Microstructure Development and Superplasticity in (α+γ) Microduplex Alloys with Different Matrix Phases

Abstract

Formation process and superplastic deformation of (α+γ) 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 γ (fcc) is matrix and α (bcc) is the secondary phase, α precipitates from deformed γ matrix with a volume fraction of 14% at 1273 K. Subsequently recrystallization of γ takes place and the (γ+α) microduplex structure with high angle boundaries is formed. In Fe-26Cr-Ni alloys, in which α is matrix and γ is the secondary phase, α matrix recovers and subgrain structure with low angle boundaries is formed. Subsequently, γ phase precipitates at α subgrain boundaries, resulting in the (α+γ) microduplex structure. The (γ+α) microduplex specimens of Ni-40Cr-6Fe-2Ti-1Al exhibit superior superplasticity at 1273 K even at a high strain rate of 1.7 × 10 -1 s - because most of grain boundaries are of high angle despite of small α volume fraction. Contrarily, higher γ fractions (40∼50%) is necessary for the appearance of superplasticity in the (α+γ) microduplex structure of Fe-26Cr-Ni in order to suppress a subgrain growth and to promote the transition of α/α (low-angled) and α/γ (coherent) boundary structures to highly angled ones suitable for boundary sliding through dynamic continuous recrystallization of α matrix during tensile deformation.