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Published byLorin Hopkins Modified over 9 years ago
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PLASTIC DEFORMATION Dislocations and their role in plastic deformation
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What are dislocations? n Dislocations are line defects that exist in metals n There are two types of dislocations: edge and screw n The symbol for a dislocation is The dislocation density in annealed metals is normally = 10 6 /cm 2
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Types of dislocations Edge Screw
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Dislocation motion plastic deformation Note: Dislocations normally move under a shear stress
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How does a dislocation move?
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Stress field of a dislocation
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Analog to an electric charge
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Modes of deformation n Slip n Twinning n Shear band formation
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Slip n Dislocations move on a certain crystallographic plane: slip plane n Dislocations move in a certain crystallographic direction: slip direction n The combination of slip direction and slip plane is called a slip system
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Slip….. n Slip planes are normally close-packed planes n Slip directions are normally close-packed directions Recall for fcc close-packed planes are {111} Close-packed directions are
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Slip systems
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Dislocation interaction Repulsion Attraction & Annihilation Positive Negative Note: More positive-positive interactions in reality
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Positive-positive dislocation interaction n Results in more stress to move dislocations (or cause plastic deformation):called work hardening n This type of interaction also leads to dislocation multiplication which leads to more interactions and more work hardening
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Twinning n Common in hcp and bcc structures n Limited deformation but help in plastic deformation in hcp and bcc crystals n Occurs on specific twinning planes and twinning directions
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Compare slip and twinning
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Shear band formation n Limited non-homogeneous deformation Very large localized strain ~1 or 100% n Occurs especially under high strain rates n Mechanism of deformation still unclear
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Plastic deformation movement of dislocations Strengthening methods
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Cold working n Deformation at temperatures below 0.4 T m n Dislocation density increases from 10 6 /cm 2 to 10 10-12 /cm 2 n High dislocation density results in a large number of dislocation interactions which results in high strength and hardness
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Solid solution strengthening n Interaction between stress fields of alloy atoms and dislocations n This is the purpose of alloying
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Grain size refinement n Small grains result in higher strength n Small grains is equivalent to a large number of grain boundaries in the same volume n Grain boundaries act as barriers to dislocation motion
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Mechanism Strength is inversely proportional to grain size = 0 + k y d -1/2 Hall-Petch equation Smaller grains have more boundary area and hence more barriers to dislocation motion
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Precipitation hardening n Precipitates are second-phase particles n Hard precipitates act as barriers to dislocation motion n Applicable only to some alloy systems
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