Microstructure, microhardness and oxidation behavior of Mo-Si-B alloys in the Moss+Mo2B+Mo5SiB2 three phase region

A series of Mo-Si-B alloys in the Moss+T2 (Mo5SiB2)+Mo2B three phase region were designed to examine the effect of the lower Si solubility limit in the Moss phase on the microstructure, hardness and oxidation behavior. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Vickers hardness tests and Electron Probe Microanalysis (EPMA) were carried out for the evaluation. The XRD results show an unusual lattice constant expansion for the Moss that is consistent with B as an interstitial solute. SEM images establish the microstructure evolution during the annealing. The analysis of the indentations produced by the Vickers hardness tests demonstrate that the samples in the Moss + Mo2B + T2 three phase region have about a 50% higher fracture toughness than comparable samples in Moss + Mo3Si + T2 three phase region due to the lower Si solubility in the Moss phase. The thermogravimetric analysis (TGA) results show that the oxidation resistance is directly related to T2 phase fraction in the microstructure. Alloys with a Moss + Mo2B + T2 microstructure exhibit a similar oxidation behavior to those with a Moss + Mo3Si + T2 microstructure when the T2 phase fraction in the two alloys is similar. Mo-Si-B alloys in the Moss + Mo2B + T2 three phase region could achieve better mechanical properties than alloys in the Moss + Mo3Si + T2 three phase region and are attractive candidates for high temperature applications.