Effects for Microstructure on the Oxidation Behavior of Mo-rich Mo-Si-B IntermetallicsVoramon Supatarawanich - Purdue Graduate Student - Materials Engineering
The multiphase alloys of Mo-Mo5SiB2-Mo3Si were studied because Mo5SiB2 and Mo3Si may form the protective glass scale and bcc-Mo may help to improve the fracture toughness. The effects of volume fractions and morphologies of the phases on the oxidation behavior of the multiphase alloys were examined.
Pesting is a transformation of cast MoSi2 back to a powder state by oxidation. MoSi2 exhibits this behavior at around 500oC for extended periods in air. The process is accelerated at intermediate temperatures (800oC and 1300oC).
Cyclic oxidation tests were performed with weighings at 24 hours intervals. Compostions evaluated included "eutectic (approx 25 at% Si, 35 at% B, and 40 at% Mo); a 60 vol% Boron rich version, and a 50 vol% Mo version. Three phases expected in the system are BCC-Mo, Mo3Si, and Mo5SiB2.
The multiphase alloys showed poor oxidation resistance at 800 oC while a protective glass scale could form on the surface at 1300oC. Overall, the near eutectic alloys with a fine dispersion of Mo, Mo5SiB2, and Mo3Si phases displayed the best oxidation resistance for the tested temperatures. The results indicate that the oxidation resistances of specimens with significant fraction of Mo dendrites or T2 dendrites were comparable. The alloy with 50 vol.% bcc-Mo solid solution formed a protective scale at 600 and 1300 oC. However, the glass scale that formed on the surface of the alloys with the lowest Mo3Si and highest Mo5SiB2 volume fraction was NOT protective at 1300oC. The results indicate that, along with Mo5SiB2, the Mo3Si phase is an important source of silicon for controlling the composition of the protective glass scale at high temperature.