Chapter7c

Chapter7c - Chapter Outline Introduction Dislocations and...

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1 1 MSE 2090: Introduction to Materials Science Chapter 7, Strengthening Dislocations and Strengthening Mechanisms What is happening in material during plastic deformation? Chapter Outline ± Dislocations and Plastic Deformation 9 Motion of dislocations in response to stress 9 Slip Systems 9 Plastic deformation in ± single crystals ± polycrystalline materials ± Strengthening mechanisms 9 Grain Size Reduction 9 Solid Solution Strengthening 9 Strain Hardening ± Recovery, Recrystallization, and Grain Growth Not tested: 7.7 Deformation by twinning, Direction and plane nomenclature in §7.4. 2 MSE 2090: Introduction to Materials Science Chapter 7, Strengthening Why metals could be plastically deformed? Why the plastic deformation properties could be changed to a very large degree by forging without changing the chemical composition? Why plastic deformation occurs at stresses that are much smaller than the theoretical strength of perfect crystals? Introduction These questions can be answered based on the idea proposed in 1934 by Taylor, Orowan and Polyani: Plastic deformation is due to the motion of a large number of dislocations. Plastic deformation – the force to break all bonds in the slip plane is much higher than the force needed to cause the deformation. Why? 3 MSE 2090: Introduction to Materials Science Chapter 7, Strengthening If the top half of the crystal is slipping one plane at a time then only a small fraction of the bonds are broken at any given time and this would require a much smaller force. The propagation of one dislocation across the plane causes the top half of the crystal to move ( to slip ) with respect to the bottom half but we do not have to break all the bonds across the middle plane simultaneously (which would require a very large force). The slip plane – the crystallographic plane of dislocation motion. Dislocations allow deformation at much lower stress than in a perfect crystal 4 MSE 2090: Introduction to Materials Science Chapter 7, Strengthening Direction of the dislocation motion For mixed dislocations, direction of motion is in between parallel and perpendicular to the applied shear stress Edge dislocation line moves parallel to applied stress Screw dislocation line moves perpendicular to applied stress
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2 5 MSE 2090: Introduction to Materials Science Chapter 7, Strengthening Strain field around dislocations Edge dislocations introduce compressive, tensile, and shear lattice strains, screw dislocations introduce shear strain only. Dislocations have strain fields arising from distortions at their cores - strain drops radially with distance from the dislocation core 6 MSE 2090: Introduction to Materials Science Chapter 7, Strengthening Interactions between dislocations The strain fields around dislocations cause them to interact (exert force on each other). When they are in the same plane, they repel if they have the same sign (direction of the Burgers vector) and attract/annihilate if they have opposite signs. 7
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Chapter7c - Chapter Outline Introduction Dislocations and...

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