Topics Up to Midterm & Crystal Structures Flashcards
metals
malleable (can be deformed permanently), metallic bonds
ceramics
brittle - no permanent deformation and they crack instead, has poor strength in tension
polymer
has permanent deformation, large molecule composed of repeating structural units called monomers, which are covalently bonded together in a chain-like or network structure
stress
strength, force/area
strain
ductility, change in length/initial length
young’s modulus
measure of elasticity, structure independent. the higher, the stiffer. stress = youngs modulus x strain
elastic strain
reversible/recoverable – atoms return to their original positions after unloading
tensile test
used to determine material properties and mechanical behavior for metals and polymers. stress is highest in reduced sections because cross-sectional area is small
why are tensile tests not used for ceramics
difficult w low strain (it will fracture), the shape of tensile test is difficult to make with ceramic, difficult to grip
plastic deformation
permanent change in the shape or size of a material when subjected to a stress that exceeds its yield strength. occurs from the step-by-step movement of dislocations
elastic deformation
where the material returns to its original shape upon the removal of the applied stress
3 point bending
to test material properties and mechanical behavior for ceramics
Crystal
solid materials whose atoms, ions, or molecules are arranged in a highly ordered, repeating three-dimensional pattern. This regular arrangement extends in all directions, creating a structure known as a crystal lattice
tempered glass
toughened glass. it is heated and cooled to solidify the surface; the surface cools much faster than the core. residual stresses – tensile at core, compression at surface
Face Centered Cubic (FCC)
most metals are cubic FCC
- diagonal 4r, r = radius
- 4 total atoms
- a = 2 x sqrt2 R
- CN = 12
- APF = 0.74
- stacking sequence = ABCABC
- 4 unique close packed planes ( 4 distinct non-parallel directions exist in the crystal )
- 3 close packed directions per close-packed plane
- octahedral interstitial site
Atomic Packing Factor (APF)
max fraction of a volume that can be filled with spheres, the volume of spheres/volume of the unit cell
Rock salt structure
- number of anion (-ve) = 4
- number of cations (+ve) = 4
- cation coordination number = 6
- volume of cube = (2Ra+2Rc)^3
- octahedron interstitial site. Rc/Ra = 0.414
Interstitial Sites
space between other atoms
Body-Centered Cubic (BCC) Crystal Structure
metallic crystal structure, body-centered position, contact along the body diagonal
- CN = 8
- a = 4/sqrt3 x R
- APF = 0.68, thus not closed packed as that is less than FCC
- does not have true close-packed directions because the atomic packing factor (APF) is lower than FCC or HCP
Hexagonal Closed Packed (HCP)
- ABAB stacking sequence of closed-packed planes
- APF = 0.74
- tetrahedra
- CN = 12
- has 6 total number of atoms in unit cell
proportional limit
end of the linear elastic region
ultimate tensile strength
the highest value of stress/strain
yield strength
where plastic deformation begins. yield strength and proportional limit are not the same. 0.2% offset yield
necking
when the cross-sectional area reduces, nonuniform plastic is where necking starts. After ultimate tensile strength
dislocation
crystalline linear imperfection responsible for step-wise breaking and reforming of bonds during plastic deformation, 1 dimensional defect, plastic deformation of a metal increase number of dislocations which inhibits dislocation movement
dislocation density
to lower dislocation density, heat metal. to increase dislocation density, plastically deform. to strengthen a metal, inhibit dislocation movement
zero dimensional imperfections
point defects, interstitial impurities. more difficult for a dislocation to move through the lattice close to an impurity thus an increase of impurities make it harder to deform
one dimensional imperfections
linear imperfections/dislocations
two dimensional imperfections
interfacial imperfections - surface defects and planar defects, ex. grain boundaries
three dimensional imperfections/second phase particles
volume defects, second phase - region of a solid that has a different crystal structure. dislocations moving in a crystal that encounter second phase particles must deal with obstacles discussed in grain boundaries
interstitial defect
zero dimensional imperfection. point defect produced when an atom is placed into the crystal at a site that is normally not a lattice point
substitutional impurity
atom of a different element replaces a host atom in the crystal
vacancies
zero dimensional. atoms vibrate in their lattices trying to jump out of their sites and some atoms are successful leaving a vacancy. when atoms don’t jump out, its because binding energy is stronger than thermal energy. number of vacancies increase with temp
boltzman distribution
describes the probability distribution of particles among various energy states in a system at thermal equilibrium
free surfaces imperfection
2 dimensional imperfection occurring at the external surface of a material where the atomic structure is disrupted because it lacks neighboring atoms on one side. It doesn’t increase the strength of a metal
internal interfaces/grain boundaries
form of internal surface that occurs when one crystal contacts another. if dislocation moves during plastic deformation, it has to go through grain boundary, changing its direction. More difficult for dislocation travel through grain boundary as it requires more energy. can be used to inhibit dislocation movement, strengthening a metal. decreasing grain size would create more grain boundaries, strengthening a metal
octahedral interstitial sites
- CN = 6
- RC/RA = 0.414 ( ratio between cations and anions )
- FCC has octahedral interstitial sites
tetrahedral interstitial site
- CN = 4
- RC/RA = 0.225
simple cubic interstitial site
- CN = 8
- RC/RA = 0.732
- exists at center of the cube but only one site per unit cell is available
diamond cubic crystal structure
- 8 subcubes
- CN = 4
- has a tetrahedral interstitial site
