ENS 167 Fundamentals of Materials Science and Engineering PDF

ENS 167 Fundamentals of Materials Science and Engineering Source (book and ppt): Callister, Materials Science and Engineering: An Introduction, 8th Edition  Chapter 4: Imperfections in Solids ISSUES TO ADDRESS... What are the solidification me...

ENS 167 Fundamentals of Materials Science and Engineering Source (book and ppt): Callister, Materials Science and Engineering: An Introduction, 8th Edition  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?  Imperfections in Solids Solidification- result of casting of molten material – 2 steps Nuclei form Nuclei grow to form crystals – grain structure Start with a molten material – all liquid nuclei crystals growing grain structure liquid Adapted from Fig. 4.14(b), Callister & Rethwisch 8e. Crystals grow until they meet each other  Polycrystalline Materials Grain Boundaries regions between crystals transition from lattice of one region to that of the other slightly disordered low density in grain boundaries – high mobility – high diffusivity – high chemical reactivity Adapted from Fig. 4.7, Callister & Rethwisch 8e.  Solidification Grains can be -equiaxed (roughly same size in all directions) -columnar (elongated grains) ~ 8 cm heat flow Shell of Columnar in equiaxed grains area with less due to rapid undercooling cooling (greater Adapted from Fig. 5.17, ΔT) near wall Callister & Rethwisch 3e. Grain Refiner - added to make smaller, more uniform, equiaxed grains.  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.  Types of Imperfections Vacancy atoms Interstitial atoms Point defects Substitutional atoms Line defects Dislocations Area defects Grain Boundaries  Point Defects in Metals Vacancies: -vacant atomic sites in a structure. Vacancy distortion of planes Self-Interstitials: -"extra" atoms positioned between atomic sites. self- interstitial distortion of planes  Equilibrium Concentration: Point Defects Equilibrium concentration varies with temperature! No. of defects Activation energy Nv ⎛ ⎟ v⎞ No. of potential = −Q N ⎝ kT ⎠⎜ exp defect sites Temperature Boltzmann's constant (1.38 x 10-23 J/atom-K) (8.62 x 10-5 eV/atom-K) Each lattice site is a potential vacancy site  Observing Equilibrium Vacancy Conc. Low energy electron microscope view of Click once on image to start animation a (110) surface of NiAl. 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 Reprinted with permission from Nature (K.F. join the island. McCarty, J.A. Nobel, and N.C. Bartelt, "Vacancies in Solids and the Stability of Surface Morphology", Island grows/shrinks to maintain Nature, Vol. 412, pp. 622-625 (2001). Image is equil. vancancy conc. in the bulk. 5.75 μm by 5.75 μm.) Copyright (2001) Macmillan Publishers, Ltd.  Imperfections in Metals (i) Two outcomes if impurity (B) added to host (A): Solid solution of B in A (i.e., random dist. of point defects) OR Substitutional solid soln. Interstitial solid soln. (e.g., Cu in Ni) (e.g., 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.  Imperfections in Metals (i) Solid solution of B in A plus particles of a newphase (usually for a larger amount of B). Second phase particle has different composition and often different structure.  Imperfections in Metals (ii) Conditions for substitutional solid solution (S.S.) W. Hume – Rothery rule – 1. Δr (atomic radius) < 15% – 2. Proximity in periodic table i.e., 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  Imperfections in Metals (iii) Application of Hume–Rothery rules – Solid Solutions Element Atomic Crystal Electro- Valence Radius Structure nega- (nm) tivity 1. Would you Cu 0.1278 FCC 1.9 +2 C 0.071 predict more Al or H 0.046 O 0.060 Ag Ag 0.1445 FCC 1.9 +1 to dissolve in Zn? Al Co 0.1431 0.1253 FCC HCP 1.5 1.8 +3 +2 Cr 0.1249 BCC 1.6 +3 Fe 0.1241 BCC 1.8 +2 2. More Zn or Ni 0.1246 FCC 1.8 +2 Pd 0.1376 FCC 2.2 +2 Al in Cu? Zn 0.1332 HCP 1.6 +2 Table on p. 118, Callister & Rethwisch 8e.  Impurities in Solids Specification of composition m1 – weight percent C1 x = m1 + m2 100 m1 = mass of component 1 = n x – atom percent C 1' m1 nm1+m2n 100 nm1 = number of moles of component 1  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 slip steps  Imperfections in Solids Linear Defects (Dislocations) – Are one-dimensional defects around which atoms are misaligned Edge dislocation: – extra half-plane of atoms inserted in a crystal structure – b perpendicular (⊥) to dislocation line Screw dislocation: – spiral planar ramp resulting from shear deformation – b parallel (||) to dislocation line Burger’s vector, b: measure of lattice distortion  Imperfections in Solids Edge Dislocation Fig. 4.3, Callister & Rethwisch 8e.  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.  Imperfections in Solids Screw Dislocation Screw Dislocation b Dislocation line Burgers vector b (b) (a) Adapted from Fig. 4.4, Callister & Rethwisch 8e.  Edge, Screw, and Mixed Dislocations Mixed Edge Adapted from Fig. 4.5, Callister & Rethwisch 8e. Screw  Imperfections in Solids Dislocations are visible in electron micrographs Fig. 4.6, Callister & Rethwisch 8e.  Dislocations & Crystal Structures Structure: view onto two close-packed planes & close-packed planes. directions are preferred. close-packed directions close-packed plane (bottom) close-packed plane (top) Comparison among crystal structures: FCC: many close-packed planes/directions; HCP: only one plane, 3 directions; BCC: none Specimens that Mg (HCP) were tensile tested. tensile direction Al (FCC)  Planar Defects in Solids One case is a twin boundary (plane) – Essentially a reflection of atom positions across the twin plane. Adapted from Fig. 4.9, Callister & Rethwisch 8e. Stacking faults – For FCC metals an error in ABCABC packing sequence – Ex: ABCABABC  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.  Optical Microscopy Useful up to 2000X magnification. Polishing removes surface features (e.g., scratches) Etching changes reflectance, depending on crystal orientation. crystallographic planes Adapted from Fig. 4.13(b) and (c), Callister & Rethwisch 8e. (Fig. 4.13(c) is courtesy of J.E. Burke, General Electric Co.) Micrograph of brass (a Cu-Zn alloy) 0.75mm  Optical Microscopy Grain boundaries... are imperfections, are more susceptible to etching, may be revealed as polished surface dark lines, change in crystal surface groove orientation across grain boundary (a) boundary. Adapted from Fig. 4.14(a) and (b), Callister & ASTM grain Rethwisch 8e. (Fig. 4.14(b) is courtesy size number of L.C. Smith and C. Brady, the National Bureau of Standards, Washington, DC n-1 [now the National Institute N=2 of Standards and Fe-Cr alloy Technology, Gaithersburg, MD].) number of grains/in2 (b) at 100x  Optical Microscopy Polarized light – metallographic scopes often use polarized light to increase contrast – Also used for transparent samples such as polymers  Microscopy Optical resolution ca. 10-7 m = 0.1 μm = 100 nm For higher resolution need higher frequency – X-Rays? Difficult to focus. – Electrons wavelengths ca. 3 pm (0.003 nm) – (Magnification - 1,000,000X) Atomic resolution possible Electron beam focused by magnetic lenses.  Summary Point, Line, and Area defects exist 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.)  QUIZ Dislocations: are line defects, slip between crystal planes result when dislocations move, produce permanent (plastic) deformation.