Ch 4 notes -Materials Eng.
Ch 4 notes -Materials Eng. EGN 3365
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This 11 page Class Notes was uploaded by Kathleen Quijada on Friday January 30, 2015. The Class Notes belongs to EGN 3365 at Florida International University taught by Kinzy Jones in Spring2015. Since its upload, it has received 99 views. For similar materials see Materials Engineering in Engineering and Tech at Florida International University.
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Chapter f Chapter 4 Imperfections in Solids ISSUES TO ADDRESS What are the solidification mechanisms What types of defects arise in solids Can the number and type of defects be varied and controlled How do defects affect material properties Are defects undesirable Chapter4 1 Imperfections in Solids result of casting of molten material 2 steps Nuclei form Nuclei grow to form crystals grain structure Start with a molten material all liquid itjl 7 ed from Fig 414b Calisteramp RethWisch 8e Chapter4 2 liquid Crystals grow until they meet each other Aap ml Polycrystalline Materials Big angle boundaries quotI I quotjAngle of misalignment 39 b t t I ltgtltgt gtlt E gt lt lt regions e ween crys a s H A fwvvzx i quot200 quotquot H h re tranSItion from lattice of AxgtOgtl 399 3 9 l I r N I 4 I grail one region to that of the A My I wars31gt boundary quotquot m fI l I in 4 Other 1 r ijLLFfffs eq Smallangle cg 1 vlquot rain 397 I 39 39 11 L39 39J slightly disordered I733Ef gy m b0undary I 1quot h k 7 39L39 low denSIty in grain AU Vikki73 Vivian boundaries 39 I L h quot 39Iquotr 1 I Ir l 1quotI Is l t high mobility x gg v czgv uj 11 mgr5 I 15 high diffuswity Fr z u n 1 I r o Angle of misalignment Chapter4 3 high chemical reactivity Adapted from Fig 47 Calister amp Rethwisch 8e 8212012 Nuclei 9 CVVS39I39OIIS L gram shral Stvvclu is fit Same Small angle boundaries Physical 4 muvaml in materials Solidification equiaxed roughly same size in all directions columnar elongated grains 8 cm gt Grains can be heat flow Shell of Columnar in equiaxed grains area with less due to rapid undercooling cooling greater Adapted from Fig 517 near W8 Calister amp Rethwisch 36 Grain Refiner added to make smaller more uniform equiaxed grains Chapter4 4 8212012 Eqvioixcd when all grains are at am Sm size doricrHahon IS random impoth For CotsHm Heat ow OUl39 Imperfections in Solids There is no such thing as a perfect crystal What are these imperfections Why are they important Many of the important properties of materials are due to the presence of imperfections Chapter4 5 Cool WW1 in Coastal 0 m3 Unique man main crustals or glimmels are mil Pa a in Shape no such 39Hninj as o parka O JS MI Types of Imperfections Vacancy atoms Interstitial atoms Substitutional atoms Lela Point defects Dislocations Line defects Grain Boundaries Area defects 7D Chapter4 6 mensicmai amp Point Defects in Metals Vacancies vacant atomic sites in a structure 39 33 go JL 39 A v f 1 v A v A 0 oxtuozo t33 Selflnterstitials quotextraquot atoms positioned between atomic sites Vacancy distortion of planes self interstitial Chapter4 7 8212012 Happen nalwally IF Ht spawns not nal His or QLqul Equilibrium Concentration Point Defects Equilibrium concentration varies with temperature Activation energy N v expl 0V defect sites No of defects Boltzmann39s constant 138 x 103923 JatomK 862 x10395eVatomK qggggggp 00000000 4 qggggggp 00000000 is a potential vacancy site NV qu kff an lc39 V Dl l39 par MW Each lattice site Chapter4 8 Measuring Activation Energy We can get Q from an experiment Measure this exponen m dependence T defect concentration Chapter4 9 Nv cltFCc con ca fa h39on N Cam calCulalc 39an allwa M X Int ROMP N gtlt P A39cu SH CS 1 WW 0v 0 2 Hum Estimating Vacancy Concentration Find the equil of vacancies in 1 m3 of Cu at 1000 C Given p 84 gcm3 ACu 635 gmol QV 09 eVatom