On the Crystallography of Peritectic Austenite and the Role of Primary Ferrite in High Manganese and Aluminum Steels

Abstract

Two high manganese and aluminum steels with duplex microstructures of δ-ferrite and austenite, having different weight fractions of primary δ-ferrite (0.35 vs. 0.66), were used to study the crystallography of austenite formed during the peritectic reaction. Electron backscattered diffraction (EBSD) was used to characterize the crystallographic growth directions and orientation relationships between the two phases. For a primary δ-ferrite weight fraction of 0.66, the peritectic austenite product was seven times more likely to have the Kurdjumov-Sachs (K-S) orientation relationship. In contrast, a lower weight fraction of δ-ferrite produced a peritectic reaction where both δ-ferrite and the peritectic austenite product grew with the same or parallel to the heat flux. The K-S orientation was observed for only 5% of the peritectic interfaces. Manganese and aluminum solute profiles across the peritectic interface were used to show that the mechanism of the peritectic reaction remains the same for each alloy. Thermal analysis was used to compare the two alloys at the dendrite coherency point and the alloy with the smaller fraction of primary δ-ferrite had a greater solid fraction (0.4 versus 0.34) at the dendrite coherency point.