Louis G. Hector

First-principles data for solid-solution strengthening of magnesium: From geometry and chemistry to properties

Abstract Solid-solution strengthening results from solutes impeding the glide of dislocations. Existing theories of strength rely on solute/dislocation interactions, but do not consider dislocation core structures, which need an accurate treatment of chemical bonding. Here, we focus on strengthening of Mg, the lightest of all structural metals and a promising replacement for heavier steel and aluminum alloys. Elasticity theory, which is commonly used to predict the requisite solute/dislocation interaction energetics, is replaced with quantum-mechanical first-principles calculations to construct a predictive mesoscale model for solute strengthening of Mg. Results for 29 different solutes are displayed in a “strengthening design map” as a function of solute misfits that quantify volumetric strain and slip effects. Our strengthening model is validated with available experimental data for several solutes, including Al and Zn, the two most common solutes in Mg. These new results highlight the ability of quantum-mechanical first-principles calculations to predict complex material properties such as strength.

Threefold Increase in the Young’s Modulus of Graphite Negative Electrode during Lithium Intercalation

Density functional theory (DFT) is used to reveal that the polycrystalline Youngs modulus (E) of graphite triples as it is lithiated to LiC 6 . This behavior is captured in a linear relationship between E and lithium concentration suitable for continuum-scale models aimed at predicting diffusion-induced deformation in battery electrode materials. Alternatively, Poissons ratio is concentration-independent. Charge-transfer analyses suggest simultaneous weakening of carbon-carbon bonds within graphite basal planes and strengthening of interlayer bonding during lithiation. The variation in bond strength is shown to be responsible for the differences between elasticity tensor components, C ij , of lithium-graphite intercalation (Li-GIC) phases. Strain accumulation during Li intercalation and deintercalation is examined with a core-shell model of a Li-GIC particle assuming two coexisting phases. The requisite force equilibrium uses different Youngs moduli computed with DFT. Lithium-poor phases develop tensile strains, whereas Li-rich phases develop compressive strains. Results from the core-shell model suggest that elastic strain should be defined relative to the newest phase that forms during lithiation of graphite, and Li concentration-dependent mechanical properties should be considered in continuum level models.

Adhesion, stability, and bonding at metal/metal-carbide interfaces: Al/WC

We examine the relative stability and adhesion of the polar Al(1 1 1)/WC(0 0 0 1) interface using density functional theory. Relaxed atomic geometries and the ideal work of adhesion were calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. The interfacial electronic structure was analyzed to determine the nature of metal/carbide bonding. Based on the surface and interfacial free energies, we find that both the clean WC(0 0 0 1) surface and the optimal interface geometry are W-terminated. Although both terminations yield substantial adhesion energies in the range 4–6 J/m 2 , bonding at the optimal C-terminated structure is nearly 2 J/m 2 stronger, consistent with an argument based on surface reactivity. In addition, we examine the effects of Li and Mg alloying elements at the interface, and find that they result in a strain-induced reduction of metal–ceramic adhesion. 2001 Elsevier Science B.V. All rights reserved.

Basal and prism dislocation cores in magnesium: comparison of first-principles and embedded-atom-potential methods predictions

A binary embedded-atom method (EAM) potential is optimized for Cu on Ag(111) by fitting to ab initio data. The fitting database consists of DFT calculations of Cu monomers and dimers on Ag(111), specifically their relative energies, adatom heights, and dimer separations. We start from the Mishin Cu-Ag EAM potential and first modify the Cu-Ag pair potential to match the FCC/HCP site energy difference then include Cu-Cu pair potential optimization for the entire database. The optimized EAM potential reproduce DFT monomer and dimer relative energies and geometries correctly. In trimer calculations, the potential produces the DFT relative energy between FCC and HCP trimers, though a different ground state is predicted. We use the optimized potential to calculate diffusion barriers for Cu monomers, dimers, and trimers. The predicted monomer barrier is the same as DFT, while experimental barriers for monomers and dimers are both lower than predicted here. We attribute the difference with experiment to the overestimation of surface adsorption energies by DFT and a simple correction is presented. Our results show that the optimized Cu-Ag EAM can be applied in the study of larger Cu islands on Ag(111).The core structures of screw and edge dislocations on the basal and prism planes in Mg, and the associated gamma surfaces, were studied using an ab initio method and the embedded-atom-method interatomic potentials developed by Sun et al and Liu et al. The ab initio calculations predict that the basal plane dislocations dissociate into partials split by 16.7 angstrom (edge) and 6.3 angstrom (screw), as compared with 14.3 angstrom and 12.7 angstrom (Sun and Liu edge), and 6.3 angstrom and 1.4 angstrom (Sun and Liu screw), with the Liu screw dislocation being metastable. In the prism plane, the screw and edge cores are compact and the edge core structures are all similar, while ab initio does not predict a stable prismatic screw in stress-free conditions. These results are qualitatively understood through an examination of the gamma surfaces for interplanar sliding on the basal and prism planes. The Peierls stresses at T = 0K for basal slip are a few megapascals for the Sun potential, in agreement with experiments, but are ten times larger for the Liu potential. The Peierls stresses for prism slip are 10-40MPa for both potentials. Overall, the dislocation core structures from ab initio are well represented by the Sun potential in all cases while the Liu potential shows some notable differences. These results suggest that the Sun potential is preferable for studying other dislocations in Mg, particularly the textless c + a textgreater dislocations, for which the core structures are much larger and not accessible by ab initio methods.

