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10-14-Lecture - Defo rm a tion and Dislocations Part B ENGR...

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1 Defo rm a tion and Dis locations Part B ENGR 213
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STRENGTHENING MECHANISMS IN METALS Aim: to impede dislocation motion ductility usually sacrificed recover w/ heat treatment Four methods grain size reduction (refinement) -- impeding dislocation movement alloying (solid-solution) -- adding soluble impurities strain hardening -- increasing numbers of dislocations precipitation hardening -- adding more phases adjust cooling path produces multiple phases (Chap 11)
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IMPEDING DISLOCATIONS edge dislocation Block movement along slip plane CLIMB: requires diffusion (temp) screw dislocation Detour around obstacle CROSS-SLIP
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4 Method #1 Strengthening by Grain Reduction During deformation, slip or dislocation must take place across a common boundary Grain boundary Change of direction of motion across boundaries is more difficult if misorientation increases Atomic disorder within grain region results in a discontinuity of slip planes from one grain to the other
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5 Motion of Dislocation as It Encounters a Grain Boundary
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GRAIN SIZE REDUCTION dislocation must change direction = f (angle) discontinuity of slip planes OR dislocation source stronger, tougher grain size (d = avg grain diameter) intercept method ASTM standard comparison charts controlled by rate of solidification OR plastic deformation + heat treatment d k y y + = 0 σ σ
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7 Influence of Grain Size on Yield Strength of 70 Cu – 30 Zn Brass Alloy YS Grain size
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