Plastic deformation of nickel single crystals at low temperatures

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Abstract
Single crystals of nickel (purities 99.98% and 99.4%) have been deformed in tension at various temperatures between 4.2°K and 300°K. Shear stress-shear strain relations have been determined as well as the reversible change of flow stress with temperature. The work-hardening parameters obtained are discussed in terms of dislocation theory that has been developed mainly on the basis of data on Al and Cu. From the temperature dependence of the stress at the beginning of dynamical recovery the activation energy of cross-slip is estimated to be slightly lower than that of copper, implying a somewhat higher stacking fault energy. The advantages of the temperature change method for an investigation of the flow stress are pointed out following a theoretical analysis of the relation between this method and that of the work-hardening parameters. The present results on nickel confirm this relation. Metallographic and x-ray evidence has been obtained in favour of deformation twinning in nickel at 4.2°K and 20°K although no drop in load has been observed of the kind found in copper. Serrations in the recorded load-extension curves indicate discontinuous flow at the lowest temperatures.

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