Energetics of Oxygen Interstitials in Cr and V
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Energetics of Oxygen Interstitials in Cr and V Brian S. Good and Evan Copland1 Materials Division, NASA Glenn Research Center, Cleveland OH 44135 1 Case Western Reserve University, Cleveland OH ABSTRACT Dissolved oxygen in group IIIA-VA (Nb, Ti, Zr, Y) based alloys is a fundamental problem, affecting both mechanical properties and oxidation resistance, yet details of the phenomenon are poorly understood. In these alloys, oxygen is more stable dissolved in the metal than as an oxide-compound. In contrast, alloys based on Ni, Fe, Al and Cr exhibit almost no oxygen solubility. To improve the performance of Nb and Ti based alloys it is necessary to understand the differences in oxygen solubility between these two groups of metals. As a first step we considered the energetics of interstitial oxygen in α-V and α-Cr. Both of these metals have a BCC structure, yet the oxygen solubility in V is much higher than that in Cr. We obtain total energies, densities of states and population analyses using the CASTEP plane-wave pseudopotential density functional computer code. The differences in the energetics and electronic structures of the two materials, particularly the partial densities of states associated with the interstitial oxygen, are discussed. INTRODUCTION The use of alloys in high temperature oxidizing atmospheres relies on the formation of a continuous oxide layer on the surface, separating it from the environment and limiting the oxidation reaction rate. Useful oxide compounds (e.g., Cr2O3, Al2O3 and SiO2) must have slow oxygen transport kinetics, but this is not the only criterion required. The oxide must form on the alloy surface and the alloy-scale interface must approach a condition of equilibrium. The first is a function of composition and transport kinetics, while the second is purely thermodynamic and requires that the alloy is saturated with oxygen. For Ni and Fe based alloys this does not present a problem as oxygen is more stable as an oxide and the concentration required for saturation is below levels that affect mechanical properties. In contrast, oxygen is more stable dissolved in group IIIA-VA (Nb, Ti, Zr, Y…) based alloys; oxygen concentrations of 2 to 10 at% are required for saturation and these levels are detrimental to mechanical properties. To maintain bulk mechanical properties oxygen concentration needs to remain below saturation levels, which results in a steady state condition at the alloy-scale interface where oxide is reduced and oxygen continually diffuses into the alloy. This issue needs to be addressed before suitable oxidation behavior can be obtained for these alloys. The oxygen saturation limit is determined by the stability of the oxide compound and the stability of dissolved oxygen in the alloy. As the oxide compound is typically fixed by the required transport kinetics the stability of dissolved oxygen is the fundamental issue. Therefore, it would be interesting to gain a better understanding of the stability of oxygen in these metals. Neutron diffraction studies show that o
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