The Temperature Dependence of Mechanical Properties in Cu-Ni-Zn Alloy Single Crystals

Abstract

The temperature dependence of stress-strain curves in Cu-Ni-Zn single crystals, especially of the critical resolved shear stress (CRSS), has been studied. Single crystals of composition close to Cu_(1−2x)Ni_xZn_x and Cu_(0.75−x)Ni_xZn_0.25 containing Cu₂NiZn were prepared and deformed in tension between 77 and 1060°K.
The results obtained are as follows:
(1) The temperature dependence of stress-strain curves are similar to those of binary Cu alloys, though the concentration and ratio of Ni and Zn play important and delicate roles in them.
(2) There are three temperature ranges concerning the temperature dependence of CRSS. The range I corresponds to the low temperature range where CRSS decreases with increasing temperature. The temperature-dependent part of CRSS shows the activation energies of 0.66∼0.77 eV and is almost independent of alloy composition.
(3) The range II corresponds to the middle temperature range where CRSS depends little or reversely on the temperature. In Cu-20 Ni-20 Zn and Cu-25 Ni-25 Zn alloys, in addition, a maximum in CRSS exists between 500 and 600°K.
(4) The range III corresponds to the high temperature range where CRSS decreases with increasing temperature. In the temperature range T⁄T_m 0.7∼0.8 a relation \dotε∼τⁿ holds (\dotε: strain rate, τ: CRSS). The activation energy for diffusion is found to be 1.9∼2.2 eV and is almost independent of the solute concentration.
(5) Like binary Cu alloys, the deformation substructures observed in the transmission electron microscope are composed of piled-up dislocation arrays below 773°K and a homogeneous dislocation arrangement at 873°K. In Cu-25 Ni-25 Zn deformed at 573°K, dislocation pairs of same sign are observed on the same slip plane. Then, the maximum on CRSS at 500∼600°K in Cu-20 Ni-20 Zn and Cu-25 Ni-25 Zn is considered to be closely related to the formation of long range order.