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Definition of a “phase”
- Has a distinctive temperature and
composition range
- Has a definite structure
- Usually has an easily seen interface.
- NB Grain boundaries between the same
Binary Phase Diagrams Lesley Cornish Definition of a phase Has a - - PowerPoint PPT Presentation
Binary Phase Diagrams Lesley Cornish Definition of a phase Has a - - PowerPoint PPT Presentation
Binary Phase Diagrams Lesley Cornish Definition of a phase Has a distinctive temperature and composition range Has a definite structure Usually has an easily seen interface. NB Grain boundaries between the same phases, and
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Maps?
???
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Examples!
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Must be able to understand phase diagrams
Might expect single phase…. but if as-cast, or not annealed for long enough, might be surprised!
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Good tools for analysis
SEM in backscattered electron mode → see average Z SEM in 2ndry electron mode → check for holes, by tilting X-ray diffraction → identify phases and structures Thermal analysis → reaction temperatures
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Check that all EDX peaks are accounted for….
Very light bits are contamination (gold)
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Observe in low magnification first
Very inhomogeneous specimen!
SLIDE 9
Dendrite grows into its shape because the atoms add more easily on the tips than on the sides in specific directions for different structures r* = critical radius for growth to
- ccur
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Coring: Solidification of the same phase with different compositions
Temperature
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←Facetted crystals Often compounds, and Bi Dendrites → Most metals form dendrites
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Rejection of solute…
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Hume-Rothery rules for extended isomorphous solid solutions
- Same structure
- Atom size within 15%
- Similar valencies (i.e. to bond with the same
number of atoms – else form compound)
- Similar electronegativities (else form