Summary and Terms
Terms
Slip Model: An early model of plastic deformation which suggested that atoms on close packed planes slip past one another when enough stress is applied. This model significantly overestimated the yield properties of metals. Section 12.3.2
Dislocations: 2-Dimensional line defects which explain the plastic properties of many crystals. Section 12.4.1
Edge Dislocation: A line of atoms with non-ideal coordination, which can be thought of as an extra half-plane inserted into the crystal. We use $\bot$ to represent edge dislocations. Importantly, the imperfection induced in the lattice is local - farther away than about 5 bond lengths from the dislocation the lattice is essentially perfect. Section 12.4.2
Screw Dislocation: A type of dislocation characterized by part of a plane of atoms being sheared out of alignment, like ripping a piece of paper in half by pulling forward and backward on either side. Section 12.6.3
Mixed Dislocation: A dislocation which has both edge and screw character. These are quite difficult to visualize but work out fine mathematically. Section 12.6.4
Slip Planes: The planes along which dislocations move most easily, usually close-packed planes. Section 12.7.1
Slip Direction: Directions that lie in slip planes along which dislocations are favored to slip. These will also almost always be close-packed directions. Section 12.7.1
Slip System: The combination of a slip system and slip direction which defines how a dislocation will move. Slip systems depend on atomic spacings and packings, so they are therefore material-dependent. Section 12.7.1
Different crystal structures have different numbers of low-energy slip systems; a crystal with more available slip systems, like FCC or BCC, will tend to be ductile, while one with fewer, like HCP, will tend to be more brittle. Section 12.7.3
Burgers Vector: A vector which describes the direction and magnitude of distortions to a crystal lattice caused by a dislocation. Section 12.8.1
Burgers Circuit: A construction used to identify a dislocation's burgers vector by drawing a circuit around the dislocation core and counting the atoms to identify the change from a typical lattice. Section 12.8.1
Edge dislocations are defined by burgers vectors which are perpendicular to the dislocation line, while screw dislocations are defined by burgers vectors which point parallel to the dislocation line (the axis around which the "screw" turns). Section 12.8.2
Solid Solution Strengthening: A strengthening mechanism by which impurity atoms are added to a host lattice to impede the motion of dislocations. The introduced atoms can be either substitutional or interstitial. Section 12.9.2
Precipitate Hardening: A strengthening mechanism that introduces small areas of different phases from the host lattice called precipitates. Precipitates can block dislocation motion, hardening the material. Section 12.9.4
Grain Size Reduction: A strengthening mechanism that relies on reducing grain size to increase the number of grain boundaries present in a material, which impedes the movement of dislocations. Section 12.9.5
Strain Hardening: A strengthening mechanism where more dislocations are introduced in a metal via deformation. These additional dislocations tend to impede each others' movement via the strain they induce in the lattice, resulting in a harder material. Strain hardening is also called "cold working" Section 12.9.7
Annealing (heating a metal to a significant percentage of its melting point) can have far-reaching affects on hardening mechanisms. Specifically, annealing vastly reduces cold working effects by allowing dislocations to move around and annihilate more easily. Annealing can also change the size of grains and precipitates, usually making them larger, which also affects their hardening effects. Section 12.10.1