Al-Ni

Abstract: Complex concentrated alloys (CCAs) are a new generation of metallic alloys composed of three or more principal elements with physical and mechanical properties that can be tuned by adjusting their compositions. The extensive compositional workspace of CCAs makes it impractical to perform a comprehensive search for a specific material property using experimental measurements. The use of computational methods can rapidly narrow down the search span, improving the efficiency of the design process. We carried out a high-throughput parameterization of modified embedded atom method (MEAM) interatomic potentials for combinations of Cu, Ti, Ni, Cr, Co, Al, Fe, and Mn using a genetic algorithm. Unary systems were parameterized based on DFT calculations and experimental results. MEAM potentials for 28 binary and 56 ternary combinations of the elements were parameterized to DFT results that were carried out with semi-automated frameworks. Specific attention was made to reproduce properties that impact compositional segregation, material strength, and mechanics.

Abstract: As the operating temperature of jet turbine engines increase, creep becomes the life-limiting property for turbine disks and blades. At intermediate temperatures, around 600-800 °C, microtwinning contributes significantly to creep strain in these alloys. Therefore, understanding how microtwins form and grow is critical to improving the creep life of future Ni-base superalloy components. In addition, exploring the effect of different alloying elements, such as Nb and Ta, on the formation of microtwins is critical for future alloy development. Several mechanisms of microtwinning have been proposed among which the Kolbe mechanism, based on thermally activated reordering, is believed to be dominant. In this work we employ atomistic simulation to investigate the effects of Nb solutes on the Kolbe mechanism. The simulation demonstrates that Nb atoms significantly slow down the reordering processes, explaining the experimentally observed improvement in the creep resistance.

Abstract: Second nearest neighbor modified embedded-atom method (2NN-MEAM) interatomic potentials are developed for binary aluminum (Al) alloys applicable from room temperature to the melting point. The binary alloys studied in this work are Al-Cu, Al-Fe and Al-Ni. Sensitivity and uncertainty analyses are performed on potential parameters based on the perturbation approach. The outcome of the sensitivity analysis shows that some of the MEAM parameters interdependently influence all MEAM model outputs, allowing for the definition of an ordered calibration procedure to target specific MEAM outputs. Using these 2NN-MEAM interatomic potentials, molecular dynamics (MD) simulations are performed to calculate low and high-temperature properties, such as the formation energies of stable phases and unstable intermetallics, lattice parameters, elastic constants, thermal expansion coefficients, enthalpy of formation of solids, liquid mixing enthalpy, and liquidus temperatures at a wide range of compositions. The computed data are compared with the available first principle calculations and experimental data, showing high accuracy of the 2NN-MEAM interatomic potentials. In addition, the liquidus temperature of the Al binary alloys is compared to the phase diagrams determined by the CALPHAD method.

Abstract: An interatomic potential for the Ni–Al system is presented within the third-generation charge optimized many-body (COMB3) formalism. The potential has been optimized for Ni₃Al, or the γ' phase in Ni-based superalloys. The formation energies predicted for other Ni–Al phases are in reasonable agreement with first-principles results. The potential further predicts good mechanical properties for Ni₃Al, which includes the values of the complex stacking fault (CSF) and the anti-phase boundary (APB) energies for the (1 1 1) and (1 0 0) planes. It is also used to investigate dislocation propagation across the Ni₃Al (1 1 0)–Ni (1 1 0) interface, and the results are consistent with simulation results reported in the literature. The potential is further used in combination with a recent COMB3 potential for Al₂O₃ to investigate the Ni₃Al (1 1 1)–Al₂O₃ (0 0 0 1) interface, which has not been modeled previously at the classical atomistic level due to the lack of a reactive potential to describe both Ni₃Al and Al₂O₃ as well as interactions between them. The calculated work of adhesion for this interface is predicted to be 1.85 J m⁻², which is in agreement with available experimental data. The predicted interlayer distance is further consistent with the available first-principles results for Ni (1 1 1)–Al₂O₃ (0 0 0 1).

