Book contents
- Frontmatter
- Contents
- Preface
- Note to the Reader
- 1 Stress and Strain
- 2 Elasticity
- 3 Tensile Testing
- 4 Other Tests of Plastic Behavior
- 5 Strain-Hardening of Metals
- 6 Plasticity Theory
- 7 Strain-Rate and Temperature Dependence of Flow Stress
- 8 Slip
- 9 Dislocation Geometry and Energy
- 10 Dislocation Mechanics
- 11 Mechanical Twinning and Martensitic Shear
- 12 Hardening Mechanisms in Metals
- 13 Ductility and Fracture
- 14 Fracture Mechanics
- 15 Viscoelasticity
- 16 Creep and Stress Rupture
- 17 Fatigue
- 18 Residual Stresses
- 19 Ceramics and Glasses
- 20 Polymers
- 21 Composites
- 22 Mechanical Working
- Appendix A Miller Indices
- Appendix B Stereographic Representation of Orientations
- Index
9 - Dislocation Geometry and Energy
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- Preface
- Note to the Reader
- 1 Stress and Strain
- 2 Elasticity
- 3 Tensile Testing
- 4 Other Tests of Plastic Behavior
- 5 Strain-Hardening of Metals
- 6 Plasticity Theory
- 7 Strain-Rate and Temperature Dependence of Flow Stress
- 8 Slip
- 9 Dislocation Geometry and Energy
- 10 Dislocation Mechanics
- 11 Mechanical Twinning and Martensitic Shear
- 12 Hardening Mechanisms in Metals
- 13 Ductility and Fracture
- 14 Fracture Mechanics
- 15 Viscoelasticity
- 16 Creep and Stress Rupture
- 17 Fatigue
- 18 Residual Stresses
- 19 Ceramics and Glasses
- 20 Polymers
- 21 Composites
- 22 Mechanical Working
- Appendix A Miller Indices
- Appendix B Stereographic Representation of Orientations
- Index
Summary
Introduction
It was well known in the late 19th century that crystals deformed by slip. In the early 20th century the stresses required to cause slip were measured by tension tests of single crystals. Dislocations were not considered until after it was realized that the measured stresses were far lower than those calculated from a simple model of slip. In the mid-1930s G. I. Taylor, M. Polanyi, and E. Orowan independently postulated that preexisting crystal defects (dislocations) were responsible for the discrepancy between measured and calculated strengths. It took another two decades and the development of the electron microscope for dislocations to be observed directly.
Slip occurs by the motion of dislocations. Many aspects of the plastic behavior of crystalline materials can be explained by dislocations. Among these are how crystals can undergo slip, why visible slip lines appear on the surfaces of deformed crystals, why crystalline materials become harder after deformation, and how solute elements affect slip.
Theoretical strength of crystals
Once it was established that crystals deformed by slip on specific crystallographic systems, physicists tried to calculate the strength of crystals. However, the agreement between their calculated strengths and experimental measurements was very poor. The predicted strengths were orders of magnitude too high, as indicated in Table 9.1.
The basis for the theoretical calculations is illustrated in Figure 9.1. Each plane of atoms nestles in pockets formed by the plane below (Figure 9.1a).
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- Information
- Mechanical Behavior of Materials , pp. 139 - 157Publisher: Cambridge University PressPrint publication year: 2005