The effect of electron irradiation on the "flow stress of magnesium and zinc single crystals oriented for basal slip was investigated. Tensile tests were performed before and after electron bombardment with 1 MeV energy at 4.2°K. An increase in the flow stress was observed after irradiation in the two h.c.p. metals. The flow stress increase in both metals was found to be proportional to the square root of the electron dose. This irradiation strengthening effect in the h.c.p. metals was similar to that found in our previous studies in f.c.c metals, but it was opposite to the irradiation' weakening effect found in pure iron. The present result adds support to our previous assertion that intrinsically hard crystals such as iron are softened by the presence of point defects that assist kink nucleation on screw dislo¬cations, and that intrinsically soft crystals such as f.c.c. and h.c.p. (basal slip) crystals are hardened by the presence of point defects that hinder growth of kinks on dislocations.
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