NA 602 x 1023 atomsmol 09 eVatom N WV exp Tquot 27x 1o4 A 1273 K 862 x 10395 eVatomK For1m3N p x A x1m3 80x 1028 sites Cu Answer NV 27 x 1039480 x 1028 sites 22 x 1025 vacancies Chapter4 10 Observing Equilibrium Vacancy Conc Low energy electron microscope view of a 110 surface of MN Increasing temperature causes surface island of atoms to grow Why The equil vacancy conc increases via atom motion from the crystal to the surface where they join the island Click once on image to start animation Reprinted with permission from Nature KF McCarty JA Nobel and NC Bartelt quotVacancies in Solids and the Stability of Surface Morphologyquot Nature Vol 412 pp 622625 2001 Image is 575 pm by 575 pm Copyright 2001 Macmillan Publishers Ltd Island growsshrinks to maintain equil vancan 39 he bulk 9 Chapter4 11 S M39mm L w 39 NI 1 Imperfections in Metals i Two outcomes if impurity B added to host A Solid solution of B in A Le random dist of point defects mursm Carlotta Substitutional solid soln Interstitial solid soln eg Cu in Ni eg C in Fe Solid solution of B in A plus particles of a new phase usually for a larger amount of B Second phase particle different composition often different structure l39aJ Chapter4 12 u NA bowing 8212012 51ml NV Rnxlo39qux toquot T tats IL K 762 m ev am z As you kart W mom VaCanagcl bigger ql mmdtrs 9 Types of alloys b orth am Carnationch JV 1 C Wk H atoms Imperfections in Metals ii Conditions for substitutional solid solution SS W Hume Rothery rule 1 Aratomic radius lt 15 2 Proximity in periodic table ie similar electronegativities 3 Same crystal structure for pure metals 4 Valency All else being equal a metal will have a greater tendency to dissolve a metal of higher valency than one of lower valency Chapter4 13 HI will dissova in Zinc blc 0F valan PA has 0 hi her Imperfections in Metals iii Application of Hume Rothery rules Solid Solutions Element Atomic Crystal Electro Valence Radius Structure nega nm tivity 1 Would you predict Cu 01278 FCC 19 2 more or Ag 0 71 to dissolve in Zn 2 2 More Zn or in Cu 18 16 18 18 22 16 Zn 01332 HCP Table on p 118 Calister amp Rethwisch 8e Chapter4 14 adult17j h Jigsaw Impurities in Solids Specification of composition atom percent nm1 number of moles of component 1 Chapter4 15 N WC crlttorj Hit 8212012 humor of moles am Lehmans art 1mm load 01 lne booK Line Defects Dislocations are line defects slip between crystal planes result when dislocations move produce permanent plastic deformation Schematic of Zinc HCP before deformation after tensile elongation me la WW3 gt Chapter 4 16 Imperfections in Solids SurgeK39s Vcd oif quotOISUYC O F lOFH iCC Linear Defects Dislocations Shr Dr Are onedimensional defects around which atoms are misaligned Edge dislocation extra halfplane of atoms inserted in a crystal structure 86 b perpendicular i to dislocation line Screw dislocation spiral planar ramp resulting from shear deformation 8 O b parallel H to dislocation line measure of lattice distortion U thrs the gimp Chapter4 17 Imperfections in Solids Edge Dislocation dislocation Titan Dri39l39IlI Fig 43 Calister amp Rethwisch 8e Chapter4 18 4311quot quoti is L w A39islocarhon A 8212012 mirhon do 1 his l5st Vde l is 4m dislocation Motion of Edge Dislocation 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 Atomic View of edge dislocation motion from left to right as a crystal is sheared Click once on image to start animation Courtesy PM Anderson Chapter4 19 Imperfections in Solids Smw cl isiocacbon Screw Dislocation 39 g r 39 1 39 quot f 39 39 Dislocation line Burgers vector b gt Adapted from Fig 44 Calisteramp RethWisch 8e Chapter4 20 VMSE Screw Dislocation In VMSE a region of crystal containing a dislocation can be rotated in 3D dislocation motion may be animated F V39 T V39e ront lew VMSE Screen Shots 0390 IW Chapter421 8212012 Edge Screw and Mixed Dislocations Chapter4 