Electron Microscopy of SAP Following Tensile Deformation or Quenching

Abstract

Transmission electron microscopy is used to support the mechanisms of plastic deformation for SAP-type alloys. An extruded Al-4wt % A12O3 SAP alloy, which was annealed for 2 hr at 500° C., was studied either before or after tensile deformation at various temperatures. In the extruded and annealed alloy, the grains are elongated along the direction of extrusion with a more or less homogeneous distribution of the alumina particles. The samples deformed in tensile tests at low temperatures (-196 to +20°C) reveal a high density of dislocations; the dislocations tend to arrange in cell boundaries with increasing strain. Evidence for cross-slip of dislocations is given by the direct observation of cross-slip traces in the region between the particles. For low deformations (≤ 1%) at 20°C., the dislocation density calculated from the activation volume for the dislocation intersection process (forest process) is in fair agreement with the dislocation density measured by electron microscopy. After deformation at 350°C., the dislocation density is also high, while dislocation loops can be recognized both attached to the particles and isolated. After deformation at 500° C., the dislocation density is much reduced in most of the grains, and the remaining dislocations are pinned principally by the particles. Reactions of attractive dislocations, however, can be recognized in grains where there is still a high dislocation density. A study of the same alloy after cold-rolling, annealing at 640° C., and subsequent cooling either by quenching or slow cooling leads to the conclusion that the particle-matrix interface normally is not coherent. This is supported by the fact that a full structural relationship between the larger particles and the matrix which may lead to coherency occurred only rarely and because the strain field surrounding the smaller particles is consistent with differential thermal contraction effects rather than with coherency.