Emerging mechanisms of FeCrAl(RE) oxide scale formation and permeation from 1st principles

Alumina forming alloys are important for high temperature applications due to the high stability of the alpha-Al2O3 scale which forms above 900 C. FeCrAl(RE) alloys are alumina formers with small additions of reactive elements, RE e.g. Y, Zr, Hf and Ce, added to improve among others oxidation behavior and scale adhesion. Cr is added to the binary FeAl system in the role of a 3rd element in order to decrease early Fe, and internal Al oxidation, and promote alpha-Al2O3 formation. As corrosion and oxidation processes deplete the alloy of scale forming metal alloy, the durability suffers. Here, density functional theory, DFT has been employed to study parameters controlling oxide scale growth in general and permeation of oxidants through formed scales on FeCrAl(RE) in particular. The context of oxide growth is given by Wagner theory of oxidation in which diffusion of ionic species; cations, anions, and electrons control oxide growth rate. Inasmuch as alumina is a large band-gap insulator, the conduction band is inaccessible for electron transport. Thus, electron transport utilizing oxygen vacancies has been studied here. Activation energies for electron transport were calculated to be ~0.5 eV rendering electrons mobile. Oxygen vacancy, Vo diffusion barriers range between ~2-5 eV depending on electronic charge of the vacancy. A percolative Vo and electron transport is thus proposed in alumina, rendering both species mobile.The third element effect was given a local meaning at early stages of scale growth in a systematic study comparing Sc, Ti, V, Cr, Mn, Fe, Co, and Ni employed as guest ions in an alumina lattice. Comparing the affinity to oxygen vacancies, Vo only Cr and V displayed ideal intermediate affinities, i.e. intermediate to Fe and Al. V was thus proposed alongside Cr as a third element in the Fe-TM-Al ternary alloy system.Chromia nodules embedded in the protective alumina scale formed on FeCrAl(RE) were observed to permeate nitrogen in a reducing 95% N2, 5% H2, 35 ppm H2O environment. Al in the alloy was shown to reduce the chromia particle upon which a nitrogen permeation channel through said particle is sustained and for which Al acts as nitrogen sink.Enhanced oxidation was observed around surface RE-oxide particles. Here, oxidation of Al by water is understood to be the driving force for incorporating RE into alumina grain-boundaries. This RE decoration retards grains coarsening, leading to a thicker and more adherent early scale. In O2 containing atmospheres, this defect rich "messy" scale eventually becomes oxidized, upon which hydride ions are consumed and RE precipitate. RE(III) are shown to introduce stresses into grain-boundaries leading to a faster precipitation while smaller RE(IV) can maintain enhanced oxidation for longer before precipitation occurs

Towards In Silico Mining for Superconductors -- Cutting the Gordian Knot

A random forest regression based supervised machine learning method to predict experimental critical temperature of superconductivity from the electronic band structure, as obtained from Density Functional Theory, is demonstrated. This complementarity between experiment and theory draws inspiration from the merging of Kohn-Sham and Bogoliubov-De Gennes equations [W. Kohn, W, EKU Gross, and LN Oliveira, Int. J. of Quant. Chem., 36(23), 611-615 (1989)]. Features in the Kohn-Sham Density Functional Theory band structure away from EF becoming decisive for the superconducting gap demonstrates this divide-and-conquer physical understanding. Not committing to any microscopic mechanism for the SC at this stage, it implies that in different classes of materials, different electronic features are responsible for the superconductivity. However, training on known members of a class, the performance of new members may be predicted. The method is validated for the A15 materials, including both binary A3X and ternary A6XY intermetallics, A=V, Nb, demonstrating that the two do indeed belong to the same class of superconductors.Comment: 12 pages, 4 figures and Supplementary Information containing 8 additional figure