22 ll Imperfections in Solids DISIDCG39H39DHS cLDn J Sl vprllm coranvc Dislocations are visible in electron micrographs J wt can Sc lnoQ39IHnlcmms imPur39er quot 939 39 Sewndmvl ParJileS Dislocations amp Crystal Structures 1H 0 formal plums Structure2 View t tquotquot I I 39 Wdirections are preferred closepacke planes I l closepacked directions U S closepacked plane bottom close packed plane top Comparison among crystal structures FCC many closepacked planesdirections HCP only one plane 3 directions BCC none Specimens that Mg HCP were tensile tested tensile direction Al FCC 4 Chapter4 24 l dongodion W AWN Planar Defects in Solids One case is a twin boundary plane Essentially a reflection of atom positions across the twin plane 0 o o o o c c o n o o o o o Adapted from Fig 49 Calister amp Rethwisch 8e 3 o o o o o Stacking faults For FCC metals an error in ABCABC packing sequence Ex ABCABABC Chapter4 25 8212012 di wl39 F Wmttlg bU39l Shll ktme TOW 39 1p39mlly found in FCC Catalysts and Surface Defects Ledge Terrace A catalyst increases the Crevi9 A7 rate of a chemical fieSquot 5315 reaction Without being Wand WM 7 Adam quotStep CO 1 S U m ed Fig 410 Calister amp Rethwisch 8e Active sites on catalysts quotVf si ofi W 39 are normally surface quot defects Single crystals of 090530902 used in an automotive catalytic converter L Av y Izz Fig 411 Calister amp Rethwisch 86 CD 0 t Chapter4 26 Hartman CL Melall gt Cottonle Microscopic Examination Crystallites grains and grain boundaries Vary considerably in size Can be quite large ex Large single crystal of quartz or diamond or Si ex Aluminum light post or garbage can see the individual grains Crystallites grains can be quite small mm or less necessary to observe with a microscope Chapter4 27 ari ncl Sic Papquot 8212012 Wish slimming1 l idxa 39 W 91203 Optical Microscopy W l i cal MiCVDSCDP I PDl39lShinj WOW Useful up to 2000X magnification Polishing removes surface features eg scratches SCr Etching changes reflectance depending on crystal orientation an i m i W Iquot nd 351 wt hay 391 w our p I t crystallographic planes Adapted from Fig 413b and c Calister amp Rethwisch 8e Fig 413c is courtesy ofJE Burke General Electric Co O l I Optical Microscopy Su Paco QNDVC quot m i d Grain boundaries J are imperfections are more susceptible WWW to etching 39 may be revealed as polished surface dark lines change in crystal 39 surface groove orientation across grain boundary a boundary v AdaptedfromFig414a I 39 39 z and b Calisteramp ASTM grain f39 Rethwisch 8e size number 5 quot i39E s L li h39Z t e lady i 3 2 7 7 the National Bureau of N 2n1 39 v C 39 quot Standards Washington DC 39 now the National Institute of quot Standards and Technology 39 G 39th b MD number of grainsin2 alloy al ers W at 100x b magnification Chapter4 29 Optical Microscopy metallographic scopes often use polarized light to increase contrast Also used for transparent samples such as polymers ex 39 polyNS Plastic SvnglatSScs Chapter4 30 1O Microscopy Optical resolution ca 107m 01 pm 100 nm For higher resolution need higher frequency XRays Difficult to focus Electrons wavelengths ca 3 pm 0003 nm Magnification 1000000X Atomic resolution possible Electron beam focused by magnetic lenses a Chapter4 31 j 8212012 Scanning Tunneling Microscopy STM Atoms can be arranged and imaged Photos produced from the work of CP Lutz Zeppenfeld and BM Eigler Reprinted with permission from International Business Machines Corporation copyright 1995 Carbon monoxide Iron atoms arranged molecules arranged on a copper 111 on a platinum 111 surface These Kanji surface characters represent the word atom Chapter4 32 j Sinct H 50 s 151 in atoms Summary Point Line and Area defects exist in solids The number and type of defects can be varied and controlled eg Tcontrols 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 Chapter4 33 11