MAE 3324 Ch3 Flashcards
Define “crystalline material.”
A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances.
Define “amorphous” with respect to materials.
Amorphous materials do not crystalize and have no long-range atomic order.
Describe the atomic hard-sphere model.
Atoms are considered solid spheres having well-defined diameters. The spheres are used to show the structure of atoms in a unit cell and how the atoms touch their nearest neighbors.
Define “Lattice.”
A three-dimensional array of points coinciding with atom positions.
T or F
Corners or unit cells usually coincide with the centers of hard-sphere atoms.
True
Name some common metals that have a face-centered cubic (FCC) unit cell.
Copper
Aluminum
Silver
Gold
For FCC, how many face atoms are there?
Nf = 6
For FCC, how many corner atoms are there?
Nc = 8
For FCC, how many interior atoms?
Ni =0
How many total atoms in an FCC unit cell?
Four
Define “Coordination number.”
The number of nearest neighbor touching atoms.
What is the coordination number for FCC?
12
Define atomic packing factor?
The sum of the sphere volumes of all atoms within a unit cell divided by the unit cell volume.
APF = (volume of atoms in the cell)/(total unit cell volume)
T or F
Metals typically have relatively large atomic packing factors to maximize the shielding provided by the free electron cloud.
True
Describe the body centered cubic BCC structure.
Atoms are located at all eight corners and a single atom at the center of the cube.
Give some examples of elements having BCC structures.
Chromium, iron, & tungsten.
How many corner atoms in BCC?
Nc = 8
How many center atoms in BCC?
Ni = 1
How many face atoms in BCC?
Nf = 0
What is the total number of atoms in a BCC unit cell?
N = 2.
What is the coordination number for the BCC crystal structure?
8
What is the atomic packing factor for FCC?
0.74
What is the atomic packing factor for BCC?
0.68
Describe the Hexagonal Close Packed HCC structure.
The tops and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center. Another horizontally orientated plane of three atoms is directly in the middle of the top and bottom planes.
How many corner atoms in HCP?
Nc = 12
How many face atoms for HCP?
Nf = 2
How many interior atoms for HCP?
Ni = 3
How many total atoms for HCP?
6
What is the coordination number for HCP?
12
What is the atomic packing factor for HCP
0.74
T or F
The atomic packing factor and coordination number for HCP are the same as FCC?
True
Define “Polymorphism” with respect to materials.
When a material contains more than one crystal structure.
What is “allotropy?”
When a material is polymorphic and also an elemental solid.
T or F
In an allotropic solid, the prevailing crystal structure depends on both the temperature and the external pressure?
True