EFFECTS OF GRAIN-BOUNDARY DEFORMATION ON THE CREEP TRANSITION UNDER THE POWER LAW REGIME IN AN AL-MG SOLID-SOLUTION ALLOY

Abstract

In an Al-3wt.%Mg alloy, the transition creep behaviour was investigated at 573 K (0.62 T(m)) under the stress range 10-55 MPa. The stress exponent n increases with increasing stress from about 3 to 5, and the stress required for the onset of the transition behaviour is found to decrease with decreasing grain size. Under the condition of viscous-glide-controlled creep, subgrains are developed near corrugated grain boundary and, in these deformed regions, creep deformation may be controlled by dislocation climb. Therefore the creep deformation may proceed by the parallel processes of viscous glide and dislocation climb. Subgrains developed near the boundary may promote the transition in stress exponent. Since the volume fraction of these subgrains increases with decreasing grain size, the transition may occur at lower stresses with decreasing grain size. Consequently, it can be suggested that in a polycrystalline aggregate the development of subgrains near grain boundaries may govern the transition behaviour even though, at higher stresses, dislocations locally can break away from their retarding solute clouds, as has been previously reported.