MICROSTRUCTURAL STUDY OF GRAIN-BOUNDARY CAVITATION DURING HIGH-TEMPERATURE DEFORMATION OF AL-4.5-PERCENT MG ALLOY

Microstructural processes of the formation of grain boundary cavities during deformation of an Al-4.5 mass% Mg solid solution alloy at high temperatures (373-623 K) were discussed from a viewpoint of microstructure. Structural examination was carried out mostly at 533 K at which, under a strain rate of 2.7 x 10(-5) s-1, the ductility (the reduction in area at fracture) took a minimum value (approximately 48%) and the mode of fracture was intergranular because of severe grain boundary cavitation. Transmission electron microscopy revealed that, during deformation, grain boundaries became serrated by the interaction with subboundaries developed in their vicinity, and that cavities were initially formed at cusps of the serrated boundaries (at subboundary/grain boundary junctions). These facts indicate that grain boundary irregularity acting as a site of stress concentration during its sliding is the cusp, not the deformation ledge; both the serration and the sliding of grain boundary are considered to be prerequisite for the grain boundary cavitation in the present material. The reduction of ductility with temperature in a temperature range below 533 K arose from the increasing degree of serration and sliding of grain boundary, and its recovery at temperatures above 533 K was ascribed to the local migration of grain boundary by which cavities are left behind it (in the grain interior) and the concentrated stresses are relaxed rapidly at their sites. The idea of the cavity formation at subboundary/grain boundary junctions is not new, but has been suggested hitherto. The present work deals with detailed processes of this type of cavitation.