THE EVOLUTION OF METASTABLE BF BORIDES IN A TI-AL-B ALLOY

Additions of boron to titanium aluminides in the alpha2 to gamma composition range leads to the formation of a variety of boride particles. Information available on the ternary phase diagram indicates that the equilibrium borides are TiB (B27), Ti3B4 (D7b) and TiB2 (C32). Conventional and high resolution transmission microscopy of the borides produced in a dilute Ti-40.97Al-0.97B alloy revealed the presence of numerous monoboride particles with the B(f)(CrB-type) structure. This metastable form of TiB evolves during solidification, nucleating on preexisting particles of the same compound with the normal B27 structure. In addition, the B(f) particles contain faults equivalent to nanoscale intergrowths of the D7b and C32 structures. The evolution of the boride phases in this system was analyzed in terms of crystallographic relationships elicited from structural models, all of which are based on the same trigonal building block of 6 metal atoms around each B atom. A Mie potential calculation was used to assess the relative stability of the monoboride forms. It is proposed that the B(f) structure arises in response to a change in the melt chemistry which would produce Ti3B4 or TiB2 under equilibrium conditions. The preferential formation of B(f) is ascribed to its closer crystallographic relationship with the B27 structure, which facilitates its nucleation on existing TiB crystals.