The alpha-case formation reactions between Ti and investment molds (Al_2O_3,ZrSiO_4,ZrO_2,CaO stabilized ZrO_2) were evaluated in a plasma arc melting furnace.Regardless of thermodynamic approaches,there were distinct alpha-case formations.The reaction products were characterized by electron probe micro-analysis and transmission electron microscopy.Theα-case generation between Ti and Al_2O_3 mold was not able to be explained by the conventionalα-case formation mechanism,which is known to be formed by the interstitials,especially oxygen dissolved from mold materials.However,from our experimental results and thermodynamic calculations,it was confirmed that theα-case is formed not only by an interstitial element but also by substitutional metallic elements dissolved from mold materials.Our newly establishedα-case formation mechanism will surely lead to a variety of significant applications of theα-case controlled Ti casting. The alpha-case formation reactions between Ti and investment molds (Al 2 O 3, ZrSiO 4, ZrO 2, CaO stabilized ZrO 2) were evaluated in a plasma arc melting furnace. Regardless of thermodynamic approaches, there were distinct alpha-case formations. The reaction products were characterized by electron probe micro-analysis and transmission electron microscopy. The α-case generation between Ti and Al_2O_3 mold was impossible to be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, from our experimental results and thermodynamic calculations, it was confirmed that the α-case is formed not only by an interstitial element but also by substitutional metallic elements dissolved from mold materials. Our newly established α-case formation mechanism will surely lead to a variety of significant applications of the α-case controlled Ti casting.