A Quantum Approach to Alloy Design

An Exploration of Material Design and Development Based Upon Alloy Design Theory and Atomization Energy Method

Masahiko Morinaga

• Publisher: Elsevier
• Year: 2018
• Print ISBN: 9780128147061
• eBook ISBN: 9780128147078
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

A Quantum Approach to Alloy Design: An Exploration of Material Design and Development Based Upon Alloy Design Theory and Atomization Energy Method presents a molecular orbital approach to alloy design that is based on electronic structure calculations using the DV-X alpha cluster method and new alloying parameters obtained from these calculations. Topics discussed include alloy properties, such as corrosion resistance, shape memory effect and super-elasticity that are treated by using alloying parameters in biomedical titanium alloys. This book covers various topics of not only metals and alloys, but also metal oxides, hydrides and even hydrocarbons.In addition, important alloy properties, such as strength, corrosion resistance, hydrogen storage and catalysis are treated in view of electron theory.- Presents alloy design theory and the atomization-energy method and its use for the fundamental understanding of materials and materials design and development- Discusses, for the first time, the atomization-energy analysis of the local lattice strains introduced around alloying elements in metals- Illustrates a simplified approach to predict the structure and phases stability of new alloys/materials

1

Introduction

Abstract

This book treats a quantum approach to alloy design developed on the basis of a molecular orbital method. In view of the electron bonding, it is no longer needed to distinguish between iron and steels and nonferrous alloys, and even the current classification into metals, semiconductors, ceramics and polymers is not necessarily required. The present quantum approach is useful not only for the fundamental understanding of alloys, but also for alloy design without relying on trial-and-error experiments.

In this introduction, first, characteristics of alloy properties are explained, while touching on several current topics of structural alloys. Second, previous electronic approaches to metals and alloys are reviewed concisely to understand the present status of electron theory. Information on the electronic state appears to be very limited in solute-rich alloys as compared with pure metals or impurity-containing metals. Third, a series of investigations of wide scope is outlined briefly to survey a new approach to alloy design.

Keywords

Alloy design; structural alloy; electronic structure; molecular orbital calculation; atomization energy; working processing

1.1 Alloy Properties

Superior alloy has a beautiful microstructure. For example, the micrograph shown in Fig. 1-1 is taken from a heat resistant single-crystal Ni-based superalloy for use of the blades in jet engines and gas turbines. The cuboidal γ′ (Ni₃Al) precipitates with the submicrometer size are arranged three-dimensionally and periodically in the γ (fcc Ni) matrix. The volume fraction of the γ′ phase is as high as 60%–65%. This beautiful microstructure is, however, no longer sustained, if any harmful phase (i.e., the σ phase) precipitates in the γ matrix, which deteriorates mechanical properties of alloys. Therefore, it is strongly needed to control the alloy composition to be free from the σ-phase precipitation. For this purpose, the phase computation (PHACOMP) method was developed in 1964. Since then, we have used the term “alloy design”.

Figure 1-1 The γ′ phase precipitated in a single crystal Ni-based superalloy. The γ phase exists in the interface region between adjacent cubes of the γ′ phase. By courtesy of Dr. T. Ohno (Hitachi Metals Ltd.). The micrograph used in the cover of this book is the same as this picture, that is taken from Hitachi Metals Technical Review, vol. 13, 1997.

Needless to say, mechanical strength is one of the most important properties of the structural alloys. Plastic deformation takes place readily in most alloys, and both the size and the shape change easily by either cold or hot working. This is a great advantage of metals and alloys over other materials. Even new functions could emerge in some alloy by cold working. For example, gum metals (Ti alloys, e.g., Ti-9%Nb-3%V-6%Zr-1.5%O and Ti-23%Nb-0.7%Ta-2%Zr-1.2%O (mol%)) have been developed recently [1]. A typical stress–strain curve is shown in Fig. 1-2A. By applying the 90% cold working to this alloy, nonlinear elastic deformation takes place as in gum material. Also, as shown in Fig. 1-2B, in case gum metal is 90% cold worked, elastic constant and linear thermal expansion coefficient keep constant in the temperature range of −200°C (73 K) to 400°C (673 K). Such unique properties are called the elinvar and the invar properties, respectively.

Figure 1-2 (A) Stress vs. strain curves for gum metals (Ti alloys) and (B) elinvar and invar properties emerge in them by cold working. By courtesy of Dr. T. Saito (Toyota Physical and Chemical Reserch Institute).

