In an Al-2.9%Mg solid solution alloy, the behaviors of the static and cyclic creep deformation and the modes of grain boundary deformation with the various stress amplitude ratio under the stress range of 15 to 50 MPa at 0.62 $T_m$ ($T_m$ ; the absolute melting temperature, 573 K) were investigated. For static creep, there was a transition from class A to class M creep behavior with increasing stress. The observed transition stress, $\sigma_u$, was about 42 MPa. The dominant grain boundary deformation characteristics in the class A regime and the class M regime were the grain boundary massive migration and the grain boundary serration, respectively.
However, no transition was observed for cyclic creep, instead, the creep behavior tended markedly to exhibit the class M behavior with increasing the stress amplitude ratio, δ, where δ is defined as Δσ/σ (Δσ is the difference between the applied peak stress, σ, and the applied minimum stress, $\sigma_{min}$, during cycling). The grain boundary cracking (GBC) of the cyclically crept specimen was observed only in the stress range equivalent to that of the class A creep behavior of the static creep, that is, in the stress range below the transition stress, $\sigma_u$, of the static creep. The measured internal stress, using by strain transient dip test, at the each critical stress amplitude ratio, $\delta_{GBC}$, is nearly equal to or slightly higher than the critical applied minimum stress of GBC, $\delta_{GBC}$, which is defined as (1-$\delta_{GBC}$) × σ. In other words, The GBC was observed only under the cyclic stressing in the viscous glide regime of the static creep at the conditions of both σ ＞ $\sigma_i$ and $\sigma_{min}$ ≤ $\sigma_i$ and a long enough unloading period for the dislocation recovery to take place. Consequently, the formation of the grain boundary cracking is considered to be closely related with both the recovery of dislocations in the grain boundary and the homogeneous distribution of Mg solute atoms in the grain boundaries.
