Uphill Diffusion Research Articles

Synergistic effects of Pt and Y addition in (Ni, Pt)CrAlY bond coat on oxide spallation resistance and growth of interdiffusion zone between bond coat and Ni-based single crystal superalloy

This study shows the beneficial effect of Pt addition to the (β+γ)-NiCrAlY coating. Adding Y in NiCrAl increases oxide growth kinetics but improves spallation resistance. Pt with Y does not change the oxidation rate compared to only adding Y but significantly reduces spallation when cooled to room temperature. Continuous layers of Al2O3 and Cr2O3, along with other oxides such as YAlO3, NiCr2O4, and NiAl2O4, are observed. Moreover, Pt addition reduces the thickness of the interdiffusion zone where Al goes through an uphill diffusion profile, minimizing Al loss and significantly decreasing the growth of deleterious TCP phases.

Atomic cluster expansion interatomic potential for defects and thermodynamics of Cu–W system

The unique properties exhibited in immiscible metals, such as excellent strength, hardness, and radiation-damage tolerance, have stimulated the interest of many researchers. As a typical immiscible metal system, the Cu–W nano-multilayers combine the plasticity of copper and the strength of tungsten, making it a suitable candidate for applications in aerospace, nuclear fusion engineering, and electronic packaging, etc. To understand the atomistic origin of the defects (e.g., vacancies, free surfaces, grain boundaries, and stacking faults and thermodynamical properties), we developed an accurate machine learning interatomic potential for Cu–W based on the atomic cluster expansion (ACE) method. The Cu–W ACE potential can faithfully reproduce the fundamental properties of Cu and W predicted by density functional theory (DFT) calculations. Moreover, the thermodynamical properties, such as the melting point, coefficient of thermal expansion, diffusion coefficient, and equation of the state curve of the Cu–W solid solution, are calculated and compared against DFT and experiments. Monte Carlo molecular dynamics simulations performed with the Cu–W ACE potential predict the experimentally observed phase separation and uphill diffusion phenomena. Our findings not only provide an accurate ACE potential for describing the Cu–W immiscible system but also shed light on understanding the atomistic mechanism during the Cu–W nano-multilayers formation process.

Interdiffusion mechanism between EB-PVD deposited CoCrAlY bond-coat and superalloy substrate under high-temperature oxidation service conditions in gas turbine

The interdiffusion mechanism between EB-PVD deposited CoCrAlY coating and superalloy substrate under high-temperature oxidation atmosphere in actual gas turbines was investigated both experimentally and through thermodynamics and kinetics simulation. The interdiffusion behavior differs at different regions of blade due to the different service temperatures. The elemental interdiffusion and phase transformation behavior were investigated and discussed. Needle-like topologically close-packed phase and σ-CrCoNi phase have precipitated within the secondary reaction zone due to the γ→γ’ transition and the uphill diffusion of Cr, W, and Mo. Decreasing the Co content in the MCrAlY bond-coat can inhibit the interdiffusion behavior to a certain extent.

Outstanding hot corrosion behavior of Pt modified CoNiCrAl medium-entropy alloy

Pt modified CoNiCrAl medium-entropy alloys were optimized to observe the hot corrosion performance. The results indicated that Pt solidly dissolved in the BCC phase, promoted the uphill diffusion of Al, compensated for the loss of Al caused by phase transformation, effectively repaired the oxide scale, and reduced cracks propagation and spalling of the oxide scale. Combined with DFT calculations, verifying that Pt increased the adsorption energy for S atoms, hindered the inward diffusion of S and suppressed internal sulfidation. 10Pt-MEA had a thinner internal oxidation and corrosion pit depth, which was related to the thickness of Al-depletion layer.

