Dislocations in Solids (eBook)

A Tribute to F.R.N. Nabarro

John P. Hirth (Herausgeber)

eBook Download: EPUB

2011 | 1. Auflage
650 Seiten
Elsevier Science (Verlag)
978-0-08-056498-2 (ISBN)

Infos

New models for dislocation structure and motion are presented for nanocrystals, nucleation at grain boundaries, shocked crystals, interphase interfaces, quasicrystals, complex structures with non-planar dislocation cores, and colloidal crystals. A review of experimentally established main features of the magnetoplastic effect with their physical interpretation explains many diverse results of this type. The model has many potential applications for forming processes influenced by magnetic fields.

. Dislocation model for the magnetoplastic effect
. New mechanism for dislocation nucleation and motion in nanocrystals
. New models for the dislocation structure of interfaces between crystals with differing crystallographic structure
. A unified view of dislocations in quasicrystals, with a new model for dislocation motion
. A general model of dislocation behavior in crystals with non-planar dislocation cores
. Dislocation properties at high velocities
. Dislocations in colloidal crystals

Inhalt

Front cover 1
Dislocations in Solids 4
Copyright page 5
Preface 6
Contents 8
List of Contents of Volumes 1-13 10
Chapter 81. Atomistic Simulations of Dislocations in FCC Metallic Nanocrystalline Materials 14
1. Bulk nanocrystalline plasticity 16
2. Introduction to atomistic simulation 17
3. Atomistic simulations of deformation in bulk 3D nanocrystalline metals 32
4. Experimental-computational synergy tools 44
5. Discussion and concluding remarks 49
Acknowledgements 52
References 53
Chapter 82. Influence of Grain Boundary Structure on Dislocation Nucleation in FCC Metals 56
1. Introduction 59
2. Atomistic simulation methodology 67
3. Structure and energy of tilt grain boundaries in Cu and Al 74
4. Dislocation nucleation from symmetric and asymmetric tilt boundaries in Cu and Al 92
5. Models for dislocation nucleation: single crystals and GBs 113
6. Insights and implications 130
7. Concluding remarks 146
Acknowledgements 148
References 148
Chapter 83. Interfaces Between Dissimilar Crystalline Solids 154
1. Introduction 156
2. Coherent and semicoherent interfaces in fcc/fcc bilayers 159
3. Incoherent interfaces in fcc/bcc bilayers 165
4. Vacancies and interstitials in Cu/Nb interfaces 188
5. Dislocation interaction with Cu/Nb interfaces 206
6. Summary 216
Acknowledgements 217
References 217
Chapter 84. Size Effects and Dislocation-Wave Interaction in Dislocation Dynamics 220
1. Introduction 222
2. The dislocation dynamics (DD) method 223
3. Integration of DD and continuum plasticity 234
4. Problems with size effects and the DD approach 237
5. Dislocation interaction with shock waves in small volumes 255
Acknowledgements 261
References 261
Chapter 85. Dislocations and Plasticity of Icosahedral Quasicrystals 264
1. Introduction 267
2. Quasiperiodic order and diffraction properties 268
3. Dislocations in quasicrystals 275
4. Observations of dislocations 283
5. Plasticity of quasicrystals. Experimental data 288
6. Theoretical models of plasticity 326
7. Plasticity modeling 336
References 340
Chapter 86. Magnetoplastic Effect in Nonmagnetic Crystals 346
1. Introduction 348
2. Early work 352
3. Some basic dependencies of the magnetoplastic effect on physical parameters 354
4. Preliminary kinematic scheme of the magnetoplastic effect 366
5. Magnetoplasticity and mechanical loading 372
6. Magnetoplasticity under simultaneous action of other fields 390
7. Magnetic influence on macroplastic phenomena in nonmagnetic crystals 403
8. Experimental evidences confirming a spin origin of the effect 418
9. Some estimations and theoretical considerations 433
10. Conclusions 441
Acknowledgements 443
References 443
Chapter 87. Non-planar Dislocation Cores: A Ubiquitous Phenomenon Affecting Mechanical Properties of Crystalline Materials 452
1. Introduction 454
2. Generalized stacking faults and gamma-surfaces 456
3. Body-centered-cubic metals 459
4. Hexagonal close-packed metals 468
5. A3B intermetallic compounds with L12 structure 477
6. A3B intermetallic compounds with non-cubic structures 484
7. AB intermetallic alloys and compounds with B2 structure 489
8. AB intermetallic compounds with L10 structure 494
9. Tetragonal C11b MoSi2 501
10. Miscellaneous materials 506
11. Conclusions 513
Acknowledgements 515
References 515
Author Index 528
Subject Index 544

