Interdiffusion Microstructures Research Articles

Tuning the secondary reaction zone between PtAl coating and nickel based single crystal superalloy during high-temperature oxidation through regulating aluminizing time

Microstructure degradation caused by interdiffusion between coating and superalloy is an important factor affecting the performance of thermal barrier coatings. Tuning the interdiffusion microstructure of PtAl Coatings/Superalloys interface is the key to preparing high performance thermal barrier coatings. In this paper, two β-(Ni, Pt)Al coatings with different compositions and thicknesses were first deposited on the same nickel-based single-crystal superalloy by regulating the aluminizing time. After the isothermal oxidation at 1100 °C, a typical two-layer structure forms in the two different coating/superalloy specimens but with different interdiffusion microstructures. It was found that a noticeable secondary reaction zone (SRZ) with topologically close-packed (TCP) precipitates develop in the specimen with 6 h aluminizing after oxidation for 36 h and its thickness continuously increase. By contrast, no SRZ forms in the specimen with 4.5 h aluminizing after oxidation for even 100 h, but only the microstructure transformation of the interdiffusion zone (IDZ) occurred. That is mainly due to the existence of an incomplete transformed sublayer in the as-prepared specimen with a short aluminizing time, which may affect the subsequent interdiffusion of Al, Ni, and refractory elements. It is highly expected that regulating the aluminizing time is an effective strategy to tune the interdiffusion structure between coating and substrate, which should be helpful for designing the coating/substrate system with better performance.

A grand-potential based phase-field approach for simulating growth of intermetallic phases in multicomponent alloy systems

Intermetallic phase-based alloys, in particular transition-metal aluminides, are potential structural and coating materials for high-temperature applications. Such applications usually involve interdiffusion between two dissimilar materials. To simulate interdiffusion microstructures quantitatively, it becomes essential to solve alloy phase-field models in conjunction with multicomponent CALPHAD databases. This coupling, however, still remains a challenge when considering binary or multicomponent intermetallic phases. Here, a novel method that incorporates successfully diffusion potential dependent-properties of bulk multicomponent phases into a grand-potential based multi-phase-field model is proposed. It uses phase-specific properties directly precomputed from CALPHAD-type databases as discrete functions of solute diffusion potentials. Six different alloy cases, ranging from a five-phase binary (Ni-Al) to a two-phase quaternary (Al-Cr-Ni-Fe) alloy, are simulated to illustrate the application and correctness of the method. The cases include both substitutional and intermetallic phases. Where a comparison is possible, the simulations show good agreement with DICTRA and experimental results, thus validating our proposed method. In contrast to our approach, we find that DICTRA fails in three of the simulations involving ordered intermetallics. We further show that the interface width in this model can be varied without accuracy loss, thus enabling computationally affordable simulations at experimentally comparable length and time scales.

A Grand-Potential Based Phase-Field Approach for Simulating Growth of Intermetallic Phases in Multicomponent Alloy Systems

Intermetallic phase-based alloys, in particular transition-metal aluminides, are potential structural and coating materials for high-temperature applications. Such applications usually involve interdiffusion between two dissimilar materials. To simulate interdiffusion microstructures quantitatively, it becomes essential to solve alloy phase-field models in conjunction with multicomponent CALPHAD databases. This coupling, however, still remains a challenge when considering binary or multicomponent intermetallic phases. Here, a novel method that incorporates successfully diffusion potential dependent-properties of bulk multicomponent phases into a grand-potential based multi-phase-field model is proposed. It uses phase-specific properties directly precomputed from CALPHAD-type databases as discrete functions of solute diffusion potentials. Six different alloy cases, ranging from a five-phase binary (Ni-Al) to a two-phase quaternary (Al-Cr-Ni-Fe) alloy, are simulated to illustrate the application and correctness of the method. The cases include both substitutional and intermetallic phases. Where a comparison is possible, the simulations show good agreement with DICTRA and experimental results, thus validating our proposed method. In contrast to our approach, we find that DICTRA fails in three of the simulations involving ordered intermetallics. We further show that the interface width in this model can be varied without accuracy loss, thus enabling computationally affordable simulations at experimentally comparable length and time scales.

