Effect of Yttrium Alloying on Intermediate to High-Temperature Oxidation Behavior of Mo-Si-B Alloys

The oxidation behavior of 0.2 Y-alloyed Mo-9Si-8B (at. pct) was investigated in a wide temperature range from 923 K to 1673 K (650 °C to 1400 °C). Formation of a thin yttrium-silicate scale at the outer layer along with the thick silica-rich inner layer containing Y-rich oxide inclusions was detected beyond 1573 K (1300 °C). A substantial improvement in the oxidation resistance of the alloy could be realized at 1073 K to 1273 K (800 °C to 1000 °C) with the addition of yttrium. The formation of a viscous silica-rich protective scale could prevent the permeation of MoO3 at the initial stages of oxidation at this temperature regime. The growth of the internal oxidation zone followed a parabolic rate at 1273 K to 1673 K (1000 °C to 1400 °C), and the activation energy values calculated for both the outer oxide scale and internal oxidation zone formation indicated the inward diffusion of oxygen as the dominant rate controlling mechanism. The microstructural and kinetic data obtained for internal and external oxidation indicate that yttrium-silicate scale reduces the inward diffusion of oxygen, thereby improving the oxidation resistance of the alloy at high temperatures in any oxidizing environment.