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Chapter 4- ISSUES TO ADDRESS... What types of defects arise in solids? Can the number and type of defects be varied and controlled? How do defects affect.

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Presentation on theme: "Chapter 4- ISSUES TO ADDRESS... What types of defects arise in solids? Can the number and type of defects be varied and controlled? How do defects affect."— Presentation transcript:

1 Chapter 4- ISSUES TO ADDRESS... What types of defects arise in solids? Can the number and type of defects be varied and controlled? How do defects affect material properties? sterling silver (7.5% copper) much harder and stronger than silver silicon transistors rely on controlled doping 1 Are defects undesirable? NO CHAPTER 4: IMPERFECTIONS IN SOLIDS

2 Chapter 4-2 Vacancy atoms Interstitial atoms Substitutional atoms Dislocations Grain Boundaries Point defects Line defects Area defects TYPES OF IMPERFECTIONS

3 Chapter 4-3 Vacancies: -vacant atomic sites in a structure. Self-Interstitials: -"extra" atoms positioned between atomic sites. POINT DEFECTS Much lower concentration than vacancies.

4 Chapter 4- Boltzmann's constant (1.38 x J/atom K) (8.62 x eV/atom K) N D N exp Q D kT No. of defects No. of atomic sites Activation energy Temperature Each lattice site is a potential vacancy site 4 Equilibrium concentration varies with temperature! EQUIL. CONCENTRATION: POINT DEFECTS

5 Chapter 4-5 We can get Q from an experiment. Measure this... Replot it... MEASURING ACTIVATION ENERGY

6 Chapter 4-6 Find the equil. # of vacancies in 1m of Cu at 1000C. Given: 3 Answer: ESTIMATING VACANCY CONC. Fraction means 1 in sites is a vacancy!

7 Chapter 4-14 Example: Copper Data from Table inside front cover of Callister (see next slide): crystal structure = FCC: 4 atoms/unit cell atomic weight = g/mol (1 amu = 1 g/mol) atomic radius R = nm (1 nm = 10 cm) -7 THEORETICAL DENSITY,

8 Chapter 4-7 Low energy electron microscope view of a (110) surface of NiAl. Increasing T 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. Reprinted with permission from Nature (K.F. McCarty, J.A. Nobel, and N.C. Bartelt, "Vacancies inNature Solids and the Stability of Surface Morphology", Nature, Vol. 412, pp (2001). Image is 5.75 m by 5.75 m.) Copyright (2001) Macmillan Publishers, Ltd. OBSERVING EQUIL. VACANCY CONC.

9 Chapter 4- Impurities in Solids Completely pure metal (or other substance) – Impossible!! Pay – really pay – for % - NIST standards still to impurities per m 3 Alloy – deliberately add impurity to engineer properties. Copper (7.5%) in silver

10 Chapter 4- Terminology - Alloys Solvent – component in highest concentration – also called – host Solute – component present in minor concentration

11 Chapter 4-8 Two outcomes if impurity (B) added to host (A): Solid solution of B in A (random and uniform dist. of defects) Solid solution of B in A plus particles of a new phase (usually for a larger amount of B) OR Substitutional alloy (e.g., Cu in Ni) Interstitial alloy (e.g., C in Fe) Second phase particle --different composition --often different structure. POINT DEFECTS IN ALLOYS

12 Chapter 4- Remember metals- often crystallize in close packed structures – high packing density. More interstitial or substitutional ?? Usually < 10%

13 Chapter 4-9 Low energy electron microscope view of a (111) surface of Cu. Sn islands move along the surface and "alloy" the Cu with Sn atoms, to make "bronze". The islands continually move into "unalloyed" regions and leave tiny bronze particles in their wake. Eventually, the islands disappear. Reprinted with permission from: A.K. Schmid, N.C. Bartelt, and R.Q. Hwang, "Alloying at Surfaces by the Migration of Reactive Two- Dimensional Islands", Science, Vol. 290, No. 5496, pp (2000). Field of view is 1.5 m and the temperature is 290K. ALLOYING A SURFACE

14 Chapter 4- Factors that Govern Solid Solution Atomic size - < 15% difference Crystal structure – same Electronegativity - ?? Valences – more likely to dissolve another metal of higher valence

15 Chapter 4- Copper and Nickel ElementCuNi Radii0.128 nm0.125 Unit cellFCC Electronegativity Oxidation (Valence) +1 (+2)+2 Expect to form solid solution? Ni – 3d 8 4s 2 (28) Cu - 3d 10 4s 1 (29)

16 Chapter 4- Carbon in Iron ElementCFe Radii0.017 nm0.124 Unit cellHexBCC Electronegativity Valence +4+2 (+3) Solid solution ? Interstitial or substitional? C - 2s 2 2p 2 Fe – 3d 6 4s 2

