ch5-1 材料学.pptVIP

Chapter 5. Dislocations and Strengthening Mechanisms --- 1st part New words Dislocation 位错 Strengthen 强化 slip system 滑移系 solid solution strengthening 固溶强化 strain hardening 应变强化 Recovery 回复 Recrystallization 再结晶 Forging 锻造 Outline ! Dislocations and Plastic Deformation Motion of dislocations in response to stress Slip Systems Plastic deformation in ! single crystals ! polycrystalline materials ! Strengthening mechanisms Grain Size Reduction Solid Solution Strengthening Strain Hardening ! Recovery, Recrystallization, and Grain Growth Introduction 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? Dislocations allow deformation at much lower stress than in a perfect crystal 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. Direction of the dislocation motion For mixed dislocations, direction of motion is in between parallel and perpendicular to the applied shear stress. Strain field around dislocations Dislocations have strain fields arising from distortions at their cores - strain drops radially with distance from dislocation core. Edge dislocations introduce compressive, tensile, and shear lattice strains, screw dislocations introduce shear strain only. Interactions between Dislocations The strain fields around dislocations cause them to interact (exe