Effect of chemical composition variations in refractory Hf-Nb-Ta-Ti-Zr-Mo-V-Si complex concentrated alloys on the structure, mechanical properties, and oxidation behaviour

In this study, the structure, mechanical properties, and oxidation behaviour of the arc-melted (HfNbTaTiZr)₈₄Si₁₆, (HfMoNbTaTiZr)₈₄Si₁₆, and (HfMoNbTaTiVZr)₈₄Si₁₆ refractory complex concentrated alloys (RCCAs) were investigated. All the three alloys exhibited composite-like microstructures. The (HfNbTaTiZr)₈₄Si₁₆ alloy had a dual-phase structure consisting of the bcc and hexagonal Me₅Si₃ phases. Alloying with Mo or Mo and V resulted in the formation of an additional orthorhombic HfMoSi-type phase. The (HfNbTaTiZr)₈₄Si₁₆ alloy showed the lowest strength at 22–1200 °C, but the highest room-temperature plastic strain among the alloys studied. Additions of Mo or Mo and V were found to improve strength and reduce plasticity. At 1400 °C, all the alloys softened rapidly and became highly deformable. The (HfNbTaTiZr)₈₄Si₁₆ alloy exhibited the fracture toughness of ∼15 MPa m^1/2, which was twice higher than those of the (HfMoNbTaTiZr)₈₄Si₁₆ and (HfMoNbTaTiVZr)₈₄Si₁₆ alloys. The (HfNbTaTiZr)₈₄Si₁₆ alloy demonstrated the near-parabolic oxidation kinetics, the lowest mass gain (∼66 mg cm⁻²) after 48 h, and the longest time (24 h) to edge swelling during the oxidation tests at 1200 °C. Alloying with Mo or Mo and V aggravated the oxidation resistance, decreasing the time to edge swelling or complete disintegration to 6 h. With the values of ∼15 MPa m^1/2 and ∼66 mg cm⁻², the (HfNbTaTiZr)₈₄Si₁₆ alloy was among the toughest and most oxidation-resistant RCCAs and Nb-based silicides to date. The relationships between the chemical composition, structure, mechanical properties, and oxidation behaviour of the alloys studied were thoroughly analysed and discussed.