fundamentals of engineering supplied-reference handbook - Ventech!

MATERIALS SCIENCE/STRUCTURE OF MATTER
CRYSTALLOGRAPHY
Common Metallic Crystal Structures
Body-centered cubic, face-centered cubic, and hexagonal
close-packed.
Body-
Centered
Cubic
(BCC)
Face-
Centered
Cubic
(FCC)
Hexagonal
Close-Packed
(HCP)
Number of Atoms in a Cell
BCC: 2
FCC: 4
HCP: 6
Packing Factor
The packing factor is the volume of the atoms in a cell
(assuming touching, hard spheres) divided by the total cell
volume.
BCC: 0.68
FCC: 0.74
HCP: 0.74
Coordination Number
The coordination number is the number of closest neighboring
(touching) atoms in a given lattice.
82
ATOMIC BONDING
Primary Bonds
Ionic (e.g., salts, metal oxides)
Covalent (e.g., within polymer molecules)
Metallic (e.g., metals)
CORROSION
A table listing the standard electromotive potentials of
metals is shown on page 81.
For corrosion to occur, there must be an anode and a
cathode in electrical contact in the presence of an
electrolyte.
Anode Reaction (oxidation) of a Typical Metal, M
M o → M n+ + ne –
Possible Cathode Reactions (reduction)
½ O2 + 2 e – + H2O → 2 OH –
½ O2 + 2 e – + 2 H3O + → 3 H2O
2 e – + 2 H3O + → 2 H2O + H2
When dissimilar metals are in contact, the more electropositive
one becomes the anode in a corrosion cell. Different
regions of carbon steel can also result in a corrosion
reaction: e.g., cold-worked regions are anodic to non-coldworked;
different oxygen concentrations can cause oxygendeficient
region to become cathodic to oxygen-rich regions;
grain boundary regions are anodic to bulk grain; in
multiphase alloys, various phases may not have the same
galvanic potential.
DIFFUSION
Diffusion coefficient
Q
D =
( RT)
Do e −
, where
D = the diffusion coefficient,
Do = the proportionality constant,
Q = the activation energy,
R = the gas constant [1.987 cal/(g mol⋅K)], and
T = the absolute temperature.
THERMAL AND MECHANICAL PROCESSING
Cold working (plastically deforming) a metal increases
strength and lowers ductility.
Raising temperature causes (1) recovery (stress relief), (2)
recrystallization, and (3) grain growth. Hot working allows
these processes to occur simultaneously with deformation.
Quenching is rapid cooling from elevated temperature,
preventing the formation of equilibrium phases.
In steels, quenching austenite [FCC (γ) iron] can result in
martensite instead of equilibrium phases—ferrite [BCC (α)
iron] and cementite (iron carbide).