Microstructural development of Mo-rich Mo-B-Si alloys


Based upon extended isothermal annealing treatments on arc-cast and rapidly solidified splat samples, the high temperature phase equilibrium has been established for Mo-rich alloys in the Mo-B-Si system. At both 1600 °C and 1200 °C a prominent feature of the phase stability is the development of a two phase field involving a Mo solid solution (Mo-ss) and a Mo5SiB2 (T2) intermetallic (D8l) phase. For alloys with compositions covering the (Mo-ss and T2) two-phase field, the annealing of arc-cast samples results in the formation of the Mo-ss precipitates with a faceted morphology within the supersaturated T2 intermetallic phase. This reaction provides for the in-situ formation of a ductile phase dispersion within T2. Preliminary indentation tests demonstrate that the Mo-ss precipitates do act to impede crack growth and contribute to toughening of the two-phase microstructure. For a Mo70B20Si10 alloy, some of the overall features of the microstructural evolution have been characterized by X-ray diffraction and SEM/EDX studies. The crystallographic orientation of Mo-ss precipitates within the T2 matrix has been determined by TEM/SAED studies to show mainly the relation: Mo-ss {111}∥T2 {001}; Mo-ss 〈011〉∥T2 〈110〉 and is considered in terms of a HRTEM analysis of interfacial structure. In addition, single phase T2 can be produced in splat samples with a T2 stoichiometric composition eliminating the solidification segregation normally encountered in arc-cast samples. RSP also allows for the in-situ formation of the two-phase microstructure directly from the melt. Upon annealing at 1200 °C, the microstructures consist of homogeneous dispersed Mo phase in a T2 matrix.

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Conference Proceeding

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TMS Annual Meeting