THE EFFECT OF EXTRINSIC GRAIN-BOUNDARY DISLOCATIONS ON THE GRAIN-SIZE STRENGTHENING IN POLYCRYSTALS

The grain size dependence of flow stress at room temperature in the regime of small strains (0-2%) has been examined in as-annealed and 2% pre-strained and subsequently annealed specimens of 316L stainless steel. Grain sizes in the range of 3.4-22.4-mu-m were considered. The stress-strain curves exhibit linear hardening characteristic beyond 0.2% plastic, strain. The analysis of the Hall-Petch parameters show a linear increase in sigma-0(epsilon) and a linear decrease in K(epsilon) with strain in the as-annealed specimens. The increase in sigma-0(epsilon) has been associated with both, the work-hardening processes in the grain interior and the long range stress field of extrinsic grain boundary dislocations (EGBDs). The EGBDs also act as sites of stress concentration thereby making it easier to generate dislocations in the vicinity of grain boundaries. Therefore, K(epsilon) which is function of the stress required to generate dislocations decreases with increasing strain. The observed drop in flow stress as a result of annealing of pre-stained specimens at 800-degrees-C has been related with the annihilation of dislocations at and in the vicinity of grain boundaries. Annealing at 550-degrees-C (this temperature is sufficient for the delocalization of the cores of EGBDs) does not have any significant effect on the density of dislocation at and in the vicinity of grain boundaries. Therefore, no significant change in the flow stress was observed.