Grain boundary engineering for superplasticity in steels

The microstructure with suitable boundary characters for superplasticity is summarized for the steels which consist of two phases, i.e., ferrite (bcc α) + austenite (fcc γ) or ferrite (α) + cementite (orthorhombic θ -Fe3C). In (α + γ) duplex alloys, a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) produces the (α + γ) duplex structure through the competition of recovery/recrystallization of matrix and precipitation. In Fe-Cr-Ni (α + γ) duplex stainless steels with high γ fractions (40–50%), α matrix undergoes recovery to form α subgrain boundaries and γ phase precipitates on α subgrain boundaries with near Kurdjumov-Sachs relationship during aging. By warm deformation, the transition of α boundary structure from low-angle to high-angle type occurs by dynamic continuous recrystallization of α matrix and, simultaneously, coherency across α/γ boundary is lost. Contrarily, α phase first precipitates in deformed γ matrix in Ni-Cr-Fe based alloy during aging. Subsequently discontinuous recrystallization of γ matrix takes place and the (α + γ) microduplex structure with high-angle γ boundaries is formed. The formation of those high-angle boundaries in (α + γ) microduplex structure induces the high strain rate superplasticity. In an ultra-high carbon steel, when pearlite was austenitized in the (γ + θ) region, quenched and tempered at the temperature below A1, an (α + θ) microduplex structure in which most of α boundaries are of high-angle type is formed through the recovery of the fine (α ′ lath martensite + θ) mixture during tempering. Such (α + θ) microduplex structure with high angle α boundaries exhibits higher superplasticity than that formed by heavy warm rolling or cold rolling and annealing of pearlite which contains higher fraction of low angle boundaries.