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 alpha-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 alpha-Al2O3 formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of alpha-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 root 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 alpha-Mo, crack deflection and branching. Flexural strength is related to volume fraction of the alpha-Mo and Al content. Compression tests on the 76Mo-14Si-10B alloy between 1100 degrees C and 1350 degrees C have shown excellent strength retention, and evidence of thermally activated plastic flow. (c) 2006 Elsevier Ltd. All rights reserved.