Deformation bands and the formation of grain boundaries in a superplastic aluminum alloy

Superplastic aluminum alloys are often classified according to the mechanism of microstructural transformation during annealing after deformation processing. In Al-Cu-Zr materials, such as Supral 2004, the presence of fine (10 to 50 nm) second-phase particles retards dislocation rearrangement and the formation and migration of boundaries during either annealing or elevated temperature deformation after thermomechanical processing. This leads to predominance of recovery in the evolution of microstructure, although high-angle boundaries must still form in order to account for the superplastic response of such materials. The mechanisms of high-angle boundary formation in such circumstances have remained unclear. The term “continuous recrystallization” (CRX) has been used as a phenomenological description of recovery-dominated processes that take place uniformly through- out the microstructure and lead to the formation of fine grains with high-angle boundaries. Orientation imaging microscopy (OIM) methods have been employed to assess the as-processed microstructure of this alloy and its evolution during annealing at 450 °C, as well as during superplastic deformation at this temperature. Orientation images demonstrate the presence of deformation bands of alternating lattice orientations that corresponds to the symmetric variants of the brass, or B, texture component ((112){110} in rolled material). During annealing, the high-angle grain boundaries (disorientation of 50 to 62.8 deg) develop from transition regions between such bands while the lower-angle boundaries (i.e., up to 20 deg) separate an evolving cell structure within the bands. Further OIM results show that the bands remain distinct features of the microstructure during either annealing alone or during deformation under superplastic conditions

The evolution of grain boundary character during superplastic deformation of an Al-6 pct Cu-0.4 pct Zr alloy

The evolution of microstructure, texture, and microtexture in an Al-6 pct Cu-0.4 pct Zr alloy was studied during mechanical testing at 480 °C and a strain rate of 5·10−4 s−1. The as-processed material had an elongated, banded microstructure and a deformation texture with orientation distribution along the β-orientation fiber. The true strain vs true stress curve exhibited three stages: I, II, and III. Work hardening occurred in stages I and III, whereas nearly steady-state behavior was observed in stage II. A bimodal distribution of boundary disorientation angles was evident in as-processed material and persisted into stage I, with peaks at 5–15 deg in the low-angle boundary (LAB) regime and at 45–60 deg in the high-angle boundary (HAB) regime. An increase in strain rate sensitivity coefficient, m, in stage I was accompanied by fragmentation of the initial microstructure, leading to the formation of new grains. During stage II the strain rate sensitivity coefficient, m, attained a value of 0.5, which is consistent with the onset of grain boundary sliding. In stage III, the texture and the grain boundary disorientation distribution became randomized while the m value decreased. Grain elongation and cavity formation at second-phase particles and along grain boundaries were seen in samples deformed to failure. The as-processed microstructure is described in terms of deformation banding, and a model for the evolution of such a structure during superplastic deformation is proposed.Peer reviewe