Influence of Al contents on alpha-case formation of Ti–xAl alloys

Alpha-case (α-case) formations between Al2O3 mold and Ti–xAl alloys were investigated for the economic Ti–xAl alloys net-shape forming. The α-case formation reaction was examined in a plasma arc melting furnace. The α-case reactions between mold and Ti–xAl alloys were characterized by optical microscopy and a hardness tester. When the Al concentration of Ti–xAl alloys was more than 30 at.%, there were no α-case formation reactions. For the clear examination of the phenomenon, its α-case formation mechanism was elucidated by an electron probe micro-analyzer, transmission electron microscopy, and thermodynamic calculations. The α-case was formed not only by interstitial oxygen elements but also by substitutional metallic elements dissolved from mold materials. Therefore, in the case of more than 30 at.% Ti–xAl alloys, there were neither interstitial nor substitutional α-case formations since Ti–xAl alloys have both negligible solubility of oxygen and low activity in molten states.