ENME382 S2010 0201 ch08

ENME382 S2010 0201 ch08 - Chapter 8 Deformation...

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ISSUES TO ADDRESS. .. Why are the number of dislocations present greatest in metals ? Chapter 8: Deformation & Strengthening Mechanisms Chapter 8 - ENME 382 Spring 2010 Section 0201 1 How are strength and dislocation motion related? Why does heating alter strength and other properties?
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• Covalent Ceramics • Metals (Cu, Al): Dislocation motion easiest - non-directional bonding - close-packed directions for slip electron cloud ion cores + + + + + + + + + + + + + + + + + + + + + + + + Chapter 8 - ENME 382 Spring 2010 Section 0201 2 (Si, diamond): Motion difficult - directional (angular) bonding • Ionic Ceramics (NaCl): Motion difficult - need to avoid nearest neighbors of like sign (- and +) + + + + + + + + + + + - - - - - - - - - -
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Dislocation Motion • Metals - plastic deformation occurs by slip – an edge dislocation (extra half-plane of atoms) slides over adjacent plane half-planes of atoms. Chapter 8 - ENME 382 Spring 2010 Section 0201 3 • If dislocations can't move, plastic deformation doesn't occur! Adapted from Fig. 8.1, Callister & Rethwisch 3e.
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Dislocation Motion • A dislocation moves along a slip plane in a slip direction perpendicular to the dislocation line • The slip direction is the same as the Burgers vector direction Edge dislocation Chapter 8 - ENME 382 Spring 2010 Section 0201 4 Screw dislocation Adapted from Fig. 8.2, Callister & Rethwisch 3e.
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Slip System – Slip plane - plane on which easiest slippage occurs • Highest planar densities (and large interplanar spacings) – Slip directions - directions of movement • Highest linear densities Deformation Mechanisms Adapted from Fig. 8.6, Callister & Chapter 8 - ENME 382 Spring 2010 Section 0201 5 Rethwisch 3e. – FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed) => total of 12 slip systems in FCC – For BCC & HCP there are other slip systems.
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Stress and Dislocation Motion Resolved shear stress, τ R – results from applied tensile stresses slip plane normal, n s Resolved shear stress: τ R = F s / A s τ R Relation between σ and τ R τ R = F S / A S Applied tensile stress: = F / A σ F A Chapter 8 - ENME 382 Spring 2010 Section 0201 6 A S τ R F S F cos λ A /cos φ λ F F S φ n S A S A F φ λ σ = τ cos cos R
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• Condition for dislocation motion: CRSS τ > τ R • Ease of dislocation motion depends on crystallographic orientation 10 -4 GPa to 10 -2 GPa typically φ λ σ = τ cos cos R Critical Resolved Shear Stress σ σ σ Chapter 8 - ENME 382 Spring 2010 Section 0201 7 τ maximum at λ = φ = 45º τ R = 0 λ =90° τ R = σ /2 λ =45° φ =45° τ R = 0 φ =90°
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Adapted from Fig. 8.9,
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This note was uploaded on 01/24/2011 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.

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ENME382 S2010 0201 ch08 - Chapter 8 Deformation...

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