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

Abstract

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb ss/Nb 5Si 3 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 Nb 3Si silicide formed in the Hf-free alloys and transformed to Nb ss and αNb 5Si 3 during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb ss and Nb 5Si 3 were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb 5Si 3 only partially transformed to αNb 5Si 3, suggesting that Ti stabilises the βNb 5Si 3 to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb 5Si 3 (Mn 5Si 3-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 Nb ss 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 Nb ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb ss 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 Nb ss and was sensitive to the area fraction of Nb ss in the alloy.