Thermal Hardening of Cadmium Crystals

Abstract

BY annealing a crystal of chemically pure zinc in the temperature range 0–300° C. after a small plastic extension (∽1 per cent) had been given, Orowan1,2 found that the crystal could be made resistant to plastic deformation until a yield stress much higher than the previously determined 'normal' value was reached. The crystal would then suddenly yield and continue to extend plastically at the normal lower yield stress. On the other hand, a freshly strained crystal began to flow when retested as soon as the lower yield stress was reached. He termed the effect 'thermal hardening'. These effects remind one strongly of the yield phenomenon and strain ageing of iron, and the terms 'upper' and 'lower yield points', respectively, are suggested for the yield values belonging to the thermally hardened and freshly strained states.