Effect of Cr Additions on Toughness, Strength, and Oxidation Resistance of an Nb-4Si-20Ti-6Hf Alloy at Room and/or High Temperatures

The mechanical properties and oxidation resistance of Nb-4Si-20Ti-6Hf-yCr alloys (where y = 6, 10, 14 at. pct) with an Nb_SS-dominant Nb_SS/Nb₃Si/Nb₅Si₃ microstructure at room and/or high temperatures were primarily studied. It was found that at room temperature, fracture toughness was decreased by Cr additions, the fracture mode of the Nb_SS phase was intergranularly cleavage, and the silicides (Nb₃Si and Nb₅Si₃) fractured in a brittle manner. It is interesting that strength as a function of Cr content is dependent on temperature. Up to 1423 K (1150 °C), the strength was improved by Cr additions, whereas above that temperature, the strength decreased. As the Cr content increased from 6 to 14 at. pct, for example, the yield strength σ _0.2 increased from 198 to 258 MPa at 1423 K (1150 °C) but decreased from 109 to 83 MPa at 1623K (1350 °C). Cr additions cannot improve the oxidation resistance of this Nb_SS-dominant microstructure considerably; the weight gain at 100 hours of air exposure at 1523 K (1250 °C) was 255 mg/cm² at a nominal Cr content of 6 at. pct and 220 mg/cm² at a Cr content of 14 at. pct.