Lecture 1B: Physical Metallurgy of Aluminum part 2 Flashcards
Atomic number of aluminum
13
Valency of aluminum
3
What are some physical properties of aluminum?
Low melting point
Low density
High electrical and thermal conductivity
High latent heat of fusion
Alloying will change the properties
Which properties are related to aluminum’s binding energy (U)?
- Melting point (bond breaking)
- Coefficient of thermal expansion (bond stretching with thermal energy)
- Young’s modulus (stress stretching of bonds)
Where does the binding energy occur?
At the typical neighbor bonding energy (r0)
Which metals are FCC?
Atoms with 3 valence electrons
Ex: aluminum
Which metals are BCC?
Atoms with 1 valence electron
Which metals are HCP?
Atoms with 2 valence electrons
Unit cell
Smallest repeating unit that produces the periodic arrangement of atoms in a crystalline solid
What is the packing structure for FCC lattices?
ABC repeated
What are the closed-packed planes and directions in Aluminum?
The FCC unit cell of aluminum has 4{111} planes and three <110> directions per plant
Shear Modulus of Al
26 GPa
Young’s Modulus of Al
70 GPa
Poisson’s ratio of Al
0.34
Critical Resolved Shear Stress (tCRSS)
The threshold stress to start dislocation motion (slip) on a slip system
This is a material property
Critical Resolved Shear Stress for Aluminum
148 psi (~1 MPa) on {111}<110>
FCC metals have relatively low tCRSS
When does plastic yielding start?
Plastic yielding starts when the resolved shear stress is equal to the critical resolved shear stress
Stage 1 of plastic yielding
Single slip (easy glide) on the primary slip system (interaction between dislocations)
Stage 2 of plastic yielding
Dislocation pile-up
(absent if cross-slip is easy)
Stage 3 of plastic yielding
Dislocation intersection
What is the width of a stacking fault determined by?
It is determined by the stacking fault energy (SFE)
What happens due to aluminum having a high stacking fault energy?
Only one atom row exists between partial dislocations
Why is the activation energy for cross-slip low in aluminum (can more easily cross-slip?)
Because of the short separation distance between partial dislocations, these can more easily form a constriction in the stacking fault and cross slip
Which metal experiences all 3 stages of plastic deformation?
HCP is the only one that experiences easy glide
Which metal experiences stages 2 and 3 of plastic deformation?
FCC crystals with low stacking fault energy, like Cu, experience stages 2 and 3
Which metal experiences stage 3 only of plastic deformation?
FCC crystals with high stacking fault energy, like Al, and BCC crystals experience only stage 3 of plastic deformation
Why is stage 1 (easy glide) not present in BCC and FCC?
Due to the existence of a large number of slip systems
Why do FCC crystals with low stacking fault energy experience stage 2 of plastic deformation?
Because of low stacking fault energy, FCC crystals have wide stacking faults.
Since it cannot cross-slip its partials, it shows stage 2, dislocation pile-up
Why do BCC crystals not experience stage 2 of plastic deformation?
Because BCC has 24 independent cross-slip systems, it cross-slips really easily, and has less tendency to form partial dislocations
Why do FCC crystals with HIGH stacking fault energy experience only stage 3 of plastic deformation?
Because of cross-slip of full dislocations (5 independent slip systems) and cross-slip of partials (high SFE narrow stacking faults) is easy, dislocation pile-up does not exist
What is the proportional limit?
The proportional limit is the end of the linear portion of the stress vs. strain graph, where the elastic region ends and the plastic region begins
In Al, since the transition is gradual, this proportional limit is difficult to find
Proof stress
Maximum elastic strain in most materials is 0.001-0.002
As a result, offset/proof stress is taken at 0.001-0.002 offset from strain
How can pure aluminum be strengthened?
strain hardening
solid solution strengthening
second phase and age hardening
grain refinement and grain refinement design
How can you increase stress, and therefore strength in aluminum?
By adding obstacles to dislocation motion
tCRSS increases, and strength increases
What are examples of obstacles to dislocation motion?
Other dislocations
Solute atoms
Second phase particles
Grain boundaries
Why does the number of valence electrons determine the crystal structure of a metal?
Because of efficiency (energy minimization) in locating electrons in the interstitial spaces in the crystal structure
(note only sp electrons are considered, since d electrons are tightly held to nucleus)
