Chapter 7- Dislocations and strengthening mechanisms

Chapter 7- Dislocations and strengthening mechanisms -...

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Chapter 7: Dislocations & Strengthening Mechanisms 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 - 1
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Plastic Deformation Chapter 7 - 2 F δ linear elastic linear elastic δ Plastic deformation 1. Initial 2. Small load 3. Unload planes still sheared F δ elastic + plastic bonds stretch & planes shear δ plastic Plastic deformation most often involves the motion of dislocations
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Dislocations in Crystalline Solids Dislocations visible in electron micrographs Chapter 7 - 3 Adapted from Fig. 4.6, Callister 7e. TEM micrograph of Ti alloy (50,000x) Dislocation density in metals : from 10 3 mm -2 (carefully solidified) to 10 10 mm -2 (heavily deformed) Dislocation density in ceramics : from 10 2 mm -2 to 10 4 mm -2
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Edge Dislocation Fig. 4.3, Callister 7e. Chapter 7 - 4
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Screw Dislocation Adapted from Fig. 4.4, Callister 7e. Burgers vector b Dislocation line b (a) (b) Chapter 7 - 5
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Stress Concentration at Dislocations Adapted from Fig. 7.4, Callister 7e. Chapter 7 - 6
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Dislocation Motion Dislocations & plastic deformation Cubic & hexagonal metals - plastic deformation by plastic shear or slip where one plane of atoms slides over adjacent plane by defect motion (dislocations). If dislocations don't move, plastic deformation doesn't occur! Chapter 7 - 7 Adapted from Fig. 7.1, Callister 7e.
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Dislocation Motion Dislocation moves along the slip plane in the slip direction Burgers vector shows the direction of shear stress necessary to induce dislocation motion Chapter 7 - 8 Adapted from Fig. 7.2, Callister 7e. Edge dislocation slip b τ direction Screw dislocation slip b direction
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Deformation Mechanisms Slip System – combination of slip plane and slip direction • Slip plane - plane allowing easiest slippage of dislocation Highest planar densities - wide interplanar spacings • Slip direction - direction of dislocation movement Highest linear densities Slip plane: ( 111 ) Slip Directions: <110> Adapted from Fig. 7.6, Callister 7e. Chapter 7 - 9 – FCC: Slip occurs on {111} planes (close-packed) in <110> directions (close-packed) => total of 12 slip systems in FCC: 4 {111} x 3<111> Large number of slip systems => ductile metals
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Slip Systems in BCC BCC Slip occurs on {110} planes (near close-packed) in <111> directions (close-packed) Adapted from Fig. 3.6, Callister 7e. Slip planes: { 110 } Slip Directions: <111> => total of 12 slip systems in BCC: 6 {110} x 2<111> HCP: 3 to 6slip systems => more brittle Chapter 7 - 10
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Simple Cubic Structure (SC) Close-packed directions are cube edges. Chapter 7 - 11 (Courtesy P.M. Anderson) SC: Slip occurs on {100} planes in <010> directions (close- packed) => total of 6 slip systems in SC: 3 {100} x 2<010>
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Line Defects - Dislocations • are line defects, slip between crystal planes result when dislocations move, produce permanent (plastic) deformation.
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Chapter 7- Dislocations and strengthening mechanisms -...

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