17 Chapter 4- Defects in Ceramics Still have interstitials and vacancies

18 Chapter 4- Substitutional ions

19 Chapter 4-11 Impurities must also satisfy charge balance Ex: NaCl Substitutional cation impurity Substitutional anion impurity IMPURITIES

20 Chapter 4-8 Frenkel Defect -- a cation is out of place. Shottky Defect -- a paired set of cation and anion vacancies. Equilibrium concentration of defects Adapted from Fig , Callister 5e. (Fig is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) See Fig , Callister 6e. DEFECTS IN CERAMIC STRUCTURES

21 Chapter 4-10 Definition: Amount of impurity (B) and host (A) in the system. Weight % Two descriptions: Atom % Conversion between wt % and at% in an A-B alloy: Basis for conversion: COMPOSITION

22 Chapter dimensional defect around which some atoms are misaligned are line defects, cause slip between crystal plane when they move, produce permanent (plastic) deformation. Dislocations: Schematic of a Zinc (HCP): before deformation after tensile elongation slip steps LINE DEFECTS

23 Chapter 4-12 Dislocations slip planes incrementally... The dislocation line (the moving red dot)......separates slipped material on the left from unslipped material on the right. Simulation of dislocation motion from left to right as a crystal is sheared. (Courtesy P.M. Anderson) INCREMENTAL SLIP

24 Chapter 4-13 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. (Courtesy P.M. Anderson) BOND BREAKING AND REMAKING

25 Chapter 4- Edge dislocation Burgers vector: magnitude and direction of lattice distortion. Dislocation line: for edge dislocation above, it is to the page. Edge dislocation – extra ½ plane of atoms

26 Chapter 4- Burgers Vector

27 Chapter 4- Motion – Edge dislocation Applied shear

28 Chapter 4- Screw dislocation Screw – thought of as a result of applied shear – shifted upper Region of crystal 1 atomic distance Bergers vector b

29 Chapter 4- Motion Screw Dislocation Apply shear

30 Chapter 4- Mixed Dislocation

31 Chapter 4-14 Structure: close-packed planes & directions are preferred. Comparison among crystal structures: FCC: many close-packed planes/directions; HCP: only one plane, 3 directions; BCC: none Mg (HCP) Al (FCC) tensile direction Results of tensile testing. view onto two close-packed planes. DISLOCATIONS & CRYSTAL STRUCTURE

32 Chapter 4-15 Grain boundaries: are boundaries between crystals. are produced by the solidification process, for example. have a change in crystal orientation across them. impede dislocation motion. grain boundaries Adapted from Fig. 4.7, Callister 6e. Adapted from Fig. 4.10, Callister 6e. (Fig is from Metals Handbook, Vol. 9, 9th edition, Metallography and Microstructures, Am. Society for Metals, Metals Park, OH, 1985.) ~ 8cm Metal Ingot AREA DEFECTS: GRAIN BOUNDARIES

33 Chapter 4- Area Defects Tilt Boundary – array of edge defects Twin Boundary Twist Boundary –array of screw defects Stacking Defect - break in sequence ABDABC Pores Cracks Other phases

34 Chapter 4-16 Useful up to 2000X magnification. Polishing removes surface features (e.g., scratches) Etching changes reflectance, depending on crystal orientation. close-packed planes micrograph of Brass (Cu and Zn) Adapted from Fig. 4.11(b) and (c), Callister 6e. (Fig. 4.11(c) is courtesy of J.E. Burke, General Electric Co. 0.75mm OPTICAL MICROSCOPY (1)

35 Chapter 4-17 Grain boundaries... are imperfections, are more susceptible to etching, may be revealed as dark lines, change direction in a polycrystal. Adapted from Fig. 4.12(a) and (b), Callister 6e. (Fig. 4.12(b) is courtesy of L.C. Smith and C. Brady, the National Bureau of Standards, Washington, DC [now the National Institute of Standards and Technology, Gaithersburg, MD].) OPTICAL MICROSCOPY (2)

36 Chapter 4- Imperfeicoes nos Solidos Importancia das Imperfeicoes nos Solidos Classificacao dos Solidos - Pontuais - Linha – Discordancias - Interfaciais - Volume

37 Chapter 4-18 Point, Line, and Area defects arise in solids. The number and type of defects can be varied and controlled (e.g., T controls vacancy conc.) Defects affect material properties (e.g., grain boundaries control crystal slip). Defects may be desirable or undesirable (e.g., dislocations may be good or bad, depending on whether plastic deformation is desirable or not.) SUMMARY

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