Grain-size dependence of discontinuous yielding in strain-aged steels

Abstract

Recent studies of discontinuous yielding in polycrystalline metals support interpretations of the Hall-Petch relationship which relate k y to the local stress required to spread yielding across grain-boundaries at a yield front. Changes in yield-point behaviour due to strain-ageing can be rationalised on the assumption that the growth of k y provides a measure of the increase in the average value of this local stress. In low-carbon steels, the first phase of ordinary strain-ageing is concerned with Cottrell-atmosphere formation. In test-pieces aged within this first stage, mobile dislocations are almost certainly released by unpinning, but after later stages of ageing the magnitude of the discontinuous yield becomes insensitive to continued solute segregation to dislocations: yield-nucleation by unpinning is then unlikely. After the first stage of ageing it is probable that mobile dislocations are nucleated from grain-boundaries. Thus a reduction in solute segregation at boundaries, by quenching from ~700°C before prestraining, is accompanied by a decrease in the second-stage (“plateau”) values of Luders strain and k y . It is suggested that prestraining causes a reduction in the stress required to generate dislocations from boundaries and that the final slow increase in k y is due to a transfer of interstitial solute from grain interiors to unsaturated grain-boundaries.