The standard electrode potentials for the oxidation of Y and Mg are 2.4 and 2.7V, respectively. That's not that far apart, so it's not a stretch to assume that the potentials at 760°C would be in a proximity that allows for quite some electrochemistry. You can look if you find data on the temperature dependence of the elements.

I doubt that either SF6 or (especially) CO2 are compatible chemically with strongly reducing metals like Mg or Y at 760C. Why not use just argon? Or was your intention to add some fluoride or oxide to the system?