Textauszug

Chapter 82

Influence of Grain Boundary Structure on Dislocation Nucleation in FCC Metals

Mark A. Tschopp    School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
Air Force Research Laboratory (UTC), Wright-Patterson Air Force Base, Dayton, OH 45433, USA

Douglas E. Spearot    Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, USA

David L. McDowell    Woodruff School of Mechanical Engineering, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, USA

1 Introduction

According to materials scientist and philosopher Cyril Stanley Smith, the structure of materials is best described as a multilevel architecture, with “interplay of perfection and imperfection” among all length scales [1,2]. This point of view asserts that a clear understanding of material behavior at each length scale is imperative to elucidate the technological or economic value of a material [1]. However, many theories of macroscopic material behavior are not based on direct evaluation of micro- or nanoscale mechanisms. Two examples are the kinematic hardening model in continuum plasticity, which was posed to describe experimental observation of the Bauschinger effect [3], and the Hall–Petch relationship [4,5], which describes the increase in yield strength associated with the decrease in the grain size for metallic polycrystalline materials. Both Hall [4] and Petch [5] envisioned grain boundaries as obstacles to dislocation motion (resulting in dislocation pile-ups) and assumed that yield occurred once the stress exerted on the neighboring grain by the dislocation pile-up reached a critical value. Experimental observations of this type are of tremendous scientific importance; however, it is highly desirable to directly study the underlying material micro- and nanostructure with the aim of developing constitutive models that capture the activity and interaction of atomic-scale structure and associated mechanisms. For example, both experiments (cf. [6–8]) and simulations (cf. [9–12]) have reported that material softening occurs once the grain size is reduced below a critical grain diameter, which cannot be predicted by the classical Hall–Petch relationship. Moreover, it is understood that scale effects in plasticity occur over a range of mechanisms and scales.

This work focuses on modeling the atomic level mechanisms associated with the structure and inelastic behavior of homophase grain boundaries (GBs) on the nanoscale and developing structure–property relationships that incorporate these nanoscale observations. Several experimental studies on polycrystalline metallic samples have reported that interface structure has an effect on material properties, such as grain boundary energy, mobility, corrosion, crack nucleation and ductility (cf. [13,14]). Most of the published experimental investigations indicate that there is some correlation between the occurrence of “special” coincident site lattice boundaries and material properties; however, results published in the literature do not point to a universal relationship. In nanocrystalline materials, the grain boundaries play a more profound role in material behavior due to the increased interfacial area associated with the decrease in the grain size. In addition, nanoscale confinement severely limits the operation of traditional dislocation sources, such as Frank–Read sources [15], mandating that the grain boundaries participate directly in the accommodation of the applied strain. Thus, grain boundary dislocation emission and absorption [16–39], which may be coupled with atomic shuffling [38–40], stress assisted grain boundary sliding [38–45] and grain rotation [46], have all been observed in computational studies as potential grain boundary deformation mechanisms. Conceivably, the activation of each grain boundary mechanism depends on several factors, including the electronic details of the material (intrinsic and unstable stacking fault energies), the grain size, the structure of the grain boundaries and the deformation conditions (strain state and strain rate).

