Abstract
Superplasticity of δ/γ duplex stainless steels has been studied in relation to the microstructural change during deformation in order to elucidate the role of the second phase particles. In the case of deformation in the δ/γ duplex phase region, local strain concentration within the δ-ferrite matrix due to dispersion of relatively hard γ particles and the subsequent recrystallization occur repeatedly. When the alloys containing high Cr and Mo are deformed at temperatures around 900°C, σ phase particles precipitate via eutectoid decomposition of δ-ferrite into new γ and σ phases, and the γ/σ duplex phase structure forms. The dynamic recrystallization of γ phase matrix occurs locally and intermittently, and extremely large superplasticity is also obtained. Prior cold work largely accelerates these processes, and leads to equiaxed δ/γ or γ/σ duplex structure in the early stage of deformation. The superplasticity in both cases can be obtained by subtle balance between the local strain hardening and the subsequent recrystallization, and thus the flow stress exhibits large strain rate dependence. This model can also explain granular appearance of the fracture surface as a coalescence of microvoids induced by the individual hard phase particles. In order to obtain large superplasticity, the microstructure should consist of hard particles embedded within a soft matrix and the amount of hard particles should be at least greater than 10% in the case of γ/δ duplex structure. The structure with hard phase matrix within which the soft second phase particles disperse do not exhibits superplasticity.
