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Copper (Cu) is a catalyst broadly used in industry for hydrogenation of carbon dioxide, which has broad implications for environmental sustainability. An accurate understanding of the degeneration behavior of Cu catalysts under operando conditions is critical for uncovering the failure mechanism of catalysts and designing novel ones with optimized performance. Despite the widespread use of these materials, their failure mechanisms are not well understood because conventional characterization techniques lack the necessary time and spatial resolution to capture these complex behaviors. In order to overcome these challenges, we carried out transmission electron microscopy (TEM) with a specialized in situ gas environmental holder, which allows us to unravel the dynamic behavior of the Cu nanowires (NWs) in operando. The failure process of these nanoscale Cu catalysts under CO2 atmosphere were tracked and further rationalized based on our numerical modeling using phase-field methods.

Copper (Cu) is a catalyst broadly used in industry for hydrogenation of carbon dioxide, which has broad implications for environmental sustainability. An accurate understanding of the degeneration behavior of Cu catalysts under

Designing low-energy interface structures is crucial in growth and coarsening studies of Al2Cu (θ′) precipitates. Given new experimental insights into the θ′/Al system, we use Density Functional Theory (DFT) calculations to investigate the energetics of the coherent (001)θ′//(001)Al and semi-coherent (010)θ′//(010)Al interfaces. We find that the recently proposed occupancy of interstitial Cu atoms at the coherent interface by Bourgeois et al. increases interfacial energy, and hence, is likely due to kinetic effects. For the semi-coherent interface, the semi-coherent interfacial energy does not significantly depend on interfacial configurations or misfit strains up to 10-unit cells of Al.

Here, we design thermally stable microstructures of Al3Cu2Mg9Si7 (Q) precipitates based on commercial aluminum alloy (AS7GU) for automotive applications utilizing low-diffusivity solutes in aluminum. We find that the newly developed alloys show a ~ 20% enhancement in mechanical properties at 300 °C compared to the baseline AS7GU alloy.

The compound Al3Cu2Mg9Si7, is known as the Q-phase and forms as a thermodynamically stable precipitate during aging in the quaternary Al-Cu-Mg-Si system. We perform atomic-scale density functional theory (DFT) calculations of defect properties, solute partitioning, and interfacial stability of the Al3Cu2Mg9Si7 (Q) precipitate. We find: (i) simple native point defect (i.e., vacancies and anti-sites) thermodynamics can partially explain the experimentally observed off-stoichiometry, such as the observed variation of compositions, Al3+δCu2Mg9-δSi7 (Mg-deficient and Al-rich) in experiment. (ii) Calculated solute-partitioning energies of common solutes allow us to define general rules for site-preference in the Q-phase in terms of electronic structure and atomic radius. To validate our DFT predictions, we perform atom-probe tomography (APT) experiments for six-different elements (Zn, Ni, Mn, Ti, V, and Zr). The results show that the partitioning behavior of solutes Ni, Zn, and Mn are consistent with DFT predictions, but the transition elements (Ti, V, and Zr), which are anomalously slow diffusers in Al, partition to the Q-phase in constrast to DFT partitioning energies. (iii) For the low energy interface (11 2 ¯ 0)Q//(510)Al observed in needle shaped Q-precipitate, we survey various terminations and orientations and derive a low-energy interfacial structure. We find this low-energy interfacial model has Cu atoms nearest to the interface, which is in agreement with previous literature on Cu interfacial segregation at the Q′//α-Al interface. The computed interfacial energy (0.52 J/m2) and the corresponding structure will be useful input to future multi-scale modeling of microstructural evolution.

We propose a general theory of Ostwald ripening for prolate spheroidal particles in a nonideal nondilute multicomponent alloy. The diffusion problem of a growing or shrinking particle is solved using prolate spheroidal coordinates under the assumption that the spheroidal particle has a constant Wulff shape. The result shows that the diffusional growth rate increases with an increasing particle aspect ratio due to the increased surface area per volume. The anisotropic interfacial energy necessary to guarantee that the particles are always prolate spheroids with a given aspect ratio is also determined. We find that the chemical potential decreases with an increasing particle aspect ratio under a constant volume-equivalent radius. Based on the two correction factors, asymptotic analysis reveals that the temporal exponents for the coarsening laws for spheroid particles are identical to that for spherical particles. However, as the aspect ratio increases the amplitudes of the temporal power laws of the average equivalent radius, the matrix supersaturations, and the particle composition decrease, whereas the amplitude of the number of particles per volume increases. It is also shown that the particle shape anisotropy affects the amplitudes, but not the direction of the vector representing the matrix supersaturation and particle composition.

