The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics

A longstanding limitation of first-principles calculations of substitutional alloy phase diagrams is the difficulty to account for lattice vibrations. A survey of the theoretical and experimental literature seeking to quantify the impact of lattice vibrations on phase stability indicates that this effect can be substantial. Typical vibrational entropy differences between phases are of the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of configurational entropy differences in binary alloys (at most 0.693 k_B/atom). This paper describes the basic formalism underlying ab initio phase diagram calculations, along with the generalization required to account for lattice vibrations. We overview the various techniques allowing the theoretical calculation and the experimental determination of phonon dispersion curves and related thermodynamic quantities, such as vibrational entropy or free energy. A clear picture of the origin of vibrational entropy differences between phases in an alloy system is presented that goes beyond the traditional bond counting and volume change arguments. Vibrational entropy change can be attributed to the changes in chemical bond stiffness associated with the changes in bond length that take place during a phase transformation. This so-called ``bond stiffness vs. bond length'' interpretation both summarizes the key phenomenon driving vibrational entropy changes and provides a practical tool to model them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure

A Cluster-Based Computational Thermodynamics Framework with Intrinsic Chemical Short-Range Order: Part I. Configurational Contribution

Exploiting chemical short-range order (SRO) is a promising new avenue for manipulating the properties of alloys. However, existing modeling frameworks are not sufficient to understand and predict SRO in multicomponent (>3) alloys. In this work, we developed a hybrid computational thermodynamics framework by marrying unique advantages from CVM (Cluster Variation Method) and CALPHAD (CALculation of PHAse Diagram) method through incorporating chemical SRO into CALPHAD with a novel cluster-based solution model. The key is to use the Fowler-Yang-Li transform to decompose the cumbersome cluster chemical potentials in CVM into fewer site chemical potentials of the basis cluster, thereby considerably reducing the number of variables that must be minimized for multicomponent systems. The new framework puts more physics, primarily intrinsic SRO, into CALPHAD, while maintaining its practicality and efficiency. It leverages statistical mechanics to yield a more physical description of configurational entropy and opens the door to cluster-based CALPHAD database development. The application of this newly proposed model in the prototype FCC AB system demonstrated that this model can correctly capture the essential features of the phase diagram and thermodynamic properties. The hybrid CVM-CALPHAD framework represents a new methodology for thermodynamic modeling that enables atomic-scale order to be exploited as a dimension for materials design, which potentially leads to novel complex concentrated alloys. It achieves a balance between the accuracy and computational cost for modeling multicomponent alloys with the intrinsic SRO in the context of CALPHAD

Roadmap on multiscale materials modeling

Modeling and simulation is transforming modern materials science, becoming an important tool for the discovery of new materials and material phenomena, for gaining insight into the processes that govern materials behavior, and, increasingly, for quantitative predictions that can be used as part of a design tool in full partnership with experimental synthesis and characterization. Modeling and simulation is the essential bridge from good science to good engineering, spanning from fundamental understanding of materials behavior to deliberate design of new materials technologies leveraging new properties and processes. This Roadmap presents a broad overview of the extensive impact computational modeling has had in materials science in the past few decades, and offers focused perspectives on where the path forward lies as this rapidly expanding field evolves to meet the challenges of the next few decades. The Roadmap offers perspectives on advances within disciplines as diverse as phase field methods to model mesoscale behavior and molecular dynamics methods to deduce the fundamental atomic-scale dynamical processes governing materials response, to the challenges involved in the interdisciplinary research that tackles complex materials problems where the governing phenomena span different scales of materials behavior requiring multiscale approaches. The shift from understanding fundamental materials behavior to development of quantitative approaches to explain and predict experimental observations requires advances in the methods and practice in simulations for reproducibility and reliability, and interacting with a computational ecosystem that integrates new theory development, innovative applications, and an increasingly integrated software and computational infrastructure that takes advantage of the increasingly powerful computational methods and computing hardware

Precious metal based bulk glass-forming liquids : development, thermodynamics, kinetics and structure