The elemental uphill diffusion with micropores reduction during HIP treatment for a solution-treated nickel-based superalloy

Micropores and elemental segregation are detrimental to the high-temperature properties of nickel-based single-crystal (SX) superalloys and hot isostatic pressing (HIP) treatment has been considered as an appropriate method to reduce micropores and elemental segregation. In this study, the effect of a two-step HIP treatment on the solution-treated nickel-based SX superalloy was investigated. An elemental uphill diffusion with the micropores reduction was discovered during HIP treatment and caused the elemental segregation of Al, Cr and Ta at the dendritic scale. It indicates that the HIP treatment is not always beneficial for alloy homogenization. The results show that the first-step HIP treatment could promote the uphill diffusion and cause the elemental segregation with the significant micropores reduction. The second-step HIP treatment could decrease the elemental segregation with further micropores reduction. During subsequent heat treatments, the elemental segregation caused by HIP treatment could be basically eliminated with a slight increase of micropores. During HIP treatment, the micropores reduction and elemental segregation were correlated with the HIP temperature and pressure. This study will provide the supports for controlling micropores and homogenization of nickel-based SX superalloys.

DNS of a non-premixed CH4/O2 flame in a supercritical CO2 environment

3D Direct Numerical Simulation (DNS) is performed to investigate turbulent non-premixed methane oxy-combustion in a CO2-rich environment at supercritical conditions (300 bar). The study focuses on a shear-layer configuration typical of slot burners, where the central slot carries the fuel and the oxidant mixture O2/CO2 flows on both sides of the central jet. The simulation solves the compressible Navier–Stokes equations coupled with the translated volume formulation of the Peng–Robinson cubic equation of state (VTPR-EoS), utilizing a reduced 23-species kinetic scheme derived from the ARAMCO 2.0 complete mechanism. The extremely high pressure results in turbulent scales smaller than those observed at lower pressures. Both premixed and diffusion flame zones are identified, with heat primarily released in non-premixed rich regions, leading to a CO mass fraction of up to 0.55. The flow exhibits uphill (counter-gradient) diffusion for both CO2 and H2O, primarily influenced by the interaction of binary diffusion coefficients and individual species chemical potential gradients. Values in the range of 2.52−2.62 are obtained for the fractal dimension of temperature iso-surfaces. Mixture fraction probability density functions (PDFs) are compared with the standard presumed β−PDF, revealing an underestimation of mixing. Differently from flames at atmospheric pressures, the Dufour effect in the energy diffusion flux, is not negligible. Furthermore, fugacity coefficients must be considered in the calculation of chemical species kinetic rates.

Magma-carbonate interactions drive CO2 production and metal enrichment in shallow dikes and sills at volcanic arcs

Abstract The contribution of CO2 from crustal carbonates into arc magmas is debated, as is its role in the long-term C cycle. To better understand the contributions and mechanisms that drive CO2 production in arc magmas, we examined in detail basaltic dike and sill contacts with carbonate in the Jurassic Bonanza arc on Vancouver Island, Canada. We discovered discrete boundary melts that formed along dike and sill margins in contact with limestone, which display unique Ca, U, and Sr enrichments, Si depletion, and 87Sr/86Sr that approaches host limestone values (~0.708). Binary mixing modeling indicates ~20%–25% limestone assimilation into basalt formed the boundary melts. Contrasting viscosities between boundary and interior melts hinder mixing and chemical homogenization but appear to promote uphill diffusion and metal enrichment within systems that cool in minutes to days. While shallow dikes and sills may be volumetrically minor in an arc magma system, the open flow of magma and large surface area in channels greatly enhances magma-carbonate interactions, and ultimately CO2 production, likely over that of more common and voluminous plutons.

The crystallization kinetics analysis and the new prediction index of melilite-bearing slag under solid fuel gasification condition

