Alloy designs for high temperature Mo-base systems

For high temperature applications Mo base alloy requirements include both superior structural performance and environmental resistance. To address these requirements alloys in the Mo-Si-B system and refractory multi-principal element alloys (RMPEA) are being developed that exhibit a promising potential, 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 (Moss) Mo3Si and Mo5SiB2 (T2) phases have been the focus of attention. However, the Si solubility in the Moss phase diminishes the ductility and toughness. In order to address this issue a new design based upon Moss, Mo2B and T2 phases lowers the Si solubility in the Moss to improve ductility while the T2 phase maintains the oxidation performance. Selected additions of Al and Ti enable a density reduction to below 8 g/cm3. The RMPEA designs for Mo-rich alloys provide for excellent structural performance, but the complex oxidation products provide no protection. In this case a new coating design has been introduced that provides the required environmental resistance.