature decreases with thermal cycling. This is the same behaviour observed for our M,* and indicates that the electrical resistivity has insufficient resolution to detect the very smooth transformation beginning.…”

“…The thermal or mechanical cycling of shape-memory alloys has been reported to modify the transformation characteristics. In particular, for Cu-Zn-A1 alloys different experimental techniques, such as electrical resistivity [3,4] and differential scanning calorimetry [5,6,71, have been used to analyse such effects, leading to results which seem not to be coincident. Until now, the evolution of the thermodynamic magnitudes involved in the thermoelastic transformation has not been studied.…”

Effects of ageing on bainitic and thermally induced martensitic transformations in ductile Cu–Al–Mn-based shape memory alloys

“…Therefore, it is supposed that in Cu-Zn-Al alloys with a low T B2-DO 3 ðL2 1 Þ c temperature (<600 K), the crystal structure of the b phase in the quenched state is the B2 structure. Actually, it has been reported that a Cu 64.2 Zn 29.1 Al 6.7 alloy with a low T B2-DO 3 ðL2 1 Þ c temperature has the B2 structure in the quenched state [32]. Kuper et al [33] reported that the activation energy for self-diffusion in a Cu-(47-48) at.% Zn alloy (b brass) single crystal with the B2 structure in the temperature range 537-654 K, determined using Cu 64 and Zn 65 as the tracer elements, was $150 kJ mol À1 .…”

Grain boundary engineering of Co–Ni–Al, Cu–Zn–Al, and Cu–Al–Ni shape memory alloys by intergranular precipitation of a ductile solid solution phase

“…In the Cu-Zn-Al system, an FCC Cu-based solid solution phase, α, can coexist with austenite β ( [38], B2 [38,39] or structure depending on composition and thermal processing [40]), as can be seen in a vertical section at 4 wt% Al of the Cu-Zn-Al phase diagram in Fig. 2(c), where the chosen alloy composition is shown as the red dashed line.…”