Reactive Element Effects in High-Temperature Alloys Disentangled

Reactive elements—REs—are decisive for the longevity of high-temperature alloys. This work joins several previous efforts to disentangle various RE effects in order to explain apparently contradicting experimental observations in alumina forming alloys. At 800–1000\ua0\ub0C, “messy” aluminum oxy-hydroxy-hydride transients initially formed due to oxidation by H2O which in turn\ua0undergo secondary oxidation by O2. The formation of the transient oxide becomes supported by dispersed RE oxide particles acting as water equivalents. At higher temperatures, electron conductivity in impurity states owing to oxygen vacancies in grain boundaries (GBs) becomes increasingly relevant. These channels are\ua0subsequently closed by REs pinning the\ua0said vacancies. The universality of the emerging understanding is supported by a comparative first-principles study by means of density functional theory addressing RE(III): Sc2O3, Y2O3, and La2O3, and RE(IV): TiO2, ZrO2, and HfO2, that upon reaction with water, co-decorate a generic GB model by hydroxide and RE ions. At 100% RE coverage, the GB model becomes relevant at both temperature regimes. Based on reaction enthalpy ΔHr considerations, “messy” aluminum oxy-hydroxy-hydride transients are accessed in both classes. Larger variations in ΔHr are found for RE(III)-decorated alumina GBs as compared to RE(IV). For RE(III), correlation with GB width is found, increasing with increased ionic radius. Similarly, upon varying RE(IV), minor changes in stability correlate with minor structural variations. GB decorations by Ce(III) and Ce(IV) further consolidate the emerging understanding. The findings are used to discuss experimental observations that include impact of co-doping by RE(III) and RE(IV)

Transition metal attenuated mechanism for protective alumina formation from first principles

A mechanistic perspective on the growth of protective oxides on high temperature alloys at elevated temperatures is provided. Early, defect rich transient alumina is understood to form by outwards diffusion of oxygen vacancies and electrons. The impact of transition metal (TM) ions (Sc, Ti,\ua0V, Cr, Mn, Fe, Co, Ni) on the oxygen vacancy diffusion and electron transport in α-alumina was studied by employing density functional theory. Activation energies for electron transfer EA(ET) between oxygen vacancies in pure as well as TM doped α-alumina were subject to analysis, and similarly so for the TM and charge dependent activation energy for oxygen vacancy diffusion EA(VO). EAQ(ET) were found to be ∼0.5 eV while 2 eV < EAQ(VO) < 5 eV was obtained. The higher and lower EAQ(VO) values correspond to uncharged and doubly charged VO sites, respectively. Redox processes among VO sites, addressed by a bipolaron approach, were understood to enhance VO mobility and thus to facilitate oxide growth. TM adatoms induced asymmetry in the potential energy surface for oxygen vacancy diffusion was subject to analysis. Competition for electrons between all-Al3+surrounded oxygen vacancies and vacancies adjacent to the late 3d adatoms comes out in favor of the latter. A novel take on the 3rd element effect in FeCrAl emerges from analysis of the ternary TM-TM*-Al system

Nitrogen and Water in High Temperature Corrosion - Insights from First Principle Calculations

Service life-time of alloys used in high temperature applications is often limited by corrosion. FeCrAl(Re) alloys are partially designed to form a protective α-alumina oxide scale at elevated\ua0 temperatures in order to mitigate further oxidation. The scale growth commonly follows a parabolic rate law, which under a Wagnerian setting depends on transport of charged species; ions, vacancies and electrons. Small amounts of reactive elements (Y, Zr, Hf) are added to the alloy to improve scale properties, while also inhibiting outward Al diffusion. The oxide growth thus depends on mobility of oxygen vacancies in the scale.\ua0\ua0\ua0\ua0\ua0\ua0\ua0\ua0\ua0\ua0\ua0 A FeCrAl(Re) alloy was exposed to a 95% N2, 5% H2, and low p(O2) environment, usually used for heat-treatment, forming a predominantly protective alumina scale with nodular inclusions of chromia. Under said conditions the chromia rich nodules permeate nitrogen. Density functional theory (DFT) calculations show that reducing processes, owing to the relative stability of chromia and alumina, lead to a maintained coverage of coordinatively unsaturated sites (CUS) on chromia surfaces, acting as N2 dissociation sites. Chromium oxy-nitrides were shown metastable, offering a path for nitrogen into the alloy. H2O acts as the main oxidant in this environment. A quasi-Wagnerian context was explored in which hydrogen was preferentially disposed as H- in oxygen vacancies in hydroxylated alumina grain boundaries

Fates of Hydrogen During Alumina Growth Below Yttria Nodules in FeCrAl(RE) at Low Partial Pressures of Water

