ch08 - 2/11/2011 Chapter 8: Deformation & Strengthening...

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2/11/2011 1 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 Chapter 8 - 2 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 + + + + + + + + + + + + + + + + + + + + + + + + Chapter 8 - 3 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, Cal ister & Rethwisch 3e. Chapter 8 - 4 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, Cal ister & Rethwisch 3e. Chapter 8 - 5 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, Cal ister & 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. Chapter 8 - 6 Stress and Dislocation Motion Resolved shear stress, t R – results from applied tensile stresses slip plane normal, ns Resolved shear stress: t R = F s / A s A S t R t R F S Relation between s and t R t R = F S / A F cos l A / cos f l F F S f n S A S A Applied tensile stress: = F / A s F A F f l s t cos cos R
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2/11/2011 2 Chapter 8 - 7 • Condition for dislocation motion: CRSS t t R • Ease of dislocation motion depends on crystallographic orientation 10 -4 GPa to 10 -2 GPa typically f l s t cos cos R Critical Resolved Shear Stress t maximum at l = f = 45º t R = 0 l =90° s t R = s /2 l =45° f =45° s t R = 0 f =90° s Chapter 8 - 8 Single Crystal Slip Adapted from Fig. 8.8, Cal ister & Rethwisch 3e. Adapted from Fig. 8.9, Cal ister & Rethwisch 3e. 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 s f l s t l = 35° f = 60° t crss = 20.7 MPa a) Will the single crystal yield? b) If not, what stress is needed? s = 6500 psi Adapted from Fig. 8.7, Cal ister & Rethwisch 3e. t (45 MPa) (45 MPa) (0.41) 18.5 MPa crss (cos35 ) (cos60 ) 20.7 MPa Chapter 8 - 10 Ex: Deformation of single crystal psi 7325 41 . 0
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ch08 - 2/11/2011 Chapter 8: Deformation &amp; Strengthening...

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