Aluminum-lithium-copper alloys have a low density, high elastic modulus and high specific strength. Due to this combination of properties, alloys strengthened with the ternary (Al-Li-Cu) T 1 phase have attracted a great deal of interest especially in aerospace applications. Determining the atomic structural information of the precipitate is a fundamental step in developing a basis for advanced alloy design; however, even though many experimental studies have addressed the T 1 crystal structure, it remains the subject of some controversy. Here, we use density functional theory (DFT) calculations to investigate the structure and composition of the T 1 phase by comparing the energetic stability of five previously-proposed models of the crystal structure of T 1 . The DFT formation energy of these proposed T 1 crystal structures was calculated using a special quasi-random structure (SQS) approach to describe a disordered Al-Cu sub-lattice. In conflict with the experimental phase diagram, DFT calculations of all five proposed models result in an energetically unstable T 1 phase. We search for a new, lower-energy structure of T 1 using a cluster expansion approach, and find a new structural model with DFT energy that is stable (at T = 0 K), i.e., on the calculated convex hull of the Al-Li-Cu ternary system. However, this new predicted phase does not have a tie-line with Al, but the formation energy of the phase is very close to the energy required to make a tie-line with Al ( Δ E = 0.013 eV/atom), which could be affected by finite temperature entropic effects (i.e., vibrational entropic stabilization).

We recently proposed a new, low-energy atomic structure of the Al2CuLi (T1) phase using first-principles calculations based on density functional theory [Acta Mater. 145 (2018) 337-346]. Here, we refine the Li positions in the proposed T1 structure and find that the T1 phase has a tie-line with Al at finite-temperature by vibrational entropic stabilization on the Gibbs triangle of the ternary Al-Cu-Li system. We also derive a low-energy T1/Al coherent interfacial structure and find that Ag and Mg solutes are predicted to segregate to the coherent interface, which is in agreement with recent experimental observations by atom probe tomography.

We examine the equilibrium morphology of Al 2 Cu ( θ ′) precipitates in Al-Cu alloys using a phase field method with the parameters supplied by first-principles density functional theory (DFT) calculations. The phase field method employed allows for an interfacial energy that is highly anisotropic: there are missing high-energy orientations and corners on the Wulff shape. This high degree of anisotropic interfacial energy yields a plate-shaped equilibrium θ ′ precipitate in two-dimensions and a disk-like shape in three dimensions. Also, we consider the effects of a mismatch in elastic-moduli (elastic inhomogeneity) of Al and θ ′, elastic anisotropy, as well as tetragonal misfit strain anisotropy to gain a fuller picture of the elastic energy contributions to the morphology of θ ′ precipitates. Based on our phase-field modeling, the results show that the aspect ratio of the precipitate morphology with the anisotropy of interfacial and strain energies as given by DFT is significantly smaller than the aspect ratio observed in the experiment after long aging times (∼50 h). Specifically, the computed length (54 nm) is almost ten-times smaller than the length (∼580 nm) observed in the Al-Cu experiment with similar precipitate thickness (∼10 nm). Thus, we conclude that the experimental morphology after long aging times (∼50 h) is strongly influenced by the kinetics of precipitate growth.

Q-phase precipitation strengthened aluminum alloys have increasing interest in the automotive sector as a means of enabling higher operating temperatures and improved fuel economy. To enable the optimized design of Q-phase strengthened alloys, knowledge of precipitate composition variations is crucial. Here we use atom-probe tomography (APT) and transmission electron microscopy (TEM) to uncover the near equilibrium compositions of Q-phase precipitates in nine Al-Si-Mg-(Cu) alloys aged for 24 h–1000 h at 473 K. The composition of the Q-phase has been found to vary with bulk alloy composition, particularly in regards to changes in both Cu and Mg content. This composition variation is in contradiction to the stoichiometric line compound Al5Cu2Mg8Si6 typically found in commercially available CALPHAD databases. Our results reveal a common compositional connection between the rod-like Q-phase precipitates and the B′ variant of the β′-phase commonly found in the Cu free Al-Si-Mg system. The presence of small alloying additions of Cu determines stability of either the hexagonal Q-phase or cubic β-phase upon aging to equilibrium conditions. The energetics of Cu variation within the Q-phase has been evaluated using first-principles calculations and demonstrates that interstitial Cu atoms stabilize the metastable B′-phase and that the observed Q-phase composition variation from the ground-state is due to off-stoichiometries in the single-phase field. Our analysis provide new insight into the compositional variation of precipitates formed during aging, having implications on the intended application of the alloy, whether for room temperature applications or castings that may see elevated temperature exposure.

