Role of Grain Boundaries in the Discontinuous Yielding of Low-Carbon Steels

Abstract

The grain-size-dependence of the lower yield stress in a low-carbon steel, as defined by k y in the Petch relationship, can be reduced by rapid cooling from temperatures above ∼ 400° C. The rate of return of k y towards its saturation value, during subsequent ageing, has been measured in a partially decarburized steel quenched from 700° C, and in a low-carbon steel quenched from 450° C. The time taken to restore k y during ageing at 90° C ( > 103 min) showed that the process depends on interstitial-solute segregation to grain-boundary regions, rather than on relocking of dislocations in the body of the grains. Plastic deformation took place during quenching from 700° C and many of the new dislocations were stopped at grain boundaries; possibly such boundary dislocations play a part in nucleating slip at a yield front in samples quenched from 700° C. No significant plastic deformation occurred in quenching from 450° C; in this case the reduction in k y on quenching, and its subsequent restoration in ageing, provide evidence that k y can depend on solute segregation to the boundaries themselves. Metallographic observations of dislocation distributions in the early stages of plastic deformation support the idea that dislocations are generated from some high-angle boundaries without the help of intense stress concentrations. It is concluded that, in slowly cooled and in fully strain-aged steels, k y is probably related directly to the local stresses required to nucleate dislocations from grain boundaries at the yield front.