Chap+7+Posting - LearningObjectives: 1.

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Chapter 7: Dislocations and rengthening Mechanisms Strengthening Mechanisms Learning Objectives: 1. Describe edge and screw dislocation motion from an atomic perspective. escribe how plastic deformation occurs by the motion of edge and screw 2. Describe how plastic deformation occurs by the motion of edge and screw dislocations in response to applied shear stresses. 3. Define slip system and cite one example. 4. Describe how the grain structure of a polycrystalline metal is altered when it is plastically deformed. 5. Explain how grain boundaries impede dislocation motion and why a metal having small grains is stronger than one having large grains. 6. Describe and explain solid solution strengthening for substitutional impurity atoms in terms of lattice strain interactions with dislocations. 7. Describe and explain the phenomenon of strain hardening (or cold working) in terms of dislocations and strain field interactions. 8. Describe recrystallization in terms of both the alteration of microstructure and mechanical characteristics of the material. 9. Describe the phenomenon of grain growth from both macroscopic and atomic perspectives.
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DISLOCATION MOTION
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ISLOCATIONS & MATERIALS CLASSES + • Metals: DISLOCATIONS & MATERIALS CLASSES + + + + + electron cloud ion cores • Covalent Ceramics (Si, diamond): • Ionic Ceramics (NaCl): - - - - - - - - - -
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STRESS AND DISLOCATION MOTION lip plane slip plane normal, n s A s τ R F s τ R
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ip bands Slip bands
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ip Systems Slip Systems
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ip systems Slip systems
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CRITICAL RESOLVED SHEAR STRESS σ σ σ λ force direction and slip direction φ force direction and normal to slip plane
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oncept Check Concept Check uestion: plain the difference between Question: Explain the difference between resolved shear stress and critical resolved ear ress shear stress.
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ample Example We wish to produce a rod composed of a single crystal of pure aluminum, which has a critical resolved shear stress of 148 psi. We would like to orient the rod in such a manner that, when an axial stress of 500 psi is applied, the ddf b li i di ti 45 i ti t th i f th d d rod deforms by slip in a direction 45° direction to the axis of the rod and actuates a sensor that detects the overload. At what orientation should the rod be fabricated?
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This note was uploaded on 10/26/2009 for the course MSE 250 taught by Professor Jone during the Summer '09 term at University of Michigan-Dearborn.

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Chap+7+Posting - LearningObjectives: 1.

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