The characteristics of a dislocation structure in a deformed metal are believed to influence a number of mechanical properties. Depends on the test conditions cyclic creep may accelerate the deformation rate compared to that of static creep. The fact that the creep rate of cyclic creep is faster than that of static creep suggests that there might be conspicuous difference between the dislocation structures of statically and cyclically crept specimens. Unfortunately, little is known quantitatively about the nature of the dislocation structures in the specimens cyclically and statically crept.
Because of this cause, the relationship between the creep properties and dislocation structures is our concern. In polycrystalline 99.99% Aluminum, the creep characteristics are compared with dislocation structures produced by cyclic (2cpm, $\frac{t_u}{t_1}$=0.5, 1) and static creep at $0.33T_m$ and under the peak stress of 54Mpa. Transmission electron microscopy was used to study the dislocation structures formed during the creep deformation processes.
In the present work, no difference was detected between the cell sizes, but the marked difference was observed in the thickness of cell boundary and the number of dislocations within the cell interior produced under the same peak stress by steady state static and cyclic creep. It is observed that the annihilation of dislocations occurs during cyclic unloading creep (stage). And there is evidence that the recovery by the climb of dislocations occurs during cyclic unloading stage. From these results, it is considered that the recovery process generates mobile dislocations and the movement of glissile dislocations contributes to the cyclic creep acceleration. The activation energy for cyclic creep, $Q_{cy}$=63.5KJ/mole, is lower than that for static creep, $Q_{st}$=88.5KJ/mole. It is suggested that the activation energy for cyclic creep is the activation energy of vacancy migration in pure Aluminum at $0.33T_m$.
From the above results, it is considered that cyclic creep acceleration occurs due to the enhanced climb of edge dislocations by excess vacancies during cyclic unloading stage.
