Microstructure development and superplasticity in (α+γ) microduplex alloys with different matrix phases

Formation process and superplastic deformation of (alpha+gamma) microduplex structure by thermomechanical processing (solution treatment+heavy cold rolling+aging) have been studied in the cases for different matrix phases. In Ni-40Cr-6Fe-2Ti-1Al, in which gamma (fcc) is matrix and alpha (bcc) is the secondary phase, or precipitates from deformed gamma matrix with a volume fraction of 14% at 1273 K. Subsequently recrystallization of gamma takes place and the (gamma+alpha) microduplex structure with high angle boundaries is formed. In Fe-26Cr-Ni allays, in which alpha is matrix and gamma is the secondary phase, alpha matrix recovers and subgrain structure with low angle boundaries is formed. Subsequently, gamma phase precipitates at alpha subgrain boundaries, resulting in the (alpha+gamma) microduplex structure. The (gamma+alpha) microduplex specimens of Ni-40Cr-6Fe-2Ti-1Al exhibit superior superplasticity at 1273 K even at a high strain rate of 1.7 x 10(-1) s(-1) because most of grain boundaries are of high angle despite of small alpha volume fraction. Contrarily, higher gamma fractions (40 similar to 50%) is necessary for the appearance of superplasticity in the (alpha+gamma) microduplex structure of Fe-26Cr-Ni in order to suppress alpha subgrain growth and to promote the transition of alpha/alpha (low-angled) and alpha/gamma (coherent) boundary structures to highly angled ones suitable for boundary sliding through dynamic continuous recrystallization of alpha matrix during tensile deformation.