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MSU / Engineering and Tech / EGR 375 / which of the following systems (i.e., pair of metals) would you expect

which of the following systems (i.e., pair of metals) would you expect

which of the following systems (i.e., pair of metals) would you expect

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School: Murray State University
Department: Engineering and Tech
Course: Materials Science
Term: Fall 2016
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Cost: 50
Description: Imperfections in Solids IV – 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? The crystalline materials do not exhibit a perfect order – all contain various defects called imperfections 1)
Uploaded: 07/24/2017
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• How do defects affect material properties?




• What types of defects arise in solids?




• What are the solidification mechanisms?



Imperfections in Solids IV – 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? The crystalline materials do not exhibit a perfDon't forget about the age old question of What is an arc minute and an arc second?
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ect order – all contain various defects called imperfections 1) Solidification The result of casting of molten material. Three steps: a) Nuclei form within a molten material – all liquid b) Nuclei grow to form crystal c) Crystal grow until they meet each other – grain structure nuclei crystals growing grain structure liquidhttps://www.youtube.com/watch?v=uG35D_euM-0 EGR 375/MSE201  Spring 2017 IV. 1 Dr. George Bunget  Murray State University Imperfections in Solids 1) Type of Imperfections • Point defects • Vacancy atoms • Interstitial atoms • Substitutional atoms • Line defects – dislocations  • Area defects – grain boundaries A. Point Defects 1) Vacancies Vacant atomic sites in a crystal structure distortion  of planes Vacancy in a (111) plane  for silicon micrograph. Vacancy EGR 375/MSE201  Spring 2017 IV. 2 Dr. George Bunget  Murray State University Imperfections in Solids a) Equilibrium Defect Concentration Equilibrium number of vacancies N varies with temperature where 2) Interstitial Atoms “Extra” atoms positioned between atomic sites. distortion  of planesself interstitial EGR 375/MSE201  Spring 2017 IV. 3 Dr. George Bunget  Murray State University Imperfections in Solids 3) Substitutional Atoms – Impurities in solids • Pure metal – only one type of metal  • Impossible to obtain • Refining limits are 99.9999% • Alloy – impurity atoms have been added intentionally to increase certain properties of the material • Solvent – the element/compound of greatest amount • Solute – the impurities – minor concentration • Solid solution: • Solid solution of B in A • The impurity atoms are randomly and uniformly dispersed within the solid • Two types: • Substitutional solid solution • Interstitial solid soln. Substitutional solid soln. (e.g., Cu in Ni) Interstitial solid soln. (e.g., C in Fe) Second phase particle within the original phase matrix -- different composition -- often different structure.EGR 375/MSE201  Spring 2017 IV. 4 Dr. George Bunget  Murray State University Interstitial Sites Imperfections in Solids • Specification of composition • Weight percent • Atom percent • Conversion from weight to atom percent • For two elements EGR 375/MSE201  Spring 2017 IV. 5Dr. George Bunget  Murray State University Imperfections in Solids Impurities in Solids – Example 1 (4.6) Atomic radius, crystal structure, electronegativity, and the most common valence are given in the following table for several elements; for those that are nonmetals, only atomic radii are indicated. Which of these elements would you expect to form the following with nickel: (a) a substitutional solid solution having complete solubility (b) a substitutional solid solution of incomplete solubility (c) an interstitial solid solution EGR 375/MSE201  Spring 2017 IV. 6 Dr. George Bunget  Murray State UniversityImperfections in Solids 1) Impurities in Solids – Example 2 (4.7) Which of the following systems (i.e., pair of metals) would you expect to exhibit complete solid solubility? Explain your answers. (a) Cr-V (b) Mg-Zn (c) Al-Zr (d) Ag-Au (e) Pb-Pt Four Hume-Rothery rules must be satisfied a) Cr – V EGR 375/MSE201  Spring 2017 IV. 7Dr. George Bunget Murray State University Imperfections in Solids b) Mg-Zn c) Al – Zr EGR 375/MSE201  Spring 2017 IV. 8Dr. George Bunget Murray State University Imperfections in Solids d) Ag – Au e) Pb – PtEGR 375/MSE201  Spring 2017 IV. Dr. George Bunget 9 Murray State University Imperfections in Solids B. Linear Defects – Dislocations A line defect or one dimensional defect.  