THE INFLUENCE OF GRAIN-STRUCTURE ON THE DUCTILITY OF THE AL-CU-LI-MN-CD ALLOY-2020

The effect of various grain structures produced by thermomechanical treatments on monotonic tensile properties was investigated. Materials having a completely or partially recrystallized structure exhibit elongations between 4 and 8 pct when aged to peak strength. For both cases low ductility is associated with (a) planar deformation, (b) random texture, (c) the presence of large intermetallic compounds along the recrystallized grain boundaries, and (d) precipitate free zones. The first three enhance crack nucleation at high angle grain boundaries, and subsequent crack propagation occurs along the precipitate free zones. The completely unrecrystallized materials have elongations between 10 and 13 pct in both longitudinal and transverse directions. High ductility is associated with a sharp texture and transgranular fracture mode. The maximum ductility is obtained by reducing the unrecrystallized grain size. The results suggest that improved properties may be obtained by lowering the Fe and Si contents to remove coarse constituent phases, eliminating Cd, and replacing Mn with Zr in order to obtain a highly textured, unrecrystallized structure.