Microstructure and mechanical properties of uncoated Nb-18.7Si and Nb-18.7Si-5Ti alloys and their improved oxidation resistance after application of silicide coating

Abstract The near-eutectic Nb-Si alloys are promising refractory alloys for strategic high temperature applications. In the present study, the microstructure of cast Nb-18.7Si and Nb-18.7-5Ti (in at.%) alloys is examined. The effect of Ti on the compression properties and fracture toughness of the cast binary alloy is evaluated at room temperature. Ti addition caused about 120 MPa increase in strength and improvement in the fracture toughness by ∼ 4.5 MPa.m 0.5 . The oxidation resistance, ascertained in air at 1400°C, is extremely poor for both the alloys. Protective Fe-Cr modified slurry silicide coating prevents extensive dimensional degradation of the alloys during cyclic oxidation for at least 4-5 h. The oxidation resistance of the coating is ascribed to the formation of the oxide layer which spreads on the surface as well as heals the coating cracks. Therefore, the silicide coated near eutectic Nb-Si alloys hold the potential for use in hostile and extreme environments.