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.%) a dditions, were investigated by polarized light optical microscopy, har dness measurements and transmission electron microscopy (TEM). A previ ous study (Ratchev et al., Mater. Sci, Eng. A222 (1997) 189) on the ho t ductility of these materials was Further extended to include the rol e 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 dime nsion) may correspond to a small-equiaxed grain visible (with relative ly sharp contrast) under polarized light optical microscopy. Developme nt of long-range misorientations in the cell-forming regions is sugges ted to be the mechanism for their easy optical visibility. Much higher dislocation density and total absence of high angle boundaries iu the cell-forming regions rule out possibilities of dynamic recrystallizat ion. Decreased cell size and increased cell misorientation (both local and long range) were observed with increasing strain. This in turn ma y indicate a combination of deformation/recovery as the formation mech anism. Observed higher recoverability in the material with larger inte rparticle 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., Act a 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 interpa rticle spacings. (C) 1998 Elsevier Science S.A. All rights reserved.