Solute strengthening from first principles and application to aluminum alloys

Alloys containing substitutional solutes exhibit strengthening due to favorable solute fluctuations within the alloy that hinder dislocation motion. Here, a quantitative, parameter-free model to predict the flow stress as a function of temperature and strain rate of such alloys is presented. The model builds on analytic concepts developed by Labusch but introduces key innovations rectifying shortcomings of previous models. To accurately describe the solute/dislocation interaction energies in and around the dislocation core, density functional theory and a flexible-boundary-condition method are used. The model then predicts the zero temperature flow stress, the energy barrier for dislocation motion, and thus the finite temperature flow stresses. The model is used to predict the flow stresses of various Al alloys. Excellent results are obtained for Al–Mg and Al–Mn. Al–Fe with ppm levels of Fe is not predicted well but, using experimental results for Fe, results for the quasi-binary Al–Cr–(Fe) and Al–Cu–(Fe) alloys agree well with experiments. The model is also consistent with the “stress equivalency” postulate of Basinski. This parameter-free model using first-principles input thus provides a basis for achieving the long-sought goal of computational design of alloys, within the context of solute-strengthening mechanisms.     

Calculation of solubility in titanium alloys from first principles

We present an approach to calculate the atomic bulk solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model considers all the appropriate ground states of the alloy and results in a simple Arrhenius-type temperature dependence determined by a “low-solubility formation enthalpy”. This quantity, directly obtainable from first principles, is defined as the composition derivative of the compound formation enthalpy with respect to nearby ground states. We apply the framework and calculate the solubility of the A solutes in A–Ti alloys (A = Ag, Au, Cd, Co, Cr, Ir, W, Zn). In addition to determination of unknown low-temperature ground states for the eight alloys, we find qualitative agreements with solubility experimental results. The presented formalism, correct in the low-solubility limit, should be considered as an appropriate starting point for estimation of whether a more computationally expensive formalism is needed.     

铝合金应力／应变时效机制的第一性原理研究

基于第一性原理计算方法探讨了铝合金应力／应变时效的可能机制,综合评估了时效温度和外加应力／应变对Al-Sc合金中Al3Sc固溶边界和A1-Cu合金中Al／θ＇＇界面能的潜在影响。计算结果表明：在传统时效过程中引入外加拉应力／应变,声子态密度在高态区有红移现象,可以明显降低溶解熵;同时,导致相图中固溶线上移,表明外加拉应力／应变可降低Al3-Sc在Al-Sc合金中的极限固溶度,从而增加析出相的最大可能体积分数。外加应力／应变对Al-Cu合金中不同取向的Al／θ＇＇,界面形成能有不同程度的影响,这种差别可以通过泊松效应进一步放大,从而影响到Al-Cu合金中析出相的择优取向。这2种机制在应力／应变时效中均可能发挥重要作用。     

Theory-guided bottom-up design of β-titanium alloys as biomaterials based on first principles calculations: Theory and experiments

In this study we present a new strategy for the theory-guided bottom up design of β-Ti alloys for biomedical applications using a quantum mechanical approach in conjunction with experiments. Parameter-free density functional theory calculations are used to provide theoretical guidance in selecting and optimizing Ti-based alloys with respect to three constraints: (i) the use of non-toxic alloy elements; (ii) the stabilization of the body centered cubic β-phase at room temperature; (iii) the reduction of the elastic stiffness compared to existing Ti-based alloys. Following the theoretical predictions, the alloys of interest are cast and characterized with respect to their crystallographic structure, microstructure, texture, and elastic stiffness. Due to the complexity of the ab initio calculations, the simulations have been focused on a set of binary systems of Ti with two different high melting body-centered cubic metals, namely, Nb and Mo. Various levels of model approximations to describe mechanical and thermodynamic properties are tested and critically evaluated. The experiments are conducted both, on some of the binary alloys and on two more complex engineering alloy variants, namely, Ti–35 wt.% Nb–7 wt.% Zr–5 wt.% Ta and Ti–20 wt.% Mo–7 wt.% Zr–5 wt.% Ta.     

Nanophases of binary and multicomponent alloys

I present a theory which explains the appearance of an amorphous-like, disordered phase in nanoparticles of binary alloys. The theory claims that this phase represents a transition state between two bulk phases of the binary system. While the transition state is completely unstable in the conditions of an open system, where exchange of the species between the particle and its surrounding is not limited, it may become stabilized in the closed system where the species exchange is prohibited. I derive the material-parameters criterion, which is the condition for the stabilization. The transition state represents a shallow minimum of the molar Helmholtz free energy as a function of the order parameter and is not significant in a large system because the global optimum there is delivered by the heterogeneous mixture of the two bulk phases connected by the common-tangent construction. However, in a particle of the size below the critical, the transition state becomes the global optimizer because of the prohibitively large energy “cost” of the phase separating interface. Thus the theory explains the effect as due to the free energy of the phase separating interface, not the energy of the free surface of the particle. The theory sheds light on the structure of the critical nuclei in the process of nucleation of one phase from another one. New experiments to verify the theory are suggested.     

Grain-boundary segregation in binary alloys

Results of experimental studies of grain-boundary segregation in binary alloys have been considered and analyzed. The model of grain-boundary segregation and an isotherm that allows for the possibility of the formation of complexes in grain boundaries have been suggested.     

