Effect of Al, Cr and Ta additions on the oxidation behaviour of Nb–Ti–Si in situ composites at 800 °C

The oxidation behaviour of as-cast Nb–24Ti–18Si and heat-treated (1500 or 1400 °C/100 h) Nb–24Ti–18Si–5Al, Nb–24Ti–18Si–5Cr, Nb–24Ti–18Si–5Al–5Cr, Nb–24Ti–18Si–6Ta–5Al–5Cr, Nb–24Ti–18Si–8Cr–4Al and Nb–24Ti–18Si–6Ta–8Cr–4Al (at.%) alloys in static air at 800 °C for 100 h has been studied using TG, XRD and EPMA. All the alloys exhibited linear oxidation kinetics. The as-cast (Nb,Ti)3Si containing Nb–24Ti–18Si alloy suffered from catastrophic pest oxidation. The other alloys, in which the Nb5Si3 was present, did not pest. Al addition reduced the oxidation rate significantly. Higher oxidation rates and lower adherence of the oxide scale were observed with increase of Cr concentration and/or Ta addition in the Al-containing alloys. In the oxide scales, Si-containing and non-Si-containing oxides of the stoichiometry MO2 were present together with remnants of 5-3 silicide and/or Laves phase. The oxygen solubility in the latter two phases was not high and these phases did not oxidise easily compared to the (Nb,Ti)ss. Below the oxide scale of the Nb–24Ti–18Si–5Al alloy a thick (∼100 μm) internal oxidation zone was formed where the (Nb,Ti)ss contained up to 30 at.% dissolved oxygen. In the Nb–24Ti–18Si–5Cr alloy this zone was thinner and leaner in oxygen. In the Cr- and Al-containing alloys the internal oxidation zone was almost eliminated and the diffusivity of oxygen through the (Nb,Ti)ss was reduced owing to the synergy of the Ti, Cr and Al additions, which was not affected by the addition of Ta to the alloy. The presence of Cr-rich C14 silicide Laves phase in the microstructure of some alloys was not beneficial regarding their oxidation at 800 °C.