Polycrystalline yielding—perspectives on its onset

Abstract

It is proposed that the onset of yielding in polycrystalline materials must be considered in terms of three grain size ranges: (1) nano-crystals; (2) large grain sizes, approaching 3 mm and larger grains; and (3) the range of grain sizes in between. In region 1 there is no elastic stress enhancement. Regions 2 and 3 are differentiated by having different sources of elastic stress enhancement. In region 3 elastic stress enhancement arises from elastic incompatibility at grain boundaries, which enhancement causes slip close to grain boundaries. At small grain sizes in this region the elastic stress enhancement at the grain boundaries approaches zero. At large grain sizes in this region there is a smaller relative portion of the grain affected by increased stress at the grain boundaries and an increasing contribution of grain shape to stress enhancement within the grains. Region 2 is dominated by the effect of grain shape. It is proposed that, if bicrystal results can be considered representative of polycrystalline behavior, the grain size at which the elastic stress enhancement at the grain boundary is zero would provide the highest yield strength. The work of Meyers ( Phil. Mag., 1982, 46, 737), although not complete, represents a reasonable approach to the onset of yielding. Suggestions for a change in the model are made. A question is raised as to the viability of relating the onset of yielding to a straight line plot of yield stress to d n , since such plots do not take into a account a number of factors which must impact on yielding.