In an Al-3at%Mg alloy, the transition creep behavior was investigated at 573K∼613K($0.62T_m\sim0.66T_m$) under the stress range of 10 to 50MPa with the grain sizes of 150 and 500㎛, respectively. The stress exponent, n, increases with increasing stress from about 3 to 5, and the stress required for the onset of the transition behavior 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 grain boundaries promote the transion behavior. Since, the volume fraction of these subgrains increases with decreasing grain size, the transion behavior may occur at lower stresses with decreasing grain size, but this trend is similar to different test temperatures.
Consequently, it can be suggested that in a polycrystalline aggregate, the development of subgrains near grain boundaries may govern the transition behavior, even though at higher stresses dislocations locally can break away from their retarding solute clouds, and then the transition stress is independent of test temperatures but increases with increasing grain size.
