Temperature and Orientation Dependence of the Yield Stress in Ni3Ge Single Crystals

Flow stress measurements were made on single crystals of Ni3Ge with several different orientations. Slip systems were determined by two-surface trace analysis, and dislocation arrangements due to deformation were observed by transmission electron microscopy. The yield stress increases with increasing temperature in the temperature range of −196∼800°C where {111} slip operates (positive temperature dependence), but it decreases as {001} slip commences. Critical resolved shear stress for {111}〈10\bar1〉 slip is orientation-dependent. The positive temperature dependence is pronounced even below room temperature, and the yield stress nearly doubles as the temperature is raised from −196 to 27°C. Electron microscope observation on dislocation arrangements in the specimens deformed at −196 and 27°C has revealed that the mobility of screw dislocations decreases with increasing temperature. These observations indicate that the positive temperature dependence of the yield stress is controlled by the mobility of screw dislocations. This decrease of mobility leading to the positive temperature dependence of the yield stress can be explained by thermally activated cross-slip of screw dislocations from the (111) plane to the (010) plane.