Deformation microstructure and positive temperature dependence of flow stress in Ni3Ge

Abstract

Single crystalline specimens of Ni3Ge were compressed along [0 0 1] at different temperatures. Temperature dependence of the critical resolved shear stress (τ0(T)) has been obtained. Detailed measurements at cryogen temperatures (4.2-77 K temperature interval) showed that the increase of the yield stress with temperature starts at liquid helium temperature. The values of the apparent activation volume (V*) have been obtained as a result of stress relaxation tests repeated along the stress-strain curve at different stress levels at seven temperatures (77, 293, 373, 473, 573, 673, 773 K). Transmission electron microscopy (TEM) methods were used to analyze the evolution of dislocation structure with temperature and strain. The measurements of the straight dislocations fraction determined as δ = ρsd / ρ, (ρsd is the straight dislocation density, ρ is a total dislocation density) have been performed. The experimental data obtained in different tests in Ni3Ge confirm two-stage nature of the positive temperature dependence of the flow stress of alloys with the L12 structure. The model of thermal strengthening considering two different types of mechanisms, first acting at low temperatures and second at high has been proposed. Using this model the temperature dependence of the critical resolved shear stress has been approximated and the activation parameters of thermal strengthening of Ni3Ge have been obtained.