Phase transformations in XD TM TiB 2-reinforced near-γ Ti [sbnd]48Al [sbnd]2Nb [sbnd]2Mn

Abstract

The elevated temperature phase transformations occurring in investment cast and hot-isostatically pressed near-gamma Ti 48Al 2Nb 2Mn containing 0, 0.5, 1, 2 and 7 vol.% TiB 2 have been investigated utilizing in situ thermal analysis, high temperature X-rays and optical and transmission electron microscopies. A series of reversible transformations were observed during heating and cooling:α 2 + γ → α + γ → α + β + γ → β + γ → β +L where, on heating, the low temperature eutectoid transformationα 2 + γ→α was followed by β precipitation and γ dissolution, a high temperature reverse peritectoid reaction,→αβ + γ, and, finally, an inverse peritectic reaction,γ→β+ L. Furthermore, the variable TiB 2 morphologies observed suggest that TiB 2 redissolves during XD TM processing, reprecipitating during subsequent solidification. Its morphology then evolves from lacey to plate- or needle-like, to blocky as the B content increases or the solidification rate decreases. Finally, it was shown that, rather thatn the TiB 2 content per se, the elemental B content and, by extension, the total interstitial content were the primary factors controlling the differences in the phase transitions temperatures observed in these materials. As the interstitial content increased, the eutectoid transformation temperature increased, while the peritectoid transformation temperature was not affected and the peritectic transformation temperature decreased.