A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti, Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nbss/Nb5Si3 base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb3Si silicide formed in the Hf-free alloys and transformed to Nbss and αNb5Si3 during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nbss and Nb5Si3 were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb5Si3 only partially transformed to αNb5Si3, suggesting that Ti stabilises the βNb5Si3 to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb5Si3 (Mn5Si3-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nbss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nbss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nbss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nbss and was sensitive to the area fraction of Nbss in the alloy.