Pre-yield Relaxation and the Yielding of Single Crystals of Iron

Abstract

IN tensile tests on polycrystalline iron at the usual rates of strain, if the process of strain is repeatedly interrupted for intervals of three minutes at stresses below the usual upper yield stress, the load immediately falls because of relaxation. If, after this, the process of strain is halted at roughly the upper yield stress of an uninterrupted test, there is no immediate relaxation; instead the load is maintained for a delay period of 2.3–90 sec, when there is sudden yielding and relaxation to the lower yield stress¹. I accounted for such a period without load drop by a reduction of the number of mobile dislocations (or of operative dislocation sources) by the series of relaxations. This explanation was strengthened by the observation that if after relaxation from a pre-yield stress, the stress is brought back to its original value by further straining, a further relaxation gives a smaller load drop. It was expected that single crystals would behave differently from polycrystalline material, and this has now been verified.