Crystal structure I Flashcards
What is crystalline material?
Material where atoms are arranged in a repeating array, and
exhibit long-range order
What is crystal structure? What does it govern and what types of materials do they exist in?
the spatial arrangement of atoms in crystalline solids
* Governs material properties
* Exists in metals, many ceramics and some polymers
What is crystal morphology
shape and size of crystals or grains (e.g. coarse,
elongated or columnar, equiaxed, fine)
What is the unit cell?
The repeating entity that defines the structure
in a crystalline solid
What are the characteristics of the unit cell?
-Number of atoms per unit cell
-Coordination number
-Close packed directions
-Atomic packing factor
What is coordination number?
The number of nearest-neighbor or
touching atoms in a crystal structure
What are close-packed directions?
Directions within the unit cell with
highest linear density of atoms
What is the atomic packing factor?
The volume of atoms in a unit cell per
volume of a unit cell
Define density
ρ = mass per unit volume (kg/m3
)
What is theoretical density?
ρTh = mass of atoms in unit cell per
volume of unit cell
What type of properties does each phase or crystal structure have?
Mechanical, thermal, and electrical properties.
What properties change as iron changes between BCC (body centered cubic) and FCC (face centered cubic) structure?
Increasing temperature, with corresponding
changes in atomic packing densities
and corresponding stiffness (Young’s
modulus)
What mechanical properties do bond strength and crystal structure affect?
- Modulus of Elasticity
(Young’s modulus) - Resilience
- Yield Strength
- Ultimate Tensile Strength
- % elongation
- % reduction in area
- Toughness
What is the number of atoms per unit cell for the BCC structure?
2 atoms per unit cell
What is the coordination number for the BCC structure?
8
What is the number of atoms per unit cell for FCC?
4
What is the coordination number for the FCC structure?
12
What is the number of atoms per unit cell for the HCP structure?
6 atoms per unit cell
What is the coordination number for the HCP structure?
12
What are the three-unit cells discussed in class?
BCC, FCC, and HCP.
What are the close packed directions in a BCC unit cell?
The cube
diagonals
Where are the close packed directions in a FCC unit cell?
Along the
diagonals of the cube faces
What are the close packed directions for HCP?
Along the diagonals of the hexagonal face
Name examples of metals that have the BCC crystal structure.
Chromium, iron (α, below
910 0C), molybdenum,
tantalum
