Vacancy-mediated Diffusion Research Articles

Oxygen Vacancy-Mediated Synthesis of Inter-Atomically Ordered Ultrafine Pt-Alloy Nanoparticles for Enhanced Fuel Cell Performance.

Pt-based intermetallics are expected to be the highly active catalysts for oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells but still face great challenges in controllable synthesis of interatomically ordered and ultrafine intermetallic nanoparticles. Here, we propose an oxygen vacancy-mediated atomic diffusion strategy by mechanical alloying to reduce the energy barrier of the transition from interatomic disordering to ordering, and to resist interparticulate sintering via strong M-O-C bonding. This synthesis results in a nanosized core/shell structure featuring an interatomically ordered PtM core and a Pt shell of two to three atomic layers in thickness and can be extended to the multicomponent PtM (M = Co, FeCo, FeCoNi, FeCoNiGa) systems. The electron enrichment in the Pt outer shell induced by the compressive strain leads to the enhanced antibonding orbital occupation below the Fermi level and accelerated OH* desorption kinetics. The optimized PtCo-O/C-6 catalyst presents excellent ORR activity (mass activity = 1.28 A mgPt-1 at 0.9 ViR-free, peak power densities = 2.38/1.25 W cm-2 in H2-O2/-air) and durability (∼1% activity loss in over 50 h in air condition) in fuel cells at a total Pt loading of 0.1 mgPt cm-2. Furthermore, we establish a systematic correlation to elucidate the formation mechanisms of highly ordered intermetallic catalysts underlying oxygen vacancies. This study provides a general approach for the large-scale production of highly ordered and nanosized Pt-dispersed intermetallic catalysts.

Machine learning-augmented modeling on the formation of Si-dominated Non-β″ early-stage precipitates in Al-Si-Mg alloys with Si supersaturation induced by non-equilibrium solidification

Recent experiments have shown that Al-Si-Mg alloys solidified under high cooling rates may lead to the nucleation of Si-enriched clusters that are remarkably different from the conventional Mg-Si co-clusters (e.g. β″ particles), and yet the responsible mechanism remains to be elucidated. Here we tackle the problem using a multiscale modeling framework that integrates atomistic modeling, energy landscape sampling, and lattice-based kinetic Monte Carlo (kMC) simulation. The migration energy barriers for vacancy-mediated diffusion amid complex local chemical environments are predicted on-the-fly using a surrogate machine learning model. We discover that the actual vacancy-Si migration barriers are much lower than those assumed in the classic linear interpolation approximation. Such a strong deviation from conventional wisdom, in conjunction with differing Si solute composition, can lead to a great variety in the nucleated early-stage precipitates. More specifically, a high-level supersaturation of Si solute (i.e.xSi/(xSi+xMg)>0.75) would lead to an unexpectedly high enrichment of Si in the nucleated clusters with the Si:Mg ratio up to 5∼6; while at a lower-level supply of Si solute the Mg-Si co-clusters (i.e. Si:Mg ratio around 1∼2) are nucleated instead. These findings provide a viable explanation for the diverse types of early-stage precipitates observed in various experiments, from Si-enriched precipitates in high-pressure die cast Al alloys to β″ particles in conventional casting and/or heat-treated alloys. The implications of our findings are also discussed.

First-principles investigation of lanthanides diffusion in HCP zirconium via vacancy-mediated transport

