On the Electronic Structure and Thermodynamics of Alloys

A free energy formalism is developed in order to describe phase equilibria in binary alloys. The proposed phenomenological approach uses a limited number of experimental data to provide a global thermodynamic description of a system including its equilibrium and metastable phase diagrams. Emphasis is placed on the description of short range order by means of the cluster variation method. A microscopic theory is also developed in order to predict the enthalpies of formation of transition metal alloys as well as the short range order dependence of these enthalpies. The theory uses a tight-binding Hamiltonian together with the generalized perturbation method. 0ff~diagonal disorder is taken into account, and charge transfer is treated self consistently in the random alloy. All input parameters to the theory are obtained from ab-initio calculations for the pure elements. In this regard, the model can be considered parameter free. The phenomenological approach has been used to analyzed the Al-Ni, Ni~Cr, and Al~Li systems. It is found that the vibrational entropy of‘ formation plays an important role in the thermodynamics of the Al-Li and Ni-Cr alloys. The approach allows an accurate description of stable and metastable order-disorder or order-order equilibria existing in the Ni-Al or AL~Li systems. The model is used to predict a metastable clustering tendency in Al-Li alloys which appears to have been recently confirmed by experiment. The microscopic theory has been applied to the VB-VIE and IVB-VIIIB (Ni, Pt, Pd) alloys. The calculations are in good agreement with the available experimental data and phase diagram information. It is shown that off-diagonal disorder and electronic self-consistency play a crucial role in the accuracy of the results

On the Electronic Structure and Thermodynamics of Alloys

A free energy formalism is developed in order to describe phase equilibria in binary alloys. The proposed phenomenological approach uses a limited number of experimental data to provide a global thermodynamic description of a system including its equilibrium and metastable phase diagrams. Emphasis is placed on the description of short range order by means of the cluster variation method. A microscopic theory is also developed in order to predict the enthalpies of formation of transition metal alloys as well as the short range order dependence of these enthalpies. The theory uses a tight-binding Hamiltonian together with the generalized perturbation method. 0ff~diagonal disorder is taken into account, and charge transfer is treated self consistently in the random alloy. All input parameters to the theory are obtained from ab-initio calculations for the pure elements. In this regard, the model can be considered parameter free. The phenomenological approach has been used to analyzed the Al-Ni, Ni~Cr, and Al~Li systems. It is found that the vibrational entropy of‘ formation plays an important role in the thermodynamics of the Al-Li and Ni-Cr alloys. The approach allows an accurate description of stable and metastable order-disorder or order-order equilibria existing in the Ni-Al or AL~Li systems. The model is used to predict a metastable clustering tendency in Al-Li alloys which appears to have been recently confirmed by experiment. The microscopic theory has been applied to the VB-VIE and IVB-VIIIB (Ni, Pt, Pd) alloys. The calculations are in good agreement with the available experimental data and phase diagram information. It is shown that off-diagonal disorder and electronic self-consistency play a crucial role in the accuracy of the results

Extracting free energy of clusters in concentrated binary alloys from atomistic Monte Carlo simulations

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Precipitation in Al-Zr-Sc alloys: a comparison between kinetic Monte Carlo, cluster dynamics and classical nucleation theory

Zr and Sc precipitate in aluminum alloys to form the Al\_3Zr\_xSc\_{1-x} compound which, for low supersaturations of the solid solution, exhibits the L1\_2 structure. The aim of the present study is to model at an atomic scale the kinetics of precipitation and to build mesoscopic models so as to extend the range of supersaturations and annealing times that can be simulated up to values of practical interest. In this purpose, we use some ab initio calculations and experimental data to fit an Ising type model describing thermodynamics of the Al-Zr-Sc system. Kinetics of precipitation are studied with a kinetic Monte Carlo algorithm based on an atom-vacancy exchange mechanism. Cluster dynamics is then used to model at a mesoscopic scale all the different stages of homogeneous precipitation in the two binary Al-Zr and Al-Sc alloys. This technique correctly manages to reproduce both the kinetics of precipitation simulated with kinetic Monte Carlo as well as experimental observations. Focusing on the nucleation stage, it is shown that classical theory well applies as long as the short range order tendency of the system is considered. This allows us to propose an extension of classical nucleation theory for the ternary Al-Zr-Sc alloy.Comment: submitted for publication in "Solid-Solid Phase Transformations in Inorganic Materials", edited by TMS, 200

