Microstructure change during superplastic deformation in (α+γ) duplex stainless steel

Superior superplasticity is obtained in ( a (bcc)+ T (fee)) duplex stainless steel with the ( α + r ) microduplex structure in which fine T phase is formed at o; subgrain boundaries. In this microduplex structure, a / or boundaries are low-angle ones before superplastic deformation. These low-angle boundaries are changed to high-angle ones by dynamic continuous recrystallization of the a matrix in the early stage of deformation. T phase has nearly the Kurdjumov-Sachs (K-S) orientation relationship with respect to a subgrain before deformation. Orientation relationship between T and ot deviates gradually from the K-S relationship in the early stage of deformation mainly due to dynamic continuous recrystallization of the a matrix and much less coherent a/7 interphase boundaries are formed. Thus, the structure suitable for the grain boundary and interphase boundary sliding is formed by this process in the early stage of deformation, leading to the subsequent fine-structure superplasticity.