Melilite represents a prevalent crystalline phase precipitated from the gasification slag produced by various solid fuels, including coal and biomass. The crystallization of melilite can lead to gasifier blockage, posing significant challenges. This study investigated the crystallization behavior of melilite-bearing slags during slag tapping. To achieve this, FactSage, XRD, SEM-EDS, and DSC were employed to investigate the crystallization thermodynamics and kinetics of 12 synthetic melilite-bearing slag samples. The results revealed that the crystallization temperatures of slags with the mass fraction of SiO2 + Al2O3 is 50 wt% (SiO2 + Al2O3 = 50 wt%) ranged between 1150 and 1100 °C. Slags with the SiO2 + Al2O3 = 60 wt% and mass ratio of SiO2 to Al2O3 > 1.0 (Si/Al > 1.0) showed no crystals precipitate, which is conducive to slag tapping. Thermodynamics plays a crucial role in melilite crystallization, as it involved the uphill diffusion of Ca and Al. To quantify the thermodynamic crystallization ability, a parameter called Thermodynamic Crystallization Ability (TCA) is defined. Increasing Si/Al contributes significantly to reducing TCA. When the TCA is less than 0.33 wt%/K, the melilite-bearing slag will not crystallize. Furthermore, the kinetic analysis results demonstrated that the crystallization activation energy (EC) of melilite-bearing slag ranged from 310.64 to 458.54 kJ/mol. Moreover, the rate-determining step is the surface reaction, which was influenced by cluster structure. The essential short-range ordered structure that composes the melilite crystal is Al-O-Al; however, due to higher Si/Al, the concentration of the Al-O-Al structure in the slag decreases, resulting in higher EC value. These findings provide theoretical guidance for the stable operation of the gasifier.

Phase-field simulation of the microstructure and properties evolution in cast austenitic stainless steels for nuclear power plants during thermal aging and annealing

The microstructural evolution of the Z3CN20.09M stainless steel used for primary pipes of the pressure water reactor (PWR) during thermal aging and annealing was discussed via the phase-field method. The evolution law of the amplitude and wavelength of the one-dimensional chromium (Cr) concentration curve representing the degree of spinodal decomposition and their connection regarding nanohardness was analyzed. Result shows that elemental concentration and temperature markedly affect the simulation. Cr elements demonstrate up-hill diffusion, that is, the ferrite decomposes into the Cr-enriched phase (α′ phase) and the Fe-enriched phase (α phase), when the initial Cr concentration is in the spinodal area. Meanwhile, The temperature increase within a certain temperature range (553 K–773 K) accelerates the progress of spinodal decomposition. The α′ phase redissolves into the matrix when the temperature reaches the critical value (823 K). The amplitude and the wavelength take on a growth trend from rapid to tardy during thermal aging. The amplitude rapidly decreases and the wavelength continuously increases during annealing. The relationship between amplitude–wavelength and nanohardness was established. The simulation using the phase-field method is consistent with the experiment of the material nanohardness.

Carbon effect on thermo-kinetics of Co-Cr-Fe-Mn-Ni high entropy alloys: A computational study validated by interdiffusion experiments

Interstitial alloying is nowadays an important direction for further developments of High Entropy Alloys. In this work, the impact of interstitial carbon on the thermodynamics–kinetics coupling in CoCrFeMnNi-based HEAs is studied through CALPHAD and continuum approaches. First, purely thermodynamic characteristics (phase diagrams and chemical potentials) are calculated. Then purely kinetics properties, i.e., carbon-content dependent mobilities of substitutional elements are modeled, based on experimental data. Consequently, a continuum model is applied to combine these properties in order to simulate interdiffusion in carbon-free/carbon-bearing HEA couples, underlining mutual influence of carbon and matrix elements diffusion on each other. The results are benchmarked against the experimental composition profiles and a very good agreement is observed, specially when carbon effect for all elements is explicitly included. A non-monotonous dependence of the chromium up-hill diffusion on the carbon content is explained by a strong thermodynamic interaction between carbon and chromium elements.

Revealing the evolution mechanism from spot segregation to banded defect above the austenitization temperature

Spot segregation and banded defect are unsolved technical problems of high-quality steel. In this paper, spot segregation of different sizes and irregular morphology were found in equiaxed crystal zones of slab. The characteristics of speckle type and porosity type spot segregation were compared, where the segregation degree of large-size spot segregation was worse. After heat treatment and hot rolling processes, the concentration changes of carbon, manganese, and iron were obtained by EPMA and SEM-EDS. The results showed that the carbon distribution became more even but the change in manganese concentration was not obvious, and the maximum concentration of carbon was almost unchanged, which indicated that severe segregation cannot be alleviated by the existing processes. Circular and irregular atoms were found in the banded defect. These atoms cooperate with thermodynamic coupling caused by hot rolling and contribute to the composition homogenization and void healing of the banded defect. In addition, the connection between holding time and diffusion efficiency was studied by simulating the uphill diffusion process of carbon and manganese at a holding temperature of 1250 °C. When the diameter of the spot segregation was less than 3000 μm, the carbon could uniformly diffuse before 12,000 s, and then if the spot segregation was kept warm for 36,000 s, the mean concentration of manganese was still less than 1.0 wt%, which is too high to be considered homogenized. Finally, an evolution model was established to explain the formation process and evolution mechanism from spot segregation to band defect.

