3 Atomic Packing

Question 1

For FCC, what are

atoms per unit cell
cube length a
APF
Coordination number?

Answer 1

FCC:
4 atoms per cell
a = (2 root 2)R
APF = 0.74
Coord = 12

Question 2

For BCC, what are:

atoms per unit cell
cube length a
APF
Coordination number?

Answer 2

BCC:

2 atoms per unit cell
a = 4R / sqrt(3)
APF = 0.68 = pi * sqrt(3)/8
Coord = 8

Question 3

For HCP, what are:

atoms per unit cell
c/a ratio
APF
Coordination number?

Answer 3

HCP:

6 atoms per unit cell
c/a of 1.633
APF of 0.74
Coord number of 12

Question 4

What’s the difference between amorphous and crystalline?

Answer 4

Crystalline structures are ordered, while an amorphous structure is disordered.

Amorphous are isotropic, amorphous are anisotropic

Question 5

How do you make amorphous metals?

Answer 5

Cool the metal extremely rapidly, to prevent it from solidifying.

Amorphous copper can be made by spewing it out of a spinning wheel

Question 6

What materials are normally crystalline, and what are normally amorphous?

Answer 6

Simple compounds consisting of one or two elements are typically crystalline.

Complex compounds (or simply constraint on atom positions) tend to be amorphous

Question 7

What are the two rules to creating a unit cell?

Answer 7

Highest level of geometric symmetry, or fewest number of atoms.

Remember that this unit cell must be tesselateable

Question 8

What are some examples of HCP?

Answer 8

Ti, Zn, Co

Question 9

What are the important things to remember about HCP?

Answer 9

c/a = sqrt(8/3) = 1.633
APF = pi / sqrt (18) = 0.74
Three interior atoms are fully within the hexagon.
Both things come from geometric arguments. c/a from two constructed triangles, and APF from 6-sphere volume divided by volume of hexagonal unit cell.

Question 10

What are the important things to remember about FCC?

Answer 10

There’s 1/8th of an atom on each corner, and half an atom on each face. That is all.
a = 2root(2)*R from pythagoras on cube face.

Question 11

What are some examples of FCC?

Answer 11

Cu, Al, Ag

Question 12

What are some examples of BCC?

Answer 12

Fe, Cr, W

Question 13

What are the important things to remember?

Coord number?
APF?
a?

Answer 13

Coord 8
APF pi root(3/8) = 0.68
a = r/root(3) * r

Question 14

OTHER CRYSTAL SYSTEMS:

How many dimensions needed to define a parallelepiped?

Answer 14

Six. Three length and three angles.

Question 15

What are these parallelepipeds called?

Answer 15

Bravais lattices.

Question 16

What are the four main ways of changing the crystal structure?

Answer 16

Temperature, pressure, plastic deformation, and alloying.

Question 17

How does stainless steel work?

Answer 17

18-8 Stainless steel - Cr Ni % weight

Cr easily forms an oxide tenacious thin film
Ni has a natural corrosion resistance

Question 18

Why is stainless steel not magnetic?

Answer 18

Iron is BCC at room temp, and FCC over 900C.

BCC Fe is magnetic, FCC Fe isn’t.

Nickel is FCC at room temp, and stabilizes Fe.

Question 19

What’s the difference between Austenitic and ferritic stainless steel?

Answer 19

Austenitic has nickel and ferritic doesn’t.

Question 20

Why is nickel present in stainless steel?

Answer 20

Corrosion resistance. Or strength.

Question 21

CRYSTAL NOMENCLATURE

Answer 21

Yay!

Question 22

What’s the difference between allotropes, polymorphs, and phases?

Answer 22

Allotropes generally for pure elements. Phases generally for packing configurations.

Question 23

What’s a miller index?

Answer 23

A method to describe planes and directions.

Question 24

What are some ways of quantifying different planes and different lines?

Answer 24

By how many atoms it intersects. Planar atomic density and linear atomic density.

Question 25

What does “anisotropic” mean?

Answer 25

That a material has different properties in different directions

Question 26

Does a single crystal exhibit isotropy or anisotropy?

Answer 26

A single crystal exhibits anisotropy. Different planes exhibit different arrangements and resistances.

Question 27

Name the two types of interstitial sites.

Answer 27

Octahedral and tetrahedral.

Question 28

On a BCC crystal, point out the octahedral sites and the tetrahedral sites.

Answer 28

Cheat! (Draw diagram)

Question 29

On an FCC crystal, point out the octahedral and tetrahedral sites.

Answer 29

(draw diagram)