Mechanism and regularities of oxidation in Nb-Ti and Nb-Ti-Si systems

We study the processes of high-temperature oxidation in Nb-Ti, Nb-Ti-Si, and Nb-Ti-Si-Mo systems and propose a mechanism of oxidation of VN-10 niobium alloy highly alloyed with titanium (up to 50%), a Ti-Nb-Ti composite multilayer material, and unalloyed niobium and VN-10 alloy subjected to diffusion saturation of their surface with silicon and molybdenum. It is shown that high alloying and (to a lesser extent) Ti-cladding of niobium allow one to suppress the process of catastrophic high-temperature oxidation of it and increase significantly the heat resistance of the material. Additional formation of diffusion silicide and molybdenum-silicide layers on the surface of niobium alloys increases their heat resistance by more than a factor of ten.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 107–115, January–February, 1995.     

Effect of admixtures of oxygen on the oxidation of iron and Fe−Cr alloys in lead melts

We study the process of oxidation of Armco iron and Fe−16Cr and Fe−16Cr−1Al model alloys held in lead melts with different concentrations of oxygen for 1000 h at 650°C. It was discovered that the intensity of oxidation, the structure, and phase composition of oxide layers are determined by the activity of oxygen in the liquid metal. By the methods of layer-by-layer X-ray diffraction analysis and microscopic X-ray diffraction analysis, it was shown that, for low concentrations of oxygen in lead (C ₀≤10⁻⁶ wt.%), a thin (1–5μm) oxide [magnetite (Fe₃O₄] film is formed on the surface of iron. If alloys are held under the same conditions, then we also observe an increase in the concentration of chromium in the subsurface layers. For higher concentrations of oxygen (up to 10⁻⁵ wt.%), a film of magnetite (with inclusions of pure lead) is formed on the surface of unalloyed iron. In alloys, under the layer of magnetite, we detect the formation of oxide layers with the same composition as a solid solution of Fe₃O₄ and FeCr₂O₄ and the structure of spinel. These layers efficiently suppress the process of penetration of lead but do not completely terminate the process of diffusion of oxygen into the bulk of the material, which eventually leads to the internal oxidation of alloy. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 97–101, May–June, 1997.