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Chapter 8 Deformation and Strengthening Mechanisms

Chapter 8 Deformation and Strengthening Mechanisms -...

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Chapter 8 Deformation and Strengthening Mechanisms Basic Concepts of Dislocations Edge and screw are the two fundamental dislocation types Edge dislocation = localized lattice distortion exists along the end of an extra half-plane of atoms, which also defines the dislocation line Screw dislocation = shear dislocation with the dislocation line passing through the center of a spiral, atomic plane ramp Mixed dislocations= crystalline materials with screw and edge dislocations Plastic deformation corresponds to the motion of large number of dislocations o no movement means no plastic deformation Edge dislocation moves by applying a shear stress perpendicular to the dislocation line Slip = process by which plastic deformation is produced by dislocation motion Slip plane = the crystallographic plane along which the dislocation line transverses Dislocation density = number of dislocations in a material is expressed as the total dislocation length per unit volume o OR the number of dislocations that intersect a unit area of a random section Characteristics of Dislocations About 5% of deformation energy is retained internally for metals that are plastically deformed; the rest dissipates as heat Lattice strain = slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations and interstitial and impurity atoms Atomic lattice distortion exists around dislocation line because of extra half plane of atoms Slip Systems Slip Plane = plane on which easiest slippage occurs o Highest planar densities and large interplanar spacings Slip Directions = directions of movement o Highest linear densities Burger vectors: (a=unit cell edge length) o b(FCC) = (a/2)*<110> o b(BCC) = (a/2)*<111> o b(HCP) = (a/3)*<11-20> Slip in Single Crystals Resolved shear stress o Results from applied tensile stresses τ R (max) = σ[cos(φ)cos(λ)] o sum of angles never equals 90 Critical Resolved Shear Stress o If both are 45 degrees yield strength = 2*Crit Shear Stress Condition for dislocation motion Plastic Deformation of Polycrystalline Metals Because of random crystallographic orientation of the number of grains direction of slip varies from one grain to another φ λ τ σ cos cos crss = y
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