MATS 2001 Exam 1

metals

• repeating crystalline structure

• metallic bonds (sea of electrons)

• strong and ductile

• conductors

ceramics

• mix of metal and non-metal; ionic and covalent bonds

• regularly arranged atoms (except glasses, which are non-crystalline solids)

• strong but brittle, high melting point

• high % ionicity → insulators, low % ionicity

polymers

• composed primarily of carbon and hydrogen

• semicrystalline or amorphous

• low melting temp

• mostly poor conductors

• covalent and van der Waals forces

Energy Well Diagram

• deeper curve indicates stronger bonds and therefore higher melting temperatures

• also means higher stiffness (E) and lower thermal expansion coefficient

melting temperature

• adding thermal heat increases vibration in atoms (which is what temperature measures)

• at the melting temperature, bonds break

ionic bonds

• “giving” electrons

• strongest

• ≥ 1.6 electronegativity difference is ionic

covalent bonds

• “sharing” electrons

• similar electronegativities (or same)

• electronegativity difference between 1.0 and 1.6 means covalent with ionic tendencies

metallic bonds

• sharing loosely held electrons

• high electron mobility → conductive

crystal lattice terms

• unit cell: smallest repeat entity

• lattice parameter (a): unit cell dimensions

• coordination number (CN): number of nearest neighbors

• atomic packing factor (APF): % volume filled by atoms

isotropy

uniformity in each direction

from anisotropic to isotropic: single crystals, polycrystals, amorphous

stiffness

resistance to any deformation

strength

• force or stress a system can withstand before failing

• how difficult it is to move dislocations

ductility

distance a material can stretch before failing

toughness

energy a material can absorb before failing

Theoretical Density

complete atomic weight in unit cell / volume of unit cell

• n = number of atoms

• A = atomic weight

• V_c = volume of unit cell

• N_A = Avogadro’s number

cleaving

• occurs ALONG high density planes

• common in anisotropic single crystals

• isotropic materials typically fracture → random cracks and breakage

diffraction

occurs when a wave encounters regularly spaced obstacles

constructive (double amplitude) and destructive (zero amplitude) interference

0 Dimension defects

point defects

• intrinsic → vacancies

• extrinsic → impurities, alloys

1 Dimensional defects

line defects

• edge

• screw

• mixed (most common)

2 Dimensional defects

planar defects

• grain boundaries

• phase boundaries (in multi-phase materials)

• twin boundaries

• stacking faults (in close packed planes - FCC)

3 Dimensional defects

bulk/volume defects

• pores

• cracks

• foreign inclusions

solubility between metals

• atomic size factor

• crystal structure

• electronegativity

• valences

surface hardening

• machine softer metal, then harden surface

• interstitial diffusion

• carburizing

steady state diffusion

flux (J) does not change over time; concentration profile remains constant

non-steady state diffusion

flux J and concentration gradient change with time

• enrichment

  • carburizing

• depletion

strain

how much you deform a material

strain hardening

elastic recovery after plastic deformation

• dislocations have formed and remain

• less ductile from now on, due to lattice strain fields

• existing dislocations impede further dislocations

strengthening metals

inhibit dislocation motion (increase the force needed to move one), in other words: make crystal less perfect

• grain boundaries

• impurity atoms

• other dislocations