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Materials Science & Engineering I

by: Dr. Kelton Rau

Materials Science & Engineering I MSE 2101

Dr. Kelton Rau
GPA 3.88


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This 5 page Class Notes was uploaded by Dr. Kelton Rau on Thursday September 17, 2015. The Class Notes belongs to MSE 2101 at University of Connecticut taught by Staff in Fall. Since its upload, it has received 26 views. For similar materials see /class/205838/mse-2101-university-of-connecticut in Material Science and Engineering at University of Connecticut.

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Date Created: 09/17/15
Lecture 6 amp7 IMPERFECTIONS IN SOLIDS ISSUES TO ADDRESS and controlled What types of defects arise in solids How do defects affect material properties Are defects undesirable Can the number and type of defects be varied How do defects affect mechanical properties INTERSTITIAL SOLID SOLUTION Main consideration Impurity atoms fill the voids or interstices in host lattice So atomic diameter of interstitial impurity must be substantially smaller than that of host atoms Normally maximum allowable concentration of interstitial impurity atoms is lt 10 as even very small atoms introduce huge lattice distortions Example Carbon when added up to 2 to iron forms an interstitial solution Atomic radii of C and Fe are 0071 and 0124 nm SIZE OF INTERSTITIALS FOO r a 2R 2126 213 J5 1R 2 2 r 0414R SIZE OF INTERSTITIALS BOO Interstitial sites tetrahedral site 22 32 ltR rgt2 2 2 4R 4R j j R22Rrr2 2J5 4J I2 2Rr 0667R2 o r 0291R ALLOYING A SURFACE Low energy electron microscope view of a 111 surface of Cu Sn islands move along the surface and quotalloyquot the Cu with Sn atoms to make quotbronzequot The islands continually move into quotunalloyedquot regions and leave tiny bronze particles in their wake Eventually the islands disappear Reprinted with permission from A K Schmid NC Bartet and RC Hwang quotAlloying at Surfaces by the Migration of Reactive TwoDimensional Islandsquot Science Vol 290 No 5496 pp 156164 2000 Field of view is 15 mm and the temperature is 290K COMPOSITION Definition Amount of impurity B and host A in the system Two descriptions Weight Atom mass of Bx 1 00 C atoms of I3x 100 total mass total atoms Conversion between wt and at in an AB alloy C39 A C IA CB39 3339 x100 03 B B 0 AAA C BAB CAAA CBAB atomic weight of B mass of B moles of B x Ayatomio weight ofA mass of A moles of A x AA Basis for conversion LINE OR LINEAR DEFECTS Dislocations are line defects 3 types edge screw amp mixed cause slip between crystal plane when they move produce permanent plastic deformation Schematic of a Zinc HCP bar before deformation after tensile elongation ELASTIC DEFORMATION 1 Initial 2 Small load 3 Unload bonds stretch return to initial Elastic means reversible PLASTIC DEFORMATION METALS 1 Initial 2 Large load 3 Unload bonds stretch 39amp planes shear Plastic means permanent Dislocations help this process EDGE DISLOCATION Extra half plane of atoms Burgers Vector Florin 43 The atom positions around an edge dislocation extra halfplane of atoms I i DIET shown in perspective Adapted from A G Guy Essentials of Materials Science MCGfaWHlll BOOk COmpany New York IIll39Illl39llIWIIIMI39TIII 197612 153 d39s39liit39o 0 0 0 ciao llllllgllillllllll 1739 39 ritr r vrr 1 quotl 39illi iquot aa aa39 II 0 Symbol l7 mill W l A Edge dislocation line Directed into the page Atoms above dislocation line are in compression and those below are in tension SCREW DISLOCATION Ficum 44 a A screw dislocation within a crystal b The screw dislocation in a as Viewed from above The dislocation line extends along line AB Atom positions above the slip plane are designated by open circles those below by solid circles Flgure b from W T Read Jr Dislocations in Crystals McGraw 0 Hill Book Company New York 1953 gt gtlt gtlt gt X D lt Q Z Dislocation gtlt line 2 Burge t h v Symbol a AB is screw dislocation line MIXED DISLOCATIONS hum 43 a Schematic re resentation of a disliocation that has edge screw and mixed character b Top View where 0 en circles enote atom positions a P ozo ozo above the slip plane Solid circles atom positions below At point A the dislocation is pure screw while at point B it is pure edge A For regions in between where there is curvature in the A dislocation line the gtgt 2 b gtltgtlt character is mixed edge gtltgtlt J and screw Figure b gtlt from W T Read Jr V Dislocations in Crystals McGrawHill Book Company New York 1953 a BU RGER S VECTOR Edge dislocation Screw dislocation Burger s vector gt I is 79 x 7 f Mng m 39 hi fr l 1 I I I rg V104 Dislocation e W line Dislocation line into page s 39 1 Edge dislocation Burger s vector perpendicular to dislocation line Screw dislocation Burger s vector parallel to dislocation line Mixed dislocation Orientation