Carelyn E. Campbell

Development of a diffusion mobility database for Ni-base superalloys

Abstract For the fcc phase of the Ni–Al–Co–Cr–Hf–Mo–Re–Ta–Ti–W system, diffusion data in various constituent binary systems were assessed to establish a multicomponent diffusion mobility database. The diffusion assessment relied on an existing thermodynamic database for the calculation of needed thermodynamic factors. The mobilities determined for the self-diffusion of the components in the fcc phase (a metastable state for some components) were consistent with the correlation of the diffusivity with the melting point. The general agreement of calculated and measured diffusion coefficients in the Ni–Co–Cr–Mo and Ni–Al–Cr–Mo quaternary systems demonstrated the ability of the database to extrapolate to higher order systems. Finally, the mobility database, in conjunction with an available thermodynamic database and a finite-difference diffusion code, was used to simulate a multicomponent diffusion couple between two commercial Ni-base superalloys.

Transient liquid-phase bonding in the Ni-Al-B system

Transient liquid-phase (TLP) bonding experiments were performed using a Ni-10.3 at. pct Al alloy and a Ni-10 at. pct B filler material, and the results were compared to simulations performed using the finite-difference diffusion code, DICTRA. For the simulations, a thermodynamic assessment of the Ni-Al-B system was used to define the phase diagram and the thermodynamic factors of the diffusion coefficients. Composition-dependent diffusion mobilities were assessed for the ternary system. Predicted liquid widths as functions of time were in good agreement with the experiments. The calculated and experimental Al composition profiles agree in the matrix but not in the liquid. The simulations qualitatively predicted the observed precipitation and later dissolution of the intermetallic τ phase (Ni20Al3B6) in the base material. This research demonstrated the potential for modeling the formation of spurious phases during TLP bonding of practical superalloy systems.

A thermodynamic assessment of the Ni-Al-B system

The thermodynamics of the Ni-Al-B system are assessed based on available literature data. Both the Ni-B and Al-B systems were revised to treat B as an interstitial element in a face-centered cubic structure, rather than as a substitutional element. In addition, the Al-B binary description was modified to account for the recent experimental work on the melting properties of AlB12. Two Ni-Al assessments are considered for the ternary assessment. For the same optimized ternary parameters, the two Ni-Al assessments produced similar results, with the differences increasing at high temperatures near the liquidus.

On the properties of α/α+β ternary diffusion couples

Abstract The properties of diffusion couples in a ternary system are examined theoretically when one end member is composed of single-phase material (α) and the other end member is composed of two-phase material (α+β). Using the model of Morral and co-workers, the cases in which the two-phase region either grows or dissolves are examined. It is proved that when the two-phase material grows, there can be no jump in concentration (or phase fraction of β) at the boundary between the two materials. When it dissolves a jump is necessary. Numerical examples are presented to demonstrate this result. It is also shown how the diffusion path varies depending on the composition of the single-phase α end member for a fixed α+β end member composition. Under some conditions, three solutions are found for the same initial end member compositions. When these multiple solutions exist, either two or three are virtual (path involves supersaturation). Whenever all three solutions are virtual, a single solution with three interfaces can be found. In this case an intermediate layer of α phase forms within the α+β material to relieve the supersaturation.

Assessment of the Cr-B system and extrapolation to the Ni-Al-Cr-B quaternary system

The Cr-B system is re-assessed treating boron as an interstitial element in the disordered solid solution fcc and bcc phases. The binary assessment is in good agreement with the available experimental phase diagram data and thermochemical data. The Cr-B binary assessment is then used in the assessment of the Ni-Cr-B and Al-Cr-B systems. The ternary assessments are in reasonable agreement with the limited experimental data for the systems. Finally, the ternary assessments are combined with the previous ternary assessment of the Ni-Al-B system to extrapolate to the quaternary Ni-Al-B-Cr system.

Invited review: Modelling of thermodynamics and diffusion in multicomponent systems

Abstract The availability of reliable materials data is key to the successful design of materials and manufacturing processes. Commercial alloys seldom consist of only two or three elements, but rather may contain a large number of elements for which the needed data are rarely available. The CALPHAD (calculation of phase diagrams method), as implemented in a number of software tools, enables the development of thermodynamic and diffusion databases and the extrapolation of these property data from binary and ternary systems to higher order systems. The computational methods used to calculate thermodynamic and diffusion properties can be invaluable in the design of new materials. In addition, the databases and software tools provide an efficient method of storing a wealth of data and allow efficient retrieval of the needed information. The present paper reviews the development and application of multicomponent thermodynamic and diffusion mobility databases using the CALPHAD method.

