To investigate the creep behavior of pure Al under static and cyclic stress at $0.4T_m$, the compressive creep tests are performed. The behavior of compressive creep are similar to that of tensile creep and for polycrystalline Al, the threshold stree (σ/E) is observed to be $6.78×10^{-4}$ under compressive stress.
To study crystallographic aspect of creep behavior, the creep test is performed by using the single crystal and the results of creep test are as follows;
Irrespective of crystallographic orientation, mostly one slip system is operated at steady state under cyclic creep acceleration and the slip step spacing of operating slip system is narrower in cyclic than in static creep.
Another result is that cyclic creep activation energy of single crystal is very similar to that of polycrystalline, this consequence shows that deformation process of single and poly-crystal is same. Also cyclic creep activation energy is 80kJ/mol which is similar to that for pipe diffusion.
These results show that cyclic creep acceleration is due to dislocation climb enhanced by mechanically generated excess vacancies under cyclic stress. The lower value of the activation energy of cyclic creep than that of static creep also supports those results.
