Chapter 7 Notes

Chapter 7 Notes - Chapter 7 Dislocations and Strengthening...

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Unformatted text preview: Chapter 7 Dislocations and Strengthening Mechanisms Basic Concepts- Hardness and strength are measures of a materials resistance to this deformation.- During deformation, interatomic bonds must be ruptured and then reformed.- In crystalline solids, plastic deformation most often involves the motion of dislocations, linear crystalline solids, plastic deformation most often involves the motion of dislocations, linear crystalline defects that were introduced in Section 4.5- Edge and screw are the two fundamental dislocation types- Plastic deformation corresponds to the motion of large numbers of dislocations- An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line?- It moves right to left by successive and repeated breaking of bonds and shifting by interatomic distances of upper half-planes. Before and after the movement of a dislocation through some particular region of the crystal, the atomic arrangement is ordered and perfect; it is only during the passage of the extra half-plane that the lattice structure is disrupted- Dislocation motion is analogous to the mode of locomotion employed by a caterpillar. - All metals and alloys contain some dislocations that were introduced during solidification, during plastic deformation, and as a consequence of thermal stresses that result from rapid cooling. - The number of dislocations, or dislocation density in a material, is expressed as the total dislocation length per unit volume or, equivalently, the number of dislocations that intersect a unit area of a random section Characteristics of Dislocations- Strain fields that exists around dislocations which are influential in determining the mobility of the dislocations, as well as their ability to multiply- When metals are plastically deformed, some fraction of the deformation energy is retained internally; the remainder is dissipated as heat. Some atomic lattice distortion exists around the dislocation line because of the presence of the extra half-plane of atoms- For a screw dislocation, lattice strains are pure shear only. These lattice distortions may be considered to be strain fields that radiate from the dislocation line. The strains extend into the surrounding atoms, and their magnitude decreases with radial distance from the dislocation. - Two dislocations of opposite sign and having the same slip plane will be attracted to one another, and dislocation annihilations will occur when they meet- That is the two extra half-planes of atoms will align and become a complete plane Slip Systems- Dislocations do not move with the same degree of ease on all crystallographic planes of...
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This note was uploaded on 04/11/2008 for the course MSE 209 taught by Professor Kelly during the Spring '08 term at UVA.

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Chapter 7 Notes - Chapter 7 Dislocations and Strengthening...

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