The objective of this work is to use atomistic simulations to examine the structure and dislocation nucleation/emission behavior of symmetric and asymmetric tilt grain boundaries in FCC copper and aluminum. Discrete atomic scale mechanisms associated with dislocation nucleation are incorporated into a first-order constitutive model for the tensile strength of tilt grain boundaries. This atomistically motivated constitutive model explicitly incorporates the orientation dependence of the opposing lattice regions and the influence of porosity within the interface region through an average measure based on coordination. Furthermore, this work provides a detailed understanding of the influence of grain boundary structure on dislocation nucleation which is critical for the advancement of grain boundary engineering concepts. Recall that the objective of grain boundary engineering [47] is to increase the percentage of “special” grain boundaries and to reduce the connectivity of “random” grain boundaries through material processing. Reducing the connectivity of random boundaries is found to be particularly important, as polycrystalline samples with a properly oriented continuous path of weak boundaries would be susceptible to failure regardless of the percentage of special interfaces [48]. Schuh et al. [14] reported that the fraction of special grain boundaries can be increased through sequential cycles of straining and annealing. As a result, enhancements in corrosion resistance, creep resistance and crack nucleation and growth resistance under various loading conditions have been observed experimentally. Of particular effectiveness is the introduction of annealing twins [49], which are essentially highly coherent GBs. Molecular dynamics simulations in this work provide a more refined definition of “special” with regard to how grain boundary structure affects dislocation nucleation.

This chapter is organized as follows. The remainder of this section focuses on concepts related to grain boundary geometry and structure in metallic crystalline materials. Section 2 provides a brief overview of atomistic simulation techniques (equations of motion, interatomic potentials, etc.) and the specific simulation geometries used in this work to model grain boundary structure and dislocation nucleation. Section 3 discusses the dependence of grain boundary structure and energy on the misorientation angle/axis in the case of symmetric tilt grain boundaries and the inclination angle for asymmetric tilt grain boundaries. Section 4 investigates the relationship between interface structure and dislocation nucleation in symmetric and asymmetric tilt GBs. Section 5 presents a first-order, atomistically-inspired model designed to correlate the strength required for dislocation nucleation with interface structure via the interface free volume and the resolved stresses on the primary slip plane for uniaxial loading. To capture the influence of the lattice orientation, atomistic simulations of homogeneous dislocation nucleation in single crystals are also discussed. Section 6 discusses how atomic-level information of dislocation nucleation from grain boundaries can impact research in plasticity, micromechanics and grain boundary engineering techniques. A summary of major contributions in this work is provided in Section 7.

1.1 Overview of grain boundary geometry

In general, solid–solid interfaces between crystalline regions may be classified into two categories: homophase and heterophase [50]. The set of homophase interfaces includes grain boundaries, twins and stacking faults in pure metals, whereas heterophase interfaces exist in binary and other material systems in which the composition and/or the Bravais lattice change across the interface plane. For homophase boundaries (such as those in this work), the degree of coherency is a function of the misorientation angle of the interface, the boundary plane orientation, and the nanoscale translations that exist to minimize the interface energy in the local neighborhood of the boundary. From a macroscopic perspective, planar interfaces between two crystal regions have five degrees of freedom [13,50,51], i.e., an interface is fully characterized by a misorientation angle, θ, a misorientation axis vector, M, and the normal vector to the interface plane, N. Fig. 1 shows a schematic of the misorientation scheme. Boundaries for which the normal to the interface plane is perpendicular to the misorientation axis (⊥N) are defined as “tilt” interfaces, as shown in Fig. 1(a). Similarly, boundaries for which the normal to the interface plane is parallel to the misorientation axis are defined as “twist” interfaces. Grain boundaries in actual polycrystalline materials may have both tilt and twist character, as shown in Fig. 1(b). From a microscopic perspective, interfaces between crystal lattices have three addition degrees of freedom associated with the mutual nanoscale...

Produkt-Details

• Erscheint lt. Verlag: 22.9.2011
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Mineralogie / Paläontologie
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 0-08-056498-4 / 0080564984
• ISBN-13: 978-0-08-056498-2 / 9780080564982