Exam 1: Chapters 3 & 4 Flashcards
True or false?
Seeding a casting by adding small inert particles promotes homogeneous nucleation throughout the casting and results in smaller grain size than casting without seeding.
False
True or false?
Dislocations moving cause plastic deformation, ductility, and the existence of a yield strength.
True
True or false?
The center of a casting is generally the first region to freeze.
False
True or false?
Heterogeneous nucleation is easier than homogeneous nucleation and is what usually happens in any real materials process where a liquid transforms to a solid.
True
FCC materials are more ductile than BCC or HCP materials because?
FCC materials have close packed planes in four directions
True or false?
The critical radius for nucleation of a solid phase in a liquid to solid transformation gets larger as the temperature is decreased more and more below the solidification temperature.
False
True or false?
An interstitial solid solution generally results when there is little difference in size between the solvent and the solute atoms of the solid solution
False
What is the atomic packing factor of a BCC cell?
0.68
Substitution solid solution
a solid solution in which solute atoms of one element can replace those of solvent atoms of another element
Interstitial solid solution
a solid solution formed in which the solute atoms can enter the voids or holes in the solvent atom lattice
Vacancy
a point imperfection in a crystal lattice where an atom is missing from an atomic site
Frenkel imperfection
a point imperfection in an ionic crustal in which a positive cation moves into an interstitial site in an ionic crystal, and a cation vacancy is created in the normal ion site
Schottky imperfection
A point imperfection in an ionic crystal in which two oppositely charged ions are missing from an ionic crystal creating a cation-anion divacancy.
Grain boundry
A 2D planar defect that separates crystals of different orientations in a polycrystalline aggregate.
Grain size number
A nominal number of grains per unit area at a particular magnification
Atomic packing factor
The volume of atoms per unit cell divided by the volume of the unit cell.
Metallic glass
metals with an amorphous atomic structure
Nuclei
Small particles of a new phase formed by a phase change that can grow until the phase change is complete.
Homogeneous nucleation
The formation of very small regions of a new solid phase in a pure metal that can grow until solidification is complete. The pure metal itself provides the atoms that make up the nuclei.
Embryos
Small particles of a new phase formed by a phase change that are not of critical size and that can redissolve.
Critical radius
The minimum size that a particle of a new phase formed by nucleation must have to become a stable nucleus.
Heterogeneous nucleation
The formation of very small regions of a new solid phase at the interfaces of solid impurities. These impurities lower the critical size at a particular temperature of stable solid nuclei.
Grain
a single crystal in a poly-crystalline aggregate
Equiaxed grains
Grains that are approximately equal in all directions and have random crystallographic orientations.