The dislocation strongly influence many of the material properties Several types: a) Edge dislocation – a linear defect along the edge of a plane having an extra portion of atoms with an edge terminating within the crystal. • Symbol: slip steps Before  deformationAfter tensile elongation Burgers vector • It is a crystal vector that is required to close the circuit • It represents the magnitude and direction of shear the lattice distortion • It is perpendicular to the line of dislocation • Determine the Burgers vector: • Trace around the end of the dislocation plane to form a closed loop. • Record the number of lattice vector travelled along each side of the loop EGR 375/MSE201  Spring 2017 IV. 10 Dr. George Bunget  Murray State University Imperfections in Solids • In a perfect lattice, trace out the same path, moving the same number of lattice vectors along each direction as before. This loop will not be complete, and the closure failure is the Burgers vector: b) Screw dislocation – formed by shear stress. • The Burgers vector is parallel to the screw dislocation line EGR 375/MSE201  Spring 2017 IV. 11Dr. George Bunget  Murray State University Imperfections in Solids Front View Top View c) Mixed Dislocations – most dislocations found in metals are not pure edge or pure screw dislocations – mixed MixedEdge Screw EGR 375/MSE201  Spring 2017 IV. 12 Dr. George Bunget  Murray State University Imperfections in Solids Dislocations are visible in electron micrographs a) Dark lines are dislocations EGR 375/MSE201  Spring 2017 IV. 13Dr. George Bunget  Murray State University Imperfections in Solids C. Interfacial Defects – Plane Defects 3) Grain Boundaries • Regions between crystals • Transition from lattice on one region to that of another • Slightly disordered • Low density in grain boundaries  • High mobility • High diffusivity • High chemical reactivity Grains can be: • Equiaxed – roughly same size in all directions • Columnar – elongated grains ~ 8 cmheat  flow Columnar in  area with less  undercooling Shell of  equiaxed grains  due to rapid  cooling (greater  ΔT) near wall EGR 375/MSE201  Spring 2017 IV. 14 Dr. George Bunget  Murray State University Imperfections in Solids D. Microscopic Examination 1) Optical Microscopy • Useful up to 2000X magnification. • Polishing removes surface features (e.g., scratches) • Etching changes reflectance, depending on crystal orientation. crystallographic planes Courtesy of J.E. BGrain Boundaries • More susceptible to etching • Revealed as dark lines Micrograph of urke, brass (a Cu-Zn alloy) General Electric Co. 0.75 mm polished surface • Change in crystal orientation  across boundary • ASTM grain number size (a)surface groove grain boundary EGR 375/MSE201  Spring 2017 IV. 15 Fe-Cr alloy (b) Dr. George Bunget  Murray State University Imperfections in Solids Example of Measuring Grain Size  Determine: • Mean intercept length • ASTM grain size number G • Magnification is  EGR 375/MSE201  Spring 2017 IV. 16Dr. George Bunget  Murray State University EGR 375/MSE201  Spring 2017 Imperfections in Solids IV. Dr. George Bunget  17Murray State University Imperfections in Solids 1) Grain Size ASTM Chart EGR 375/MSE201  Spring 2017 IV. 18Dr. George Bunget  Murray State University Imperfections in Solids 2) Scanning Electron Microscopy 3) Microscopy Comparison EGR 375/MSE201  Spring 2017 IV. 19Dr. George Bunget  Murray State University Imperfections in Solids a) Structural Feature Comparison EGR 375/MSE201  Spring 2017 IV. 20Dr. George Bunget  Murray State University

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