Materials Science Test #1

Classifications of Materials

Metals, Polymers, Ceramics, and Composites

Material Relationship

Processing ---affects--> Structure ---affects---> Properties

Major Properties of Materials

Mechanical, Electrical, Thermal, Optical, Magnetic, and deteriorative (chemical stability)

Advanced Materials

Semiconductors, Biomaterials, Smart Materials, and Nanomaterials

Primary Bonds

Covalent, Metallic, and Ionic

Pauli-Exclusion Principle

Each electron has its own unique combination of numbers

Valence Electrons

Those in the outer-most shell

Ionic Bond

Ex: NaCl; transfer electrons; normally non-directional

Covalent Bond

Carbon-Carbon; share electrons; directional; similar electronegativity

Metallic Bond

All Metals; 3 or fewer valence electrons; non-directional

Mixed Bonding

Covalent-Ionic bond; intermetallic materials;

Secondary Bonding

arises from interactions between dipoles; weaker than primary bonds;

Electropositive

Donate electrons; metal

Electronegative

Accepts electrons; non-metals

Net Energy in Ionic Bond

En = Ea + Er = A/r + B/r^n

Percent Ionic Character

EN < .4 --> non-polar covalent;

.4 <= EN <= 1.7 --> polar covalent;

EN > 1.7 --> Ionic

"Amorphus"

Noncrystalline structure

Unit Cell

smallest representative volume which contains the complete lattice pattern of the crystal

Basis

Number of atoms located on the lattice points

Simple Cubic Structure (SC)

very rare; only contains 1 atom; APF = .52; coordination # = 6

Atomic Packing Factor

(volume of atoms in unit cell) / (volume of unit cell)

Body Centered Cubic (BCC)

coordination # = 8; APF = .68; contains 2 atoms;

Face Centered Cubic (FCC)

Most common in metals; coordination # = 12; APF = .74 (maximum possible); contains 4 atoms

Hexagonal Close-Packed Structure (HCP)

Very common in metals; ABAB... stacking sequence; coordination # = 12; APF = .74; contains 6 atoms;

Theoretical Denisty

(mass in unit cell) / (volume of unit cell)

Order of densities of classes

P(metal) > P(ceramics) > P(polymers)

Crystallographic Directions

Obtain head & tail coordinates;

Subtract tail from head;

Adjust to smallest integer value;

Enclose in brackets w/o commas and negative signs;

Family of Directions

Equivalent Directions grouped together;

Linear Density

(# of atoms) / (unit length of direction vector)

Miller Indices

Reciprocals of the axial intercepts for a plane, cleared of fractions & common multiples

Planar Density

(# of atoms centered on plane) / (area of plane)

Isotropic

Looks the same; independent of direction

Anisotropic

Direction to grains; determines properties of single crystals

Polymorphism

Two of more crystal structures for the same material; allotropy in elements; polymorphism in compounds

Anisotropy

Dependent on direction;

Defects

Point, Line, Volume, and Area

Point Defects

Vacanceis: missing an atom; (most common)

Self-Interstitials: atom stuch between planes

Point Defects in Alloys

1) Solid Solution of B in A; Substitutional solid or Interstitial solid solutions

2) Solid Solution of B in A plus particles of a new phase

Hume-Rothery Rules

1) similar atomic size; |ra-rb|/ra x 100

2) similar crystal structure for pure metals

3) similar electronegativity

4) valencies; higher valence = greater tendency

Compositions

Weight percent and Atom percent

Weight Percent

C1 = m1/(m1+m2) x 100;

Atom Percent

C1 = n(m1) / (n(m1) + n(m2)) x 100;

Linear Defects: Dislocations

line defects around which some atoms are misaligned; produce permanent deformation

Edge Dislocation

extra column/row of atoms;

Screw Dislocation

spiral planar ramp resulting from sheer deformation

Twin Boundary

reflections of atoms at a plane

External Surfaces

surface atoms are not bounded as in the crustal matrix; high energy

Planar Defects

Twin boundary, stacking faults, external surfaces, and phase boundaries

Volume Defects

Pores, cracks, etc. (larger than atom defects); occur during processing and fabrication

Fine Grain

Grain Size: 8 - 10

Course Grain

Grain Size: 2 - 3