Types of Imperfections 1. Point Defects Vacancies Interstitial - PowerPoint PPT Presentation

Kasetsart University 213211: Imperfection Types of Imperfections 1. Point Defects – Vacancies – Interstitial Atoms – Substitutional Atoms 2. Linear (line) Defects – Dislocations 3. Interfacial (area) Defects – Grain Boundaries Dr.Peerapong Triyacharoen 58 Department of Materials Engineering

Kasetsart University 213211: Imperfection Point Defects • Vacancies: missing atoms from their lattice sites. Vacancy distortion of planes • Self-Interstitial: extra atoms positioned between atomic sites. self- interstitial distortion of planes Dr.Peerapong Triyacharoen 59 Department of Materials Engineering

Kasetsart University 213211: Imperfection Vacancy Concentration Energy formation Equilibrium number of a vacancy of vacancies ⎛− ⎞ N Q = ⎜ ⎟ v v exp ⎝ ⎠ N kT Total number Absolute temperature of atomic sites Boltzmann’s constant: 1.38x10 -23 J/atom-K, 8.62x10 -5 eV/atom-K Gas constant (R): 8.31 J/mol-K, 1.987 cal/mol-K Dr.Peerapong Triyacharoen 60 Department of Materials Engineering

Kasetsart University 213211: Imperfection Example: Vacancy Conc. Design a heat treatment that will provide 1000 times more vacancies in copper than are normally present at room temperature. About 20,000 cal/mol are required to produce a vacancy in copper. Cu: FCC, a Cu = 0.36151 nm. Solution 4atoms/cel l 22 3 = = × 8.47 10 Cu atoms/cm N × - 8 3 (3.6151 10 cm) − ⎛ ⎞ 20 , 000 3 = × 22 = × 8 ⎜ ⎟ At T room ; vacancies/ cm ( 8 . 47 10 ) exp 1 . 815 10 N v × ⎝ ⎠ 1 . 987 298 3 = × 11 Wish to produce vacancies/ cm 1 . 815 10 N v − ⎛ ⎞ 20 , 000 × 11 = × 22 ⎜ ⎟ 1 . 815 10 ( 8 . 47 10 ) exp × ⎝ ⎠ 1 . 987 T T = 375 K = 102°C Dr.Peerapong Triyacharoen 61 Department of Materials Engineering

Kasetsart University 213211: Imperfection Point Defects in Alloys • Alloy = A metallic substance that is composed of two or more elements. Solvent = element present in the greatest amount Solute = element present in a minor concentration Two outcomes if impurities (B) added to host (A): 1. Solid solution 2. New second phase Dr.Peerapong Triyacharoen 62 Department of Materials Engineering

Kasetsart University 213211: Imperfection Point Defects in Alloys (con.) Substitutional alloy Interstitial alloy (e.g., Cu in Ni) (e.g., C in Fe) Second phase particle --different composition --often different structure. Dr.Peerapong Triyacharoen 63 Department of Materials Engineering

Kasetsart University 213211: Imperfection Composition Definition: Amount of impurity (B) and host (A) in the system. Two descriptions: • Weight % element • Atomic % element mass of B # atoms of B = × = × wt% B 100 at% B 100 total mass total # atoms • Conversion between wt% and at% in an A-B alloy: × at% B AW = × B wt% B 100 × + × at% A AW at% B AW ( ) ( ) A B ÷ wt% B AW = × B at% B 100 ÷ + ÷ wt% A AW at% B AW ( ) ( ) A B Dr.Peerapong Triyacharoen 64 Department of Materials Engineering

Kasetsart University 213211: Imperfection Example: Composition Determine the composition of a compound Fe 3 C in at% and wt%. Solution: at% Fe = (¾)x100 = 75 at% Fe at% C = (¼)x100 = 25 at% C × 75 55.85 = × = wt% Fe 100 93.32 wt% Fe × + × 75 55.85 25 12 ( ) ( ) × 25 12 = × = wt% C 100 6.68 wt% C × + × 75 55.85 25 12 ( ) ( ) Dr.Peerapong Triyacharoen 65 Department of Materials Engineering

Kasetsart University 213211: Imperfection Defects in Ceramic Structures • Frenkel Defect -- a cation is out of place. • Shottky Defect -- a paired set of cation and anion vacancies. Shottky Defect: Frenkel Defect ~ e − Q D /kT • Equilibrium concentration of defects Dr.Peerapong Triyacharoen 66 Department of Materials Engineering

Kasetsart University 213211: Imperfection Impurity • Impurities must also satisfy charge balance Na+ Cl- • Ex: NaCl cation • Substitutional cation impurity vacancy Ca2+ Na+ Na+ Ca2+ Ca2+ impurity initial geometry resulting geometry • Substitutional anion impurity anion vacancy O2- Cl- Cl- O2- impurity initial geometry resulting geometry Dr.Peerapong Triyacharoen 67 Department of Materials Engineering

Kasetsart University 213211: Imperfection Linear Defects • Dislocations: – one-dimensional defect around which some of the atoms are misaligned – cause slip between crystal plane when they move (along the close packed direction) – produce permanent (plastic) deformation. Schematic of a Zinc crystal (HCP): • before deformation • after tensile elongation slip steps Dr.Peerapong Triyacharoen 68 Department of Materials Engineering

Kasetsart University 213211: Imperfection Dislocation: Edge Dislocation Burger vector, b: magnitude and direction of the lattice distortion Dr.Peerapong Triyacharoen 69 Department of Materials Engineering

Kasetsart University 213211: Imperfection Dislocation: Screw Dislocation Screw dislocation line Dr.Peerapong Triyacharoen 70 Department of Materials Engineering

Kasetsart University 213211: Imperfection Dislocation: Mixed Dislocation Dr.Peerapong Triyacharoen 71 Department of Materials Engineering

Kasetsart University 213211: Imperfection Dislocations & Crystal Structure • Structure: close-packed view onto two planes & directions close-packed planes. 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; Mg (HCP) BCC: none tensile direction Al (FCC) Dr.Peerapong Triyacharoen 72 Department of Materials Engineering

Kasetsart University 213211: Imperfection Interfacial Defects 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. Angle of misalignment grain boundaries Angle of misalignment Dr.Peerapong Triyacharoen 73 Department of Materials Engineering

Kasetsart University 213211: Imperfection Optical Microscope Dr.Peerapong Triyacharoen 74 Department of Materials Engineering