Lecture11

Lecture11 - ENGR 2220 Structure & Properties of...

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ENGR 2220 ENGR 2220 – Structure & Properties of Materials Lecture 11 Dislocations & Strengthening Mechanisms hapter 7 Chapter 7
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Dislocation Motion • Dislocation moves along slip plane in slip direction perpendicular to dislocation line • Slip direction same direction as Burgers vector Edge dislocation Adapted from Fig. 7.2, Callister 7e. Screw dislocation
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Dislocations & Materials Classes • Metals: Disl. motion easier. -non-directional bonding + + + + + + + + -close-packed directions for slip. electron cloud ion cores + + + + + + + + + + + + + + + + • Covalent Ceramics (Si, diamond): Motion hard. irectional (angular) bonding -directional (angular) bonding • Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and - - neighbors. + + + + + + + ++++ --- - - - -
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Dislocation Density
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Effects of Stress at Dislocations Adapted from Fig. 7.5, Callister 7e.
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ELASTIC DEFORMATION 1. Initial 2. Small load 3. Unload bonds stretch turn to δ return to initial F F Linear- lastic Elastic means reversible ! elastic Non-Linear- elastic δ
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PLASTIC DEFORMATION (METALS) 1. Initial 2. Small load 3. Unload nes bonds planes still sheared stretch & planes shear lastic + plastic δ plastic F δ elastic + plastic Plastic means permanent ! linear linear F δ elastic elastic δ plastic
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SIMULATION: DISLOCATION OTION/GENERATION Tensile loading (horizontal dir.) of a FCC metal with MOTION/GENERATION notches in the top and bottom surface. Over 1 billion atoms modeled in 3D block. Note the large increase in disl. density. Simulation courtesy f Farid Abraham. Used with of Farid Abraham. permission from International Business Machines Corporation.
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φ Angle between normal to the ip plane and applied stress slip plane and applied stress l b t th li di ti λ Angle between the slip direction and applied stress φ λ σ = τ cos cos R
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Stress and Dislocation Motion • Crystals slip due to a resolved shear stress, τ R . • Applied tension can produce such a stress. lip plane Resolved shear stress: τ R = F s / A s Relation between σ and τ R Applied tensile stress: = F / A σ F slip plane normal, n s A S τ R τ R = F S / A S F cos λ A /cos φ A τ R F S λ F F S φ n S A A S F φ λ σ = τ cos cos R
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Critical Resolved Shear Stress • Condition for dislocation motion: CRSS τ > τ R Crystal orientation can make • Crystal orientation can make it easy or hard to move dislocation Material Prop. φ λ σ = τ cos cos R σ σ σ τ R = 0 τ R = σ /2 τ R = 0 λ =90° λ =45° φ =45° φ =90° τ maximum at λ = φ = 45º
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Single Crystal Slip Adapted from Fig. 7.9, Callister 7e. Adapted from Fig. 7.8, Callister 7e.
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Slip Motion in Polycrystals Stronger - grain boundaries pin deformations σ Slip planes & directions ( λ , φ ) change from one rystal to another Adapted from Fig. 7.10, Callister 7e. (Fig. 7.10 is crystal to another. τ R will vary from one rystal to another (g courtesy of C. Brady, National Bureau of Standards [now the National Institute of crystal to another.
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Lecture11 - ENGR 2220 Structure & Properties of...

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