Study on role of dynamic recrystallization in fine-structure superplasticity.

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Structural change during superplastic flow has been studied in order to clarify the deformation mechanism and the role of dynamic recrystallization in superplasticity of a duplex stainless steel (Fe-25%Cr-7%Ni-3%Mo) and a high carbon steel (Fe-1%C-1.4%Cr). Main results obtained are as follows.1. A microduplex structure with fine ferrite and austenite grains (about lmum) was obtained in the duplex stainless steel by thermomechamical processing. Ferrite matrix before tensile deformation were subgrains and ferrite/ferrite boundaries were low-angle ones and unsuitable for grain boundary sliding. The specimen with this microduplex structure showed, however, over 1700% elongation at 1273K and epsilon=1.7x10^<-2>/s. In the early stage of deformation, dynamic recrystallization took place in ferrite matrix and ferrite/ferrite boundaries changed to high-angle ones. After the recrystallization of ferrite, dislocations were scarcely observed in both phases of ferrite and austenite during deformation, indicating that grain boundary sliding is the predominant mode of superplastic flow in the present steel. It was concluded that the role of dynamic recrystallization is to make the structure suitable for grain boundary sliding in the early stage of deformation.2. A fine ferrite grain structure with spheroidized cementite particles was obtained in the high carbon steel by warm rolling by 90% at 923K after pearlite transformation. Most of ferrite grains were subgrains even after 90% warm rolling. After superplastic deformation, ferrite grain boundaries were high-angle ones. This result suggests that ferrite phase dynamically recrystallized and became the suitable structure for grain boundary sliding during the deformation.