Alpha-case formation mechanism on titanium investment castings

The aim of this study is to clarify of the alpha-case formation mechanisms for the economic net-shape forming of Ti and Ti alloys. The alpha-case formation reaction between Ti and Al2O3 mold was examined in a plasma arc melting furnace. The reaction products were characterized by electron probe micro-analyzer and transmission electron microscopy. The alpha-case generation between Ti and Al2O3 mold could not be explained by the conventional alpha-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, from the experimental results and thermodynamic consideration, it could be confirmed that the alpha-case is formed by not only interstitial oxygen atoms but also substitutional metal atoms dissolved from mold materials.