Improvement of Oxidation Resistance for Ni-Base Single Crystal Superalloy TMS-238 by Suppression of Sb Segregation at Oxide/Substrate Interface Using CaO

Impurity Sb degrades the oxidation resistance of Ni-base superalloys after Al2O3 crucible melting. The purpose of this study is to clarify the effect of CaO on the oxidation resistance of Ni-base single crystal superalloy TMS-238 containing Sb and the reaction mechanism. Single crystal alloys containing Sb were cast after melting using an Al2O3 crucible and a CaO crucible. In the cyclic oxidation tests, the mass did not decrease for the alloy after CaO crucible melting. In STEM observations for the alloy after CaO crucible melting, Sb-Ca-O inclusion was found. In 3DAP for the oxide/substrate interface of the alloy after CaO crucible melting, Sb segregation was not detected. Moreover, the CaO rod was inserted in the TMS-238 melt to observe the CaO/melt interface. In EPMA observations of the surface of the CaO rod, Sb was observed. Therefore, the higher oxidation resistance of the alloy after CaO crucible melting than that of Al2O3 crucible melting is due to the suppression of Sb segregation at the oxide/substrate interface by removing Sb from the melt and fixing Sb in the alloy. These occur by the formation of Sb-Ca-O on the surface of CaO, and the formation of Sb-Ca-O inclusions in the alloy, respectively.