Order in Solids

Question 1

Amorphous

Answer 1

• 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

Question 2

Crystalline

Answer 2

Dense + regular packing - lower energy

Question 3

Polycrystals

Answer 3

Several crystals packed together

Question 4

Why are metallic crystal structures densely packed?

Answer 4

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

Question 5

Which crystal structures are the simplest?

Answer 5

Metallic

Question 6

APF

Answer 6

Volume of atoms in unit cell/ total unit cell volume

Question 7

Coordination number

Answer 7

first touching neighbours in hard sphere model

Question 8

Simple cubic structure

Answer 8

CN=6, 1 atom/ unit cell, a = 2R, APF=0.52

Question 9

BCC structure

Answer 9

2 atoms/unit cell, a = 4R/sqrt(3), APF=0.68, CN=8

Question 10

FCC structure

Answer 10

a=2Rsqrt(2), CN=12, APF=0.74, 4 atoms/unit cell

Question 11

HCP structure

Answer 11

APF=0.74, CN=12, 6 atoms/unit cell

Question 12

What are the voids in FCC structure?

Answer 12

4 octahedral voids w/CN=6 and 8 tetrahedral voids w/CN=4

Question 13

Polymorphism

Answer 13

When metals + non-metals have >1 crystal structure

Question 14

Anisotropic

Answer 14

When properties vary w/direction + grains are oriented

Question 15

Isotropic

Answer 15

When grains are randomly oriented

Question 16

What happens before and after melting temperature in crystalline solid transitions?

Answer 16

Atomic vibes increase w/T + volume expansion

Question 17

What happens at melting in crystalline solid transitions?

Answer 17

Crystal formation + high APF + sudden volume decrease

Question 18

What happens below glass transition temperature in amorphous solids?

Answer 18

No further rearrangements of atoms

Question 19

How does glass transition temperature affect the properties of polymers + ceramics?

Answer 19

If T<Tg then material is hard + brittle = if T>Tg then material becomes flexible + rubbery

Question 20

Do polymers most often have both crystalline + amorphous zones?

Answer 20

Yuh

Question 21

Self-interstitials

Answer 21

Extra atoms positioned between atomic sites (less common than vacancies)

Question 22

Why is there a small difference in thermal expansion as temperature increases for materials w/varying amounts of vacancies?

Answer 22

Increasing # of vacancies increases w/T which affects total length but not size a of unit cell

Question 23

What is diffusion within a material largely governed by?

Answer 23

of vacancies

Question 24

Types of dislocations

Answer 24

Linear defects - 1D defects around which atoms are misaligned

Question 25

Edge dislocation

Answer 25

Extra 1/2 plane of atoms inserted in crystal structure - b perpendicular to dislocation line

Question 26

Screw dislocation

Answer 26

Spiral planar ramp resulting from shear deformation - b parallel to dislocation line

Question 27

Why does dislocation move preferentially in (1 1 1) plane of FCC structure rather than in (1 0 0) plane?

Answer 27

Smaller Burgers vector means less energy needed for motion

Question 28

Grain boundaries

Answer 28

• 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

Question 29

Equiaxed vs columnar grains

Answer 29

Equiaxed are roughly same size in all directions while columnar are elongated grains

Question 30

What does large number of boundaries + defects lead to?

Answer 30

Harder material

Question 31

Do small grains make a stronger or weaker material?

Answer 31

Stronger

Question 32

Is total interfacial energy of large grain materials greater than or less than that of fine grain materials?

Answer 32

Less than b/c boundary area/unit volume is smaller for large grains so large grains eat small grains

Question 33

Cored vs equilibrium structure

Answer 33

Fast rate of cooling leads to cored while slow rate of cooling leads to equilibrium

Question 34

Eutectic microstructure

Answer 34

Alternating layers of alpha and beta crystals

Question 35

Definition of eutectic

Answer 35

Liquid in equilibrium w/2 solids

Question 36

Eutectoid

Answer 36

Solid phase in equilibrium w/2 solid phases

Question 37

Peritectic

Answer 37

Liquid + solid 1 –> solid 2