ON THE MICROSTRUCTURAL ASPECTS OF THE NONHOMOGENEITY OF SUPERPLASTIC DEFORMATION AT THE LEVEL OF GRAIN GROUPS

The surface relief of a superplastically deformed magensium alloy (Mg-1.5%Mn-0.3%Ce) was studied. Zones/bands of localized deformation were detected by means of vacuum etching, Between the localized deformation bands were less-deformed regions. After 20% elongation in vacuum (1.33 x 10(-3) Pa), zones of intensive vacuum etching were observed as two intersecting bands of localized deformation oriented at 35-60-degrees to the tensile axis. Spacing between the localized deformation bands is 6-8 grain diameters after a tensile elongation of 20% and 3-4 grain diameters after an elongation of 160%. The observed bands of strain localization are explained from the viewpoint of cooperative grain boundary sliding, i.e. shifting of grain groups as a whole unit along grain boundary surfaces oriented close to the maximum shear stress direction. It is suggested that the cooperative nature of GBS be taken into account when evaluating the real local strain rate and the real strain-rate sensitivity of grain boundary sliding process.