Processing and mechanical properties of a molybdenum silicide with the composition Mo-12Si-8.5B (at.%)

Alloys with the nominal composition Mo-12Si-8.5B (at.%) were prepared by ar e-melting or powder-metallurgical processing. Cast and annealed alloys cons isted of approximately 38 vol.% alpha -Mo in a brittle matrix of 32 vol.% M o3Si and 30 vol.% Mo5SiB2. Their flexure strengths were approximately 500 M Pa at room temperature, and 400-500 MPa at 1200 degreesC in air. The fractu re toughness values determined from the three-point Fracture of chevron-not ched specimens were about 10 MPa m(1/2) at room temperature and 30 MPa m(1/ 2) at 1200 degreesC in air. The relatively high room temperature toughness is consistent with the deformation of the alpha -Mo particles observed on f racture surfaces. Three-point flexure tests at 1200 degreesC ill air and a tensile test at 1520 degreesC in nitrogen indicated a small amount of high temperature plasticity. Extrusion experiments to modify the microstructure of cast alloys were unsuccessful due to extensive cracking, However, using powder-metallurgical (PM) techniques, microstructures consisting of Mo3Si a nd Mo5SiB2 particles in a continuous alpha -Mo matrix were fabricated. The room temperature Fracture toughnesss of the PM materials was on the order o f 15 MPa. m1/2. (C) 2001 Elsevier Science Ltd. All rights reserved.