Crystallography and interphase boundary of (MnS + VC) complex precipitate in austenite

Crystallography and interphase boundary of (MnS + VC) complex precipitates formed in austenite (γ) matrix are studied by transmission electron microscopy (TEM) in an austenitic Fe-36 mass pct Ni alloy containing small amounts of manganese, sulfur, vanadium, and carbon. When VC is formed directly within the γ matrix grain, it displays a cube-cube orientation relationship (OR) with respect to γ. When VC is formed on MnS, which precipitated in γ with a cube-on-edge OR, three distinctive VC/γ ORs are found: (1) , (2) the cube-cube OR, and (3) the cube-on-edge OR. The MnS/γ OR becomes irrational after γ recrystallization. When VC forms on such incoherent MnS particles, which hold irrational ORs with respect to γ matrix, a wide variety of VC/γ ORs are observed. Geometrical analysis by near coincidence site lattice (NCS) model achieves reasonable success in explanation of the observed planar facets for VC precipitates having rational ORs with respect to γ. However, as for the VC formed on the incoherent MnS with irrational ORs, there is rather poor agreement between observation and prediction. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.