Concepts for estimating all types of diffusion coefficients of NiCoFeCr multi-principal element alloys using two dissimilar or a combination of ideal and non-ideal pseudo-ternary diffusion couples

The estimation of diffusion coefficients in multicomponent system is facilitated by strategically designing diffusion couples with constrained diffusion paths. We have demonstrated methods of estimating tracer diffusion coefficients following three different combinations of diffusion profiles: (i) By intersecting a pseudo-ternary diffusion path with a rectilinear pseudo-binary diffusion path, (ii) by considering a combination of two dissimilar pseudo-ternary diffusion paths, and (iii) considering a combination of ideal and non-ideal pseudo-ternary diffusion paths with uphill diffusion characteristics. Equation schemes required for such analyses depending on the type of diffusion profiles are explained in detail. This approach relies on the availability of reliable thermodynamic data for the estimation of the tracer diffusion coefficients. Following, as demonstrated, one can calculate the intrinsic and interdiffusion coefficients of all the elements for a conventional diffusion couple in which all the elements would produce the diffusion profiles.

Uphill in Reaction-Diffusion Multi-species Interacting Particles Systems

We study reaction-diffusion processes with multi-species particles and hard-core interaction. We add boundary driving to the system by means of external reservoirs which inject and remove particles, thus creating stationary currents. We consider the condition that the time evolution of the average occupation evolves as the discretized version of a system of coupled diffusive equations with linear reactions. In particular, we identify a specific one-parameter family of such linear reaction-diffusion systems where the hydrodynamic limit behaviour can obtained by means of a dual process. We show that partial uphill diffusion is possible for the discrete particle systems on the lattice, whereas it is lost in the hydrodynamic limit.

The Optimized Homogenization Process of Cast 7Mo Super Austenitic Stainless Steel.

Super austenitic stainless steels are expected to replace expensive alloys in harsh environments due to their superior corrosion resistance and mechanical properties. However, the ultra-high alloy contents drive serious segregation in cast steels, where the σ phase is difficult to eliminate. In this study, the microstructural evolution of 7Mo super austenitic stainless steels under different homogenization methods was investigated. The results showed that after isothermal treatment for 30 h at 1250 °C, the σ phase in steels dissolved, while the remelting morphologies appeared at the phase boundaries. Therefore, the stepped solution heat treatment was further conducted to optimize the homogenized microstructure. The samples were heated up to 1220 °C, 1235 °C and 1250 °C with a slow heating rate, and held at these temperatures for 2 h, respectively. The elemental segregation was greatly reduced without incipient remelting and the σ phase was eventually reduced to less than 0.6%. A prolonged incubation below the dissolution temperature will lead to a spontaneous compositional adjustment of the eutectic σ phase, resulting in uphill diffusion of Cr and Mn, and reducing the homogenization efficiency of ISHT, which is avoided by SSHT. The hardness reduced from 228~236 Hv to 220~232 Hv by adopting the cooling process of "furnace cooling + water quench". In addition, the study noticed that increasing the Ce content or decreasing the Mn content can both refine the homogenized grain size and accelerate diffusion processes. This study provides a theoretical and experimental basis for the process and composition optimization of super austenitic stainless steels.

