The Evolution of Grain Boundary Character during Superplastic Deformation of an Al-6 Pct Cu-0.4 Pct Zr Alloy

摘要

The evolution of microstructure, texture, and microtexture in an Al-6 pet Cu-0.4 pct Zr alloy was studied during mechanical testing at 480 deg C and a strain rate of 5 centre dot 10~(-4) s~(-1). The as-processed material had an elongated, banded microstructure and a deformation texture with orientation distribution along the #beta#-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 lithe 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.