The diffusion of lanthanide fission products plays an important role in the growth of the fuel-cladding chemical interaction (FCCI) region in metallic fuels. The use of a Zr interdiffusion barrier may mitigate the transport of lanthanides from the fuel to the cladding, but the efficacy of such a liner is not yet known. In this paper, the stability and vacancy-mediated diffusion of La, Ce, Pr, and Nd in hexagonal close-packed (HCP) Zr is investigated via density functional theory (DFT) calculations and self-consistent mean field (SCMF) analysis. DFT is used to calculate the formation, binding, and migration energies of vacancies and vacancy-solute pairs. The DFT energetics are used in the KineCluE code to calculate the Onsager transport coefficients. La is found to be the fastest diffusing species in HCP Zr and experiences an almost isotropic diffusion behavior. The other three species (Ce, Pr, and Nd) demonstrate anisotropic diffusion where the diffusion in the basal planes is significantly faster than that along the c-axis. The calculated lanthanide diffusivities in HCP Zr are fitted to an Arrhenius relation and the activation energies and prefactors are reported for the first time. Furthermore, the vacancy drag and the segregation tendencies were analyzed using the calculated off-diagonal transport coefficients. According to our vacancy-mediated diffusion model, lanthanides are expected to be enriched at vacancy sinks at low temperatures, while at high temperatures, lanthanides are depleted at sinks and will preferably diffuse into the bulk. The enrichment/depletion transition temperature depends on the diffusion direction (basal or axial) and hence will be controlled by the grain texture and orientation.

Layered Monoclinic Perrierite Oxo-Silicate La4Mn5Si4O22+Δ: A New Interstitial Oxide Ion Conductor for Low Temperature Applications

Oxide-ion conductors are important for various energy applications such as solid oxide fuel cells, solid oxide electrolyser cells, oxygen separation membranes, oxygen sensors, etc. State-of-the-art stable oxygen active materials often have adequate oxygen ion diffusion and chemical surface exchange only at high temperatures (> 700 oC). A promising strategy to improve the oxygen ion transport is to use the interstitial oxygen ion conductors as the interstitial diffusion barriers are often lower than vacancy-mediated diffusion barrier, potentially enabling reduced operation temperatures. However, due to the large size of the oxide ion, the formation of an adequate concentration of interstitials is difficult and hence the number of interstitial oxygen conductors is currently limited to just a handful of compounds, e.g., Ruddlesden-Popper, apatite, melilite, scheelite, fluorite and disordered hexagonal perovskites.We screened a large number of oxide materials using high-throughput ab initio methods (Please see separate poster from Jun Meng on this screening) and predicted stable formation and fast diffusion of interstitial oxygen in a new class of material La4Mn5Si4O22+δ (LMS), motivating us to synthesize this material and experimentally study its oxide ion transport properties. The room temperature X-ray diffraction study followed by Rietveld refinement confirmed its layered monoclinic perrierite structure. Electron probe micro-analysis (EPMA) and iodometric titration revealed the presence of the hyper stoichiometric oxygen anion (δ ~ + 0.5) confirming the interstitial oxygen ions within LMS lattice. Thermogravimetric analysis showed a small mass increment in LMS when heated at high temperatures suggesting the high temperature stability of interstitial oxygen ions. The measured ionic conductivity of LMS is higher than the yttria stabilized ZrO2, perovskite oxide La1-xSrxCo1-yFeyO3 and Ruddlesden-Popper oxide materials. Electrical conductivity relaxation studies demonstrated that self-diffusion and surface exchange coefficients of oxygen ion are comparable to or higher than many state-of-the-art oxide ion conductors. Finally, the high temperature stability of the synthesised material was confirmed using X-ray diffraction after each measurement.Overall, this experimental study confirms computational predictions that layered perrierite LMS is a new class of interstitial oxide ion conductors and suggests our overall approach may help the rational design of interstitial mediated oxide ion conductors-based devices suitable for efficient and stable applications at low temperature.

Effect of n- and p-Doping on Vacancy Formation in Cationic and Anionic Sublattices of (In,Al)As/AlAs and Al(Sb,As)/AlAs Heterostructures.

The vacancy generation dynamics in doped semiconductor heterostructures with quantum dots (QD) formed in the cationic and anionic sublattices of AlAs is studied. We demonstrate experimentally that the vacancy-mediated high temperature diffusion is enhanced (suppressed) in n- and p-doped heterostructures with QDs formed in the cationic sublattice, while the opposite behavior occurs in the heterostructures with QDs formed in the anionic sublattice. A model describing the doping effect on the vacancy generation dynamics is developed. The effect of nonuniform charge carrier spatial distribution arisen in heterostructures at high temperatures on the vacancy generation and diffusion is revealed.

