Interphase boundary structure with irrational orientation relationship formed in grain boundary precipitation

Interphase boundary structure of bcc precipitates formed at fcc-matrix grain boundaries in a Ni-43 mass pct Cr alloy has been studied experimentally using transmission electron microscopy (TEM) and theoretically analyzed using a geometrical model (near-coincidence site (NCS) lattice model). On both sides of the grain boundary, regardless of the presence of near-rational orientation relationship Kurdjumov-Sachs(K-S) or Nishiyama-Wassermann(N-W), precipitate(bcc)/matrix(fcc) interphase boundaries exhibit planar facets that often contain ledges and regularly aligned, line defects. The analysis indicates that the facet planes correspond to the planes of higher NCS densities for either of the near-rational orientation relationships or the irrational orientation relationships. Similar results are obtained for the precipitate/matrix interface, which lost its original orientation relationship through matrix recrystallization. It is suggested that some partial coherency is expected even for interfaces with an irrational orientation relationship formed during grain boundary precipitation.