Engineering grain boundary sliding and cavitation effects in superplastic alloys employing thermodynamics

Abstract

Plastic deformation by grain boundary sliding in superplastic alloys is described by a novel thermostatistical approach. The Gibbs free energy for cavity formation at moving grain boundaries is obtained. It equals the competition between the stored energy at the boundaries and the energy dissipated by grain boundary sliding. The latter is approximated by an entropy term induced by moving dislocations to facilitate boundary displacement. Strength loss evolution is estimated from the cavity evolution rate. The theory describes superplastic behaviour of Zn22Al, Zn21Al2Cu and Mg3Al1Zn for various temperatures, strain rates, grain sizes, and specimen geometries. Transition maps are defined for finding the optimal conditions for achieving superplastic behaviour in terms of composition, temperature, geometry and strain rate.

Effect of strain on cavity development during Al–Zn–Mg–Cu alloy superplastic flow

“…In the present work, cavities intended to appear at the triple junction and elongated along the tensile direction observed by Lhuissier [29] used X-ray micro-tomography as shown in Figure 3(b). The similar results were reported in other alloys [30–32]. The sliding of the grain-groups causes stress concentrations large enough to rupture atomic bonds at triple junctions and thus cavity begins to form as shown in Figure 4.…”