Lecture 10 - Dislocations & Strengthening Mechanisms...

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1 Dislocations & Strengthening Dislocations & Strengthening Mechanisms Mechanisms ISSUES TO ADDRESS. .. • Why are dislocations observed primarily in metals and alloys? • How are strength and dislocation motion related? • How do we increase strength? • How can heating change strength and other properties? Dislocations & Strengthening Mechanisms
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2 Dislocations & Materials Classes Dislocations & Materials Classes • Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding • Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and - - neighbors. ++++ + + + --- - - - - • Metals: Disl. motion easier. -non-directional bonding -close-packed directions for slip. electron cloud ion cores + + + + + + + + + + + + +++++ + + + + + + + Dislocation Motion 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, deformation doesn't occur! Adapted from Fig. 7.1, Callister 7e.
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3 Dislocation Motion Dislocation Motion Dislocation moves along slip plane in slip direction perpendicular to dislocation line Slip direction same direction as Burgers vector Edge dislocation Screw dislocation Adapted from Fig. 7.2, Callister 7e. Slip System Slip plane - plane allowing easiest slippage Wide interplanar spacings - highest planar densities Slip direction - direction of movement - Highest linear densities FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed) => total of 12 slip systems in FCC in BCC & HCP other slip systems occur Deformation Mechanisms Deformation Mechanisms Adapted from Fig. 7.6, Callister 7e.
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4 Stress and Dislocation Motion Stress and Dislocation Motion • Crystals slip due to a resolved shear stress, τ R . • Applied tension can produce such a stress. slip plane normal, n s Resolved shear stress: τ R = F s / A s s l i p d r e c t o n A S τ R τ R F S Relation between σ and τ R τ R = F S / A S F cos λ A /cos φ λ F F S φ n S A S A Applied tensile stress: = F / A σ F A F φ λ σ = τ cos cos R • Condition for dislocation motion: CRSS τ > τ R • Crystal orientation can make it easy or hard to move dislocation 10 -4 GPa to 10 -2 GPa typically φ λ σ = τ cos cos R Critical Resolved Shear Stress Critical Resolved Shear Stress τ maximum at λ = φ = 45º τ R = 0 λ =90° σ τ R = σ /2 λ =45° φ =45° σ τ R = 0 φ =90° σ
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5 Single Crystal Slip Single Crystal Slip Adapted from Fig. 7.8, Callister 7e.
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Lecture 10 - Dislocations &amp; Strengthening Mechanisms...

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