Atomic diffusion and microstructural evolution at the Nb/TiAl heterogenous interfaces during annealing at elevated temperatures

The relevant phase transformations induced by atomic diffusion at the vicinity of Nb/TiAl heterogenous interface were systematically investigated by establishing a novel system of TiAl-Nb couple via metallurgical bonding of Nb block and γ-TiAl powders by hot pressing sintering and post-annealing treatments. As a result, the resultant diffusion layer, acting as adhesive region linking the Nb and TiAl substrates, thickened with the increase of annealing temperature and the prolongation of holding time, which could be well described by the developed power function. A series of intermediate phases initially nucleated at the Nb/TiAl interface and grew toward both substrates, which was the result of the exchange, partitioning and recombination of solute atoms in the light of vacancy-mediated substitutional diffusion pattern. The phase transformation path on the Nb side was β→η→η + σ→σ→β0→β0 + γn, while that on the TiAl side was γ→γss→γss+ α→(γ + α)+ (α + β0)→α + β0→β0 + γn. Then the disorder-order transition of α→α2 and β0→B2 occurred during cooling. The progress of phase transformation slowed down as the diffusion proceeded, which was attributed to the decrease in atomic substitution efficiency due to the reduction of atomic concentration gradient and the extension of migration distance. In such a case, the banded σba split into spike σsp along the direction of maximum concentration gradient, and then shrunk to isolated σis until completely dissolved in β0 regime, principally dominated by Ti occupying Nb sublattices. The lenticular γn existed mainly in the form of twins, resulting from the lattice reconstruction in accordance with the orientation relationship of 111γ//110β0 and 1̅10γ//11̅1β0 in the Al partitioning region during the spontaneous fluctuation of atomic concentration of β0 regime. These findings filled some fundamental gaps with respect to TiAl-Nb diffusion reaction system, and afforded profound insights into the development of high-performance TiAl/Nb composites with tailorable microstructure.

Tailoring the radiation tolerance of eutectic high-entropy alloy via phase-composition control

Eutectic high-entropy alloys (EHEAs) take advantage of heterogeneous crystal structures, numerous interfaces, and multicomponent composition designs, implying a potential radiation tolerance in the nuclear environment. In this work, we employed the He ion irradiation to study the irradiation behavior of two types EHEAs (Fe42Ni31Al17Cr10, abbreviated as Fe42, and Fe47Ni26Al17Cr10, abbreviated as Fe47) with different phase designs. The results showed that the FCC + B2 constituted Fe42 EHEA and BCC + B2 constituted Fe47 EHEA retained their general phase-composition features up to ∼ 1.2 dpa at 1073 K, while the long-range order of B2 phase was slightly disrupted in both alloys and more severe in the Fe42 EHEA. The quantification on He bubbles formation in different phases further indicated that the ordered B2 phase in Fe47 EHEA has a superior radiation resistance with smaller bubble size and less population. The differences in phase compositions and structures were believed to influence the vacancy-mediated helium diffusion during irradiation, which can account for the He bubble behaviors in the two kinds of alloys. The analysis on the width of bubble denuded zone along the phase boundary further suggested a higher energy barrier for the vacancy migration in the B2 phase, which consistently supported our inference on the underlying mechanism of bubble formation resistance of Fe47 EHEA. We believe that this work will provide a new strategy for designing EHEAs with excellent resistance to irradiation at elevated temperatures.

