Solidification microstructure of supercooled Ti-Al alloys containing intermetallic phases

Abstract

Titanium-aluminum alloys (45, 50 and 55 at.% Al) containing the intermetallic phases α 2- Ti 3 Al and γ-TiAl were melted, supercooled and solidified in an electromagnetic levitation device. The thermal history was recorded with the aid of a fast-response two-color pyrometer and the supercooling achieved was determined from the thermal excursion during recalescence. Maximum supercoolings were 262, 286 and 348 K for the 45, 50 and 55 at.% Al alloys, respectively. Microstructural analysis reveals that the 45 at.% Al alloy selects the primary β-(Ti) phase at all supercoolings, while the 50 at.% Al alloy changes from primary α-(Ti) to primary β-(Ti) at supercoolings of the order of 85 K. The relative amount of second phase segregate (γ) decreases with increasing supercooling in the 50 at.% Al alloy, and is absent in Ti-45 at.% Al. The 55 at.% Al alloy microstructure consists of primary a dendrites in a matrix of γ segregate, but in this case the amount of γ increases with increasing supercooling. The primary β α dendrites normally transform upon cooling to an α 2 + γ lath microconstituent, although the transformation is largely suppressed by increasing the cooling rate and/or decreasing the aluminum content.