Rasterelektronenmikroskopie

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Phase Diagrams
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Phase Equilibria Gmin
min
GH-TS
Gibbs free energy
f(T,c,p)
entropy (disorder of molecules entropy of mixing)
internal energy (bonding enthalpy)
metastable
stable
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Phase Equilibrium
phase composition ltgt G minimization
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Cu-xNi Phase Diagram (pconst)isomorphous
phase composition f (T,c)
liquid phase
Gibbs phase rule FK-P2 (F degrees of
freedom depends on number of components K
and number of phases P)
liquidsolid phase
solid phase
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Equilibrium Cooling
first solid nuclei
two-phase solidification
polycrystalline solid
Cu
Ni
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Phase Composition
Ni
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Non-Equilibrium Cooling
first solid nuclei
concentration changes in solid AND liquid phase
required!! by diffusion
Ni concentration in solid nuclei
remains high low Dsolid
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Diffusion
flux
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Non-Steady-State DiffusionFickss 2nd Law
diffusion coefficient
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Binary Eutectic Phase Diagrams (Pb-Sn)
liquid phase
Sn
Pb
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Lamellar Eutectic Microstructure
Tm, similar to pure components, but two phases!!
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Sub-Eutectic Microstructure
lead-rich a phase
eutectic white b dark a
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Microstructural Examination - Metallography
preparation of cross section -cutting -grinding
(SiC paper)-polishing (e.g. 1µm diamond
suspension) -etching (selective chemical attack
of grains/grain boundaries)
optical microscopy electron microscopy
(transmission (TEM) or scanning (SEM))
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Metallography (SEM)
detector
electron beam
secondary electrons
specimen
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Binary Eutectic Phase Diagrams
a phase
solvus line precipitation of b phase
Pb
Sn
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Intermetallic Phases/Compounds(e.g. ceramics)
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The Iron-Carbon Phase Diagram
eutectic system
bcc d ferrite
fcc g austenite
bcc a ferrite
cementite Fe3C
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Hypoeutectoid Steels (CClt0.76)(? hypereutectoid)
Ck45Cc0.45
dark pearlite lamellae of Fe3C and ferrite
light a ferrite
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Hypoeutectoid Steels (CClt0.76)(? hypereutectoid)
Ck15Cc0.15
dark pearlite
light a ferrite