Readings (3) Flashcards
Single Phase
All parts of an alloy microstructure with the same atomic structure
The constitution of an alloy is described by:
1) The phases present
2) The weight fraction of each phase
3) The composition of each phase
Equilibrium Constitution
The state of lowest Gibb’s Free Energy for a given composition, temperature, and pressure.
An alloy in this state is thermodynamically stable
Solvus Boundaries
Boundaries between single- and two-phase regions
Eutectic Point
Lower limit of the single-phase liquid field formed by the intersection of two liquidus lines
Lowest temperature for which 100% liquid is stable - which is why casting, brazing, soldering etc. are often near eutectic in composition
Constitution Point
Point of interest, consisting of multiple compositions/phases
Intermediate Phases
Compounds that form between components
Stoichiometry
The atomic % of components in a compound
Eutectoid Point
The lower limit of a single-phase solid field formed by two falling phase boundaries intersecting in a V
Peritectoid Point
inverted V corresponding to an upper limit of formation of a single solid phase - two-phase field above is formed of two solid phases
-tic vs. -toid
-tic means a liquid phase is involved, -toid means all phases are solid
Dilatometry
Measurement of dimensional changes at high resolution to detect changes of density and symmetry caused by phase change
Electrical Resistivity
Measurement of electrical resistivity changes associated with difference in electron mean free path before and after phase change
Calorimetry
Sensitive differential measurement of release or take-up of latent heat associated with phase change
Optical Microscopy
Differential reflection of light, either in color or intensity, by phases or by a surface film created by chemical or electro-etching
X-ray Diffraction
Diffraction of the X-radiation by the crystal lattice of each phase, giving diffraction patterns from which crystal structure and volume fraction of phases can be inferred
Scanning Electron Microscopy (SEM)
Differential back-scattering of electrons by differing phases giving both an image and compositional information
Transmission Electron Microscopy (TEM)
Diffraction of an electron beam by the crystal lattice of each phase, giving both an image of the structure and, from the diffraction pattern, its crystal structure
Gibb’s Free Energy at phase boundaries
ΔG = 0
Homogeneous Nucleation
- Spontaneous clusters/colonies of solids form within the melt
- These grow stably provided they can reach a critical radius
Heterogeneous Nucleation
Solid nuclei form more readily on a pre-existing solid in contact with the liquid
Phase Reaction
When any phase compositions change with temperature
Segregation
The center of a grain will be of more pure substance due to cooling of the solution. As the temperature hits the liquidus line, the solid is as pure as possible (solid form), and then cooling will cause a composition gradient from the center to the outside of the grain of decreasing composition.
Solidification Cracking
Segregation concentrates impurities on the grain boundaries, generating high enough thermal stresses to crack along the weakened grain boundaries
Hybrids
Woven fabrics consisting of two or more types of reinforcing fibers
Anisotropic
Varies with direction
Discrete Fibers
Looks like “sprinkles”
Plasticizer
Additive used to soften a polymer
Thermoset
Become hard and rigid upon heating
Thermoplastic
Become soft and deformable upon heating
Elastomers
Polymers with mechanical behavior analogous to natural rubber
Aspect Ratio
L/d
Resin
Matrix component to help fiber interaction
