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20. Line, Interface & Bulk Defects

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Session Overview

Module Crystalline Materials
Concepts defects in crystals: line defects, interfacial defects, grain boundaries, and voids, motion of dislocations, effect of impurities on solid-state material properties
Keywords yield stress, strain, shear stress, line defect, surface energy, edge dislocation, screw dislocation, dislocation motion, catalysis, corrosion, grain boundary, annealing, vacancy, single-crystal, polycrystalline, precipitation strengthening, ductility, slip, voids, solution hardening, elastic deformation, plastic deformation, chemical metallurgy, physical metallurgy, Hooke’s law, fracture, close-packed, dislocation glide, toughness, hardness, brittle
Chemical Substances steel, aluminum-copper alloy (Al-Cu), silica (SiO2), calcia (CaO), alumina (Al2O3)
Applications aluminum can, steel production, aluminum-copper for airplanes, rivets on the Titanic

Prerequisites

Before starting this session, you should be familiar with:

Looking Ahead

The amount and composition of precipitates in alloys can be predicted using binary phase diagrams, as described in Session 34 and Session 35. Point defects and grain boundaries give atoms space to move through the lattice, a key factor in Session 24: Diffusion.

Learning Objectives

After completing this session, you should be able to:

  • Given a crystal under stress, determine the slip planes.
  • Sketch the movement of dislocations through the lattice, and explain how this motion contributes to plastic deformation and work hardening.
  • Given a specific material, consider its processing history and application, and identify what defects are likely to be present and how they affect the properties of interest.

Reading

Archived Lecture Notes #6 (PDF), Sections 3-4

Book Chapters Topics
[A&E] 12.4, "Defects in Crystals." Defects in metals, memory metal, defects in ionic and molecular crystals, non-stoichiometric compounds
Amazon logo [JS] 4.3, "Linear Defects, or Dislocations – One-Dimensional Imperfections." Burgers vector; edge, screw, mixed, and partial dislocations
Amazon logo [JS] 4.4, "Planar Defects – Two-Dimensional Imperfections." Twin boundaries, crystal surfaces, and grain boundaries; tilt boundaries, coincident site lattices, and dislocations; grain-size number

Lecture Video

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Resources

Lecture Slides (PDF)

Transcript (PDF)

Lecture Summary

Experimental values for the yield strengths of metals are roughly 1/10th those given by theoretical calculations based on breaking entire planes of atomic bonds. The discrepancy is explained by dislocations, introduced at the end of the last session, which allows slipping planes to break single bonds in sequence, lowering the yield stress. Two-dimensional defects can occur at the surface of crystals or at internal interfaces between zones with different lattice alignments, called grain boundaries. Macroscopic clusters of vacancies (voids) weaken metals, while clusters of impurities (precipitates) may weaken or strengthen them. The failure of rivets on the hull of the Titanic is attributed to brittle pockets of slag mixed into the steel, based on examination of the microstructure.

Homework

Problems (PDF)

Solutions (PDF)

Textbook Problems

[A&E] Sections Conceptual Numerical
[A&E] 12.4, "Defects in Crystals." 2, 3, 4, 5 none

For Further Study

Textbook Study Materials

See the [A&E] companion website from Pearson for PowerPoint outlines of each chapter, plus online quizzes, interactive graphs and 3D molecular animations:

Supplemental Readings

Amazon logo Hull, Derek, and David J. Bacon. Introduction to Dislocations. Boston, MA: Butterworth-Heinemann, 2001. ISBN: 9780750646819.

People

Egon Orowan

Geoffrey Ingram Taylor

Robert Hooke

Tim Foecke

Culture

Amazon logo Glass, Philip. Koyaanisqatsi. Orange Mountain Music, 2009.

Amazon logo Horner, James, and Will Jennings. "My Heart Will Go On (Theme from Titanic)." Titanic: Music from the Motion Picture. Performed by Celine Dion. Columbia Records, 1997.

Other OCW and OER Content

Content Provider Level Notes
Introduction to Dislocations DoITPoMS Undergraduate  
3.14/3.40J/22.71J Physical Metallurgy MIT OpenCourseWare Undergraduate (elective) / Graduate  
Crystal Structure Connexions Undergraduate  

 

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