The creep behavior of pure fcc metals under repeated stressing (cyclic creep) is investigated under constant peak stress conditions in the temperature range of 0.36-0.61 $T_m$.
In pure aluminum, cyclic creep acceleration is found to occur up to a temperature of about 0.5 $T_m$. This phenomenon is more significant with increasing stress at a given temperature. The measured activation energy for cyclic creep deformation, $Q_{cy}$, is found to be lower than that of static creep, $Q_{st}$. It may be interpreted by adding the term, expressed by the excess vacancies athermally generated during cycling, to the effective diffusivity suggested by sherby. Cyclic creep acceleration behavior can be explained by the proposed acceleration parameter, K exp $[\gamma(\sigma/E)]$ exp $(ΔQ/RT)$, where K and $\gamma$ are the constants and ΔQ is a difference between $Q_{st}$ and $Q_{cy}$.
Using pure nickel (99.99%) specimens, electrical resistivity was measured to detect the difference in vacancy concentrations in the crept specimens tested cyclically and statically. It is observed that the concentration of vacancies generated under the cyclic stressing is about twice of that formed under the static stressing. This higher vacancy concentration generated athermally under cyclic creep may assist the recovery process through dislocation climb to give cyclic creep acceleration.
The effect of quenching retards the static creep deformation but accelerates the cyclic creep deformation in pure aluminum. This accelerated cyclic creep behavior in quenched aluminum is interpreted from the role of excess vacancies generated by the non-conservative motion of jogs under repeated stressing.
From the test results of mode change, it can be said that the excess vacancies influence more predominently on the climb of edge dislocations under repeated stressing than under static stressing. This is explained by 'dislocation sweeping' phenomenon due to the backward motion of the edge dislocations.
At relatively low stress ranges, cyclic creep acceleration is observed after certain creep time (transition time). This transition time means that cyclic creep acceleration predominently occurs on the work-hardened states. This trend is more systematically examined by prestraining the specimens prior to creep testing. The phenomenon in which prestraining accelerated cyclic creep rate can be explained by the increment of jog density due to prestraining.
From the above mentioned phenomena, it may be concluded that cyclic creep acceleration occurs due to the enhanced climb of edge dislocations assisted by excess vacancies.
