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

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 degrees C and 1200 degrees C were studied for the as-cast and heat-treated materials. After heat treatment at 1500 degrees C, the microstructures of all the alloys consisted only of the mobium solid solution (Nb-ss) and Nb5Si3 phases. The addition of Ti and the decrease of the Mo concentration to 2 at.% improved the oxidation resistance of the alloys at 800 degrees C dramatically. The Hf addition had no significant effect on the oxidation behaviour of Nb-24Ti-18Si-5Al-5Cr-2Mo-5Hf(at.%) at 800 degrees 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,, phase in the heat-treated alloys were mainly responsible for the degradation of their oxidation resistance. Preferential attack of the Nb,, phase was observed in all the alloys. Pesting oxidation behaviour was exhibited at 800 degrees C by the Nb-18Si-5Al-5Cr-5Mo (as-cast), Nb-24Ti-18Si-5Al-5Cr-5Mo (as-cast), Nb-24Ti-l8Si-5Al-5Cr-2Mo (heat-treated) and Nb-24Ti-18Si-5A1-5Cr-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-I 8Si-5Al-5Cr-2Mo-5Hf alloys at 1200 degrees C. In the diffusion zones formed in the alloys without Hf addition, the 5-3 silicide was not oxidised. The TiNbO4, TiNb2O7, Ti2Nb10O29 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 degrees C and Hf oxide was also present in the latter alloy. At 1200 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.