Microstructure, mechanical properties and oxidation behavior of a multiphase (Mo,Cr)(Si,Al)2 intermetallic alloy–SiC composite processed by reaction hot pressing

A multiphase (Mo,Cr)(Si,Al)2 intermetallic alloy matrix-composite comprising body-centered tetragonal, C11b (tI6) structured (Mo0.23Cr0.10)(Si0.67−xAlx) and hexagonal, C40 (hP9) structured (Mo0.16Cr0.17)(Si0.67−xAlx) (where x ≤ 0.0008) phases as well as a dispersion of α-Al2O3 and β-SiC phases has been processed by reaction hot pressing of Mo, Si, Al, MoSi2 and Cr3C2 powders. A noticeable increase could be observed in the hardness of the matrix as well as that of the composite. The indentation cracks follow a more tortuous path and have shorter lengths compared to those in MoSi2, although the testing of Single Edge Notch Bend specimens has not shown any improvement in fracture toughness. Oxidation at 1873 K has shown the formation of a scale, containing the crystalline SiO2 identified as crystoballite, Cr2O3, α-Al2O3 and Cr2MoO6 phases. With increase in the time of oxidation, a relatively smooth and thick SiO2 film appears to overlap the other crystalline phases, and cracks develop in the scale due to development of internal stresses during growth of different oxides or cooling to room temperature.