2010-02-10 Chapter07 Dislocations and Strengthening Mechanisms

2010-02-10 Chapter07 Dislocations and Strengthening Mechanisms

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ISSUES TO ADDRESS. .. How are strength and dislocation motion related? How do we increase strength? How can heating change strength and other properties? CHAPTER 7: DISLOCATIONS AND STRENGTHENING MECHANISMS
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Tensile Strength (TS), Ductility, Toughness Important C Engineering tensile strain, ε E ngineering tensile stress, σ A B Material Tensile Strength (TS) Ductility Toughness Modulus A High Low Low High B Medium Medium High Medium C Low High Medium Low When Strength Ductility Toughness Many engineering applications require materials having high strength yet some ductility and toughness.
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Chapter 7- 2 Vacancy atoms Interstitial atoms Substitutional atoms Dislocations Grain Boundaries Point defects Line defects Area defects TYPES OF IMPERFECTIONS Dislocations play dominant role in plastic deformation. If dislocations don’t move there is no deformation.
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Chapter 7- EDGE DISLOCATION (LINE DEFECT) Extra portion of a plane of atoms Vertical planes of atoms bend around the extra half-plane (distortion) Magnitude of distortion decreases away from dislocation. Far away from dislocation crystal lattice is perfect.
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SCREW DISLOCATION (LINE DEFECT) This dislocation can be imagined by assuming shear load applied to produce the distortion shown in figure. Upper front region of the crystal is shifted one atomic distance to the right relative to the bottom portion. There is helical path that can be traced around the dislocation line by atomic planes of atoms. Hence name screw dislocation.
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Plastic deformation is a permanent deformation. Strength and hardness indicates material's resistance to plastics deformation. Plastic deformation corresponds to a net movement of a large number of atoms in the response of applied stress. During plastic deformation bonds are ruptured and reformed. Dislocations play dominant role in plastic deformation. If dislocations don’t move there is no deformation. All metals and alloys contain some dislocations: during solidification , during plastic deformation , due to thermal stresses during rapid cooling. Important
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Produces plastic deformation, Depends on incrementally breaking bonds. Adapted from Fig. 7.1, Callister 6e. (Fig. 7.1 is adapted from A.G. Guy, Essentials of Materials Science , McGraw-Hill Book Company, New York, 1976. p. 153.) DISLOCATION MOTION
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DISLOCATION CHARACTERISTICS The extra half-plane of atoms in edge dislocation creates lattice distortion. Atoms above the edge dislocation line are squeezed together and experience compressive strain , whereas atoms below experience tensile strain . In the case of screw dislocation, the strains are of pure shear only. Important
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The strain fields surrounding dislocations in close proximity to one another may interact.
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2010-02-10 Chapter07 Dislocations and Strengthening Mechanisms

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