Materials Science

Atomic Structure and Bonding

Ionic bonds: transfer of electrons between atoms (ceramics, salts); strong, brittle
Covalent bonds: sharing of electrons (diamond, silicon); strong, directional
Metallic bonds: electron cloud shared among atoms (metals); good conductivity, ductile
Van der Waals: weak secondary bonds (polymers); low melting points
Bond type determines material properties: strength, conductivity, melting point

BCC: 2 atoms per unit cell, APF = 0.68, CN = 8 (iron below 912 C, chromium, tungsten)
FCC: 4 atoms per unit cell, APF = 0.74, CN = 12 (aluminum, copper, nickel, gold)
HCP: 6 atoms per unit cell, APF = 0.74, CN = 12 (zinc, magnesium, titanium)
Miller indices describe crystallographic planes and directions
Defects: point (vacancies), line (dislocations), planar (grain boundaries)
Dislocations enable plastic deformation at stresses below theoretical strength

Tensile test: stress-strain curve reveals elastic modulus, yield strength, UTS, ductility
Hooke's law region: sigma = E*epsilon (linear elastic)
0.2% offset yield strength: intersection of stress-strain curve with a line at 0.002 strain
Hardness tests: Brinell (large sphere), Rockwell (cone or sphere), Vickers (diamond pyramid)
Impact testing: Charpy and Izod measure toughness at various temperatures
Ductile-to-brittle transition temperature: critical for structural steels
Fatigue: failure under cyclic loading; S-N curve shows endurance limit

Binary phase diagrams show phases present as a function of temperature and composition
Lever rule: determines the fraction of each phase in a two-phase region
Iron-carbon: most important engineering diagram
Eutectic (liquid to two solids): occurs at 4.30% C, 1147 C
Eutectoid (one solid to two solids): austenite to pearlite at 0.76% C, 727 C
Phases: ferrite (BCC, soft), austenite (FCC, hot), cementite (Fe3C, hard, brittle)
Steel: less than 2.14% C; Cast iron: 2.14% to 6.67% C

Galvanic corrosion: two dissimilar metals in electrical contact in an electrolyte
More active metal (anode) corrodes; nobler metal (cathode) is protected
Galvanic series: ranks metals from active to noble in seawater
Uniform corrosion: even material loss over entire surface
Pitting: localized, often more damaging than uniform corrosion
Prevention: coatings, cathodic protection, material selection, inhibitors

Ductile fracture: significant plastic deformation before failure; cup-and-cone pattern
Brittle fracture: little or no plastic deformation; flat fracture surface
Fatigue fracture: beach marks and striations visible on fracture surface
Stress concentration: cracks, notches, and holes amplify local stress
Stress intensity factor: K = sigma*sqrt(pi*a)*Y (a = crack length, Y = geometry factor)
Fracture toughness (K_Ic): critical value of K at which fast fracture occurs