Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown α-Mo, Mo3Si, and Mo5SiB2 phases. In addition, particles of SiO2 formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while α-Al2O3 formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of α-Mo and Mo3Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by α-Mo, crack deflection and branching. Flexural strength is related to volume fraction of the α-Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.