Microstructure and mechanical properties of Mo-Mo3Si-Mo5SiB2 silicides

Molybdenum boron silicides consisting of 20-50 vol.% of alpha-Mo and differ ent ratios of the intermetallic phases Mo5SiB2 ('T2') and Mo3Si were prepar ed by are-melting followed by dropcasting into Cu chill molds. For alpha-Mo volume fractions of approximate to 50% sound castings were obtained. For l ower a-Mo volume fractions such as 25% macroscopic cracks were often observ ed. Preliminary oxidation tests verified the expected increase in the oxida tion resistance as the B-concentration increases and the a-Mo volume fracti on decreases. Also, the formation of glass films was observed. Depending on composition and heat treatment, the room temperature flexure strengths var ied 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 microstruclural 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 abo ve 2400 K. Depending on the composition, Anal freezing of Mo-Si-B alloys wa s found to occur at temperatures as low as 2200 K. Care is therefore requir ed during thermomechanical processing to avoid the formation of liquid phas es. (C) 1999 Elsevier Science S.A. All rights reserved.