SUMMARY

Refractory metals and their alloys offer higher temperature alternatives to Ni-base superalloys. In particular, Mo-Si and Mo-Si-B intermetallics offer excellent oxidation and creep resistance at temperatures up to 1400C. However, these intermetallics present a significant design challenge due to their low ductility and low fracture toughness at room temperature. A balance of high temperature and low temperature mechanical properties may be achieved in Mo-Si-B alloys by including the alpha-Mo phase in addition to the intermetallic phases.

Balancing the mechanical properties requires proper microstructure optimization. Through the use of finite element simulations, microstructure-sensitive structure-property modeling allows for this optimization to be done faster and less expensively than traditional methods. Three modeling tools are required for microstructural modeling: microstructure generators to re-create statistically realistic microstructures, crystal viscoplasticity constitutive equations implemented for use with finite element solvers, and post-processing tools to evaluate important mechanical properties.

A proposed investigation into the development and application of these tools for triplex Mo-Si-B alloys is presented here by first developing these tools for the alpha-Mo phase and calibrating the constitutive equations for the alpha-Mo phase as a function of Si content, temperature, and strain rate. Following the calibration of the alpha-Mo phase constitutive equations, an initial model for the fully triplex microstructure may be achieved by treating the intermetallic phases as purely elastic. Finally, the triplex microstructure will be evaluated for an optimized microstructure balancing strength, fatigue, and ductility.