Two-phase and three-phase crystallographic relationships in white-solidified and nitrided Fe-C-Si cast iron

Abstract

Crystallographic orientation relationships (ORs) among α-Fe, cementite θ-Fe3C1-z, and hexagonal close-packed iron carbide/(carbo)nitride ε-Fe3(N,C)1+x are highly important to phase transformations and microstructure evolution during heat-treating of cast irons and steels, and during nitriding of these materials. Several α/θ, ε/θ, and α/ε two-phase ORs have been controversially discussed in literature. In that regard, an α/ε/θ three-phase crystallographic relation has been hypothesized, yet hitherto hardly verified, which justifies the existence of two distinct α/θ ORs by making use of an ε intermediate in the θ formation.In this study, ORs in a white-solidified and gas-nitrided hypoeutectic Fe-3.5 wt%C-1.5 wt%Si model alloy were investigated by electron backscatter diffraction (EBSD). The specific alloy composition and nitriding conditions allow simultaneously examining α/θ, ε/θ and α/ε ORs, and particularly the α/ε/θ three-phase relationship, which can be hardly assessed simultaneously in conventionally treated cast irons and steels.Eutectic cementite and pearlite constitute the white-solidified microstructure. Eutectic cementite and pearlitic α-Fe frequently fulfill the Isaichev OR and certain ORs, which are close to the Pitsch-Petch OR. Upon nitriding, ε forms from surface-adjacent cementite, whereas α has largely been retained due to the presence of Si. About 70% of the ε grains fulfill the OR 010θ∥112¯2¯ε/100θ∥112¯3ε. The orientation of α adjacent to the newly formed ε determines which variant of that ε/θ OR is selected. Thereby, irrespective of the two main α/θ ORs, basically the same α/ε OR is established. The α-affected variant selection proves the existence of an α/ε/θ three-phase crystallographic relationship given by 031¯θ∥101¯1¯ε∥101¯α/100θ∥112¯3ε∥131α or 1¯11¯α.