crystal structures notes

ObjectivesObjectives1.1. Identify and describe the three Identify and describe the three

primary metallic crystal structures.primary metallic crystal structures.2. Explain the terms:

Anisotropic, anisotropyPolycrystalline, polycrystallinityPolymorphic, polymorphismAllotropic, allotropy

Crystal StructuresCrystal StructuresCrystal:Crystal:

Lattice ConstantsLattice Constants

a,b,c – lattice vectorsa,b,c – lattice vectors

alpha, beta, gamma – interaxial anglesalpha, beta, gamma – interaxial angles

Why Crystals?Why Crystals?

Atoms arrange themselves into Atoms arrange themselves into crystals in order to _______ their free crystals in order to _______ their free energy. energy.

Structures that deviate from the Structures that deviate from the crystalline ideal will result in crystalline ideal will result in _________ energies in the atoms._________ energies in the atoms.

Bravais

Lattices

FCC: Face Centered CubicFCC: Face Centered Cubic

FCC: Face Centered CubicFCC Metals

Al

Cu

Au

Pb

Ni

Pt

Ag

Face Centered Cubic

2 a

2 a

General CharacteristicsGeneral CharacteristicsFCC MetalsFCC Metals

Fairly softFairly soft Malleable (easily formed) and do not Malleable (easily formed) and do not

harden much as they deformharden much as they deform Will deform a great deal before Will deform a great deal before

failurefailure

BCC: Body Centered Cubic BCC: Body Centered Cubic

BCC: Body Centered Cubic

BCC MetalsCrFeMoP

NaTaWV

Unit Cell:

APF:

Body Centered Cubic

2 a

3 a

Unit Cell:

APF:

General CharacteristicsGeneral CharacteristicsBCC MetalsBCC Metals

Generally harder than FCC metalsGenerally harder than FCC metals Will not deform as much before Will not deform as much before

failurefailure Tend to get harder as they deformTend to get harder as they deform

Hexagonal Close PackedHexagonal Close Packed

Hexagonal Close Packed

HCP Metals

Cd

Zn

Mg

Co

Zr

Unit Cell:

APF:

General CharacteristicsGeneral CharacteristicsHCP MetalsHCP Metals

Tend to be______Tend to be______

Generally will harden rapidly as they Generally will harden rapidly as they deformdeform

Cannot usually deform a lot without Cannot usually deform a lot without failurefailure

AnisotropicAnisotropic- “Not Isotropic”; - “Not Isotropic”; properties depend on ______properties depend on ______

Wood

IsotropyIsotropyIsotropic Particle Board: Properties Isotropic Particle Board: Properties are ________ of ___________ are ________ of ___________

PolycrystallinePolycrystalline – containing many – containing many crystals.crystals.

PolymorphicPolymorphic – having different – having different ________________. (eg Iron is BCC ________________. (eg Iron is BCC FCC FCC BCC upon cooling from liquid BCC upon cooling from liquid to room temperature)to room temperature)

PolymorphicPolymorphic – having different crystal – having different crystal structures.structures.

AllotropicAllotropic – same as polymorphic but an – same as polymorphic but an elemental solid (many alloys are elemental solid (many alloys are polymorphic, only a few elements are)polymorphic, only a few elements are)

AmorphousAmorphous – no structure – no structure

Self-TestSelf-Test

What do BCC, FCC and HCP stand What do BCC, FCC and HCP stand for?for?

If a metal turns from BCC to HCP If a metal turns from BCC to HCP structure as it is heated, is it structure as it is heated, is it polymorphic or polycrystalline?polymorphic or polycrystalline?

If a metal turns from BCC to FCC, If a metal turns from BCC to FCC, what happens to the volume of the what happens to the volume of the metal?metal?