Impact of d-states on transition metal impurity diffusion in TiN

In this work, we studied the energetics of diffusion-related quantities of transition-metal impurities in TiN, a prototype ceramic protective coating. We use ab-initio calculations to construct a database of impurity formation energies, vacancy-impurity binding energies, migration, and activation energies of 3d and selected 4d and 5d elements for the vacancy-mediated diffusion process. The obtained trends suggest that the trends in migration and activation energies are not fully anti-correlated with the size of the migration atom. We argue that this is caused by a strong impact of chemistry in terms of binding. We quantified this effect for selected cases using the density of electronic states, Crystal Orbital Hamiltonian Population analysis, and charge density analysis. Our results show that the bonding of impurities in the initial state of a diffusion jump (equilibrium lattice position), as well as the charge directionality at the transition state (energy maximum along the diffusion jump pathway), significantly impact the activation energies.

Theoretical studies on the stabilization and diffusion behaviors of helium impurities in 6H-SiC by DFT calculations

In fusion environments, large scales of helium (He) atoms are produced by a radical transformation along with structural damage in structural materials, resulting in material swelling and degradation of physical properties. To understand its irradiation effects, this paper investigates the stability, electronic structure, energetics, charge density distribution, PDOS and TDOS, and diffusion processes of He impurities in 6H-SiC materials. The formation energy indicates that a stable, favorable position for interstitial He is the HR site with the lowest energy of 2.40 eV. In terms of vacancy, the He atom initially prefers to substitute at pre-existing Si vacancy than C vacancy due to lower substitution energy. The minimum energy paths (MEPs) with migration energy barriers are also calculated for He impurity by interstitial and vacancy-mediated diffusion. Based on its calculated energy barriers, the most possible diffusion path includes the exchange of interstitial and vacancy sites with effective migration energies ranging from 0.101 eV to 1.0 eV. Our calculation provides a better understanding of the stabilization and diffusion behaviors of He impurities in 6H-SiC materials.

Atomic diffusion, segregation, and grain boundary migration in nickel-based alloys from molecular dynamics simulations

Grain boundary diffusion and metal mobility in alloys control material performance in many applications and yet remain poorly understood at a mechanistic level. With advances in accessible time and length scales for computational molecular simulations, and recent force field developments, we now possess tools to help unravel those mechanisms. Using large-scale molecular dynamics simulations, here we examined vacancy-mediated diffusion processes in Ni-5Cr alloy with low and high-energy grain boundaries. We show that atomic diffusion inside the grain boundary plane is about four times higher than bulk diffusion, at any temperature, and exhibits a typical Arrhenius behavior with a very small energy barrier (∼0.8 eV for Cr and 0.7 eV for Ni within 1300–1600 K). Additionally, the fastest diffusing species inverts; Cr diffusion was faster than Ni in the bulk but slower in the grain boundaries. This is attributed to the creation of high cohesive energy clusters of Cr at the grain boundary. Grain boundary migration was also observed to be temperature dependent and appears to be two times higher in the 5% Cr alloy than in pure Ni, highlighting the important role of the alloying element on grain boundary motion.

A chemo-mechanical coupling model of oxidation and interlayer cracking of copper nanowires

Copper nanowires have received extensive attention for their potential applications in optics, electricity and catalysis, while oxidation erosion has become the biggest obstacle to their widespread application. Here, we present a chemo-mechanical coupling model to investigate the interlayer cracking behaviors of nanowires during oxidation. In contrast to existing chemo-mechanical models, the present model emphasizes that the process of oxygen entry into copper nanowires is chemical reaction-mediated layer-by-layer replacement rather than vacancy-mediated diffusion, which means the oxygen ions in the outer layer would not cross over the oxygen atoms in the inner layer during their entry. These conclusions are validated by molecular dynamics simulations. We then discuss the effect of the chemical reaction-induced free energy on the stress state of the nanowire during the reaction. Finally, a self-developed finite difference procedure is used to solve the control equations and the fracture location is determined according to the energy release rate analysis. We find the fracture of nanowires is closely dependent on the size of nanowire, the reaction rate and the oxygen concentration. This work deepens our understanding of the mechanism of chemo-mechanical coupling and fracture behavior of metals due to oxidation, thus has implications in other areas involving chemo-mechanical coupling.

