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34 слайда
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1,2,3,4,5,6,7,8,9,10,11
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Слайды и текст к этой презентации:
№1 слайд
Содержание слайда: Chapter 4:
Imperfections in Solids
№2 слайд
Содержание слайда: 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
№3 слайд
Содержание слайда: 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
№4 слайд
Содержание слайда: Solidification
Grains can be - equiaxed (roughly same size in all directions)
- columnar (elongated grains)
№5 слайд
Содержание слайда: 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.
№6 слайд
Содержание слайда: Types of Imperfections
№7 слайд
Содержание слайда: Point Defects in Metals
№8 слайд
Содержание слайда: Equilibrium Concentration:
Point Defects
№9 слайд
Содержание слайда: Measuring Activation Energy
№10 слайд
Содержание слайда: Estimating Vacancy Concentration
№11 слайд
Содержание слайда: Observing Equilibrium Vacancy Conc.
№12 слайд
Содержание слайда: Imperfections in Metals (i)
№13 слайд
Содержание слайда: Imperfections in Metals (ii)
Conditions for substitutional solid solution (S.S.)
W. Hume – Rothery rule
1. r (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
№14 слайд
Содержание слайда: Imperfections in Metals (iii)
Application of Hume–Rothery rules – Solid Solutions
1. Would you predict
more Al or Ag
to dissolve in Zn?
2. More Zn or Al
in Cu?
№15 слайд
Содержание слайда: Impurities in Solids
Specification of composition
weight percent
№16 слайд
Содержание слайда: Line Defects
№17 слайд
Содержание слайда: 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
№18 слайд
Содержание слайда: Imperfections in Solids
Edge Dislocation
№19 слайд
Содержание слайда: Motion of Edge Dislocation
№20 слайд
Содержание слайда: Imperfections in Solids
Screw Dislocation
№21 слайд
Содержание слайда: VMSE: Screw Dislocation
In VMSE:
a region of crystal containing a dislocation can be rotated in 3D
dislocation motion may be animated
№22 слайд
Содержание слайда: Edge, Screw, and Mixed Dislocations
№23 слайд
Содержание слайда: Imperfections in Solids
Dislocations are visible in electron micrographs
№24 слайд
Содержание слайда: Dislocations & Crystal Structures
№25 слайд
Содержание слайда: Planar Defects in Solids
One case is a twin boundary (plane)
Essentially a reflection of atom positions across the twin plane.
Stacking faults
For FCC metals an error in ABCABC packing sequence
Ex: ABCABABC
№26 слайд
Содержание слайда: Catalysts and Surface Defects
A catalyst increases the rate of a chemical reaction without being consumed
Active sites on catalysts are normally surface defects
№27 слайд
Содержание слайда: 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.
№28 слайд
Содержание слайда: Optical Microscopy
№29 слайд
Содержание слайда: Optical Microscopy
№30 слайд
Содержание слайда: Optical Microscopy
Polarized light
metallographic scopes often use polarized light to increase contrast
Also used for transparent samples such as polymers
№31 слайд
Содержание слайда: Microscopy
Optical resolution ca. 10-7 m = 0.1 m = 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.
№32 слайд
Содержание слайда: Scanning Tunneling Microscopy
(STM)
№33 слайд
Содержание слайда: Summary
№34 слайд
Содержание слайда: ANNOUNCEMENTS