High temperature performance and interface evolution of a NiCoCrAlY coating with Pt modification and Re-based diffusion barrier

The cyclic and early isothermal oxidation behaviors at 1050 °C of the NiCoCrAlY, Pt-modified NiCoCrAlY and Pt-modified NiCoCrAlY with Re-based diffusion barrier (DB) have been evaluated. The atomic immigration and electron location function (ELF) at the Ni-Re interface were simulated by density function theory (DFT) calculations. After isothermal oxidation for 1 h, island NiCoCrAlY particles with extremely poor Al were observed in the oxide scale of normal NiCoCrAlY coating, and then the oxide scale transformed into a double-layer structure after oxidation for 20 h. The initial “island-oxide” scale model was proposed to illustrate the oxidation mechanisms for normal NiCoCrAlY coating. The oxide scale of Pt-modified NiCoCrAlY coating was single-layer Al2O3 with rapid coverage at the surface, because the modification of Pt changed the initial oxidation mechanism by promoting the uphill diffusion of Al. The Re-based DB reduced the thickness of the secondary reaction zone (SRZ) by inhibiting the element interdiffusion between the NiCoCrAlY coating and the substrate, thus the Pt-modified NiCoCrAlY with Re-based DB exhibited the lowest interdiffusion extent. Interestingly, the thermal stability of Re-based DB in NiCoCrAlY coating system was poorer than that in NiCrAlY system. The DFT simulation shows that the incorporation of Co made the energy difference more negative, which confirmed that Co promoted the diffusion of Re in the NiCoCrAlY coating, thus accelerating the degradation of Re-based DB.

The Mechanism of Dendrite Formation in a Solid-State Transformation of High Aluminum Fe-Al Alloys.

The mechanism of solid-state dendrite formation in high-aluminum Fe-Al alloys is not clear. Applying an in-situ observation technique, the real-time formation and growth of FeAl solid-state dendrites during the eutectoid decomposition of the high-temperature phase Fe5Al8 is visualized. In-situ experiments by HT-CSLM reveal that proeutectoid FeAl usually does not preferentially nucleate at grain boundaries regardless of rapid or slow cooling conditions. The critical radii for generating morphological instability are 1.2 μm and 0.9 μm for slow and rapid cooling, respectively. The morphology after both slow and rapid cooling exhibits dendrites, while there are differences in the size and critical instability radius Rc, which are attributed to the different supersaturation S and the number of protrusions l. The combination of crystallographic and thermodynamic analysis indicates that solid-state dendrites only exist on the hypoeutectoid side in high-aluminum Fe-Al alloys. A large number of lattice defects in the parent phase provides an additional driving force for nucleation, leading to coherent nucleation from the interior of the parent phase grains based on the orientation relationship {3¯30}Fe5Al8//{1¯10}FeAl, <111¯>Fe5Al8//<111¯>FeAl. The maximum release of misfit strain energy leads to the preferential growth of the primary arm of the nucleus along <111¯> {1¯10}. During the rapid cooling process, a large supersaturation is induced in the matrix, driving the Al atoms to undergo unstable uphill diffusion and causing variations in the concentration gradient as well as generating constitutional undercooling, ultimately leading to morphological instability and the growth of secondary arms.

A new Cu-W bionic shell pearl multilayer structure

Different microstructure Cu-W films can satisfy various properties requirements. In this current work, a new Cu-W bionic shell pearl multilayer structure films were prepared by the DC dual-target magnetron co-deposition method. The connections between the multilayer films are formed by the regular zigzag weaving of slender filamentary structures in the Cu-W bionic shell pearl structure. We discovered that the influence of atomic diffusion resulted in the construction of a bionic shell pearl multilayer structure under the combined effect of W content and uphill diffusion of Cu. The Cu-10.7 at.%W films with this structure consist mainly of the face center cubic(fcc) structure of Cu and a small amount of stacking faults, but its mechanical properties are better than those of the same composition film prepared by other methods. This method can be extended to a general strategy for manufacturing other immiscible systems.