Microstructure and properties of Mg-Al binary alloys

O ver the past decade, the large-scale development and application of magnesium alloys have been greatly stimulated by the demand for lightweight and environment- friendly materials in the automotive industry [1-4]. Most commercial casting magnesium alloys are mainly Mg-Al alloys, such as AZ91D, AM60B, AM50A, AE42, and AS21 alloys. Since AZ91D, AM60B and AM50A alloys contain relatively high Al content they offer a good combination of mechanical properties, corrosion resistance and ca…     

Electronic structure and properties of LaNi_5 compound from first principles

The energy,electronic structure,and magnetic and mechanical properties of LaNi5 compound have been studied by the first-principles method based on the density functional theory.The results show that the calculated lattice parameters of LaNi5 compound are almost the same with the experimental ones,and the compound is easy to form and very stable.The bond between La and Ni1 atoms is an ionic bond,and electrostatic attraction effect exists between Ni atoms.The farther the distance between Ni atoms in the LaNi5 crystal cell is,the stronger the electrostatic attraction effect is.LaNi5 intermetallic compound is ductile and ferromagnetic.The calculated hardness value of LaNi5 compound is 7.04 GPa,and the calculated elastic properties are close to the experimental results.     

Microstructure and hardness of binary Cr-Ta alloys

The dependances of the microstructure and hardness of the binary Cr-Ta alloys [ Cr-9.0, -9.2, -9.4, -9.6,-9.8, and -13.0 Ta] (mole fraction, %) were investigated. When Ta content of the alloy is less than 9.4%, there are primary dandrite grains of a Cr solid-solution phase existing in the matrix of Cr-Cr2Ta eutectic colonies in the alloy. Moreover, the regular polygon grains of the primary Cr2Ta Laves-phase are surrounded by the Cr-Cr2Ta eutectic colony in the hyper-euteetic Cr-9.4 96 Ta alloys. The scanning electron microphotograph shows that one of the Cr2Ta phase plates of an eutectic colony always connects with the primary Cr2Ta Laves-phase grain in a hyper-eutectic alloy. The eutectic colony size of Cr-Ta alloys decreases with increasing Ta. In addition, the macrohardness of Cr-Ta alloys is influanced by the chemical composition at room temperature. The binary eutectic Cr-Ta alloy presents the lowest hardness on a macrohardness scale.     

Calculated phase diagrams of aluminum alloys from binary Al–Cu to multicomponent commercial alloys

More than three decades have passed since the publication of Alan Prince’s book on multicomponent phase equilibria. The most significant development in this time has been the use of a combined computational/experimental approach to calculate multicomponent phase diagrams. This has led to important advances in the design and processing of structural and functional materials for practical applications. In this paper, we present a few examples focusing on aluminum alloys from the classical Al–Cu binary to multicomponent alloys with a view toward practical applications.     

Can the Tafel equation be derived from first principles?

A century ago, Tafel disapproved the attempts to derive the empirical equation named after him by thermodynamic methods. He noted that his observations referred to irreversible electrochemical reactions, where thermodynamics is inapplicable. This statement seems to remain valid until today. Indeed, it is impossible as yet to predict the kinetic parameters for chemical processes by determining rate constants and reaction orders from “first principles”, unless strictly specialized and, to a great extent, artificial models are developed.Nevertheless, in this paper an attempt to derive the kinetic law of mass action from “first principles” is made in macroscopic formulation. It has turned out to be possible owing to the methods of thermodynamics of irreversible processes that were unknown in Tafel’s time.     

Some properties of binary vanadium alloys

Conclusions Increasing the concentration of the alloying element in binary alloys of vanadium with titanium, chromium, tin, and aluminum at concentrations of 0–25 wt. % leads to an increase of hardness and electrical resistivity, particularly for alloys with tin and aluminum. The strength will be highest for alloys of vanadium with tin.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 62–63, July, 1971.     

The oxidation of binary iron-manganese alloys

Oxide scales formed on iron-manganese alloys are generally considered to be similar to those formed on the parent metals. Metallographic examination is used in conjunction with EMPA, XRD, and X-ray imaging to reveal the presence of the additional phases Mn₂O₃ and Mn₃O₄ in the scales formed on binary alloys containing up to 40% manganese at 700 and 800°C in 200 Torr oxygen. A mechanism is proposed to explain the apparent change in growth from iron-rich to manganese-rich scales during oxidation of Fe-40Mn at 800°C.     

Electronic and magnetic properties of Cd-doped zigzag AlN nanoribbons from first principles

The effect of Cd impurity on the electronic structure and magnetic properties of hydrogen-terminated AlN nanoribbons with zigzag edges (ZAlNNRs) was investigate using the band structure results obtained through the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT). The exchange correlation potential was treated by the generalized gradient approximation within the Perdew scheme. The calculated results show that the H-terminated zigzag AlN nanoribbon is semiconducting and nonmagnetic material with a direct band gap of about 2.78 eV, while the Cd-doped H-terminated ZAlNNR structures show complete (100 %) spin polarization very close to the Fermi level, which will result in spin-anisotropic transport. The charge transport is totally dominated by Cd spin down electrons in the H-terminated ZAlNNR. These results suggest potential applications for the development of using the AlN nanoribbons in nanoelectronics and magnetoelectronic devices as a base.