The effect of grain size on the stress-strain curve of α-iron and the connection with the plastic deformation of the grain boundaries

Abstract

It has been widely accepted that the grain size dependence of the lower yield point of mild steel is caused by the relaxation of the shear stress along the slip bands and by the ensuing concentration of this stress at the grain boundaries, which latter form an obstacle to slip. As is well known the grain size influences not only the lower yield point, but also the complete stress-strain curve i.e. the yield stress of the hardened material. Because of the strong similarity of the above size phenomena, it seems likely that also in the case of strain hardened material the effect is due to the influence of the slip bands and the grain boundaries, as mentioned above. In the present paper experimental data will be given with respect to the grain size dependence of the yield stress of decarburized (and denitrided) mild steel under uniaxial load. From the above data and with the aid of dislocation theory an estimate has been made of the grain boundary stresses. The magnitude of these stresses suggésts that during plastic deformation dislocations do not stop at a grain boundary but slip across it into the adjacent grain. A comparison of the boundary stress which occurs at a low value of the strain in the case of decarburized α-iron with the lower yield point in the case of mild steel (Hall) suggésts that the presence of carbon and nitrogen may considerably enhance the strength of the grain boundaries.