Bulk glass-forming alloy compositions are found in various noble-metal based systems. Their superior properties and their high noble metal content predestinate these alloy classes for the application as jewelry alloys. This work is separated into two parts and focuses on the gold- and platinum-based bulk glass-forming liquids. In the case of the 18 karat white gold bulk glass-forming alloy composition Au49- Ag5.5Pd2.3Cu26.9Si16.3, a fast color change is observed which is attributed to a corrosion process when worn on human skin. A strategy to modify the alloy composition was developed and evaluated that on the one hand allows the improvement of the corrosion resistance and on the other hand maintains the ability of the liquid to form a bulk metallic glass. The second part of this work comprises the evaluation of the thermophysical properties of platinum-phosphorous-based glass-forming liquids. The thermophysical properties are used in combination with high- and lowtemperature crystallization experiments to determine the origin of the high glassforming ability. Moreover, the observed differences within the platinum-phosphorous class as well as those to the compositionally related palladium-phosphorous alloys are investigated in in-situ synchrotron X-ray scattering experiments. The results suggest that the temperature dependence of the atomic dynamics in platinum-phosphorous- based liquids is reflected by structural ordering processes on certain length scales. The differences in the representative structural units and their connection scheme between platinum-phosphorous- and palladium-phosphorous-based liquids might be responsible for the different sensitivity to annealing induced embrittlement which was already reported in literature.Massivglasbildende Legierungszusammensetzungen existieren in verschiedenen edelmetallbasierten Systemen. Ihre überlegenen Eigenschafen und ihr hoher Edelmetallgehalt prädestinieren diese Legierungsklassen für die Anwendung als Schmucklegierungen. Der Schwerpunkt der zweiteiligen Arbeit liegt auf Gold- und Platin-basierten massivglasbildenden Legierungen. Im Fall der massivglasbildenden 18 Karat Weißgoldlegierungen Au49Ag5.5Pd2.3Cu26.9Si16.3 wurde eine rasche Farbänderung beobachtet, die auf einen Korrosionsprozess durch das Tragen auf der Haut zurückzuführen ist. Es wurde ein Anpassungskonzept entwickelt und umgesetzt, das einerseits die Verbesserung der Korrosionsresistenz bewirkt und andererseits die Fähigkeit der Schmelze ein metallisches Massivglas zu bilden erhält. Der zweite Teil der Arbeit beschäftigt sich mit der Bestimmung der thermophysikalischen Eigenschaften von Platin-Phosphor-basierten glasbildenden Schmelzen. Die thermophysikalischen Eigenschaften werden in Kombination mit Hoch- und Tieftemperaturkristallisationsexperimenten verwendet um den Ursprung der hohen Glasbildungsfähigkeit der Legierungen zu ergründen. Darüber hinaus wurden die beobachteten Unterschiede innerhalb der Platin-Phosphor Klasse sowie die zu den kompositionell verwandten Palladium-Phosphor-basierten Legierungen in strukturaufklärenden Synchrotron-Röntgenstreuexperimenten untersucht. Die Ergebnisse weisen darauf hin, dass sich die Temperaturabhänigkeit der atomaren Dynamik in Platin-Phosphor-basierten Schmelzen in strukturellen Ordnungsprozessen auf gewissen Längenskalen widerspiegeln. Die strukturellen Unterschiede zwischen Platinund Palladium-Phosphor-basierten Legierungen im Bereich der repräsentativen Struktureinheiten und deren Verbindungsart könnten die in der Literatur berichtete unterschiedliche Sensitivität für anlassinduzierte Versprödung sein

Vision 2040: A Roadmap for Integrated, Multiscale Modeling and Simulation of Materials and Systems

Over the last few decades, advances in high-performance computing, new materials characterization methods, and, more recently, an emphasis on integrated computational materials engineering (ICME) and additive manufacturing have been a catalyst for multiscale modeling and simulation-based design of materials and structures in the aerospace industry. While these advances have driven significant progress in the development of aerospace components and systems, that progress has been limited by persistent technology and infrastructure challenges that must be overcome to realize the full potential of integrated materials and systems design and simulation modeling throughout the supply chain. As a result, NASA's Transformational Tools and Technology (TTT) Project sponsored a study (performed by a diverse team led by Pratt & Whitney) to define the potential 25-year future state required for integrated multiscale modeling of materials and systems (e.g., load-bearing structures) to accelerate the pace and reduce the expense of innovation in future aerospace and aeronautical systems. This report describes the findings of this 2040 Vision study (e.g., the 2040 vision state; the required interdependent core technical work areas, Key Element (KE); identified gaps and actions to close those gaps; and major recommendations) which constitutes a community consensus document as it is a result of over 450 professionals input obtain via: 1) four society workshops (AIAA, NAFEMS, and two TMS), 2) community-wide survey, and 3) the establishment of 9 expert panels (one per KE) consisting on average of 10 non-team members from academia, government and industry to review, update content, and prioritize gaps and actions. The study envisions the development of a cyber-physical-social ecosystem comprised of experimentally verified and validated computational models, tools, and techniques, along with the associated digital tapestry, that impacts the entire supply chain to enable cost-effective, rapid, and revolutionary design of fit-for-purpose materials, components, and systems. Although the vision focused on aeronautics and space applications, it is believed that other engineering communities (e.g., automotive, biomedical, etc.) can benefit as well from the proposed framework with only minor modifications. Finally, it is TTT's hope and desire that this vision provides the strategic guidance to both public and private research and development decision makers to make the proposed 2040 vision state a reality and thereby provide a significant advancement in the United States global competitiveness

Synchrotron X-ray operando studies of atomic structure evolution of multi-component Al alloys in liquid state

