Mechanism of superplastic deformation in a magnesium alloy. II. The role of grain boundaries

The change in the grain-boundary structure during superplastic deformation of a magnesium alloy (Mg + 1.5% Mn + 0.3% Ce) is investigated by diffraction electron microscopy. The presence of linear defects, extrinsic grain-boundary dislocations (EGBD), is established, whose density depends weakly upon the degree of extension but sharply increases with increasing strain rate. The scheme of the development of sliding according to which the interaction of lattice dislocations with grain boundaries increases their defectiveness, is proposed based on an analysis of the nature and behaviour of EGBD. In such non-equilibrium boundaries kinetic processes activate and grain-boundary sliding is facilitated.