A Trial to Design γ/γ′ Bond Coat in Ni–Al–Cr Mode TBCs Aided by Phase-Field Simulation

Phase-field modeling coupled with calculation of phase diagram (CALPHAD) database was utilized to perform a series of two-dimensional phase-field simulations of microstructure evolution in the γ + γ′/γ + γ′ Ni–Al–Cr mode bond coat/substrate systems. With the aid of phase-field simulated microstructure evolution, the relationship between the interdiffusion microstructure and the cohesiveness/aluminum protective property with different alloy compositions and bond coat thicknesses was fully discussed. A semi-quantitative tie-line selection criteria for alloy composition of the bond coat/substrate system with the identical elements, i.e., that the equilibrium Al concentrations of γ′ and γ phases in the bond coat should be similar to those in substrate, while the phase fraction of γ′ in the bond coat tends to be higher than that in the substrate, was then proposed to reduce the formation of polycrystalline structure and thermal shock from the temperature gradient.

Interdiffusion Databanks of γ, γ′ and β Phases in NiAl-Based Ternary Systems

Advanced modern gas-turbine engines strongly rely on high-temperature thermal barrier coatings (TBCs) for the improved efficiency and power. Interdiffusion between the bond coat and the underlying Ni-based superalloy is one key factor limiting the lifetime of TBCs. In order to assist the engineering-oriented lifetime assessment and even design new TBCs, reliable composition- and temperature-dependent interdiffusivity databanks for γ, γ′ and β phases in different types of bond coats and Ni-based superalloys are the prerequisite. This chapter starts from a very brief introduction of the state-of-art experimental techniques and calculation methods for interdiffusivity determination in ternary systems. After that, the status of the interdiffusion databanks of γ, γ′ and β phases in NiAl-based ternary systems is then summarized, with a special focus on the demonstration of interdiffusivity data measured by means of single-phase diffusion couple/multiple techniques in combination with Matano-Kirkaldy method or numerical inverse method. Several typical results for NiAl-based γ, γ′ and β phases are also given. Finally, two examples of successful applications of the available interdiffusion databanks of ternary NiAl-based γ, γ′ and β phases are presented. One lies in the Re-substitutional element searching in potential new-generation Ni-based superalloys, while the other is the phase-field modeling of interdiffusion microstructure in ternary mode NiAlCr-based TBCs without/with the effect of temperature gradient.

Effects of Cr on the interdiffusion between Ce and Fe–Cr alloys

Fuel cladding chemical interaction (FCCI) has been a long-standing issue for the metallic fuel with a steel cladding in a sodium-cooled fast reactor, particularly for a high burnup fuel. Although the FCCI has been largely improved by alloying the fuels with Zr or Pd elements, applying a physical diffusion barrier between fuel and cladding, and employing advanced ferritic/martensitic (F/M) claddings, there is a scientific knowledge gap in understanding the behavior of chromium and its effects on the interdiffusion between lanthanides and advanced F/M steels that contain 9–12wt.% Cr. In this paper, we systematically studied the interdiffusion between cerium and Fe–Cr model alloys with Cr contents of 6, 9 and 12wt.%. Following the thermal annealing at 560°C for up to 100h, detailed microstructural characterizations were performed to determine the interdiffusion microstructures, compositional distributions, diffusion kinetics, and phase structures in the interdiffusion zone. This study unambiguously disclosed that, as the Ce diffuses into Fe–Cr model alloys, Cr segregates and precipitates into Cr-rich σ phase consisted of Fe and Cr instead of forming a ternary phase together with Fe and Ce. The precipitation of those nano-sized σ phase particles at the Ce diffusion front would effectively slow down the interdiffusion.