A multiscale ion diffusion framework sheds light on the diffusion-stability-hysteresis nexus in metal halide perovskites.

Stability and current-voltage hysteresis stand as major obstacles to the commercialization of metal halide perovskites. Both phenomena have been associated with ion migration, with anecdotal evidence that stable devices yield low hysteresis. However, the underlying mechanisms of the complex stability-hysteresis link remain elusive. Here we present a multiscale diffusion framework that describes vacancy-mediated halide diffusion in polycrystalline metal halide perovskites, differentiating fast grain boundary diffusivity from volume diffusivity that is two to four orders of magnitude slower. Our results reveal an inverse relationship between the activation energies of grain boundary and volume diffusions, such that stable metal halide perovskites exhibiting smaller volume diffusivities are associated with larger grain boundary diffusivities and reduced hysteresis. The elucidation of multiscale halide diffusion in metal halide perovskites reveals complex inner couplings between ion migration in the volume of grains versus grain boundaries, which in turn can predict the stability and hysteresis of metal halide perovskites, providing a clearer path to addressing the outstanding challenges of the field.

Unveiling the synergistic effects of Re-Mo alloying on diffusion behaviors in γ-Ni: From a theoretical perspective

Fine-tuning of alloy composition is crucial for improving the temperature capacity of Ni-based superalloys, which is required for deeper understanding of the strengthening mechanisms of complex alloying. However, the typical strengthening elements Re and Mo still lack clear physical explanations on their synergistic effects on diffusion behaviors. They directly control the dislocation climb and further influence the steady creep rate under high temperatures. In this work, we perform a theoretical investigation on the vacancy-mediated diffusion behaviors regulated by Re and Mo conjunctively in the γ-Ni phase. Combined with the high-accuracy first-principles calculations, a kinetic Monte Carlo model is applied to Ni-Re-Mo ternary systems to evaluate their diffusion kinetics for the first time. The results verify that the 1Re1Mo additions exhibit comparable hindering effects on vacancy formation. The most increased effects on Ni self-diffusion barrier have also been found for 1Mo1Re substitutions compared with 2Re and 2Mo cases. Besides, Mo could elevate the diffusion barriers of Re to some extent and Re would dramatically improve the diffusion barriers of Mo. Especially, the quantified predictions of vacancy diffusion constants of Ni-Rex-Moy (x + y = 5 at.%) systems vary slightly within ∼5% compared with Ni-Re5 system. From the viewpoints of diffusion kinetics, we provide the theoretical interpretations for the synergistic effects of Re–Mo co-alloying and verify the feasibility of partly substituting Mo for Re as an alloy design strategy.

Thermodynamics and kinetics of Pb intercalation under graphene on SiC(0001)

SiC-supported graphene intercalated by a two-dimensional Pb monolayer can provide an appealing platform for spintronic applications. Such a monolayer structure is thermodynamically ultrastable, as observed in recent experiments. However, important fundamentals such as the structure of intercalated phases, locations of intercalated atoms, thermodynamic preference for intercalation, and intercalation pathways for this system have not yet been understood conclusively. In this work, extensive density functional theory calculations are performed to assess Pb intercalation thermodynamics and kinetics under graphene on SiC(0001). We find that intercalation of isolated Pb atoms is strongly disfavored over adsorption on top of graphene. However, intercalation of complete Pb layers in the gallery between SiC and graphene buffer layer is strongly favored over supported Pb monolayers and moderately favored over formation of supported large three-dimensional Pb islands. We also find that initiation of intercalation either by individual Pb atoms directly penetrating graphene or by hopping under a static graphene step edge is energetically prohibitive at experimental temperatures. Consequently, more complex intercalation pathways are operative and further analyzed. We demonstrated that once an intercalated Pb monolayer forms around a graphene step edge, a facile Pb mass transport by Pb vacancy-mediated diffusion can be triggered for continued growth of the intercalated monolayer.

