THE STRUCTURE OF COMPLEX MONOBORIDES IN GAMMA-TIAL ALLOYS WITH TA AND B-ADDITIONS

Fibrous primary borides grown from the melt in a Ti-48Al-9Ta-4.3B at.% alloy were characterized by conventional and high resolution transmission electron microscopy. The as-cast borides show predominantly the B27 crystal structure of TiB, but with twice as much Ta as Ti in the metal sublattice. The borides contained numerous planar faults parallel to the (100)TiB planes, which multiplied and evolved into long platelike precitates with the structure of TaB (B(f)) upon prolonged heat treatment. The initial faults could be described as rotational twins on the (100)TiB plane, giving rise to monolayers of the B(f) structure; migration of planar diffusion regions along the needle axis subsequently coarsens the TaB regions. Experimental and computer-generated diffraction patterns were consistent with the orientation relationship anticipated from the twinning model, (100)TiB parallel-to (110)TiB and [010]TiB parallel-to [001]TiB. High resolution images showed that the B(f) precipitates are fully coherent with the parent B27 matrix and only a few atomic layers thick, but extend over many micrometers along the axis of the boride needles. Moreover, microchemical analysis revealed that the structural transformation occurs without apparent solute partitioning. The evolution of the boride structures is discussed in terms of phase stability, growth kinetics and lattice energy considerations.