of Burger s vector with respect to dislocation changes with position HOW DOES A DISLOCATION HELP PLASTIC DEFORMATION 8elasticplasti Edge o quotl dislocation r 3950 oo I III IIILlllJIIIQAIIIJ39II N 39 J quotk g I 39L a II I till K K 1 I 7wa frlg fl For plastic deformation without dislocations several bonds have to be broken simultaneously and remade after sliding Dislocations allow for a step by step incremental breaking and creation of bonds much less energetic cost BOND BREAKING AND REMAKING Dislocation motion requires the successive bumping of a half plane of atoms from left to right here Bonds across the slipping planes are broken and remade in succession Ititoooooog Iliiioooiog UOOIIIGOGO Clottiouioo Atomicviewofedge dislocation motion from 00000 IGfttorIghtasacrystal IOIDGOIOI Swmi O I O O I I l I l I I 39 Slipplane Courtesy PM Anderson INCREMENTAL SLIP Simulation of dislocation motion from left to right as a crystal is sheared Slip plane Courtesy PM Anderson view onto two closepacked planes Preferred slip planes amp directions closepacked planes amp directions closepacked plane bottom losepacked plane top Comparison among crystal structures FCC many closepacked planesdirections HCP only one plane 3 directions BCC none Results of tensile Mg HCP testing Typically amount of I ensile direction deformation figfrr 3235565 I 332jrr r AI FCC FCC gt HCP gt BCC ANOTHER TYPE OF EDGE DISLOCATION MOTION CLIMB 0 O COOLOO 0 O I 0 O OOOOEI COCO Vacancy capture by dislocation line dislocation climbs up COOLOO 0 O 0 O I OOEI O 0 0 Nearby atom moves into dislocation leaving a vacancy nearby Dislocations are the major source amp sink for vacancies Vacancies allow dislocations to climb updown TYPES OF GRAIN BOUNDARIES TWIN Grain boundary is a mirror plane Region of material on either side of boundary are called twins Twin plane boundary Fini39m 49 Schematic diagram showing a twin plane or boundary and the adjacent atom positions dark circles OTHER TYPES OF DEFECTS Stacking fault An interruption or disruption in the stacking sequence of planes Example Stacking sequence of FCC is ABCABCABCABC Stacking fault ABCABCABABC Phase boundaries Boundary between two different phases that differ in chemical composition Domain walls Boundary between regions of different magnetization in magnetic materials Bulk or Volume defects Pores cluster of vacancies inclusions cluster of impurities etc MICROSCOPIC EXAMINATION TECHNIQUES Optical microscopy Useful up to ZOOOX magnification Limited by wavelength of visible light 1 micron Electron microscopy Wavelength of electrons lt 1 Angstrom So can resolve atomic level features Scanning probe microscopy Probe maps out surface contours OPTICAL MICROSCOPY 1 Sample polished amp etched Polishing removes unwanted surface features eg scratches Etching changes reflectance depending on crystal orientation closepacked planes Adapted from Fig 411b and c Calister 6e Fig 411c is courtesy of JE Burke General Electric Co micrograph of Brass Cu and Zn j lt O75mm gt FILL IH 412 a Section of a grain boundary and its surface groove produced by etching the light re ection OPTICAL MICROSCOPY 2 Grain boundaries are imperfections are more susceptible are also shown b Photomicrograph of the surface of a polished and etched polycrystalline specimen of an iron chromium alloy in which the grain bound 5 appear dark 100x Photom rogr ph courtesy of L C Smith dCB dthNt lB f to etching 333 Nation 1 Instit of Standard 1 O m d Techno gy G thersburg MD d a rk II n e s characteristics in the Vicinity of the groove ELECTRON MICROSCOPY e we Nickel alloy Juan Villegas PhD thesis UConn MSE IMPORTANCE OF GRAIN BOUNDARIES l Grain Boundaries block dislocations 1 strength l ductility La 2 Small grain size strengthens metals Since dislocations move less before being blocked by a Grain Boun ary 3 lmpurities often segregate there due to extra space Some impurities Weaken materials like sulfur ll l steel 4 Grain Growth heating causes small grains to combine ll ito l rgerones heated Question How does this altermechanical properties Stimulus Questions Identify the crystal defects in the schematic below BOWM VEBMquotVquot39 V 39 w 000986me 5 Begum I 0m Stimulus Questions Compare Slip and Climb of dislocations which one is easierto happen Why are metals ductile and ceramics brittle Defects Good or Bad Give your two cent SUMMARY Point Line and Area defects arise in solids The number and type of defects can be varied and controlled eg T controls vacancy conc Defects affect material properties eg grain boundaries control crystal slip Defects may be desirable or undesirable eg dislocations may be good or bad depending on whether plastic deformation is desirable or not ANNOUNCEMENTS Reading Chapter 5 351 15 points 356 15 points 426 2 points 431 1 points


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