Morphology and structure of borides in as-cast Ti and gamma-TiAl based alloys

In this study, the morphology and structure of the borides in boron-modified Ti- and γ-TiAl-based alloys have been investigated using SEM, TEM, and HRTEM. A variety of different boride morphologies was observed including plates, needles, and ribbons. ^ For the plate and needle borides, the major boride phase is B27 TiB. The needle borides have their major axis parallel to [010], and are bounded by (100) and {101} type-facets. The plate borides develop the same types of facets as the needles and have habit planes parallel to the (100). There are high densities of intrinsic stacking faults on (100) in these borides and these correspond to thin embedded layers of the Bf structure. The plate borides do not exhibit well-defined ORs with respect to the surrounding phases, suggesting that they develop in the liquid melt and were then trapped by the growing solid. Needle borides are observed mostly at boundaries between lamellar colonies: these needles tend to occur in groups lying nearly parallel to one another and, in some cases, to adopt well-defined ORs with respect to the surrounding phases. ^ Cored borides with metallic phases such as β, α, ω and α 2+γ in the center are frequently observed, especially in the Ti-based alloy. These core phases usually adopt well-defined ORs with respect to the surrounding boride which enable low-energy coherent interfaces to form between the phases. ^ The ribbon borides are comprised of thin boride flakes interspersed with thin metallic layers. The major boride phase in these flakes is Bf TiB. The habit plane of the flakes is (010) and there are high densities of faults on this plane corresponding to intergrowths of the Ti3B 4 and TiB2 phases, together with thin layers or occluded pockets of metallic B2 phase. Occasional faults are observed on {110} corresponding to embedded slabs of B27 TiB. There is a well-defined OR between the boride flakes and the B2 phase within the ribbons, but not with the surrounding matrix. ^ The characteristics of these various borides are consistent with them forming as eutectic reaction products, with the exception of the finest needles and plates observed in Ti-based alloy.