Lecture 352014.pdf

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Materials Science
MATS 3011
Bharat Jalan

Lecture 352014Tuesday March 4 20141228 PMTransmission Electron Microscopecant use light to see electrons using visible lightshoot electrons down column through lenses like induction coilshits screen on bottom can do diffraction or visualizationDislocations dont have to be straitcan be curved linesmixture of pure edge and pure screw dislocationsDislocation Motioneasier to move the dislocation rather than moving the plane of atomslike moving a carpet or a worm movingHow does a single plane of atoms cause steel to elongate by 20a single dislocation might just cause a shear to shift off by 1hard to seebut if have many different planes see the actual shearnot individual atomic planesHow a material deforms by Resolved shear stress onto a glide plane due to applied stressarea of slip planeDislocation might not move along plane of shear but maybe along a diagonal planethe slip planeredonly the amt force projected on the plane can move the dislocationAAcosphiResolved shear stressFcoslambdaAFcoslambdacosphiAsigmacoslambdacosphiRwhere lambdaangle between F and slip directionphiangle btwn F and slip plane normalWhen does the material yield when starts deforming plasticallymust reach a critical value for plastic deformationRThis critical value critical resolved shear stresssigmacoslambdacosphiCRyThis eqnimportant bc can be measured experimentallyUsually one of the slip systems will minimize sigmayslip system gives coslambdacosphi that one that minimizesweakest slip in systemwe assumed that this materialsingle crystalPolycrystalline materialscomplex randomly oriented grainsfor each grain dislocations move in most favorable direction slip directionbut for random grains move randomlygrain boundaries dont open updislocation motions stop at grain boundaries the smaller the grain sizes the stronger the materialHallPetch equation MatSci Page 1
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