The development of phase-based property data using the CALPHAD method and infrastructure needs

Initially, the CALPHAD (Calculation of Phase Diagrams) method was established as a tool for treating thermodynamics and phase equilibria of multicomponent systems. Since then the method has been successfully applied to diffusion mobilities in multicomponent systems, creating the foundation for simulation of diffusion processes in these systems. Recently, the CALPHAD method has been expanded to other phase-based properties, including molar volumes and elastic constants, and has the potential to treat electrical and thermal conductivity and even two-phase properties, such as interfacial energies. Advances in the CALPHAD method or new information on specific systems frequently require that already assessed systems be re-assessed. Therefore, the next generation of CALPHAD necessitates data repositories so that when new models are developed or new experimental and computational information becomes available the relevant low-order (unary, binary, and ternary) systems can be re-assessed efficiently to develop the new multicomponent descriptions. The present work outlines data and infrastructure needs for efficient CALPHAD assessments and updates, highlighting the requirement for data repositories with flexible data formats that can be accessed by a variety of tools and that can evolve as data needs change. Within these repositories, the data must be stored with the appropriate metadata to enable the evaluation of the confidence of the stored data.

Homogenization kinetics of a nickel-based superalloy produced by powder bed fusion laser sintering

Additively manufactured (AM) metal components often exhibit fine dendritic microstructures and elemental segregation due to the initial rapid solidification and subsequent melting and cooling during the build process, which without homogenization would adversely affect materials performance. In this letter, we report in situ observation of the homogenization kinetics of an AM nickel-based superalloy using synchrotron small angle X-ray scattering. The identified kinetic time scale is in good agreement with thermodynamic diffusion simulation predictions using microstructural dimensions acquired by ex situ scanning electron microscopy. These findings could serve as a recipe for predicting, observing, and validating homogenization treatments in AM materials.

Enhanced mass transport in ultrarapidly heated Ni/Si thin-film multilayers

We investigated multilayer and bilayer Ni/Si thin films by nanodifferential scanning calorimetry (nano-DSC) at ultrarapid scan rates, in a temperature-time regime not accessible with conventional apparatus. DSC experiments were completed at slower scan rates as well, where it was possible to conduct parallel rapid thermal annealing experiments for comparison. Postexperimental characterization was accomplished by x-ray diffraction, and by transmission electron microscopy (TEM) and energy-filtered TEM of thin cross sections prepared by focused ion beam milling. We found that rate of heating has a profound effect on the resulting microstructure, as well as on the DSC signal. After heating to 560 °C at 120 °C/s, the general microstructure of the multilayer was preserved, in spite of extensive interdiffusion of Ni and Si. By contrast, after heating to 560 °C at 16 000 °C/s, the multilayer films were completely homogeneous with no evidence of the original multilayer microstructure. For the slower scan rates, we...

In situ structural characterization of ageing kinetics in aluminum alloy 2024 across angstrom-to-micrometer length scales

The precipitate structure and precipitation kinetics in an Al-Cu-Mg alloy (AA2024) aged at 190 °C, 208 °C, and 226 °C have been studied using ex situ Transmission Electron Microscopy (TEM) and in situ synchrotron-based, combined ultra-small angle X-ray scattering, small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) across a length scale from sub-Angstrom to several micrometers. TEM brings information concerning the nature, morphology, and size of the precipitates while SAXS and WAXS provide qualitative and quantitative information concerning the time-dependent size and volume fraction evolution of the precipitates at different stages of the precipitation sequence. Within the experimental time resolution, precipitation at these ageing temperatures involves dissolution of nanometer-sized small clusters and formation of the planar S phase precipitates. Using a three-parameter scattering model constructed on the basis of TEM results, we established the temperature-dependent kinetics for the cluster-dissolution and S-phase formation processes simultaneously. These two processes are shown to have different kinetic rates, with the cluster-dissolution rate approximately double the S-phase formation rate. We identified a dissolution activation energy at (149.5 ± 14.6) kJ mol-1, which translates to (1.55 ± 0.15) eV/atom, as well as an activation energy for the formation of S precipitates at (129.2 ± 5.4) kJ mol-1, i.e. (1.33 ± 0.06) eV/atom. Importantly, the SAXS/WAXS results show the absence of an intermediate Guinier-Preston Bagaryatsky 2 (GPB2)/S″ phase in the samples under the experimental ageing conditions. These results are further validated by precipitation simulations that are based on Langer-Schwartz theory and a Kampmann-Wagner numerical method.