Fundamentals of interdiffusion microstructure maps for dual-alloy systems

Interdiffusion microstructure maps (IMMs) are drawn on isothermal phase diagrams. They predict how the interdiffusion microstructure varies in a dual alloy when one alloy composition is fixed while the other is varied across a region of the phase diagram. The microstructure is defined in terms of its layers and the velocity direction of boundaries between layers. Previous work involved maps based on experimental, calculated and simulated interdiffusion microstructures. In the current work IMMs for four model systems were obtained from phase field simulations. The IMMs in this study differ from previous work in that one contains a more complicated IMM with seven areas separated by seven lines that meet at a seven-line node and one contains a less complicated IMM with only one area. Also, it was found that the microstructures at all nodes investigated to date are similar by having the microstructure of the initial dual alloy. During interdiffusion treatments the initial interface is stationary and only the amounts of phases on either side of the interface will vary with time. A list of fundamental properties of IMMs is given that combines findings from the current and previous work. In the list are three mechanisms by which microstructures change. One of these had not been seen before. In this mechanism an inversion of the microstructure occurs as the composition crosses from one side to the other of a two-phase region. These fundamentals are the foundation on which IMMs are constructed and can be a guide for their interpretation.

Diffusion Barrier Selection from Refractory Metals (Zr, Mo and Nb) Via Interdiffusion Investigation for U-Mo RERTR Fuel Alloy

U-Mo alloys are being developed as low enrichment monolithic fuel under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Diffusional interactions between the U-Mo fuel alloy and Al-alloy cladding within the monolithic fuel plate construct necessitate incorporation of a barrier layer. Fundamentally, a diffusion barrier candidate must have good thermal conductivity, high melting point, minimal metallurgical interaction, and good irradiation performance. Refractory metals, Zr, Mo, and Nb are considered based on their physical properties, and the diffusion behavior must be carefully examined first with U-Mo fuel alloy. Solid-to-solid U-10 wt.%Mo versus Mo, Zr, or Nb diffusion couples were assembled and annealed at 600, 700, 800, 900 and 1000 °C for various times. The interdiffusion microstructures and chemical composition were examined via scanning electron microscopy and electron probe microanalysis, respectively. For all three systems, the growth rate of interdiffusion zone were calculated at 1000, 900 and 800 °C under the assumption of parabolic growth, and calculated for lower temperature of 700, 600 and 500 °C according to Arrhenius relationship. The growth rate was determined to be about 103 times slower for Zr, 105 times slower for Mo and 106 times slower for Nb, than the growth rates reported for the interaction between the U-Mo fuel alloy and pure Al or Al-Si cladding alloys. Zr, however was selected as the barrier metal due to a concern for thermo-mechanical behavior of UMo/Nb interface observed from diffusion couples, and for ductile-to-brittle transition of Mo near room temperature.

Interdiffusion Between Potential Diffusion Barrier Mo and U-Mo Metallic Fuel Alloy for RERTR Applications

U-Mo alloys are being developed as low enrichment uranium fuels under the Reduced Enrichment for Research and Test Reactor Program. Previous investigation has shown that the interdiffusion between U and Mo in γ(bcc)-U solid solution is very slow. This investigation explored interdiffusional behavior, especially in regions with high Mo concentration, and the potential application of Mo as a barrier material to reduce the interaction between U-Mo fuel and Al alloys matrix. Solid-to-solid U-10wt.%Mo versus Mo diffusion couples were assembled and annealed at 600, 700, 800, 900 and 1000 °C for 960, 720, 480, 240, 96 h, respectively. The interdiffusion microstructures and concentration profiles were examined via scanning electron microscopy and electron probe microanalysis, respectively. As the Mo concentration increased from 22 to 32 at.%, the interdiffusion coefficient decreased while the activation energy increased. The growth rate constant of the interdiffusion zone between U-10wt.%Mo versus Mo was also determined and compared to be 104-105 times lower than those of U-10wt.%Mo versus Al and U-10wt.%Mo versus Al-Si systems. Other desirable physical properties of Mo as a barrier material, such as neutron adsorption rate, melting point and thermal conductivity, are also highlighted.