China’s “Going Global” Policy in Africa: An Exploratory Overview of an Evolving Policy Framework

The pace of economic engagement between China and Africa has increased exponentially in recent years, fueled mostly by burgeoning trade and investment ties. This impressive transformation of Sino-African relations has been complimented significantly by China’s “Going Global” policy, resulting in a huge number and diversity of Chinese multinational companies on the African continent. This proliferation of Chinese companies in Africa has generated new opportunities and prospects for all stakeholders, but has also engendered a host of challenges. This has no doubt had a significant impact on both the making and shaping of China’s foreign policy in Africa, and subsequently China Africa relations as a whole. This study relies on an extensive review of available qualitative and quantitative data to gain insights into the complex institutional and operational framework behind Chinese government support for the international ambitions of Chinese companies in Africa. What are the motives behind Beijing’s “Going Global” Policy? Which key institutions are involved, and what policy tools are employed to encourage the overseas investments of Chinese companies? What are the key drivers behind Chinese companies’ foray into the African market and what are the implications for China Africa relations, both in the short-term and long-term? These and other essential questions are addressed in this paper

Advances in Microstructural Understanding of Wrought Aluminum Alloys

From Springer Nature via Jisc Publications RouterHistory: received 2020-03-12, registration 2020-06-24, online 2020-07-08, pub-electronic 2020-07-08, pub-print 2020-09Publication status: PublishedAbstract: Wrought aluminum alloys are an attractive option in the quest for lightweight, recyclable, structural materials. Modern wrought aluminum alloys depend on control of complex microstructures to obtain their properties. This requires an understanding of the coupling between alloy composition, processing, and microstructure. This paper summarizes recent work to understand microstructural evolution in such alloys, utilizing the advanced characterization techniques now available such as atom probe tomography, high-resolution electron microscopy, and synchrotron X-ray diffraction and scattering. New insights into precipitation processes, deformation behavior, and texture evolution are discussed. Recent progress in predicting microstructural evolution using computer modeling is also summarized

Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory

Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and Al3Sc which for low supersaturations of the solid solution have the L12 structure. The aim of the present study is to model at an atomic scale this kinetics of precipitation and to build a mesoscopic model based on classical nucleation theory so as to extend the field of supersaturations and annealing times that can be simulated. We use some ab-initio calculations and experimental data to fit an Ising model describing thermodynamics of the Al-Zr and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy exchange mechanism. This allows us to simulate with a kinetic Monte Carlo algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then used to test the classical nucleation theory. In this purpose, we deduce from our atomic model an isotropic interface free energy which is consistent with the one deduced from experimental kinetics and a nucleation free energy. We test di erent mean-field approximations (Bragg-Williams approximation as well as Cluster Variation Method) for these parameters. The classical nucleation theory is coherent with the kinetic Monte Carlo simulations only when CVM is used: it manages to reproduce the cluster size distribution in the metastable solid solution and its evolution as well as the steady-state nucleation rate. We also find that the capillary approximation used in the classical nucleation theory works surprisingly well when compared to a direct calculation of the free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004

First-principle solubilities of alkali and alkaline earth metals in Mg-B alloys

By devising a novel framework, we present a comprehensive theoretical study of solubilities of alkali (Li, Na, K, Rb, Cs) and alkaline earth (Be, Ca, Sr, Ba) metals in the he boron-rich Mg-B system. The study is based on first-principle calculations of solutes formation energies in MgB$_2$, MgB$_4$, MgB$_7$ alloys and subsequent statistical-thermodynamical evaluation of solubilities. The advantage of the approach consists in considering all the known phase boundaries in the ternary phase diagram. Substitutional Na, Ca, and Li demonstrate the largest solubilities, and Na has the highest (0.5-1 % in MgB$_7$ at $T=650-1000$ K). All the considered interstitials have negligible solubilities. The solubility of Be in MgB$_7$ can not be determined because the corresponding low-solubility formation energy is negative indicating the existence of an unknown ternary ground state. We have performed a high-throughput search of ground states in binary Mg-B, Mg-$A$, and B-$A$ systems, and we construct the ternary phase diagrams of Mg-B-$A$ alloys based on the stable binary phases. Despite its high temperature observations, we find that Sr$_{9}$Mg$_{38}$ is not a low-temperature equilibrium structure. We also determine two new possible ground states CaB$_{4}$ and RbB$_{4}$, not yet observed experimentally.Comment: 5 figure