Microstructure and mechanical properties of Mo–Mo3Si–Mo5SiB2 silicides

Molybdenum boron silicides consisting of 20–50 vol.% of α-Mo and different ratios of the intermetallic phases Mo5SiB2 (‘T2’) and Mo3Si were prepared by arc-melting followed by dropcasting into Cu chill molds. For α-Mo volume fractions of ≈50% sound castings were obtained. For lower α-Mo volume fractions such as 25% macroscopic cracks were often observed. Preliminary oxidation tests verified the expected increase in the oxidation resistance as the B-concentration increases and the α-Mo volume fraction decreases. Also, the formation of glass films was observed. Depending on composition and heat treatment, the room temperature flexure strengths varied usually between 300 and 600 MPa. To some extent, these variations could be rationalized by differences in the microstructures. Annealing for 1 day at 1873 K in vacuum caused distinct microstructural coarsening. Annealing was sometimes accompanied by microcracking in the Mo3Si and the T2 phases. Cooling curves suggest that the liquidus temperature of the T2 phase is above 2400 K. Depending on the composition, final freezing of Mo–Si–B alloys was found to occur at temperatures as low as 2200 K. Care is therefore required during thermomechanical processing to avoid the formation of liquid phases.