(See related pages)
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| Nuclei | small particles of a new phase formed by a phase change (e.g., solidification)
that can grow until the phase change is complete. (See page(s) 172, Sec. 4.1) |
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| Homogeneous nucleation | (as pertains to the solidification of metals): the formation of
very small regions of a new solid phase (called nuclei) in a pure metal that can grow until solidification is complete. The pure homogeneous metal itself provides the atoms
that make up the nuclei. (See page(s) 172, Sec. 4.1) |
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| Embryos | small particles of a new phase formed by a phase change (e.g., solidification)
that are not of critical size and that can redissolve. (See page(s) 173, Sec. 4.1) |
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| Critical radius of nucleus | r*: the minimum radius that a particle of a new phase formed
by nucleation must have to become a stable nucleus. (See page(s) 173, Sec. 4.1) |
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| Heterogeneous nucleation | (as pertains to the solidification of metals): the formation
of very small regions (called nuclei) of a new solid phase at the interfaces of solid im-purities.
These impurities lower the critical size at a particular temperature of stable
solid nuclei. (See page(s) 173, Sec. 4.1) |
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| Grain | a single crystal in a polycrystalline aggregate. (See page(s) 173, Sec. 4.1) |
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| Equiaxed grains | grains that are approximately equal in all directions and have random
crystallographic orientations. (See page(s) 173, Sec. 4.1) |
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| Columnar grains | long, thin grains in a solidified polycrystalline structure. These grains
are formed in the interior of solidified metal ingots when heat flow is slow and uniaxial
during solidification. (See page(s) 173, Sec. 4.1) |
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| Polycrystalline structure | a crystalline structure that contains many grains. (See page(s) 173, Sec. 4.2) |
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| Alloy | a mixture of two or more metals or a metal (metals) and a nonmetal (nonmetals). (See page(s) 173, Sec. 4.3) |
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| Solid solution | an alloy of two or more metals or a metal(s) and a nonmetal(s) that is a
single-phase atomic mixture. (See page(s) 173, Sec. 4.3) |
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| Substitutional solid solution | a solid solution in which solute atoms of one element can
replace those of solvent atoms of another element. For example, in a Cu–Ni solid
solution the copper atoms can replace the nickel atoms in the solid-solution crystal
lattice. (See page(s) 173, Sec. 4.3) |
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| Interstitial solid solution | a solid solution formed in which the solute atoms can enter
the interstices or holes in the solvent-atom lattice. (See page(s) 173, Sec. 4.3) |
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| Vacancy | a point imperfection in a crystal lattice where an atom is missing from an
atomic site. (See page(s) 173, Sec. 4.4) |
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| Interstitialcy (self-interstitial) | a point imperfection in a crystal lattice where an atom
of the same kind as those of the matrix lattice is positioned in an interstitial site be-tween
the matrix atoms. (See page(s) 173, Sec. 4.4) |
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| Frenkel imperfection | a point imperfection in an ionic crystal in which a cation vacancy
is associated with an interstitial cation. (See page(s) 173, Sec. 4.4) |
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| Schottky imperfection | a point imperfection in an ionic crystal in which a cation vacancy
is associated with an anion vacancy. (See page(s) 173, Sec. 4.4) |
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| Dislocation | a crystalline imperfection in which a lattice distortion is centered around a
line. The displacement distance of the atoms around the dislocation is called the slip
or Burgers vectorb. For an edge dislocation the slip vector is perpendicular to the dislocation
line, while for a screw dislocation the slip vector is parallel to the dislocation
line. A mixed dislocation has both edge and screw components. (See page(s) 173, Sec. 4.4) |
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| Grain boundary | a surface imperfection that separates crystals (grains) of different orientations
in a polycrystalline aggregate. (See page(s) 173, Sec. 4.4) |
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| Grain-size number | a nominal (average) number of grains per unit area at a particular
magnification. (See page(s) 173, Sec. 4.4) |
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| Activation energy | the additional energy required above the average energy for a thermally
activated reaction to take place. (See page(s) 174, Sec. 4.5) |
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| Arrhenius rate equation | an empirical equation that describes the rate of a reaction as a
function of temperature and an activation energy barrier. (See page(s) 174, Sec. 4.5) |
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| Substitutional diffusion | the migration of solute atoms in a solvent lattice in which the
solute and solvent atoms are approximately the same size. The presence of vacancies
makes the diffusion possible. (See page(s) 174, Sec. 4.6) |
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| Self-diffusion | the migration of atoms in a pure material. (See page(s) 174, Sec. 4.6) |
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| Interstitial diffusion | the migration of interstitial atoms in a matrix lattice. (See page(s) 174, Sec. 4.6) |
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| Volume diffusion | atomic migration in the grain interiors of a polycrystalline aggregate. (See page(s) 174, Sec. 4.6) |
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| Grain boundary diffusion | atomic migration at the grain boundaries of a polycrystalline
aggregate. (See page(s) 174, Sec. 4.6) |
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| Fick’s first law of diffusion in solids | the flux of a diffusing species is proportional to
the concentration gradient at constant temperature. (See page(s) 174, Sec. 4.6) |
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| Fick’s second law of diffusion in solids | the rate of change of composition is equal
to the diffusivity times the rate of change of the concentration gradient at constant
temperature. (See page(s) 174, Sec. 4.6) |
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| Diffusivity | a measure of the rate of diffusion in solids at a constant temperature. Diffusivity
D can be expressed by the equation D = D0e-Q/RT, where Q is the activation energy and T is the temperature in kelvins. D0 and R are constants. (See page(s) 174, Sec. 4.6) |
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| Steady-state conditions | for a diffusing system there is no change in the concentration
of the diffusing species with time at different places in the system. (See page(s) 174, Sec. 4.6) |
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| Non–steady-state conditions | for a diffusing system the concentration of the diffusing
species changes with time at different places in the system. (See page(s) 174, Sec. 4.6) |