Oxidation of FeCrAl(Re), when exposed to similar to 35 ppm of water as sole supply of oxygen in predominantly nitrogen atmosphere, has two characteristic signatures. One is the internal nitridation owing to chromia nodules acting windows toward nitrogen permeation locally short-circuiting the protective alpha-Al2O3 scale. The second remarkable feature is the growth of thick, apparently defect-rich alumina scale under yttria-rich nodules. Hence, one part of the present study comprises exploratory DFT calculations to discriminate between the impacts of chromia and yttria viz. nitrogen permeation. The second part concerns boundary conditions for apparent rapid growth of alumina under yttria nodules. Yttria-associated surface energy stabilization of defect-rich alumina in presence of water was argued to involve hydrolysis-driven hydroxylation of said interface. Subsequent inward growth of the alumina scale was associated with outward diffusion of oxygen vacancies to be accommodated by the remaining proton producing a hydride ion upon surfacing at yttria-decorated alumina interfaces. The latter comprises the cathode process in a quasi-Wagnerian context. Two fates were discussed for this surface ion. One has H--H+ recombination to form H-2 at the interface in conjunction with OH- accommodation upon hydration, while the second allows hydrogen to be incorporated at V-O sites in hydroxylated grain boundaries of the growing alumina scale. The latter was taken to explain the experimentally observed rapid oxide growth under yttria-rich nodules. Space charge due to proton reduction was proposed to cause transient inward cationic drag

Retraction; Surface Reaction and Transport in Oxides Formed on FeCrAl Alloys in High Temperature Nitridation Environments

An extended experimental study on corrosion of commercial FeCrAl alloys in predominantly N-2 atmosphere at high temperatures raises fundamental questions concerning nitrogen permeation through oxide scale components, each commonly perceived to constitute a robust barrier. State-of-the-art microscopy combined with DFT calculations is employed to unravel the underlying mechanisms for how chromia nodules support nitrogen dissociation and subsequently act windows for internal nitridation. A similar analysis addressed the role of yttria nodules shown to exhibit apparent expulsion of nitrogen, while favoring rapid alumina growth underneath the nodules. In this case, as for the slowly growing bare alpha alumina scale, the origin for the protection towards nitrogen permeation was identified as the inability to satisfy the necessary conditions for N-2 dissociation

On Water Induced Sensitization of Ni (Fe,Cr) alloys towards Stress Corrosion Cracking in LWR Piping from 1st Principles Modelling

In Swedish Light Water Reactors (LWR), stress corrosion cracking of reactor components and welds occurs from time to time. As the nuclear power plants are ageing, it is essential to study and further understand the mechanism for environmentally induced sensitization. Natural cracking is a phenomenon that is difcult to predict and very hard to study since it occurs suddenly and often unexpectedly. In order to study the crack initiation and growth, the crack is traditionally experimentally provoked and it is not known to what degree these experimental cracks correspond to those that occur naturally. The environment in an LWR contributes to material ageing through chemical reactions with the environment. An in-depth examination has shown that the microstructures of oxide flms changes along the crack path and the oxide flm in the crack tip is signifcantly diferent from what one detects at the crack opening. In this study, 1st principles modelling is used to articulate\ua0an environment induced sensitization mechanism for stress corrosion cracking of Ni(Fe,Cr) alloys in LWR conditions

On aliovalent cations control of α-alumina growth on doped and undoped NiAl

Alumina forming Ni-base superalloys are essential due to their oxidation resistance at elevated temperatures. A two-step procedure allows to assess the outward growth of the oxide scale from the resulting oxide ridges that form at 1100\ub0C and cap the alumina grain boundaries. Employing undoped 50Ni50Al (at%) as reference, the impact of reactive elements on the diffusion processes, here Zr and Hf, is quantified using atom probe tomography. Unexpectedly, we find that up to one monolayer of Ni may co-decorate the alumina grain boundaries. Additionally, a decrease in Al-, and Ni-diffusivity of two orders of magnitude is observed owing to the reactive element effect. We employ density functional theory calculations to better understand the role of aliovalent cations, here Ni(II), Zr(IV), and Hf(IV) in the α−alumina scale. The calculations show that Ni may not only decorate the alumina grain boundaries but also facilitates transport of electrons as well as oxygen vacancies. Thereby oxide scale growth becomes enhanced. In turn, the dual impact of reactive elements, i.e. to annihilate oxygen vacancies and to remove impurity states in the band gap, explains the reduced scale growth rate