One of the interesting phenomena, related cyclic load or stress, is the acceleration of creep deformation rate (cyclic creep acceleration) although the time of appling stress is smaller than that of static stress. During past two decades, many efforts have been performed to find the cyclic creep acceleration mechanism and some theories are suggested but still now, some controversys exist between them.
In this work, the creep deformation behaviors of pure Cu and Al under the static and cyclic stress were studied in macro-and microscopical aspects for the better understanding of cyclic creep acceleration mechanism.
As the results of static and cyclic creep experiments, it is found that both the cyclic creep acceleration and retardation can be occured depending on the condition of peak stress and temperature combination. The prominent differences between static and cyclic creep behaviors are the increase of stress exponent and the decrease of creep activation energy under the cyclic stress. These experimental results mean that larger amount of athermal work is done by cyclic stress.
The differences in dislocation microstructures developed during static and cyclic creep deformation were also investigated to find the effects of cyclic stress. The cell structure, developed during cyclic creep, showes some evidences of enhanced recovery of cell boundary than those of static creep (i.e. larger cell size and more sharp and condenced cell boundary developed during cyclic creep) and as the peak stress is increased, these differences is increased. These experimental results were interpreated as the role of mechanically generated excess vacancies by cyclic stress because the highly tangled dislocation cell boundary can not be recovered without diffusion al process. The increase of cell size and decrease of stress dependence of cell size under the cyclic stress seems to be the results of enhanced recovery with mechanically generated excess vacancies. Another evidence of excess vacancy is the decrease of creep activation energy under the cyclic stress than that of static creep.
With the two apparent parameters (the difference in stress exponent "Δn" and activation energy "ΔW"), the criterion of cyclic creep acceleration can be represented as the following relationship.
$σ th/G = Ath exp(-ΔW/ΔnRT)$
In this, σth is the threshold stress for cyclic creep acceleration at a given temperature T and Ath is the material constant. Using this relation and the deformation mechanism map, the condition of cyclic creep acceleration can be represented graphically.
The comparison of creep behavior of pure Cu and Al reveals that the threshold stress for Al is much larger than that of Cu at a given temperature. This result seems to be due to the large difference in stacking fault energy because the thermal recovery of high stacking fault energy material is faster than that of low stacking fault energy material and the contribution of athermal recovery effect by cyclic stress is decreased.