Type n boundaries in n-component diffusion couples

When multicomponent alloys are in contact at elevated temperatures, complex interdiffusion microstructures with distinctive multiphase regions may form around the contact interface. The boundaries between these regions can be classified according to the number of phases that change on crossing the boundary. Accordingly, a type n boundary is a boundary in an n-component system across which n phases change. Such boundaries are shown to require that diffusion paths must pass through special features on a phase diagram. Special features are those that cannot be intersected by a random line, such as the corners of tie-triangles or the edges of tie-tetrahedra. Therefore such boundaries are expected to be seen infrequently and only when there is a kinetic benefit for a diffusion path to intersect the feature. Also it is shown that the number of ways that a type n boundary can form is given by the floor function ⌊n/2⌋. Examples are given using phase-field simulations of a model system for n=3 and 4. Simulated diffusion paths with type 3 boundaries are shown to be comparable to those reported for Ni–Cr–Al diffusion couples. In addition, it is shown that the eigenvector direction of the diffusivity and the composition of the initial alloys relative to the phase diagram can dictate whether or not a type n boundary will form in an n-component system.

Interdiffusion, Intrinsic Diffusion, Atomic Mobility, and Vacancy Wind Effect in γ(bcc) Uranium-Molybdenum Alloy

U-Mo alloys are being developed as low enrichment uranium fuels under the Reduced Enrichment for Research and Test Reactor (RERTR) Program. In order to understand the fundamental diffusion behavior of this system, solid-to-solid pure U vs Mo diffusion couples were assembled and annealed at 923 K, 973 K, 1073 K, 1173 K, and 1273 K (650 °C, 700 °C, 800 °C, 900 °C, and 1000 °C) for various times. The interdiffusion microstructures and concentration profiles were examined via scanning electron microscopy and electron probe microanalysis, respectively. As the Mo concentration increased from 2 to 26 at. pct, the interdiffusion coefficient decreased, while the activation energy increased. A Kirkendall marker plane was clearly identified in each diffusion couple and utilized to determine intrinsic diffusion coefficients. Uranium intrinsically diffused 5-10 times faster than Mo. Molar excess Gibbs free energy of U-Mo alloy was applied to calculate the thermodynamic factor using ideal, regular, and subregular solution models. Based on the intrinsic diffusion coefficients and thermodynamic factors, Manning’s formalism was used to calculate the tracer diffusion coefficients, atomic mobilities, and vacancy wind parameters of U and Mo at the marker composition. The tracer diffusion coefficients and atomic mobilities of U were about five times larger than those of Mo, and the vacancy wind effect increased the intrinsic flux of U by approximately 30 pct.

Atomic mobilities and diffusivities in Al alloys

Knowledge of diffusivity is a prerequisite for understanding many scientific and technological disciplines. In this paper, firstly major experimental methods, which are employed to provide various diffusivity data, are briefly described. Secondly, the fundamentals of various computational methods, including first-principles method, embedded atomic method/molecular dynamic simulation, semi-empirical approaches, and phenomenological DICTRA technique, are demonstrated. Diffusion models recently developed for order/disorder transitions and stoichiometric compounds are also briefly depicted. Thirdly, a newly established diffusivity database for liquid, fcc_A1, L12, bcc_A2, bcc_B2, and intermetallic phases in the multicomponent Al alloys is presented via a few case studies in binary, ternary and quaternary systems. And the integration of various computational techniques and experimental methods is highlighted. The reliability of this diffusivity database is validated by comparing the calculated and measured concentration profiles, diffusion paths, and Kirkendall shifts in various binary, ternary and quaternary diffusion couples. Next, the established diffusivity databases along with thermodynamic and other thermo-physical properties are utilized to simulate the microstructural evolution for Al alloys during solidification, interdiffusion and precipitation. A special discussion is presented on the phase-field simulation of interdiffusion microstructures in a series of Ni-Al diffusion couples composed of γ, γ′, and β phases under the effects of both coherent strain and external compressive force. Future orientations in the establishment of next generation of diffusivity database are finally addressed.