The kinetics of oxidation is examined using a phase-field model of electrochemistry when the oxide film is smaller than the Debye length. As a test of the model, the phase-field approach recovers the results of classical Wagner diffusion-controlled oxide growth when the interfacial mobility of the oxide-metal interface is large and the films are much thicker than the Debye length. However, for small interfacial mobilities, where the growth is reaction controlled, we find that the film increases in thickness linearly in time, and that the phase-field model naturally leads to an electrostatic overpotential at the interface that affects the prefactor of the linear growth law. Since the interface velocity decreases with the distance from the oxide vapor, for a fixed interfacial mobility, the film will transition from reaction- to diffusion-controlled growth at a characteristic thickness. For thin films, we find that in the limit of high interfacial mobility we recover a Wagner-type parabolic growth law in the limit of a composition-independent mobility. A composition-dependent mobility leads to a nonparabolic kinetics at small thickness, but for the materials parameters chosen, the deviation from parabolic kinetics is small. Unlike classical oxidation models, we show that the phase-field model can be used to examine the dynamics of nonplanar oxide interfaces that are routinely observed in experiment. As an illustration, we examine the evolution of nonplanar interfaces when the oxide is growing only by anion diffusion and find that it is morphologically stable.

Discovering novel, multicomponent crystalline materials is a complex task owing to the large space of feasible structures. Here we demonstrate a method to significantly accelerate materials discovery by using a machine learning (ML) model trained on density functional theory (DFT) data from the Open Quantum Materials Database (OQMD). Our ML model predicts the stability of a material based on its crystal structure and chemical composition, and we illustrate the effectiveness of the method by application to finding new quaternary Heusler (QH) compounds. Our ML-based approach can find new stable materials at a rate 30 times faster than undirected searches and we use it to predict 55 previously unknown, stable QH compounds. We find the accuracy of our ML model is higher when trained using the diversity of crystal structures available in the OQMD than when training on well-curated datasets which contain only a single family of crystal structures (i.e., QHs). The advantage of using diverse training data shows how large datasets, such as OQMD, are particularly valuable for materials discovery and that we need not train separate ML models to predict each different family of crystal structures. Compared to other proposed ML approaches, we find that our method performs best for small $(l{10}^{3})$ and large $(g{10}^{5})$ training set sizes. The excellent flexibility and accuracy of the approach presented here can be easily generalized to other types of crystals.

The “black pad” phenomenon, which refers to the blackening of electroless-plated nickel–phosphorus [Ni(P)] films during the immersion Au process, is reproduced using pure chemicals and its fundamental mechanisms are investigated. In the present analysis, under bump metallurgy (UBM) materials have profound effects on the black pad susceptibility, and the presence of abnormally large nodules (ALNs) is essential to the black pad occurrence. The Ni(P) films over Cu, Ag, and Au substrates all exhibit ALNs and are susceptible to black pads, while those over Ni and Co substrates do not have ALNs and therefore are not susceptible to black pad. In the former cases, submicron scale nodular variations of the surface curvature lead to variations in the P concentration in the Ni(P) films, which induces sufficiently large potential differences to drive galvanic corrosion when exposed to the electrolyte, which is a gold cyanide solution in this study. The UBM effect is ascribed to differences in the Ni(P) film growth mode, where the transition from a layer-by-layer growth mode to an island growth mode is easier over Cu, Ag, and Au UBMs.

The stretch formability of twin-roll cast (TRC) Mg alloys at room temperature has been investigated in relation to their work-hardening capacity. It is shown that there is a linear relationship between stretch formability and the work-hardening capacity, represented by the inverse of the yield ratio. The randomization of texture and the increase in grain size increase the work-hardening exponent, resulting in an improvement in the stretch formability of TRC Mg alloys at room temperature.