The structure of ribbon borides in a Ti-44Al-4Nb-4Zr-1B alloy

Abstract

The structure of ribbon borides in an as-cast Ti–44Al–4Nb–4Zr–1B (at.%) alloy has been studied using high-resolution transmission electron microscopy. The ribbon borides have a laminar structure comprised of thin faceted flakes of borides interspersed with layers of ordered B2 matrix. The dominant phase in the boride flakes is TiB with the B f structure, containing a high density of planar faults on (010). Lattice images from these faults reveal that they correspond to nanoscale intergrowths with the C32 and D7 b structures. The B f flakes also contain thin embedded layers with the B27 structure lying on {110}B f. The orientation relationships between each of these phases have been identified. It is shown that these observations are consistent with the ribbon borides having formed as a sequence of eutectic phase mixtures in the liquid between primary β dendrites.