an interphase boundary

A grain boundary separates two crystals that are the SAME material at different angles. An interphase boundary is the seam between two DIFFERENT phases: different crystal structures, or different compositions, or both. Think of an ice cube floating in water, or a raisin embedded in a cake: the surface where one substance ends and a genuinely different one begins is the interphase boundary. In a two-phase alloy, it is the interface wrapping each particle of the second phase.

An interphase boundary is a planar defect between two phases, and its key extra feature, compared with a grain boundary, is that the two crystals are usually not even the same structure, so their atoms and spacings differ. What matters most is how well the two lattices MATCH across the seam. That degree of matching sorts interphase boundaries into three kinds: coherent (lattices line up point-for-point, low energy), semicoherent (they mostly line up but the leftover misfit is soaked up by periodic misfit dislocations), and incoherent (no matching at all, high energy). Interphase-boundary energy therefore ranges from very low to very high depending on the fit.

Interphase boundaries govern how strong an alloy is and how its microstructure evolves. In precipitation-hardened alloys, tiny second-phase particles with low-energy coherent interfaces resist coarsening and pin dislocations, giving strength; the total interfacial energy is the driving force for those particles to grow and coarsen over time. Honest note: keep the distinction sharp. Same phase, different orientation is a grain boundary; a different phase is an interphase boundary, and the coherent / semicoherent / incoherent classification applies to interphase boundaries.