Interface Movement and Microstructure Evolution of Interdiffusion in the Binary Alloy Diffusion Couples

The phase field simulation of interface movement and interdiffusion microstructure in a binary diffusion couples was developed. The diffusion couples with nonequilibrium concentration for single phase or single phase and two-phase including the temperature and mobility effects were studied. It’s shown that the interface movement and the atoms diffusion direction were determined by the magnitude of relative concentration difference between the initial concentration and the equilibrium concentration, the distance of interface movement and interdiffusion flux increases as the temperature or the mobility increasing, and the large mobility makes the particles coarsening faster.

Phase-field simulation of diffusion couples in the Ni–Al system

AbstractBy linking thermodynamic and atomic mobility databases with two-dimensional phase-field simulation, the evolution of interdiffusion microstructures in a series of Ni–Al diffusion couples associated with the γ, γ′, and β-phases was studied. The formation and subsequent growth of the γ′-phase layer in β/γ and γ′ + β/γ diffusion couples reproduced the experimental observations well. Moreover, the effect of coherent strain on the γ – γ′ microstructural evolution, as well as that of an external compressive force on the γ + γ′/γ + γ′ diffusion couple, was investigated. The phase-field simulated concentration profiles of some of the Ni–Al diffusion couples were also compared with the corresponding experimental data and the results of one-dimensional DICTRA (DIffusion Controlled TRAnsformations) simulations. A discussion of the rafting direction was also made by comprehensively comparing the phase-field simulations with the predicted results from an elastic model.

Polymer bilayer films with semi-interpenetrating semiconducting/insulating microstructure for field-effect transistor applications

We report an unexpected improvement in the microstructural and electrical properties of regioregular poly(3-hexylthiophene) (P3HT) as an active layer in field-effect transistors (FETs) by introducing soft insulating chains from poly(methyl methacrylate) (PMMA) as a cover layer in a solvent mode-dependent manner. A joint experimental and theoretical spectroscopic and electrical analysis is provided to study the P3HT/PMMA bilayer films with semi-interpenetrating semiconducting/insulating interlocked interdiffusion microstructures. Absorption and Raman studies reveal that disordered P3HT chains in films cast from a low-boiling point (bp) solvent favor reorganization into more ordered structures during PMMA covering, thus enhancing the electrical and switch behaviors of FETs. Although high-bp solvents initially create highly crystalline P3HT films, the films are significantly destroyed after the deposition of the PMMA layer, thus dramatically degrading FET characteristics. The semi-interpenetrating semiconducting/insulating microstructures in the designed bilayer films indicate the possibility of ameliorating field-effects in polymer FETs, especially in subthreshold swing and switch on–off behaviors.

Phase Field Modeling of Interdiffusion Microstructure in Ni-Cr-Al Diffusion Couples

Evolution of interdiffusion microstructures was examined in ternary Ni-Cr-Al solid-tosolid diffusion couples using two-dimensional (2D) phase field simulation. Utilizing Cahn-Hilliard and Allen-Cahn equations, multiphase diffusion couples containing of fcc-γ and B2-β solid solution phases were simulated with alloys of different compositions and phase contents. Chemical mobility as a function of composition with constant gradient energy coefficients was used in the simulation. Simulated microstructures in γ+β/γ and γ+β/γ+β diffusion couples were compared with the experimental microstructures reported in literature. As observed experimentally, the model predicted the recession of γ+β region in the γ+β/γ couple and a stationary interface in γ+β/γ+β couple. Concentration profiles developed across the diffusion couples demonstrated that the interdiffusion occurs in the γ phase as well as in the γ+β region. Formation of single-phase γ and β layers near the interface of γ+β/γ+β couples was also investigated using the volume fraction profile obtained from the simulated microstructure.

