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10.16 - Chapter 8 Deformation Strengthening Mechanisms...

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Chapter 8 - 1 ISSUES TO ADDRESS... Why are the number of dislocations present greatest in metals ? How are strength and dislocation motion related? Why does heating alter strength and other properties? Chapter 8: Deformation & Strengthening Mechanisms
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Chapter 8 - 2 Dislocation Motion Dislocation motion & plastic deformation Metals - plastic deformation occurs by slip – an edge dislocation (extra half-plane of atoms) slides over adjacent plane half-planes of atoms. If dislocations can't move, plastic deformation doesn't occur! Adapted from Fig. 8.1, Callister & Rethwisch 3e.
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Chapter 8 - 3 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 Screw dislocation Adapted from Fig. 8.2, Callister & Rethwisch 3e.
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Chapter 8 - 4 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 & 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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Chapter 8 - 5 Dislocations & Materials Classes • Covalent Ceramics (Si, diamond): Motion difficult - directional (angular) bonding • Ionic Ceramics (NaCl): Motion difficult - need to avoid nearest neighbors of like sign (- and +) + + + + + + + + + + + - - - - - - - - - - • Metals (Cu, Al): Dislocation motion easiest - non-directional bonding - close-packed directions for slip electron cloud ion cores + + + + + + + + + + + + + + + + + + + + + + + +
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Chapter 8 - 6 Stress and Dislocation Motion Resolved shear stress, τ R results from applied tensile stresses slip plane Resolved shear stress: τ = F s / A s slip direction A τ τ F slip direction Relation between σ and τ R = F F cos λ A /cos φ λ F F φ A Applied tensile stress: = F / A σ slip direction F A F φ λ σ = τ cos cos R
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Chapter 8 - 7 • Condition for dislocation motion: CRSS τ τ Ρ • 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 τ maximum at λ = φ = 45º = 0 λ =90° σ = σ /2 λ =45° φ =45° σ = 0 φ =90° σ
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Chapter 8 - 8 Single Crystal Slip Adapted from Fig. 8.8, Callister & Rethwisch 3e. Adapted from Fig. 8.9, Callister & Rethwisch 3e.
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Chapter 8 - 9 Ex: Deformation of single crystal So the applied stress of 6500 psi will not cause the crystal to yield. MPa 20.7 cos cos = σ φ λ σ = τ λ = 35° φ = 60° τ crss = 20.7 MPa a) Will the single crystal yield? b) If not, what stress is needed? σ = 6500 psi Adapted from Fig. 8.7, Callister & Rethwisch 3e. τ = (45ΜΠα 29 = (45ΜΠα 29( 0.41 29 τ = 18.5ΜΠα < τ χρσσ = (χοσ35 ο 29 (χοσ60 ο 29 20.7ΜΠα
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Chapter 8 - 10 Ex: Deformation of single crystal psi 7325 41 . 0
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