Order in Solids Flashcards
Amorphous
- Non dense + random packing - without long range order of crystallinity
- Fast solidification doesn’t allow time to organize crystal structure so result is this - a solid w/liquid-like appearance
Crystalline
Dense + regular packing - lower energy
Polycrystals
Several crystals packed together
Why are metallic crystal structures densely packed?
Only 1 element present so all same radii, metallic bonding not directional, nearest neighbour distances small so that bond energy lowered + electron cloud shields cores from each other
Which crystal structures are the simplest?
Metallic
APF
Volume of atoms in unit cell/ total unit cell volume
Coordination number
first touching neighbours in hard sphere model
Simple cubic structure
CN=6, 1 atom/ unit cell, a = 2R, APF=0.52
BCC structure
2 atoms/unit cell, a = 4R/sqrt(3), APF=0.68, CN=8
FCC structure
a=2Rsqrt(2), CN=12, APF=0.74, 4 atoms/unit cell
HCP structure
APF=0.74, CN=12, 6 atoms/unit cell
What are the voids in FCC structure?
4 octahedral voids w/CN=6 and 8 tetrahedral voids w/CN=4
Polymorphism
When metals + non-metals have >1 crystal structure
Anisotropic
When properties vary w/direction + grains are oriented
Isotropic
When grains are randomly oriented
What happens before and after melting temperature in crystalline solid transitions?
Atomic vibes increase w/T + volume expansion
What happens at melting in crystalline solid transitions?
Crystal formation + high APF + sudden volume decrease
What happens below glass transition temperature in amorphous solids?
No further rearrangements of atoms
How does glass transition temperature affect the properties of polymers + ceramics?
If T<Tg then material is hard + brittle = if T>Tg then material becomes flexible + rubbery
Do polymers most often have both crystalline + amorphous zones?
Yuh
Self-interstitials
Extra atoms positioned between atomic sites (less common than vacancies)
Why is there a small difference in thermal expansion as temperature increases for materials w/varying amounts of vacancies?
Increasing # of vacancies increases w/T which affects total length but not size a of unit cell
What is diffusion within a material largely governed by?
of vacancies
Types of dislocations
Linear defects - 1D defects around which atoms are misaligned
Edge dislocation
Extra 1/2 plane of atoms inserted in crystal structure - b perpendicular to dislocation line
Screw dislocation
Spiral planar ramp resulting from shear deformation - b parallel to dislocation line
Why does dislocation move preferentially in (1 1 1) plane of FCC structure rather than in (1 0 0) plane?
Smaller Burgers vector means less energy needed for motion
Grain boundaries
- Regions between single crystals - transition from lattice of 1 region to that of another + slightly disordered
- Low density in grain boundaries leads to high mobility + high diffusivity + high chem reactivity
Equiaxed vs columnar grains
Equiaxed are roughly same size in all directions while columnar are elongated grains
What does large number of boundaries + defects lead to?
Harder material
Do small grains make a stronger or weaker material?
Stronger
Is total interfacial energy of large grain materials greater than or less than that of fine grain materials?
Less than b/c boundary area/unit volume is smaller for large grains so large grains eat small grains
Cored vs equilibrium structure
Fast rate of cooling leads to cored while slow rate of cooling leads to equilibrium
Eutectic microstructure
Alternating layers of alpha and beta crystals
Definition of eutectic
Liquid in equilibrium w/2 solids
Eutectoid
Solid phase in equilibrium w/2 solid phases
Peritectic
Liquid + solid 1 –> solid 2
