Enhanced oxidation resistance of Mo-12Si-8.5B alloys with ZrB2 addition at 1300 °C

Mo-12Si-8.5B and Mo-12Si-8.5B-1.0wt%ZrB2 alloys were fabricated using mechanical alloying, followed by hot-pressing. Both alloys exhibited uniform microstructure, with the Mo3Si and Mo5SiB2 phases distributing dispersedly in the α-Mo matrix. Mo-12Si-8.5B-1.0wt%ZrB2 showed a finer-grained microstructure than Mo-12Si-8.5B alloy owing to the addition of ZrB2. The results of isothermal oxidation tests at 1300 °C in air revealed that Mo-12Si-8.5B and Mo-12Si-8.5B-1.0wt%ZrB2 alloys initially suffered a transient stage with high mass loss due to the volatilization of MoO3, and then achieved a steady stage owing to the formation of a protective borosilicate scale on the alloy surface. Especially, the transient stage of Mo-12Si-8.5B-1.0wt%ZrB2 alloy was shortened to be less than 300 s, and the mass loss of this stage was reduced by at least 88% compared with that of Mo-12Si-8.5B alloy, indicating a significant improvement in the oxidation resistance. The addition of ZrB2 not only resulted in a continuous borosilicate scale quickly covering the entire base alloy during the transient stage, but also improved the protectiveness of the borosilicate scale of the steady stage by bringing out a large number of ZrO2/ZrSiO4 particles embedded discontinuously in the borosilicate scale, which effectively restricted the inward diffusion of oxygen by acting as diffusion barriers and decreased the thickness of inner oxide layers in particular.