Dynamics of Vacancy Formation and Distribution in Semiconductor Heterostructures: Effect of Thermally Generated Intrinsic Electrons.

The effect of thermally generated equilibrium carrier distribution on the vacancy generation, recombination, and mobility in a semiconductor heterostructure with an undoped quantum well is studied. A different rate of thermally generated equilibrium carriers in layers with different band gaps at annealing temperatures forms a charge-carrier density gradient along a heterostructure. The nonuniform spatial distribution of charged vacancy concentration that appears as a result of strong dependence in the vacancy formation rate on the local charge-carrier density is revealed. A model of vacancy-mediated diffusion at high temperatures typical for post-growth annealing that takes into account this effect and dynamics of nonequilibrium vacancy concentration is developed. The change of atomic diffusivity rate in time that follows on the of spatial vacancy distribution dynamics in a model heterostructure with quantum wells during a high-temperature annealing at fixed temperatures is demonstrated by computational modeling.

Machine learning based on-the-fly kinetic Monte Carlo simulations of sluggish diffusion in Ni-Fe concentrated alloys

Defect diffusion in concentrated alloys plays a key role on governing their unique mechanical and physical properties. In such alloys, defect diffusion depends on its complex local atomic environment and varies from site to site due to the chemical disorder. On-the-fly determination of the defect migration barrier at every site using the standard nudged elastic band (NEB) method is computationally expensive and often impractical. In this work, we couple machine learning and kinetic Monte Carlo (KMC) to study vacancy-mediated sluggish diffusion in concentrated Ni-Fe model alloys. Based on about 32,000 pre-calculated NEB barriers, an artificial neural network (ANN) based machine learning model is developed to accurately predict the vacancy migration barriers for arbitrary local atomic environments, including both random solution configurations and alloys with short-range orders. The ANN model is then coupled with KMC (ANN-KMC) to determine the vacancy migration barriers on-the-fly, enabling an efficient study of the vacancy diffusion in the full composition range at a wide range of temperatures. In addition, a composition and temperature dependent jump attempt frequency model is developed. Upon calibration, the ANN-KMC modeling can predict nearly identical vacancy diffusivities as those obtained from independent molecular dynamics (MD) and temperature accelerated dynamics (TAD) simulations at their accessible temperatures. The sluggish diffusion mechanisms in this specific alloy system at both high and low temperatures are discussed based on the ANN-KMC results.

Ordered CoPt oxygen reduction catalyst with high performance and durability

A high-performance and durable polymer electrolyte membrane fuel cell cathode catalyst composed of an ordered L1 0 –CoPt core and a thin Pt shell was designed and prepared. Under practical fuel cell membrane electrode assembly testing conditions, the cathode catalyst showed an outstanding initial mass activity of 0.6 A/mg Pt , which satisfies the U.S. Department of Energy performance target while also meeting the durability target of less than 40% loss in mass activity after 30,000 accelerated stress test voltage cycles. The high structural stability of the L1 0 –CoPt@Pt-shell catalyst was confirmed by postmortem materials characterization, and the origin of this robustness was revealed by density functional theory calculations, where the barriers for diffusion of Co atoms were observed to be significantly increased in the ordered intermetallic core. • An intermetallic ORR catalyst was prepared through a scalable impregnation method • Mass activity of 0.6 A/mg Pt and high durability were achieved in fuel cell testing • H 2 /air power density of 0.87 W/cm 2 at 0.67 V was achieved with only 0.1 mgPt/cm 2 • Order-dependent energetics of vacancy-mediated diffusion were evaluated using DFT Fuel cells with high power density and high durability could contribute to the realization of net zero carbon emissions in the transportation sector. The catalyst reported here, which uses atomic-level ordering to stabilize Co within the intermetallic CoPt lattice, achieves higher Co retention and better durability compared with conventional CoPt catalysts. The origins of this high durability were investigated using density functional theory calculations, which revealed significantly higher barriers for Co diffusion in ordered catalysts compared with disordered ones. This new intermetallic catalyst is a promising candidate for deployment in transportation and other clean-energy fuel cell applications. An oxygen reduction reaction catalyst composed of core-shell CoPt nanoparticles possessing an ordered Co–Pt core and Pt shell shows excellent performance and durability in fuel cell testing.

