Compositional regions of single phases at 1800°C in Mo-rich Mo-Si-B ternary system

Single-phase regions in Mo-rich Mo-Si-B alloys at 1800 degrees C were experimentally determined using a field-emission (FE) gun-type electron probe micro-analyzer (EPMA) in this study. A quantitative analysis by FE-EPMA was conducted with the calibration method we applied in our previous study to improve the accuracy in the B measurements. The compositional ranges of the determined Mo solid solution, silicide and boride phases were in good agreement with those of Mo-Si and Mo-B binary phase diagrams. On the other hand, the determined solubility of B in Mo solid solution. Mo3Si and T-1 (Mo5Si3) were quite different from that indicated in previously reported ternary phase diagrams. The compositional region of the T-2 (Mo5SiB2) single-phase ranges from 9.7 to 13.3 at% for Si and from 23.5 to 26.8 at% for B. The following two points were made clear by the ternary phase diagram as determined in this study. One is that no stoichiometric T-2 composition is in the T-2 single-phase region, confirming no T-2 single-phase material can be obtained at the stoichiometric composition at 1800 degrees C. The another is that the T-2 region of this study expands to a Si-rich area from the stoichiometric composition. (C) 2012 Elsevier B.V. All rights reserved.