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The growth and deformation of cobalt crystals Davis, Keith Gordon

Abstract

Cobalt crystals of commercial purity have been grown in an electron beam zone refiner. They were tested in tension at temperatures between 150°C and -196°C. The resolved shear stress—shear strain curves are similar in form to those for the high stacking-fault energy hexagonal metals zinc, cadmium, and magnesium. There is an initial linear region for shear strains up to around 150%, with a ratio of work hardening slope to shear modulus of about 2 × 10[superscript -4].This is followed by an upturn, which is, however, smaller in magnitude for cobalt than for the other metals. Values for the critical resolved shear stress vary from 97 Kg/cm² at room temperature to approximately 170 Kg/cm² at -196°C. Two crystals of high purity cobalt were also tested, giving a critical resolved shear stress at room temperature of 65-70 Kg/cm². The Cottrell-Stokes law is not obeyed, either for temperature changes between -196°C and 18°C. or for changes in the strain rate at room temperature. An explanation in terms of dislocation theory has been put forward. Activation energies for plastic flow have been tentatively evaluated to be of the order of 35 kT. Twinning was never observed in crystals pulled at room temperature. Deformation twins produced by bending a crystal at -196°C. took two forms, very thin ones similar in appearance to Neumman bands in iron, and more usual lenticular twins. The thin twins have a {11 2̄̅̅1}habit plane. Metallographic examination of crystals pulled to large extensions and electro-polished to remove slip lines showed small needle-like markings, probably fine twins. In addition to the deformation work, certain observations have been made concerning transformation markings which supplement the work of previous investigators. It was found that secondary transformation markings are not a necessary part of the transformation, and that primary markings form on cooling as well as on heating.

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