To minimize deformation select a material with a

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To minimize deformation, select a material with alarge elastic modulus (EorG)large elastic modulus (EorG).Plasticbehavior:This permanent deformationbehavior occurs when the tensile (or compressive)ii lthDuctility:The plastic strain at failure.uniaxial stress reachesy.38
Dislocation MotionDislocations & plastic deformationCubic & hexagonal metals - plastic deformation byplastic shear orslipwhere one plane of atoms slides over adjacent plane by defectmotion (dislocations).39If dislocations don't move, deformation doesn't occur!Adapted from Fig. 7.1,Callister 7e.
Dislocation MotionDislocation moves alongslip planeinslip directionperpendicular todislocation lineSlip direction same direction asBurgers vectorEdge dislocationAdapted from Fig. 7.2,Callister 7e.Screw dislocation40
Slip in CrystalsSlip occurs in densely or close packed planes.Lower shear stress is required for slip to occur in densely packed planes.Lower shear stress is required for slip to occur in densely packed planes.If slip is restricted in closeplanes, then less denseClose packedplanes become operative.Less energy is requiredto move atoms alongClose packedplanedenser planes.Non-close-packed41planeFoundations of Materials Science and Engineering, 5th Edn.Smith and Hashemi
Slip SystemsSlip systems are combination of slipplanes and slip direction.Each crystal has a number ofcharacteristic slip systems.In FCC crystal, slip takes place in {111}t h dlld110ditioctahedral planes and <110> directions.4 (111) type planes and3 [110] type directions.4 x 3 = 12 slip systems.42Foundations of Materials Science and Engineering, 5th Edn.Smith and Hashemi
Stress and Dislocation Motion• Crystals slip due to aresolved shear stress,R.• Applied tension can produce such a stress.Applied tension can produce such a stress.Resolved shearstress:R=Fs/AsRelation betweenandRAppliedtensilestress:=F/Aslip planenormal,nsstress:RFs/AsRandRR=FS/ASFA/stress:F/AFAASFSFcosA/cosFnSARFSASAF43coscosR
Critical Resolved Shear Stress• Condition for dislocation motion:CRSSR• Crystal orientation can makeit easy or hard to move dislocation10-4GPa to 10-2GPatypicallycoscosRR= 0=90°R=/2=45°R= 0=90°9045=45°9044maximum at== 45º
Example 1:Calculation of resolved shear stress (1)Calculate the resolved shear stress on theslip system of a FCC nickel single110111slip system of a FCC nickel singlecrystal if a stress of 13.7MPa is applied in the [001]direction of a unit cell.110111Loading direction =[001] in a unit cell[001]45
Example 2:Calculation of resolved shear stress (2)A stress of 55MPa is applied in the [001] direction of a BCC singlecrystal. Calculatea)The resolved shear stress acting on theslip systemb)The resolved shear stress acting on theslip system111101111110Loading direction =Loading direction =[001] in a unit cell[001]46

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Term
Three
Professor
GwenaelleProust
Tags
Stress, Tensile strength, plastic deformation

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