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Unformatted text preview: Chapter 8 Highlights: 1. Plastic defomation is caused by dislocation motion, involving bond breaking and reforming. 2. Dislocations of certain orientations will move more easily than others. Slip systems(plane and direction of slip) fail in a certain hierarchy, where the weakest slips first. The planes of highest planar atomic density and the directions of highest linear atomic density slip more easily. Be able to calculate and understand the resolved shear stress. 3. Slip in polycrystalline materials occurs by a slightly different mechanism than slip in single crystal materials. Slip in te individual grains occurs as in a single crystal material, but at grain boundaries the dislocation motion gets deflected, since the next grain is misoriented. This makes polycrystalline materials stronger. 4. Materials can be strengthened by hindering dislocation motion, by solid solution hardening, by grain size reduction, and by strain hardening. 5. Understand and be able to numerically calculate grain growth. Notes: Connections to other material: 1. Chapter 8 gives the microscopic picture of plastic deformation, introduced in chapter 7. 2. Slip is easier in crystal planes of high planar atomic density crystal planes and along crystal directions of high linear atomic density. These concepts were introduced in chapter 3. 3. Annealing reduces grain size by atomic diffusion (introduced in chapter 6). In the 1930's theoretical calculations suggested that materials should be much more resistant to plastic deformation than they actually were, leading to the sugggestion that a new entity called a dislocation could exist. This theoretical prediction was confirmed experimentally in the 1950's with the development of the electron microscope. You should review edge, screw and mixed dislocations. Slip: Process by which plastic deformation is produced by dislocation motion. Slip plane: The plane along which the dislocation line travels. Dislocation moves like a caterpillar, as in figures 8.1, 8.3. Dislocation density = (Total dislocation length)/(Unit volume). Dislocations are associated with strain fields arising from tensile and compressive strain in the vicinity of the dislocation. You can think of the strain field as being a vector pointing from the region of compressive strain to that of tensile strain. The strain field stores some of the energy that is expended in plastic deformation through dislocation motion. About 95% of the energy put into plastic deformation is dissipated through heat, the remaining 5% is stored in the strain fields. the remaining 5% is stored in the strain fields....
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This note was uploaded on 04/23/2008 for the course ES 260 taught by Professor Rasmussen during the Spring '08 term at Clarkson University .
- Spring '08