Study on plastic instability in superplastic deformation through analysis of the neck shape change

Abstract

Plastic instability has been reanalyzed for superplastic materials with the strain rate sensitivity m and the flow hardening parameter γ. The condition under which a neck does not grow has been obtained as I≡(γ-1)/m≥0. Even though I<0, the neck broadens gradually provided I≥-1. In the case of -1>I>-2, the neck becomes initially sharper but finally becomes broader. If I≤-2, the neck becomes sharp. Although the usual superplastic deformation is thought to proceed in an unstable state because I<0, a large elongation can take place provided that 1>-2 and the neck becomes broader.