|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|
Before starting this session, you should be familiar with:
- Cubic crystal structures (Session 15)
- Crystal directions, planes, and Miller indices (Session 16)
- Structure of edge dislocations (Session 19)
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.
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.
Archived Lecture Notes #6 (PDF), Sections 3-4
|[Saylor] 12.4, "Defects in Crystals."||Defects in metals, memory metal, defects in ionic and molecular crystals, non-stoichiometric compounds|
|[JS] 4.3, "Linear Defects, or Dislocations – One-Dimensional Imperfections."||Burgers vector; edge, screw, mixed, and partial dislocations|
|[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|
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.
|[Saylor] 12.4, "Defects in Crystals."||2, 3, 4, 5||none|
For Further Study
Other OCW and OER Content
|Introduction to Dislocations||DoITPoMS||Undergraduate|
|3.14/3.40J/22.71J Physical Metallurgy||MIT OpenCourseWare||Undergraduate (elective) / Graduate|