Oxidation of the two-phase Nb/Nb5Si3 composite: The role of energetics, thermodynamics, segregation, and interfaces

Abstract

Oxidation behavior of the two-phase Nb/Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb/Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.

On Nb Silicide Based Alloys: Alloy Design and Selection

“…The oxidation of Nb-silicide based alloys starts with the oxidation of Nb ss grains in the surface, this is followed by the oxidation of Nb 5 Si 3 grains (and other intermetallics) in the surface. The oxidation promotes the surface segregation of Nb that subsequently oxidizes [ 15 , 57 ]. This is followed by oxidation of Nb ss and Nb 5 Si 3 below the surface and even in the bulk [ 15 ] as well as of other intermetallic phases such as the C14-NbCr 2 Laves and A15-Nb 3 X phases (this is often referred to as (phase) contamination by oxygen).…”

“…This is followed by oxidation of Nb ss and Nb 5 Si 3 below the surface and even in the bulk [ 15 ] as well as of other intermetallic phases such as the C14-NbCr 2 Laves and A15-Nb 3 X phases (this is often referred to as (phase) contamination by oxygen). The oxidation of the solid solution is more sever compared with the silicide(s) and other intermetallics [ 14 , 15 , 57 ]. The oxidation of all phases depends on their chemical composition and on the alloy microstructure (volume fractions of phases, shape, size and spatial distributions of phases) that results from processing.…”

“…If the Fermi level falls on a peak in the density of state curve of the intermetallic the D self-diffusion of the latter is increased. Considering the Ti and Hf rich Nb ss /Nb 5 Si 3 interfaces in Nb silicide-based alloys and their contamination by oxygen and formation of hafnia near Hf rich areas of the Nb 5 Si 3 [ 15 , 57 , 81 ], the changes of the Fermi level resulting from alloying will affect the stability of phases and their properties, both of which are important in creep.…”

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces

“…This is due to the repulsive interaction between oxygen atoms. 37 In contrast, the binding of oxygen on Al(111) increases dramatically with increased oxygen coverages. This is due to the formation of aluminum oxide.…”

Initial oxidation of Cr 2 Nb (1 1 1) surface: A density functional study

“…Previously, we have investigated the initial oxidation mechanisms of the single crystal surface and polycrystalline surface as well as two-phase materials [19][20][21], and proposed the oxidation thermodynamics model by comparing the surface energies in the different O adsorption surfaces. From above model, it is easier to predict the product of oxidation when alloys can form a single oxide layer by selective oxidation.…”