Exploring the influence of percolation on vacancy-mediated diffusion in CoCrNi multi-principal element alloys

Multi-principal element alloys (MPEAs), including high entropy alloys, show exceptional mechanical properties and corrosion resistance. These unusual properties are mostly attributed to their particular diffusion behaviors, such as sluggish diffusion. However, this phenomenon is still controversial. Although state-of-the-art simulations report that the percolation effect plays a crucial role in triggering sluggish diffusion in binary alloys, whether this correlation could be generalized to a broad class of MPEAs is unknown. In this work, we combine machine learning (ML) with kinetic Monte Carlo (kMC) to accurately determine the diffusion coefficients in ternary CoCrNi MPEAs. Special care is taken to understand the effects of percolation on diffusion. We find that percolation does not always lead to sluggish diffusion. Instead, the diffusion properties are exclusively governed by the potential energy landscape (PEL) that the diffusing objects experience. By comparing the reduced systems of CoNi and FeNi, we reveal that the percolation effect on triggering the sluggish diffusion strongly depends on the diffusion properties of the slow-diffuser in these systems. These results shed light on the understanding of the complex diffusion behaviors in MPEAs, highlighting the importance of tunning species-dependent PEL in order to tailor their diffusion properties.

Simulation of diffusion with non-equilibrium vacancies, Kirkendall shift and porosity in single-phase alloys

In an alloy subject to vacancy-mediated diffusion, differences in intrinsic diffusivities tend to produce vacancy excess and deficit. These are accommodated by mechanisms such as dislocation climb and pore formation. This is of concern in high temperature alloy-coating systems used in industrial applications, as pores may develop at the interface between the alloy and the coating, which is undesired. This paper presents a multicomponent diffusion model with two types of vacancy sinks/sources: one is associated with dislocation climb and generates lattice shift, the other one is associated with porosity increase/decrease. The model is designed toward a 1D implementation, and porosity is described with a local average volume fraction. Thermodynamic properties and mobility are modeled according to the Calphad method to allow future application to engineering materials. Finite-difference simulations run on two binary systems, NiCr and NiSi, illustrate the role of the two types of sinks in interdiffusion and pore development. Diffusion is found to be more sensitive to the sink strengths in the NiSi system, where intrinsic diffusivities have a stronger composition dependence. This work provides a basis for the evaluation of the parameters involved in vacancy generation/annihilation (e.g. dislocation density) from experimental data, such as concentration profiles obtained from diffusion couple experiments, and for the prediction of porosity in engineering materials.

First-principles study of 3sp impurity (S, P, Si, Al) effects on vacancy-mediated diffusion in Ni and Ni-33Cr alloys

The effect of minor impurities on vacancy-mediated processes is a critical factor causing unanticipated degradation rates in structural alloys. We investigate the 3sp impurity effect by first-principles calculations to derive impurity-modified diffusion in dilute Ni. Calculations are also performed for special quasi-random structures (SQSs) of Ni-33Cr to investigate the similarity of representative hops that affect vacancy-mediated kinetics. The results show considerable enhancement of vacancy mobility by P and S for Ni and Ni-Cr alloys due to vacancy binding and reduced migration barriers of the re-orientation hops at the vicinity of the impurity atom. The enhanced diffusion can cause variability of microstructure changes and degradation rates in industrial Ni-based alloys, particularly for structural components where local segregation of impurities can occur due to non-equilibrium conditions.