Recrystallization and Related Annealing Phenomena

Anthony Rollett, Gregory S. Rohrer, John Humphreys

• Publisher: Elsevier
• Year: 2017
• Edition: 3
• eBook ISBN:
  9780080982694
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Recrystallization and Related Annealing Phenomena, Third Edition, fulfills the information needs of materials scientists in both industry and academia. The subjects treated in the book are all active research areas, forming a major part of at least four regular international conference series. This new third edition ensures the reader has access to the latest findings, and is essential reading to those working in the forefront of research in universities and laboratories. For those in industry, the book highlights applications of the research and technology, exploring, in particular, the significant progress made recently in key areas such as deformed state, including deformation to very large strains, the characterization of microstructures by electron backscatter diffraction, the modeling and simulation of annealing, and continuous recrystallization. - Includes over 50% of new, revised, and updated material, highlighting the significant recent literature results in grain growth in non-crystallizing systems, 3D characterization techniques, quantitative modeling techniques, and all-new appendices on texture and measurements - Contains synthesized, detailed coverage from leading authors that bridge the gap between theory and practice - Includes a critical level of synthesis and pedagogy with an authored rather than edited volume

Chapter 1

Introduction

Abstract

Various processes of microstructural evolution that occur during annealing are introduced, with a focus on changes in the grain structure. Grain growth is driven by the excess free energy associated with grain boundaries. This may occur uniformly or heterogeneously, in which case it is known as abnormal grain growth. Recrystallization is driven by the excess free energy associated with dislocations stored during plastic deformation. Depending on the circumstances, this is variously known as primary recrystallization, secondary recrystallization, dynamic recrystallization, or meta-dynamic recrystallization. Recovery is the process by which dislocation density decreases without long range motion of boundaries and is often accompanied by the formation of subgrain networks that replace the tangled dislocation structures formed during deformation. Dynamic recovery is the set of processes that decrease dislocation density during the deformation, as opposed to subsequent annealing. The historical development of recrystallization as a topic of research is briefly reviewed along with relevant literature.

Keywords

Abnormal grain growth; Driving force; Dynamic recrystallization; Grain growth; Recrystallization; Stored energy

1.1. Annealing of a Deformed Material

1.1.1. Outline and Terminology

The free energy of a crystalline material is raised during deformation by the presence of dislocations and interfaces, and a material containing these defects is thermodynamically unstable. Although thermodynamics would suggest that the defects should spontaneously disappear, in practice the necessary atomistic mechanisms are often very slow at low homologous temperatures, with the result that unstable defect structures are retained after deformation (Fig. 1.1a).

If the material is subsequently heated to a high temperature (annealed), thermally activated processes such as solid state diffusion provide mechanisms whereby the defects may be removed or alternatively arranged in configurations of lower energy.

The defects may be introduced in a variety of ways. However, in this book we will mainly be concerned with those defects, and in particular dislocations, which are introduced during plastic deformation. The point defects introduced during deformation anneal out at low temperatures and generally have little effect on the mechanical properties of the material. In considering only materials that have undergone substantial plastic deformation, we necessarily limit the range of materials with which we will be concerned. Metals are the only major class of crystalline material to undergo substantial plastic deformation at low homologous temperatures, and much of this book will be concerned with the annealing of deformed metals. However, at high temperatures, many minerals and ceramics readily deform plastically, and the annealing of these is of great interest. In addition, some annealing processes such as grain growth are relevant to sintered, cast, or vapor deposited materials as well as to deformed materials.

On annealing a cold-worked metal at an elevated temperature, the microstructure and the properties may be partially restored to their original values by recovery in which annihilation and rearrangement of the dislocations occurs. The microstructural changes during recovery are relatively homogeneous and do not usually affect the boundaries between the deformed grains; these changes in microstructure are shown schematically in Fig. 1.1b. Similar recovery processes may also occur during deformation, particularly at high temperatures, and this dynamic recovery plays an important role in the creep and hot working of materials.

Figure 1.1 Schematic diagram of the main annealing processes. (a) Deformed state; (b) recovered; (c) partially recrystallized; (d) fully recrystallized; (e) grain growth; and (f) abnormal grain growth.

Recovery generally involves only a partial restoration of properties because the dislocation structure is not completely removed, but reaches a metastable state (Fig. 1.1b). A further restoration process called recrystallization may occur in which new dislocation-free grains are formed within the deformed or recovered structure (Fig. 1.1c). These then grow and consume the old grains, resulting in a new grain structure with a low dislocation density (Fig. 1.1d). Recrystallization may take place during deformation at elevated temperatures and this is then termed dynamic recrystallization.

Although recrystallization removes the dislocations, the material still contains grain boundaries, which are thermodynamically unstable. Further annealing may result in grain growth, in which the smaller grains are eliminated, the larger grains grow, and the grain boundaries assume a lower energy configuration (Fig. 1.1e). In certain circumstances this normal grain growth may give way to the selective growth of a few large grains (Fig. 1.1f), a process known as abnormal grain growth or secondary recrystallization.

Table 1.1

Examples of static annealing phenomena.

	Recovery	Recrystallization	Grain Growth
Continuous	Subgrain growth	Continuous recrystallization	Normal grain growth
Discontinuous	Discontinuous subgrain growth	Primary recrystallization	Abnormal grain growth

Recent research has shown that borderlines between the various annealing phenomena are often unclear, and it is known that recovery, recrystallization, and grain growth may occur in two ways. They occur heterogeneously throughout the material, such that they may be formally described in terms of nucleation and growth stages, and in this case, they are described as discontinuous processes. Alternatively, they may occur uniformly, such that the microstructures evolve gradually with no identifiable nucleation and growth stages. In this case, the prefix continuous is used to categorize the phenomena. It should be emphasized that this is a phenomenological categorization that does not imply the operation of any particular micromechanism. The “continuous” phenomena include recovery by subgrain growth, continuous recrystallization and normal grain growth and the “discontinuous” phenomena include discontinuous subgrain growth, primary recrystallization, and abnormal grain growth. Therefore, as shown in Table 1.1, there are at least six static annealing phenomena that need to be consider...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface to the First Edition
• Preface to the Second Edition
• Preface to the Third Edition
• Acknowledgments
• Symbols
• Abbreviations
• Chapter 1. Introduction
• Chapter 2. The Deformed State
• Chapter 3. Deformation Textures
• Chapter 4. The Structure and Energy of Grain Boundaries
• Chapter 5. Mobility and Migration of Boundaries
• Chapter 6. Recovery After Deformation
• Chapter 7. Recrystallization of Single-Phase Alloys
• Chapter 8. Recrystallization of Ordered Materials
• Chapter 9. Recrystallization of Two-Phase Alloys
• Chapter 10. The Growth and Stability of Cellular Microstructures
• Chapter 11. Grain Growth Following Recrystallization
• Chapter 12. Recrystallization Textures
• Chapter 13. Hot Deformation and Dynamic Restoration
• Chapter 14. Continuous Recrystallization During and After Large Strain Deformation
• Chapter 15. Control of Recrystallization
• Chapter 16. Computer Modeling and Simulation of Annealing
• Appendix 1. Texture
• Appendix 2. The Measurement of Recrystallization
• References
• Index