This research has studied one of the challenging scientific issues in materials science, i.e., in real time, understanding quantitatively the 3D atomic structures of multiple component alloys in the liquid state and how the atomic structures evolve with temperatures until the onset of crystal nucleation. Four Al-based alloys were used in the research: (1) Al-0.4Sc, (2) Al-1.5Fe, (3) Al-5Cu-1.5Fe and (4) Al-5Cu-1.5Fe-1Si alloy (all in weight percentage). All alloys were heated up to the liquid state and then cooled down with predefined cooling rates using a dedicated solidification apparatus. During cooling, synchrotron X-ray was used to illuminate onto the samples and the total scattering data were collected at the target temperatures. Based on the total scattering data, the empirical potential structure refinement (EPSR) method was used to model and reconstruct the 3D atomic structures in the liquid state at the selected temperatures for each alloy. The research has demonstrated that the EPSR is a computationally efficient tool for searching and finding the solutions of 3D atomic structures according to the measured total scattering data. For the studied alloys, the research reveals fully the temperature-dependent structure heterogeneity and their evolutions with temperature. The key findings of the research are: (1) For the Al-0.4Sc alloy, at the short-range scale in the liquid state, Sc-centred Al polyhedrons form icosahedral type structures with the Al coordination number in the range of 10–12. As the melt is cooled down, the Sc-centred polyhedrons become more compacted, and the connections between adjacent polyhedrons change from more vertex connection to more edge and then more face-sharing connection. At the medium-range scale, the Sccentred clusters with face-sharing are proved to be the “precursors” for the L12 Al3Sc primary phase in the liquid-solid coexisting region. (2) For the three Fe-containing alloys, atomic structural heterogeneities were found to exist in the 1st atomic shell and beyond. The degree of structural heterogeneities is related with the difference in atom radius, atomic bond length and the chemical preference between different atoms in each alloy. The competition resulted in that the Al-centred clusters expand, i.e., with larger bond length, while the solute atom-centred clusters contract, so with the reduced bond length. (3) At the short-range scale, the structural heterogeneities were characterised by the co-existence and growth of the icosahedra-like (ICO-like) and crystal-like structures. During cooling, the Fe atoms show a higher degree of crystallinity than other atoms in the liquids. At the onset of crystal nucleation, relative percentage of the Fe-centred ICO-like and crystal-like Voronoi polyhedrons (VPs) reaches 8-10%, and the others in the range of 5.8-8.5%. (4) The Fe-centred short-range orders (SROs) tend to connect together via five different modes to form larger Fe-centred medium-range orders (MROs). The percentage of the face-sharing increases almost linearly as the temperature is cooled down, approximately 18-20% at the onset of nucleation in the 3 melts. The Fe-centred MROs gradually approach to the structures of the Al13Fe4 primary phase (monoclinic structure) and are proved to be the nucleation precursors for the Al13Fe4 phases. (5) For the quaternary Al-Cu-Fe-Si alloy melt, the research found that the liquid first transfers into a quasicrystal-like, metastable monoclinic Al13Fe4 phase. Such primary phase was confirmed to have a higher degree of five-fold and crystalline symmetry than the liquid. Upon cooling, the Fe-centred five-fold and crystalline symmetry both get enhanced in liquid, leading to a smaller Al13Fe4-liquid configuration entropy difference and interfacial free energy

Annual Report 2022 - Institute of Resource Ecology

The Institute of Resource Ecology (IRE) is one of the ten institutes of the Helmholtz-Zentrum Dresden – Rossendorf (HZDR). Our research activities are mainly integrated into the program “Nuclear Waste Management, Safety and Ra-diation Research (NUSAFE)” of the Helmholtz Association (HGF) and focus on the topics “Safety of Nuclear Waste Disposal” and “Safety Research for Nuclear Reactors”. The program NUSAFE, and therefore all work which is done at IRE, belong to the research field “Energy” of the HGF

Three dimensional finite element ablative thermal response analysis applied to heatshield penetration design

Heatshield design and analysis has traditionally been a decoupled process, the designer creates the geometry generally without knowledge about how the design variables affect the thermostructural response or how the system will perform under off nominal conditions. Heatshield thermal and structural response analyses are generally performed as separate tasks where the analysts size their respective components and feedback their results to the designer who is left to interpret them. The analysts are generally unable to provide guidance in terms of how the design variables can be modified to meet geometric constraints and not exceed the thermal or structural design specifications. In general, the thermal response analysis of ablative thermal protection systems has traditionally been performed using a one-dimensional finite difference calculation. The structural analyses are generally one, two, or three-dimensional finite element calculations. In this dissertation, the governing differential equations for ablative thermal response are solved in three-dimensions using the finite element method. Darcy' Law is used to model the flow of pyrolysis gas through the ablative material. The three-dimensional governing differential equations for Darcy flow are solved using the finite element method as well. Additionally, the equations for linear elasticity are solved by the finite element method for the thermal stress using temperatures directly from the thermal response calculations. This dissertation also links the analysis of thermal protection systems to their design. The link to design comes from understanding the variation in the thermostructural response over the range of the design variables. Material property sensitivities are performed and an optimum design is determined based on a deterministic analysis minimizing the design specification of bondline temperature subject to appropriate constraints. A Monte Carlo simulation is performed on the optimum design to determine the probability of exceeding the design specifications. The design methodology is demonstrated on the Orion Crew Exploration Vehicle's compression pad design.Ph.D.Committee Chair: Dr. Robert D. Braun; Committee Member: Dr. David M. Schuster; Committee Member: Dr. Stephen M. Ruffin; Committee Member: Dr. Vitali Volovoi; Committee Member: Mr. Bernie Lau