Metals Flashcards
Define specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance
Define light metals
Metals who have specific gravities less than 5
Which 4 light metals are used in production
Magnesium
Titanium
Aluminium
Beryllium (limited use as toxic to 1/5 of population)
What structures do the 4 light metals have
Aluminium - FCC
Mg, Ti, Be - HCP
Ti at 822deg - BCC
What are the 6 main additives to make Al alloys
Cu Mn Si Mg Zn Li
Al alloy groups
1XXX - 99% Al 2XXX - Cu 3XXX - Mn 4XXX - Si 5XXX - Mg 6XXX - Mg & Si 7XXX - Zn
List disadvantages of alloying Al
- Deformability is impaired
- Corrosion resistance is best for pure Al
- Surface finish is best for pure Al
- Density increased
- Conductivity is best for pure Al
List advantages of alloying Al
- Increase strength (work/age hardening)
2. Improved castability (add Si)
What are the two families of alloys
- heat treatable (2xxx,6xxx,7xxx)
- non-heat treatable (1xxx,3xxx,5xxx)
Describe general production route for alloys
- Casting
- Pre-working treatments
- Hot working
- Cold working
- Post processing
- Finishing
Why is homogenisation important in alloying
pre hot working step, can last 24hrs at elevated temperature to reduce micro/macro segregation in the material
Describe effects of the quenching process
- can lead to residual stresses causing distortion (reduce risk by quenching in hot water)
- slower quenching leads to a reduction in achievable strength. (due to allowing time for microstructural change)
Alloy conditions.
F = as fabricated H = cold worked O = annealed T = fully heat treated W = solution heat treated
Definition of annealed
Material that is cooled slowly, allowing for micro structural change and increasing toughness. This reduces high strength potential however.
At what temperature is homogenisation and solution treatment held at
Heated to below the eutectic point
Why is quenching process important
- Used to prevent precipitates forming and to keep the microstructure that was achieved at treatment temperature.
- if it is allowed to slow cool (i.e. quenching does not take place) then small precipitates will form in between the grain boundaries - this decreases mechanical properties and the material would be more brittle
Why is natural ageing important
- Very small precipitates are formed in the grains
Describe dislocation motion
- happens on particular slip systems
- a half plane of atoms is forced to one side and dislocations will be stopped by grain boundaries, or other
Definition: slip plane
plane of greatest packing density
Definition: slip direction
closest-packed direction within a plane
Definition: creep
The movement of vacancies when material is in an elevated temperature - progressive deformation
List obstacles and assists to dislocation
Obstacles:
- other dislocations
- grain boundaries
- precipitates
- cracks
Assists:
- elevated temperature
- availability if slip system
What does the Charpy test measure
Measures the energy absorbed on impact vs. Temperature
What does Von mises criterion state about plastic deformation
For plastic deformation to occur, at least 5 independent slip systems must be operational
How many slip systems do these have:
FCC
BCC
HCP
FCC - 12 slip systems, 5 independent i.e. always ductile
BCC - 48 slip systems, highly temperature dependent
(depends on DBTT)
HCP - 12 slip systems, temperature dependent
What does DBTT stand for
Ductile Brittle Transitional Temperature
Describe process of cold working/work hardening and how it strengthens a material
- Deforming a crystal, we get dislocation movement in certain directions
- Dislocations are also generated and they all obstruct each other
- This increases the materials strength
Disadvantages to cold working/work hardening
- The increase in strength means ductility decreases
2. The material becomes much more brittle
What process is used to overcome cold working/work hardening
Annealing
What is the advantage of having small grain sizes
- harder for dislocations to move
- also improves ductility and toughness
What two ways can dislocations move past precipitates
- Shear mechanism
2. Bowing mechanism
Equation for strength with respect to precipitates for both shear and bowing mechanisms
Shear mechanism:
Strength ∝ √Precipitation size
Bowing mechanism
Strength ∝ 1 / Distance between particles
How does solid solution strengthening increase strength
Overall strain energy is reduced with solute atoms segregating around the dislocation core
Name two types of solid solution strengthening
Substitutional
Interstitial
Describe Substitutional and interstitial strengthening
Substitutional:
add atoms bigger than original material
Interstitial:
add atoms smaller than original material
hardness increases because of increasing distortion of lattice
example: C introduced in Fe to make steel
What two categories of stabiliser can alloy with Ti
α stabilisers = Al, O, N, Ga and C
β stabilisers = H, V, Cr …
Problems faced when alloying Ti
embrittlement
stress corrosion cracking
Info on Ti fully α alloys
- pure HCP materials
- moderate strength at elevated temperatures
- good ductility
What are the Oxygen/Aluminium equivalent used for when creating Ti α alloys
Work out how much O/Al can be added until embrittlement effect
Info on Ti fully α/β alloys
- dominated by Ti-6Al-4V
- higher strength than α alloys
- less ductile
- less creep resistant
List major defects in Ti α and β alloys
α alloys:
can have α rich regions causing major embrittlement
β alloys:
suffer from solute segregation
Info on Ti fully β alloys
- highest strength
- can develop burn-resistant β alloys (add Chromium)
List some applications of Titanium
- weight saving relative to steel
- space saving relative to Al
- operating temp is high
- corrosion resistant
- galvanic compatibility with carbon fibre
