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

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

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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

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SLIDE 1

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

58

Types of Imperfections

  • 1. Point Defects

– Vacancies – Interstitial Atoms – Substitutional Atoms

  • 2. Linear (line) Defects

– Dislocations

  • 3. Interfacial (area) Defects

– Grain Boundaries

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SLIDE 2

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

59

Point Defects

Vacancy

distortion

  • f planes
  • Vacancies: missing atoms from their lattice sites.

self- interstitial

distortion

  • f planes
  • Self-Interstitial: extra atoms positioned between atomic sites.
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SLIDE 3

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

60

Vacancy Concentration

⎟ ⎠ ⎞ ⎜ ⎝ ⎛− = kT Q exp N N

v v

Energy formation

  • f a vacancy

Absolute temperature Total number

  • f atomic sites

Equilibrium number

  • f vacancies

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

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SLIDE 4

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

61

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.

3 22 3 8

  • atoms/cm

Cu 10 8.47 cm) 10 (3.6151 l 4atoms/cel × = × = N

Solution Cu: FCC, aCu = 0.36151 nm. At Troom; cm vacancies/

3 8 22

10 815 . 1 298 987 . 1 000 , 20 exp ) 10 47 . 8 ( × = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × − × =

v

N Wish to produce cm vacancies/

3 11

10 815 . 1 × =

v

N

⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × − × = × T 987 . 1 000 , 20 exp ) 10 47 . 8 ( 10 815 . 1

22 11

T = 375 K = 102°C

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SLIDE 5

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

62

Point Defects in Alloys

  • Alloy = A metallic substance that is

composed of two or more elements.

Two outcomes if impurities (B) added to host (A):

1. Solid solution 2. New second phase

Solvent = element present in the greatest amount Solute = element present in a minor concentration

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SLIDE 6

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

63

Point Defects in Alloys (con.)

Second phase particle

  • -different composition
  • -often different structure.

Substitutional alloy (e.g., Cu in Ni) Interstitial alloy (e.g., C in Fe)

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SLIDE 7

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

64

Composition

100 mass total B

  • f

mass B wt% × = 100 atoms # total B

  • f

atoms # B at% × =

100 AW B at% AW A at% AW B at% B wt%

B A B

× × + × × = ) ( ) (

Definition: Amount of impurity (B) and host (A) in the system.

  • Weight % element

Two descriptions:

  • Atomic % element
  • Conversion between wt% and at% in an A-B alloy:

100 AW B at% AW A wt% AW B wt% B at%

B A B

× ÷ + ÷ ÷ = ) ( ) (

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SLIDE 8

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

65

Example: Composition

Determine the composition of a compound Fe3C in at% and wt%.

Fe wt% 93.32 100 12 25 55.85 75 55.85 75 Fe wt% = × × + × × = ) ( ) (

at% Fe = (¾)x100 = 75 at% Fe at% C = (¼)x100 = 25 at% C

Solution:

C wt% 6.68 100 12 25 55.85 75 12 25 C wt% = × × + × × = ) ( ) (

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SLIDE 9

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

66

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

  • Equilibrium concentration of defects

~ e−QD /kT

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SLIDE 10

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

67

Impurity

  • Impurities must also satisfy charge balance
  • Ex: NaCl

Na+ Cl-

  • Substitutional cation impurity
  • Substitutional anion impurity

initial geometry Ca2+ impurity resulting geometry Ca2+ Na+ Na+ Ca2+

cation vacancy

initial geometry O2- impurity O2- Cl-

anion vacancy

Cl- resulting geometry

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SLIDE 11

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

68

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

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SLIDE 12

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

69

Dislocation: Edge Dislocation

Burger vector, b: magnitude and direction of the lattice distortion

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SLIDE 13

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

70

Dislocation: Screw Dislocation

Screw dislocation line

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SLIDE 14

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

71

Dislocation: Mixed Dislocation

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SLIDE 15

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

72

tensile direction

Dislocations & Crystal Structure

  • 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

close-packed plane (bottom) close-packed plane (top) close-packed directions

Mg (HCP) Al (FCC)

view onto two close-packed planes.

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SLIDE 16

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

73

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.

grain boundaries

Angle of misalignment Angle of misalignment

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SLIDE 17

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Imperfection

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Optical Microscope