Dislocation cell formation and hot ductility in an Al–Mg–Cu alloy

Hot-torqued samples of alloy AA 5182, with and without Cu (0.5 wt.%) additions, were investigated by polarized light optical microscopy, hardness measurements and transmission electron microscopy (TEM). A previous study (Ratchev et al., Mater. Sci. Eng. A222 (1997) 189) on the hot ductility of these materials was further extended to include the role of dislocation cell formation and recovery. TEM investigations have shown that after hot torsion some of the elongated grains contained a dislocation cell structure. An average of four such cells (in one dimension) may correspond to a small-equiaxed grain visible (with relatively sharp contrast) under polarized light optical microscopy. Development of long-range misorientations in the cell-forming regions is suggested to be the mechanism for their easy optical visibility. Much higher dislocation density and total absence of high angle boundaries in the cell-forming regions rule out possibilities of dynamic recrystallization. Decreased cell size and increased cell misorientation (both local and long range) were observed with increasing strain. This in turn may indicate a combination of deformation/recovery as the formation mechanism. Observed higher recoverability in the material with larger interparticle spacings may inhibit the formation of plastic instabilities or strain localizations (Hughes, Acta Metall. Mater. 41(5) (1993) 1421; Kuhlmann-Wilsdorf, Mater Sci. Eng. A113 (1989) 1; Wagner et al., Acta Metall. Mater. 43(10) (1995) 3799; Gil Sevillano et al., Prog. Mater. Sci. 25 (1981) 379; Dillamore et al., Metal Sci. 13 (1979) 73). This explains previous observations (Ratchev et al., Mater. Sci. Eng. A222 (1997) 189) on improved hot ductility in materials with large interparticle spacings.