Superplasticity and microstructural evolution in aluminium alloys

Abstract

AbstractThe fine‐grained microstructure required for superplasticity in commercial aluminium alloys is developed in one of two ways. In alloys such as AA5083 Al‐Mg‐Mn and AA7475 Al‐Zn‐Mg‐Cr, static recrystallisation produces an equiaxed microstructure prior to superplastic forming. In alloys such as AA2004 Al‐Cu‐Zr (SUPRAL) and AA8090 Al‐Li‐Mg‐Cu‐Zr, rolling and annealing prior to forming gives a recovered, ‘banded’ microstructure in which the subgrains evolve into grains defined by high angle boundaries during superplastic deformation. That banding‐ in terms of grain orientations‐ persists to much higher strains than would be expected if significant relative grain translation were occurring, and is not consistent with the widely accepted view that grain boundary sliding is the primary deformation mechanism during superplastic flow. To investigate this apparent anomaly further, work on statically recrystallised AA5083 was carried out. Examination of micro‐grid lines on sheet tensile specimens showed that intragranular slip made an insignificant contribution to superplastic flow. Examination of boundary offsets reinforced the view from work on Al‐Cu‐Zr and AA8090 that relative grain translation does not occur to any significant extent. The absence of significant intragranular slip and relative grain translation leads to the conclusion that diffusion creep is making a major contribution to the superplastic deformation.