The conditions for cyclic creep acceleration have been investigated by altering the stress frequency, amplitude and the composition of the alloy.
The stress value (threshold stress), where the cyclic creep acceleration begins to appear, seems to be independent from the cyclic stress conditions. The threshold stress of Cu and Al are measured to be $5.34×10^{-4}$, compensated by the Young's modulus. The additions of alloy elements increases this threshold stress. From the experimental data, the threshold stress can be expressed as follows
$(σ/E) = (σ/E)_p+(σ/E)_s$
where $(σ/E)_p$ means the threshold stress of pure metals, $(σ/E)_s$ means the stress increase by the solute atoms. The $(σ/E)_s$ can be derived by using Cottrell theory, as follows
$(σ/E)_s = (1-C_o)×{W}_i/E/b$
where $W_i$ means the interaction energy between dislocations and solute, $C_o$ means the concentration of solute.
The strain burst can also be observed in the range, where the cyclic creep acceleration occurs and the serration can be observed during tensile test. In this experimental range, it can be said from the static creep data that the dislocation climb at the cell boundary may play the dominant role in creep deformation. If the cyclic stress assists the climb process at the cell boundary with the plastically generated vacancy, the average waiting time for climb may be shortened. Therefore, if the tensile deformation are resumed after the cyclic stressing, the background frequency in serrated flow can be increased. As showned form the static creep data, it can be said that the solute atmosphere can be formed around the climbing dislocation and the dislocation climb may be retarded by this dragging. The increase of effective stress, which is due to the recovery assisted by the cyclic stress, can be resulted into the abrupt motion of the aged dislocation. Therefore, the strain burst can be occured during the cyclic creep acceleration.
The difference in the grain boundary deformation between statically and cyclically crept specimens of Al-Mg solid solution has been investigated at 0.6Tm. In statically crept specimens, the grain boundaries are deformed irregularly and no crack is formed. However, in cyclically crept specimens, the grain boundaries are remained to be smooth and the wedge type crack are formed at the triple points. On the basis of the experimental observations it is believed that the cyclic stressing enhanced the grain boundary sliding through the accelerated recovery with the help of mechanically generated excess vacancies.
