The effects of B and Nb additions on microstructure and phase stability of Ti-(40~52)at%Al-2at%Cr-(4at%Nb)-(0.4at%B) alloys have been studied at temperatures ranging from 1200 to 1350℃ using OM and SEM equipped with energy dispersive spectrometer and X-ray diffractometer. The addition of B results in a refinement of the as-cast microstructures. Also, it facilitates the precipitation of Widmanststten plates and finally give rise to the basketweave Widmanstatten structure. In the heat-treated conditions, the addition of B reduces the overall γ grain size and leads to an uniform microstructure. From these B effects in the heat-treated microstructure, it is likely that lacy borides act as nucleation sites for γ phase formation and efficiently suppress the coarsening γ of phase during heat-treatment. In addition, it was found that the B modified alloys are more apt to form lamellar microstructures when the alloys are heat-treated to an single phase field and cooled down to room temperature, suggesting that the B addition played an important role in accelerating the formation kinetics of γ plates..
We were able to construct pseudo-binary phase diagrams of Ti-(40~52)Al-2Cr-4Nb alloys and Ti-(40~52)Al-2Cr-4Nb-0.4B alloys at 2Cr-4Nb(-0.4B) sections from the results of extensive phase analyses at various temperatures and Al compositions. The result showed that the B addition does not change the phase sequence in both alloys. For example, Ti-47Al-2Cr-4Nb alloys pass through β+γ→α+β+γ phase sequence with the increase in temperature. However the phase diagram of the alloys containing 0.4%B exhibits a wider single phase region than that of the alloys without 0.4%B. This result seems to be a direct evidence that B stabilizes α phase rather than β or γ phases.
We were also able to construct pseudo-binary phase diagram of Ti-(40~52)Al-2Cr-0.4B alloys at 2Cr-0.4B section. The Nb addition was found to profoundly affect the phase stability of heat-treated alloys. At temperatures ranging from 1200 to 1240℃, the alloys without Nb exhibited no (α+β+γ) three-phase region or (β+γ) two-phase region, contrarily to the Nb containing alloys. This result confirms that Nb is a strong β stabilizer in Ti-Al systems.
