Oxidation of Nb–Si–Cr–Al in situ composites with Mo, Ti and Hf additions

Abstract

The effects of Mo, Ti and/or Hf additions on the isothermal oxidation behaviour of Nb–Si–Cr–Al-based in situ composites in static air at 800 °C and 1200 °C were studied for the as-cast and heat-treated materials. After heat treatment at 1500 °C, the microstructures of all the alloys consisted only of the niobium solid solution (Nb ss) and Nb 5Si 3 phases. The addition of Ti and the decrease of the Mo concentration to 2 at.% improved the oxidation resistance of the alloys at 800 °C dramatically. The Hf addition had no significant effect on the oxidation behaviour of Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (at.%) at 800 °C. The oxidation resistance of the alloys was found to be sensitive to the volume fraction of Nb ss. The coarsening and the increase of the volume fraction of the Nb ss phase in the heat-treated alloys were mainly responsible for the degradation of their oxidation resistance. Preferential attack of the Nb ss phase was observed in all the alloys. Pesting oxidation behaviour was exhibited at 800 °C 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. Pesting of the alloys occurred due to a combination of the presence of elements that oxidised rapidly and the restricted deformation capability of the scales. Spallation of oxide scales during cooling was observed for the as-cast and heat-treated Nb–24Ti–18Si–5Al–5Cr–2Mo and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloys at 1200 °C. In the diffusion zones formed in the alloys without Hf addition, the 5–3 silicide was not oxidised. The TiNbO 4, TiNb 2O 7, Ti 2Nb 10O 29 and silicon oxide were present in the scales formed on the Nb–24Ti–18Si–5Al–5Cr–2Mo and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloys at 1200 °C and Hf oxide was also present in the latter alloy. At 1200 °C, in the Ti containing alloys, large Ti oxide particles formed at the Nb ss and 5–3 silicide interface and fine Ti oxide particles formed inside the Nb ss. Needle-like Hf oxide formed inside the 5–3 silicide in the heat-treated Hf containing alloy, which resulted in the degradation of its oxidation resistance. No protective oxide scale was formed on any of the alloys studied in this work.