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MAT2730 - Metals > Metals > Flashcards

Metals Flashcards

1
Q

Define specific gravity

A

Specific gravity is the ratio of the density of a substance to the density of a reference substance

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2
Q

Define light metals

A

Metals who have specific gravities less than 5

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3
Q

Which 4 light metals are used in production

A

Magnesium
Titanium
Aluminium
Beryllium (limited use as toxic to 1/5 of population)

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4
Q

What structures do the 4 light metals have

A

Aluminium - FCC
Mg, Ti, Be - HCP
Ti at 822deg - BCC

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5
Q

What are the 6 main additives to make Al alloys

A 
Cu
Mn
Si
Mg
Zn
Li
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6
Q

Al alloy groups

A 
1XXX - 99% Al
2XXX - Cu
3XXX - Mn
4XXX - Si
5XXX - Mg
6XXX - Mg & Si
7XXX - Zn
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7
Q

List disadvantages of alloying Al

A
  1. Deformability is impaired
  2. Corrosion resistance is best for pure Al
  3. Surface finish is best for pure Al
  4. Density increased
  5. Conductivity is best for pure Al
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8
Q

List advantages of alloying Al

A
  1. Increase strength (work/age hardening)

2. Improved castability (add Si)

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9
Q

What are the two families of alloys

A
  • heat treatable (2xxx,6xxx,7xxx)

- non-heat treatable (1xxx,3xxx,5xxx)

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10
Q

Describe general production route for alloys

A
  1. Casting
  2. Pre-working treatments
  3. Hot working
  4. Cold working
  5. Post processing
  6. Finishing
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11
Q

Why is homogenisation important in alloying

A

pre hot working step, can last 24hrs at elevated temperature to reduce micro/macro segregation in the material

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12
Q

Describe effects of the quenching process

A
  • 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)
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13
Q

Alloy conditions.

A 
F = as fabricated
H = cold worked
O = annealed
T = fully heat treated
W = solution heat treated
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14
Q

Definition of annealed

A

Material that is cooled slowly, allowing for micro structural change and increasing toughness. This reduces high strength potential however.

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15
Q

At what temperature is homogenisation and solution treatment held at

A

Heated to below the eutectic point

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16
Q

Why is quenching process important

A
  • 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
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17
Q

Why is natural ageing important

A
  • Very small precipitates are formed in the grains
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18
Q

Describe dislocation motion

A
  • 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

19
Q

Definition: slip plane

A

plane of greatest packing density

20
Q

Definition: slip direction

A

closest-packed direction within a plane

21
Q

Definition: creep

A

The movement of vacancies when material is in an elevated temperature - progressive deformation

22
Q

List obstacles and assists to dislocation

A

Obstacles:

  • other dislocations
  • grain boundaries
  • precipitates
  • cracks

Assists:

  • elevated temperature
  • availability if slip system
23
Q

What does the Charpy test measure

A

Measures the energy absorbed on impact vs. Temperature

24
Q

What does Von mises criterion state about plastic deformation

A

For plastic deformation to occur, at least 5 independent slip systems must be operational

25
Q

How many slip systems do these have:
FCC
BCC
HCP

A

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

26
Q

What does DBTT stand for

A

Ductile Brittle Transitional Temperature

27
Q

Describe process of cold working/work hardening and how it strengthens a material

A
  • 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
28
Q

Disadvantages to cold working/work hardening

A
  1. The increase in strength means ductility decreases

2. The material becomes much more brittle

29
Q

What process is used to overcome cold working/work hardening

A

Annealing

30
Q

What is the advantage of having small grain sizes

A
  • harder for dislocations to move

- also improves ductility and toughness

31
Q

What two ways can dislocations move past precipitates

A
  1. Shear mechanism

2. Bowing mechanism

32
Q

Equation for strength with respect to precipitates for both shear and bowing mechanisms

A

Shear mechanism:
Strength ∝ √Precipitation size

Bowing mechanism
Strength ∝ 1 / Distance between particles

33
Q

How does solid solution strengthening increase strength

A

Overall strain energy is reduced with solute atoms segregating around the dislocation core

34
Q

Name two types of solid solution strengthening

A

Substitutional

Interstitial

35
Q

Describe Substitutional and interstitial strengthening

A

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

36
Q

What two categories of stabiliser can alloy with Ti

A

α stabilisers = Al, O, N, Ga and C

β stabilisers = H, V, Cr …

37
Q

Problems faced when alloying Ti

A

embrittlement

stress corrosion cracking

38
Q

Info on Ti fully α alloys

A
  • pure HCP materials
  • moderate strength at elevated temperatures
  • good ductility
39
Q

What are the Oxygen/Aluminium equivalent used for when creating Ti α alloys

A

Work out how much O/Al can be added until embrittlement effect

40
Q

Info on Ti fully α/β alloys

A
  • dominated by Ti-6Al-4V
  • higher strength than α alloys
  • less ductile
  • less creep resistant
41
Q

List major defects in Ti α and β alloys

A

α alloys:
can have α rich regions causing major embrittlement

β alloys:
suffer from solute segregation

42
Q

Info on Ti fully β alloys

A
  • highest strength

- can develop burn-resistant β alloys (add Chromium)

43
Q

List some applications of Titanium

A
  • weight saving relative to steel
  • space saving relative to Al
  • operating temp is high
  • corrosion resistant
  • galvanic compatibility with carbon fibre

MAT2730 - Metals (3 decks)

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