Thermally Activated Mechanisms in Crystal Plasticity

D. Caillard, J.L. Martin

• Publisher: Elsevier Science
• Year: 2003
• Print ISBN: 9780080427034
• eBook ISBN: 9780080542782
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

KEY FEATURES: - A unified, fundamental and quantitative resource. The result of 5 years of investigation from researchers around the world - New data from a range of new techniques, including synchrotron radiation X-ray topography provide safer and surer methods of identifying deformation mechanisms - Informing the future direction of research in intermediate and high temperature processes by providing original treatment of dislocation climb DESCRIPTION: Thermally Activated Mechanisms in Crystal Plasticity is a unified, quantitative and fundamental resource for material scientists investigating the strength of metallic materials of various structures at extreme temperatures. Crystal plasticity is usually controlled by a limited number of elementary dislocation mechanisms, even in complex structures. Those which determine dislocation mobility and how it changes under the influence of stress and temperature are of key importance for understanding and predicting the strength of materials. The authors describe in a consistent way a variety of thermally activated microscopic mechanisms of dislocation mobility in a range of crystals. The principles of the mechanisms and equations of dislocation motion are revisited and new ones are proposed. These describe mostly friction forces on dislocations such as the lattice resistance to glide or those due to sessile cores, as well as dislocation cross-slip and climb. They are critically assessed by comparison with the best available experimental results of microstructural characterization, in situ straining experiments under an electron or a synchrotron beam, as well as accurate transient mechanical tests such as stress relaxation experiments. Some recent attempts at atomistic modeling of dislocation cores under stress and temperature are also considered since they offer a complementary description of core transformations and associated energy barriers. In addition to offering guidance and assistance for further experimentation, the book indicates new ways to extend the body of data in particular areas such as lattice resistance to glide.

Chapter 1

Introduction

The understanding and the prediction of mechanical properties of materials implies a detailed knowledge of the elementary mechanisms that govern plasticity. In particular, those which control dislocation mobility and how this mobility changes under the influence of stress and temperature are of key importance. This active field of physics constitutes the core of the present review.

This introductory section is divided into two parts. The first one defines the authors′ intentions, while the second one recalls some useful aspects of the theory of thermally activated dislocation dynamics, which will be used throughout the book.

1.1 Scope and Outline

The flow stress of a crystal can be decomposed into two main components. The first one reflects the long-range elastic interactions of mobile dislocations with the microstructure. It results from dislocation patterning to various extents and accordingly depends on the “sample history”. The second component is the stress necessary to push dislocations over local energy barriers, which oppose their motion. These barriers can be of different nature: small obstacles, an intrinsic lattice resistance or an unpropitious dislocation core configuration.

The first component will not be studied here in detail. Only average values are estimated. At a given strain, it is a slowly decreasing function of increasing temperature, following the change of the crystal elastic constants. The dependence of flow stress on temperature predominantly reflects the properties of the second component. Short-range interaction of dislocations with energy barriers takes place in such a small volume that it is strongly influenced by thermal vibrations. Thermal activation helps dislocations to overcome these barriers, thus resulting in a reduction of stress as the temperature rises. These short-range thermally activated processes govern almost all the temperature dependent mechanical properties of materials.

These processes are obviously of fundamental importance for the understanding and modelling of strength of structural materials. New materials have rather complex structures, in which dislocation mechanisms are more difficult to identify than in single-phase metals and alloys. Fortunately, even in complex crystals, plasticity is usually controlled by a small number of elementary dislocation mechanisms.

We also hope that the proposed improved descriptions will constitute useful guidelines for the numerous attempts of multiscale modelling of crystal behaviour— a very active and promising field nowadays (see, e.g., the MRS Symposium Proceedings, Kubin et al., 2001 on this topic).

It is thus extremely important to possess a comprehensive description of the basic mechanisms, critically assessed by the best experimental results, in fairly simple situations.

A book by Kocks et al. (1975) was, to our knowledge, the first attempt in this direction. It synthesized the understanding of mechanical properties in terms of thermally-activated dislocation glide in a crystal containing a given distribution of microscopic obstacles to flow around. However, it did not include any review of experimental results. In the concluding remarks, the authors listed a few problems “that appeared to stand out as worth solving”. More than 25 years later, we try to evaluate the answers brought to these questions, taking advantage of new techniques for investigations.

The state-of-the-art 20 years ago can be found in a book entitled “Dislocations et Déformation Plastique” (Groh et al., 1980). Several aspects of dislocation mobility mechanisms in connection with crystal plasticity are also covered in a book by Suzuki et al. (1991).

Here, we attempt to describe, in a consistent way, a variety of microscopic mechanisms of dislocation mobility. The principles of these mechanisms are recalled and the equations of dislocation motion are revisited to try and avoid reference to complex results scattered in the literature. The corresponding theoretical developments are borrowed from the inevitable reference books by Friedel (1964) and Hirth and Lothe (1992), whilst making certain important additions. The most complete and comprehensive experimental results are reviewed and analyzed in terms of these theoretical models.

The identification of the true mechanisms which operate relies on experimental techniques which have appeared or have been refined during the last 30 years. These include mainly in situ deformation experiments in the transmission electron microscope (TEM) on the one hand and accurate macroscopic transient mechanical tests on the other hand. The former experiments provide quantitative information about dislocation mobility mechanisms (viscous, jerky, with or without point obstacles), local stresses, mobile dislocation densities and velocities. The latter consist of stress relaxation experiments and transient creep tests. These tests yield average values of the activation energy of the dislocation velocity, its stress dependence. They also provide valuable information about mobile dislocation densities, a poorly documented parameter. Other types of experimental results will also be considered such as “post mortem” TEM observations of dislocation structures. Recent slip trace characterization taking advantage of the high resolut...

Table of contents

• Cover image
• Title page
• Table of Contents
• Pergamon Materials Series
• Copyright page
• Series Preface
• Preface
• Reader’s Guide
• Chapter 1: Introduction
• Chapter 2: Experimental Characterization of Dislocation Mechanisms
• Chapter 3: Interactions Between Dislocations and Small-size Obstacles
• Chapter 4: Frictional Forces in Metals
• Chapter 5: Dislocation Cross-slip
• Chapter 6: Experimental Studies of Peierls–Nabarro-type Friction Forces in Metals and Alloys
• Chapter 7: The Peierls–Nabarro Mechanism in Covalent Crystals
• Chapter 8: Dislocation Climb
• Chapter 9: Dislocation Multiplication, Exhaustion and Work-hardening
• Chapter 10: Mechanical Behaviour of Some Ordered Intermetallic Compounds
• Conclusion
• Glossary of Symbols
• Index