Recently, there have been several trials to get superior mechanical properties by introducing severe plastic deformation to metals and alloys by using special processes such as equal channel angular processing (ECAP), high pressure torsion (HPT), and accumulative roll bonding (ARB) [2–4]. No size or shape changes are produced by these processes, but instead ultrafine grained (UFG) materials with 200–20 nm in grain sizes are produced. For example, very high strength Al and Fe metals are developed by the ARB process [5,6].

Besides the mechanical properties, for example, oxidation resistance is required for high-temperature alloys such as Ni-based, Co-based, and Fe-based alloys. If these alloys are exposed to air at high temperatures, metal oxide of Cr₂O₃, Al₂O₃, or SiO₂ is formed depending on the temperature. Such oxides formed on the surface protect the alloy from further oxidation.

To understand the oxidation process, it is important to know the defect structure in the oxide. For example, as shown in Fig. 1-3A, the formation energies of lattice defects are calculated in SiO₂ and Al₂O₃ [7]. The lattice defect in the oxide is assumed here to be either the Schottky defect, or the cation or anion Frenkel defect. The Schottky defect is a pair of anion and cation vacancies, and the cation (or anion) Frenkel defect consists of a vacancy and an interstitial of cation (or anion) [8]. Judging from the formation energies listed in Fig. 1-3A, the anion (O) Frenkel defect is dominant in SiO₂, and the cation (Al) Frenkel defect is dominant in Al₂O₃ as illustrated in Fig. 1-3B. Each lattice defect assists ionic diffusion in the oxide at high temperature. As a result, as shown in Fig. 1-3C, the SiO₂ layer is formed on the Si surface by inner diffusion of anion (O), whereas the Al₂O₃ layer is formed on the Al surface by outer diffusion of cation (Al), in agreement with experiments [7]. Thus, to reduce the oxidation rate at high temperature, we may need to control the defect structure in the oxides in some way, for example, by alloying. Thus, we need to learn metal oxides as well as alloys to understand a whole feature of the oxidation behavior in a fundamental manner. In other words, wide knowledge and information are necessary to understand the alloy property.

Figure 1-3 Formation of oxide layer formed on metal surface. (A) Calculated formation energies of lattice defects, (B) lattice defects in SiO₂ and Al₂O₃, and (C) ionic diffusion mechanism during oxidation at high temperature.

In particular, in the case of structural alloys it is strongly requested to have an excellent combination of various properties such as mechanical properties (e.g., tensile strength, creep strength), corrosion resistance, and oxidation resistance. However, it is rather difficult to predict these alloy properties in a reasonable way. In this book, several trials are presented to resolve a part of such difficulties using electron theory, since most of physical and chemical properties of alloys are related closely to the electronic states.

In the next section, previous electronic approaches will be reviewed briefly to deepen our understanding of electron theory for metals and alloys.

1.2 Electronic Approaches to Metals and Alloys

Pure metals have been investigated extensively from both experiments and theories, and valuable information has been accumulated on the electronic states in pure metals. For example, the Fermi surfaces are determined for a variety of metals [9]. The Fermi surface is spherical in alkali metals such as Na and K, in which a free electron model is satisfied. A nearly free electron model is satisfied for the lightweight metals such as Al and Mg, in which the s and p electrons spread over the crystal space. On the other hand, for transition metals the d electrons are localized around the atom site and the localized d electrons cause the ferromagnetism in Fe group metals. For example, the atomic moment is about 2.2 μ_B for Fe, 1.7 μ_B for Co, and 0.6 μ_B for Ni, where μ_B is a Bohr magneton.

Also, the impurity state in metals has been investigated. For example, the magnetic impurity (e.g., Fe, Cr, Mn) in metals (e.g., Au, Ag, Cu) causes a residual resistance minimum at low temperature that is known as the Kondo effect [10]. Also, the Friedel oscillation appears in either the charge distribution or the spin polarization around an impurity atom embedded in metal [10].

By contrast, electronic information is very limited in the solute-rich alloys. In many cases alloy properties have been analyzed using the electrons-per-atom ratio, e/a. This approach seems natural, since e/a is the number of outer electrons to form the chemical bonds in alloys a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• 1. Introduction
• 2. Theory for Alloy Design
• 3. Nickel Alloys
• 4. Iron Alloys
• 5. Titanium Alloys
• 6. Aluminum Alloys and Magnesium Alloys
• 7. Hydrogen Overpotential of Alloys and Intermetallic Compounds
• 8. Crystal Structure Maps for Intermetallic Compounds
• 9. A Universal Relation between Electron Density Minima and Atomic or Ionic Radii
• 10. Atomization Energy Approach to Alloys and Metal Compounds
• 11. Local Lattice Strains Around Alloying Elements in Metals
• 12. Conclusions
• Index