Abstract: We construct an interatomic potential for the Ni-Al system within the embedded-atom method formalism. The potential is based on previously developed accurate potentials for pure Ni and Al. The cross-interactions are fitted to experimental cohesive energy, lattice parameter and elastic constants of B2-NiAl, as well as to ab initio formation energies of several real or imaginary intermetallic compounds with different crystal structures and chemical compositions. The potential accurately reproduces a variety of physical properties of the NiAl and Ni3Al phases, and shows reasonable agreement with experimental and ab initio data for phase stability across the Ni-Al phase diagram. Most of the properties reproduced by the new potential were not involved in the fitting process, which demonstrates its excellent transferability. Advantages and certain weaknesses of the new potential in comparison with other existing potentials are discussed in detail. The potential is expected to be especially suitable for simulations of heterophase interfaces and mechanical behavior of Ni-Al alloys.

Abstract: Complex equilibria and phase transformations involving diffusion can now be calculated quickly and efficiently. Detailed examples are given for cases which involve varying degrees of non-equilibrium and therefore time-dependence. Despite very good agreement between such calculations and experimental results, many potential end-users are still not convinced that such techniques could be usefully applied to their own specific problems. Friendly graphic interface versions of calculating software are now generally available, so the authors conclude that the most likely source of the reluctance to use such tools lies in the formulation of relevant questions and the interpretation of the results. Although the potential impact of such tools was foreseen many years ago [M. Hillert, Calculation of phase equilibria, in: Conference on Phase Transformations, 1968], few changes in the relevant teaching curricula have taken into account the availability and power of such techniques.
This paper has therefore been designed not only as a collection of interesting problems, but also highlights the critical steps needed to achieve a solution. Each example includes a presentation of the "real" problem, any simplifications that are needed for its solution, the adopted thermodynamic formulation, and a critical evaluation of the results. The availability of such examples should facilitate changes in subject matter that will both make it easier for the next generation of students to use these tools, and at the same time reduce the time and effort currently needed to solve such problems by less efficient methods.
The first set of detailed examples includes the deoxidation of steel by aluminum; heat balance calculations associated with ladle additions to steel; the determination of conditions that avoid undesirable inclusions; the role of methane in sintering atmospheres; interface control during the physical vapour deposition of cemented carbide; oxidation of γ-TiAl materials; and simulation of the thermolysis of metallorganic precursors for Si-C-N ceramics and interface reaction of yttrium silicates with SiC-coated C/C-SiC composites for heat shield applications.
A second set of examples, more dependent on competitive nucleation and growth, includes segregation and carburization in multicomponent steels and features a series of sophisticated simulatons using DICTRA software.
Interfacial and strain energies become increasingly important in defining phase nucleation and morphology in such problems, but relatively little information is available compared to free energy and diffusion databases. The final section therefore demonstrates how computational thermodynamics, semi-empirical atomistic approaches and first-principles calculations are being used to aid filling this gap in our knowledge.

Abstract: A new embedded-atom potential has been developed for Ni₃Al by fitting to experimental and first-principles data. The potential describes lattice properties of Ni₃Al, point defects, planar faults, as well as the γ and γ′ fields on the Ni–Al phase diagram. The potential is applied to calculate the energies of coherent Ni/Ni₃Al interphase boundaries with three different crystallographic orientations. Depending on the orientation, the interface energy varies between 12 and 46 mJ/m2. Coherent γ/γ′ interfaces existing at high temperatures are shown to be more diffuse and are likely to have a lower energy than Ni/Ni₃Al interfaces.

Abstract: An embedded-atom potential has been constructed for the intermetallic compound B2−NiAl by fitting to both experimental properties and ab initio data. The ab initio data have been generated in the form of energy-volume relations for a number of alternative structures of NiAl and Ni3Al, as well as for Ni and Al. The potential accurately reproduces the basic lattice properties of B2−NiAl, planar faults, and point-defect characteristics. It also reproduces the energetics and stability of all other structures included in the fit. The potential is applied to calculate equilibrium point-defect concentrations in B2−NiAl as functions of temperature and composition near the stoichiometry. In contrast to previous calculations, the defect formation entropies arising from atomic vibrations are included in our calculation within the quasiharmonic approximation. Such entropies tend to increase the concentrations of thermal point defects in B2−NiAl at high temperatures, but the atomic disorder mechanism remains triple-defect type up to the melting point.