New insights into the microstructural stability based on the element segregation behavior at γ/γ′ interface in Ni-based single crystal superalloys with Ru addition

A new insight into the microstructural stability was proposed in Ni-based single crystal superalloys with Ru addition, and the element segregation behavior at γ/γ′ interface was investigated by three-dimensional atom probe technology (3D-APT). After standard heat treatment, it was found that Ru addition barely altered the element partitioning coefficient between γ matrix and γ′ phase, and no element-segregation layer was observed at γ/γ′ interface. During the heat exposure at 1100 °C, Ru addition obviously promoted the rafting of the γ′ precipitates and inhibited the precipitation of topological close-packed (TCP) phases. It was more important that an element-segregation layer containing Re, Co, and Cr was formed in the γ matrix close to the γ/γ′ interface due to an “uphill diffusion” effect, and its concentration was obviously reduced after Ru addition. Finally, the microstructural stability based on the element segregation behavior at γ/γ′ interface was discussed. This element-segregation layer increased the γ/γ′ interfacial energy by increasing the absolute value of the lattice misfit of γ/γ′ interface to promote the rafting of the γ′ precipitates after Ru addition. On the other hand, the decrease of the segregation concentration of Re, Co, and Cr elements as TCP phase-forming elements near the γ/γ′ interface due to a “reverse partitioning” effect inhibits the precipitation of TCP phases in Ni-based single crystal superalloys after Ru addition.

Reactive interdiffusion of an Al film and a CoCrFeNi high-entropy alloy at elevated temperatures

The phase evolution of alloys is closely related to atomic diffusion. The influence of reactive diffusion on phase formation in high-entropy alloys (HEAs) is however still unclear. The present work systematically investigates the phase evolution of a multicomponent CoCrFeNi/Al diffusion couple through isochronous-reactive interdiffusion experiments. This provides a direct way to study the influence of enthalpy and entropy on the phase formation and element diffusion behavior. At temperatures below 1173 K, the enthalpy contribution dominates the total energy, leading to the formation of intermetallic compounds. When the temperature is in the range of 1173–1573 K, the entropy of mixing starts to play a more important role. This causes diffusion of Al towards the HEA without phase transformation, forming a more disordered state on the microscale. Even after the system reaches a disordered state, the enthalpy contribution cannot be totally ignored, which is reflected by the uphill diffusion of Ni towards Al. This demonstrates the combined effects of entropy and enthalpy on the phase formation in HEAs at elevated temperatures. Considering the homologous temperature for different equimolar alloys reveals that a multicomponent configuration does not stabilize the disordered state, while the mixing enthalpies between atomic pairs have a large impact on the transition temperature from ordered to disordered state. Finally, it is shown that surface modification of the HEA can be realized through a combination of film deposition and annealing processes. Compared to the HEA matrix, the formation of intermetallic compounds results in a hard surface layer. After the system becomes disordered, the higher hardness of the film side compared to the matrix can be attributed to the lattice distortion induced by Al.

Interdiffusivity matrices and atomic mobilities in fcc Co–Ni–Si and Cu–Co–Ni–Si alloys: Experiment and modeling

Accurate diffusion kinetics information of fcc Co–Ni–Si alloys is essential for developing high-performance Cu–Co–Ni–Si alloys. Eight diffusion couples at the Co–Ni side are assembled to determine the Co- and Ni-basis diffusivities of fcc Co–Ni–Si alloys at 1273 or 1373 K. The diffusivity matrices at the intersection compositions of diffusion paths are determined by the Matano-Kirkaldy method. In addition, the diffusivity matrices along the whole composition profiles and the atomic mobilities of fcc Co–Ni–Si alloys are evaluated by the numerical inverse approach. The reliability of the obtained atomic mobilities is verified by comparing the predicted diffusion phenomena with the experimental ones. Strong cross phenomenon effects among the components are manifested by large negative cross diffusivities, the development of zero flux planes and uphill diffusion phenomena. The uphill diffusion in the fcc Co–Ni–Si system is due to the cross phenomenon between diffusion components. The location of the zero flux plane of a component is usually the intersection between the diffusion path and the iso-activity line for that component through a terminal alloy. Combining the mobility descriptions of the fcc Co–Ni–Si system with the other three sub-ternary systems in the literature, the atomic mobilities in fcc Cu–Co–Ni–Si alloys are constructed. The model-predicted composition profiles of Cu–Co–Ni–Si alloys show a good agreement with the experimental ones, validating the accuracy of the obtained atomic mobilities.