On the strength and stress-relaxation response of fine-grain Cu–42.2 at.%Zn–0.6 at.%Pb alloy polycrystals

Abstract

Tensile tests were performed at room temperature to measure the strength and stress-relaxation response of Cu–42.2 at.%Zn–0.6 at.%Pb polycrystals of grain size 5, 7, 9 and 11 μm. The values of the strength parameters, namely yield stress, ultimate tensile strength and fracture stress, were found to decrease with the increase in grain size in accordance with the Hall–Petch law, whereas the ductility decreased with the reduction in grain size. The stress-relaxation response studied at constant strain was logarithmic in nature. For a given initial stress level σ o, from which relaxation at constant strain was allowed to start, both the amount of stress relaxed, Δ σ( t) = σ o − σ( t), and the magnitude of the stress-relaxation rate, s = [d(Δ σ)/d ln( t)], increased with the increase in grain size. However, the stress-sensitivity of relaxation rate, d s/d σ o, was independent of the grain size. The intrinsic height of the energy barrier, U o, to the movement of relaxing dislocations was found to be 5.3 eV, which points to dislocation intersection as the rate-controlling process of stress-relaxation. A comparison of the available data for copper and Cu–Zn alloy system over a wide range of solute concentration shows that stress-relaxation response is sensitive to the alloy microstructure, e.g. mode of spatial distribution of solute atoms, dispersion of second-phase particles, etc.