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

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 Bf 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 D7b structures. The Bf flakes also contain thin embedded layers with the B27 structure lying on {110}Bf. 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.