Effect of Ca Addition on the Oxidation Resistance of Ni–Al Alloy

Ni-base superalloys are known for their high temperature oxidation resistance, however,
impurity of S at ppm order can significantly decrease the oxidation resistance of the alloy.
Melting using a CaO crucible is reported to improve the oxidation resistance, but Ca can
negatively impact the oxidation resistance of the alloys, depending on the amount and method
of addition. The objectives of this research were to determine the effect of Ca addition using
Al–Ca alloy, and to understand how Ca affects the oxidation resistance of the alloy. Single
crystal Ni–9.8 wt pct Al model alloys melted in an Al₂O₃ crucible were cast, with the addition of
20 ppm of S in all samples, and 0, 20, and 100 ppm of Ca using Al–4.9 wt pct Ca alloy. Ca
addition improves the oxidation resistance of the samples, though it lacked in efficiency in the
improvement of this trait compared to when using a CaO crucible. Increasing the amount of Ca
addition led to the increase in the amount of CaS formation and desulfurization, though the
removal of slag is crucial. Although Ca was effective in terms of desulfurization, to limit the
negative effects of Ca itself, the formation of sulfides must be the dominating reaction.