On the oxidation mechanism of niobium-base in situ composites

The Metal And Silicide Composite (MASC), having the composition Nb–25Ti–8Hf–2Cr–2Al–16Si, at.%, was studied in static oxidising condition in air at 815, 1100 and 1200 °C. Phases present were Nb solid solution and Nb5Si3. Pesting did not occur at 815 °C before 100 h of exposure while internal oxidation took place at each temperature and affected the Nb solid solution. The outer oxides correspond mainly to TiNb2O7 while the internal ones correspond to TiO2, HfO2 and Al2O3. The mathematical treatment of oxidation data obtained at 815 °C proved that the mass of oxygen in the oxide layer is 2.5 times less than that in the internal oxidation layer. The volume expansion (1.15) induced by oxygen leads to the cracking of silicides. An oxidation mechanism is proposed to explain the behaviour of these niobium-base in situ composites at 815 °C in air.

Figure optionsDownload as PowerPoint slideHighlights
► MASC alloy mainly oxidises by inward diffusion of oxygen at 815 °C.
► Outward diffusion of metallic species does not occur during oxidation at 815 °C.
► Pesting results of the in situ formation of tensile stresses.
► The volume expansion is associated to nano-sized oxides formed in the NbSS.
► The silicides oxidation rate controlled the oxide–substrate interface movement.