Influence of boron content on boride characteristics in cast Ti-45Al-2Mn-2Nb-xB alloys

The morphology and crystal structure of borides in Ti-45Al-2Mn-2Nb-xB (at.%) alloys containing different amount of B have been investigated to reveal the relationship between B content and boride formation. In alloys containing 0.25 at.% and 0.5 at.% B, boride particles are curved and flake-like, and are distributed at β dendrite boundaries, resulting from late nucleation and constrained by matrix during growth. Curved, ribbon-like boride particles are observed to randomly distribute in alloy containing 0.75 at.% B, and insufficient behind-time supply of B atoms to the growth front is believed to be responsible for their irregularity and large curvature bending and branching. In alloys containing 1.0 at.% or higher B, straight needle/rod-like boride particles dominate, and a small number of blocky particles appear in alloys containing 1.25 at.% and 1.5 at.% B. Most boride particles are of C32-TiB2 and have their major axes along [0001] or [12¯10] in all alloys studied in this work, and Bf-TiB and D7b-Ti3B4 randomly appear in conjunction with C32-TiB2 to form intergrowths, while B27-TiB particles are free standing and only appear in alloys containing 1 at.% or higher B.