Quantitative prediction of solute strengthening in aluminium alloys

Despite significant advances in computational materials science, a quantitative, parameter-free prediction of the mechanical properties of alloys has been difficult to achieve from first principles. Here, we present a new analytic theory that, with input from first-principles calculations, is able to predict the strengthening of aluminium by substitutional solute atoms. Solute-dislocation interaction energies in and around the dislocation core are first calculated using density functional theory and a flexible-boundary-condition method. An analytic model for the strength, or stress to move a dislocation, owing to the random field of solutes, is then presented. The theory, which has no adjustable parameters and is extendable to other metallic alloys, predicts both the energy barriers to dislocation motion and the zero-temperature flow stress, allowing for predictions of finite-temperature flow stresses. Quantitative comparisons with experimental flow stresses at temperature T=78 K are made for Al-X alloys (X=Mg, Si, Cu, Cr) and good agreement is obtained.

First-principles study of metal-carbide/nitride adhesion: Al/VC vs. Al/VN

Abstract We have performed density functional calculations to investigate the adhesion and electronic structure at interfaces between Al and the refractory transition metal nitrides/carbides VN and VC in order to understand the significance of the ceramics metalloid component upon interfacial properties. We find that for both systems the preferred bonding site places the metal interfacial atoms above the ceramics metalloid atoms, and that adhesion energies are comparable to those found for other metals (Ti, Ag) on MgO. The differences in magnitude and rank-ordering of the adhesion energies for the two interfaces are rationalized in terms of the the surface energies of the ceramics. Analysis of the charge density and density of states reveals that covalent Al–C/N bonds constitute the dominant metal–ceramic interaction.

Electronic structure calculations for LaNi5 and LaNi5H7: energetics and elastic properties

Abstract Density functional calculations of the electronic structure and enthalpy of formation Δ H of LaNi 5 and LaNi 5 H 7 are reported. Single-crystal elastic constants and Voigt–Reuss–Hill polycrystalline moduli were calculated for both materials using a stress-based least-squares fitting methodology. We obtain Δ H (0 K)=−40 kJ/mol H 2 for the hydride at zero temperature. Incorporating a Debye estimate of the phonon contribution we find Δ H (298 K)∼−39 kJ/mol/H 2 , a value that compares favorably with experimental determinations of −32 to −35 kJ/mol/H 2 . Our results indicate that the H–Ni and H–La interactions in the hydride are primarily metallic with a small ionic component. The calculated elastic moduli are in excellent accord with single-crystal measurements on LaNi 5 and with available data for polycrystalline samples of the parent and hydride.

Tensile-Shear Forces and Fracture Modes in Single and Multiple Weld Specimens in Dual-Phase Steels

In this article, weld fracture criteria based upon low strain rate (i.e., e ∼ 10 -3 -10 -2 s -1 ) tensile-shear tests of spot welds in dual-phase (DP) steels DP600, DP780, and DP980 are developed. Three empirical equations are inferred from least-squares root-fitting analyses of tensile-shear testing data. Building upon existing results in the literature, the first equation relates the tensile-shear force to the weld diameter. The second and third equations relate, respectively, a critical weld diameter and a critical tensile-shear force for interfacial fracture to the sheet thickness and hardness extrema in the heat-affected zone. These idealized equations can serve as the basis for further development of fracture criteria resembling material flow laws that account for higher strain rates and more complicated deformation paths. The effect of spot-weld placement in specific patterns or arrays on deformation and fracture behavior was also investigated to explore underlying effects from deformation field interactions between adjacent spot welds.

Tool Surface Topographies for Controlling Friction and Wear in Metal-Forming Processes

A conceptual framework is introduced for the design of tool surface topographies in bulk metal forming processes. The objective of the design is to control friction to desired levels while minimizing wear of the workpiece and tool surfaces and adhesive metal transfer between the workpiece and tool. Central to the design framework are the tool/workpiece interface properties of lubricant retention and interface permeability. Lubricant retention refers to the capacity of an interface to retain lubricant rather than freely channel it to the exterior of the tool/workpiece conjunction. Permeability refers to the capacity to distribute lubricant to all areas within the conjunction. These properties lead to the concept of two-scale surface topography consisting of a fine scale background of interconnected channels on which is superimposed an array of coarser-scale cavities. Control of friction and wear is achieved by designing the tool surface topographies at these two scales to address the unique tribological conditions of specific bulk metal forming processes. The coarser scale is designed to ensure adequate supply of lubricant within the conjunction. The finer scale is designed to ensure adequate delivery of lubricant to all parts of the conjunction where nascent workpiece surface is being formed. The design concepts are illustrated with results from laboratory experiments using the rolling process as an example, and comparing the performance of various roll surface topographies under similar processing conditions. A two-scale surface topography consisting of hemispherical cavities distributed across a background surface of finer scale, interconnected channels was shown to reduce friction compared to a single-scale ground finish, but not as much as a single-scale coarse topography consisting of densely-packed cavities produced by an electrical discharge treatment. On the other hand, the smoother cross-sections of the cavities, especially when elongated in the direction of greatest relative motion, produced significantly less wear than either of the single-scale tool surface treatments. It is concluded that two-scale engineering of tool surface topographies based upon the concepts of lubricant retention and interface permeability can provide a broad basis for achieving desired levels of interface friction while minimizing workpiece surface wear and adhesive material transfer in many metal-forming processes.

Density functional theory for hydrogen storage materials: successes and opportunities

Solid state systems for hydrogen storage continue to be the focus of considerable international research, driven to a large extent by technological demands, especially for mobile applications. Density functional theory (DFT) has become a valuable tool in this effort. It has greatly expanded our understanding of the properties of known hydrides, including electronic structure, hydrogen bonding character, enthalpy of formation, elastic behavior, and vibrational energetics. Moreover, DFT holds substantial promise for guiding the discovery of new materials. In this paper we discuss, within the context of results from our own work, some successes and a few shortcomings of state-of-the-art DFT as applied to hydrogen storage materials.