Microstructure of Metals and Alloys Flashcards
What are the three categories of microstructures of metals and alloys?
Basic crystal structures, alloys, Single crystal and polycrystals
What are the three basic crystal structures?
Body-centered cubic (bcc), Face-centered cubic (Fcc), Hexagonal close packed cubic (hcp)
What are the two alloy forms of microstructures?
Solid solution and intermetallic compound.
What are atoms made of?
A positively charged nucleus and enough negatively charged electrons to balance the charge.
What determines the atomic number and element?
Number of protons (which is equal to the number of electrons).
What are the two types of bonds found in molecules?
Primary and secondary bonds. Primary bonds lead to the formation of molecules, while secondary bonds are found between attracting molecules.
What is the difference between an atom and a molecule.
An atom is the smallest unit of matter that still has all of the properties of its element, while a molecule is a structure that contains multiple atoms bonded together
Primary Bonds
Characterized by strong atom-to-atom attractions that involve the exchange of valence electrons.
What are the three types of primary bonds.
Ionic bonds, covalent bonds, and metallic bonding.
Ionic Bond
The atoms of one element give up their outer electrons which are then attracted to the atoms of another element. This increases their valence electron count to 8.
Covalent Bond
Electrons are shared (instead of transferred) between the outermost shells to create a stable configuration.
Metallic Bonding
Takes place in pure metals and metallic alloys. The electrons in the outer shell are shared by all atoms to create a general electron cloud. This electron cloud allows for electrical conductivity.
Secondary Bonds
Bonds that do not involve the transfer or sharing of electrons. This makes them weaker than primary bonds.
What are the three types of secondary bonds?
Dipole forces, london forces, and hydrogen bonding.
Dipole Forces
These forces can be found in a molecule made up of two atoms that have an equal and opposite charge.
London Forces
These forces can be found in non-polar molecules. Due to the rapid motion of electrons in orbit they form temporary dipoles when more electrons happen to be on one side of the molecule than the other.
Hydrogen Bonding
Occurs in molecules with hydrogen stoms that are covalently bonded to another atom.
Crystalline
Atoms are located in regular or reoccuring positions in three dimensions.
Non-Crystalline
An absence of long-range order in the molecular structure.
Body-centered cubic (BCC)
Made up of atoms organized in a cube with one atom in each corner and one atom in the center.
Has a repeating pattern of 8.
Face-centered cubic (FCC)
Made up of atoms organized in a cube with one atom in each corner and one atom in the center of each face. Is more space efficient than BCC.
Has a repeating pattern of 12.
Hexagonal close-packed
In a hexagonal shape.
Has a repeating pattern of 12.
Diamond cubic
Has a repeating pattern of 8. Is found materials like semiconductors and diamonds.
Does not follow the Bravais lattic definition.
Allotropism
When materials undergo a change in crystal structure at different temperatures. An example is Iron (Fe)
Imperfections
Arise naturally due to the solidifying material’s inability to continue the unit cell’s replication indefnitely without interruption.
Can also be introduced on purpose during the manufacturing process (adding an alloying ingredient in a metal to increase its strength).
Point defects
Imperfections in an atom or a few atoms.
E.g. vacancy, ion-pair vacancy, intersitialcy, displaced ion
Line defects
A connected group of point defects that forms a line in the lattice structure.
E.g. edge dislocation, screw dislocation
Surface defects
Imperfections that extend in two directions to form a boundary.
E.g. grain boundary
Slip
Deformation mechanism involving relative movement of atoms on opposite sides of lattice slip plane. Mostly in BCC and HCP.
Enabled by dislocation irregularities.
Twinning
Deformation mechanism involving plastic deformation where atoms on one side of the twinning plane shift and form a mirror image on the other side. Most important in HCP (because does not slip easily).
What are grain boundaries and what affects grain size?
Interference of crystals in nucleation sites that form at interface surface.
Size is inversely related to cooling rate: fast=small =higher strength & hardness. slow=large
Strain hardening
Process that makes metal stronger and harder due to plastic deformation. Occurs at a temperature low enough that atoms cannot recrystalize.
Recovery
Stress is relieved in highly deformed regions.
Takes place below re-crystallization temperature. (<0.3~0.5 Tm)
Re-crystallization
New strain free grains are formed, replacing the old grains.
(=0.3~0.5 Tm)
Grain growth
Grains begin to grow and exceed the original size and form a rought surface appearance.
Above re-crystallization temperature. (>0.3~0.5 Tm)
Cold working
Plastic deformation takng place below 0.3Tm
Warm working
Plastic deformation taking place between 0.3Tm-0.5Tm.
Hot working
Plastic deformation taking place above 0.5Tm.
Usually carried out above the re-crystallization temperature and below 0.75Tm.
Annealing
Heating metal to a certain temperature, holding (soaking), and then slowly cooling.
Reduces hardness/brittleness, alters microstructures, re-crystallizes cold-worked metals, relieves residual stresses.
Hot Shortness
When alloys fracture in a brittle manner along grain boundaries when deformed at temperatures near the melting point
Dislocation
Linear irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to slide over each other at low stress levels and is known as glide or slip.