Microstructural stability of fcc-γ+B2-β coatings on γ substrate in Ni–Cr–Al system — A phase field model study

A two-dimensional phase field model was employed to examine the evolution of interdiffusion microstructures in ternary Ni–Cr–Al solid-to-solid diffusion couples containing fcc-γ and γ + β (fcc + B2) alloys. Alloys of varying compositions and volume fractions of the second phase (β) were examined to simulate the dissolution kinetics of the β phase (i.e., recession of γ + β two-phase region) observed in γ vs. γ + β diffusion couples. Simulation results showed that the rate of recession of γ + β two-phase region was dependent on the composition of the single-phase γ alloy and the volume fraction of the β phase in the two-phase alloy of the couple. Specifically, the higher the Cr and Al content in the γ alloy and the higher the volume fraction of β in the γ + β alloy lower is the rate of dissolution. Although the volume fraction played a role, the concentration gradients of Al and Cr in the γ phase (e.g., between the matrix of γ + β alloy and single-phase γ alloy) also influenced the recession of the β phase. Simulated results were found to be in good agreement with the experimental observations in ternary Ni–Cr–Al solid-to-solid diffusion couples containing γ and γ + β alloys.

Phase-field simulation of interdiffusion microstructure containing fcc-γ and L1 2-γ′ phases in Ni–Al diffusion couples

Evolution of interdiffusion microstructures was examined for binary Ni–Al solid-to-solid diffusion couples using two-dimensional (2D) phase-field simulation. Utilizing semi-implicit Fourier-spectral solutions to Cahn–Hilliard and Allen–Cahn equations, multiphase diffusion couples of fcc Ni solid solution γ vs. L1 2 Ni 3Al solid solution γ′, γ vs. γ + γ′, γ + γ′ vs. γ + γ′ with sufficient thermodynamic and kinetic database, were simulated with alloys of varying compositions and volume fractions of second phase (e.g., γ′). Chemical mobility as a function of composition was used in the study with constant gradient energy coefficient, and their effect on the final interdiffusion microstructure was examined. The microstructures were characterized by the type of boundaries formed, i.e. Type 0, Type I, and Type II, following various experimental observations in literature and thermodynamic considerations. Volume fraction profiles of alloy phases present in the diffusion couples were measured to quantitatively analyze the formation or dissolution of phases across the boundaries. Kinetics of dissolution of γ′ phase was found to be a function of interdiffusion coefficients that can vary with composition and temperature.

Simulating interdiffusion microstructures in Ni–Al–Cr diffusion couples: a phase field approach coupled with CALPHAD database

We develop a method for phase field modeling of interdiffusion microstructures in Ni–Al–Cr by linking directly the free energy and mobility data to available databases by the CALPHAD method. Its applications are demonstrated for microstructural evolution in γ+β/γ and γ+β/γ+β diffusion couples.

Interdiffusion Microstructure Map Topology

AbstractWhen two alloys are placed in contact to form a diffusion couple, the microstructure that forms on heating is a function of alloy composition. This function can be illustrated on a phase diagram as an “Interdiffusion Microstructure Map” (IMM). An IMM was determined in the present work for Ni-Cr-Al diffusion couples at 1200°C. The couples consisted of one γ+γ' alloy bonded to a number of γ+β alloys. The IMM contained five fields corresponding to five different microstructures. The fields met at a five-line node, giving the map an unexpected topology. The IMM topology was deduced by considering the transition microstructures and kinetic equations that lead to the formation of interdiffusion microstructures.