Microstructure Designs and Processing for Mo-Si-B Alloys

Recently, considerable effort has been devoted to the development of ultra-high temperature structural materials as alternatives to Ni-based superalloys to improve the energy efficiency of gas turbine systems. Among the several potential candidates, Mo-Si-B alloys have received much attention due to their high melting point and high temperature strength, but also have some remaining challenges to improve ductility, lower density and enhance environmental resistance. In the Mo-Si-B system microstructures with a Mo solid solution (Mo_ss) Mo₃Si and Mo₅SiB₂ (T₂) phases have been the focus of attention. However, the Si solubility in the Mo_ss phase diminishes the ductility and toughness. In order to address this issue, a new series of Mo-Si-B alloys in the Mo_ss+T₂+Mo₂B three phase region has been designed to examine the effect of the lower Si solubility limit in the Mo_ss phase on the microstructure, hardness and oxidation behavior. The results showed that the Mo-Si-B alloys in the Mo_ss+T₂+Mo₂B three phase region have higher fracture toughness and the same level oxidation resistance as alloys in the Mo_ss+T₂ + Mo₃Si three phase region. Selected additions of Al and Ti enable a density reduction to below 9 gm/cm³. In order to examine the Al addition effect on the Mo_ss+T₂+Mo₂B three phase region the microstructures and oxidation of Mo-Si-B alloys at different Al addition level was examined at 800, 1100, 1200, and 1300°C for both isothermal oxidation and cyclic oxidation exposure along with an analysis of the oxidation kinetics and oxide layer morphologies.<br/>While refractory metal intermetallic alloys (RMIA) offer an attractive option to extend operational capability beyond what is currently available with Ni-base superalloys the processing of the RMIAs to manufacture dimensionally controlled shapes in a commercially cost-effective way is challenging. For RMIAs the directional solidification approach used for Ni base superalloys has serious difficulties. For example, there are very limited choices for suitable mold materials that are nonreactive with the alloy melt. The solidification path for large ingots in RMIAs involving intermetallic silicide and boride phases usually yields product phases that are not part of the equilibrium phase microstructure. Due to the high-temperature stability and sluggish diffusion in refractory metals, homogenization requires very high temperatures (&gt;1500^oC) and long annealing times (&gt;1 day) that are not commercially cost effective. A conventional powder metallurgy processing approach for RMIAs involves several steps including the hot pressing of RMIA powders, high-temperature sintering, and then usually a hot isostatic press (HIP) treatment to achieve full density. Even with these multiple processing steps, it is difficult to produce complex shapes with accurate geometric control. To circumvent these challenges, an additive manufacturing (AM) route offers a new, rapid and effective strategy. With AM methods such as directed energy deposition (DED) and liquid powder bed fusion (LPBF), alloying is accomplished by the melting and reactive synthesis of component powder mixtures to full density so that the size scale of any nonequilibrium solidification products is limited that facilitates post-alloying homogenization. This approach has been demonstrated to yield the fabrication of homogeneous alloys with well-defined shapes. Besides the successful use of AM, the guidance from computational thermodynamics to identify compositions that meet density, strength and ductility goals and the use of dimensionless numbers to guide the AM process has been essential to achieve fully dense component with the desired microstructures.