A MODEL FOR SUBGRAIN SUPERPLASTIC FLOW IN ALUMINUM-ALLOYS

Aluminum alloys that contain low angle boundaries exhibit different superplastic behavior than alloys consisting of high angle boundaries. On a relative basis, the low angle boundaries increase the flow stress, but impart a greater resistance to cavitation; the strain-rate sensitivity of this material is generally smaller and the change in the strain-rate sensitivity with strain rate shows a minimum instead of a maximum as observed in the large angle boundary materials. As a result, the subgrain material can be deformed to large tensile strains at fast strain rates. A kinetic model for subgrain superplasticity that invokes a balance between the arrival and emission rates of dislocations at low angle boundaries is presented. It explains several features of subgrain superplasticity. It also explains why ultrafine dispersoids of intermetallics appear to stabilize the subgrain structure in aluminum. Early work on the correlation between flow stress and the subgrain size in dynamic recrystallization of metals may also be consistent with the model.