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Materials > Metals > Flashcards

Metals Flashcards

1
Q

Name some metals with Hexagonal close packed (HCP), FCC and BCC structures.

A

HCP : Titanium, Zinc

FCC: Nickel, Aluminium

BCC: Chromium, titanium.

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2
Q

Name 4 crystal defects.

A
  • Point defect
  • Linear defect
  • Interfacial defect
  • Bulk or volume defect.
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3
Q

Examples of point defect:

A
  • Vacancies,
  • Self-interstitials (smaller atom between the normal atoms)
  • Interstitial impurity atoms
  • Substitutional impurity atoms
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4
Q

Line defect examples:

A

Dislocation:

  1. Edge type
  2. Screw type
  3. Mixed
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5
Q

Interfacial defect examples:

A
  • Grain boundaries
  • Twin boundaries (mirrored grain boundary)
  • Stacking faults
  • Phase boundaries
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6
Q

Volume defect examples:

A
  • Voids
  • Cracks
  • Foreign inclusions
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7
Q

Describe how metals deform plastically?

A

The movement of dislocations (extra half-plane of atoms) through the crystal structure causes plastic deformation.

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8
Q

How can you strengthen a metal or alloy?

A

Restricting dislocation motion:
1. strengthening through grain and phase boundaries. Smaller grains stronger the material.

  1. Work hardening. Causing dislocation
  2. Solid solution strengthening. Alloying with impurity atoms to stop dislocations
  3. Precipitation hardening. Very fine particles of a second phase form in the original phase matrix.
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9
Q

Define phase transformation:

A

Any re-arrangement within the assembly of atoms or molecules, which carries the system from one configuration to another.

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10
Q

How do you covert weight % (wt%) to atomic % (at%)

A

at % A= (wt%A/Ma) / ((wtA/Ma) + wtB/Mb) * 100

wt%A = (at%AMa) / ((atAMa) + atB*Mb) * 100

Ma and Mb are the atomic masses of elements A and B.

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11
Q

How to use the lever rule and calculate mass fraction.

A 
Wl = (Calpa - Co) / 
(Calpha - CL)
Capla = composition of alpha pahe
Co = compsition where you are
CL = Composition of liquid.
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12
Q

Define components:

A

they are pure metals and/or compounds of which an alloy is composed.

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13
Q

Define System:

A

A system refers to a specific body of material under consideration or a series of possible alloys consisting of the same conponents.

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14
Q

Define a phase:

A

A phase may be defined as homogeneous portion of a system that has uniform physical and chemical characteristics.

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15
Q

What are the Hume-Rothery rules for unlimited solid solubility.?

A
  1. size factor: shoiuld be similar size. (less then 15 % variance)
  2. Crystal structure: same crystal structure.
  3. Valence: inos should have the same valence .
  4. Electronegativity: similar.
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16
Q

What influences the strength of an alloy with a eutectic microstructure?

A

The amount and length-scale of the eutectic structure. Max strength when 100% eutectic.
Cooling rate increase the interlamellar spacing gets smaller which increases the number of grain boundaries.

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17
Q

How to use the AISI/SAE Steel numbering system?

XXXX(X)

A

first two numbers: indicate the major alloy addition made to iron.
The last two (possibly 3) when divided by 100 give the carbon concentration.
E.g XX20 would have Carbon 0.2 wt %

18
Q

What happens when the carbon content of steel increases?

A

The yield and tensile strength and hardness increase. But ductility reduces.

19
Q

What 3 phases do pure iron go into?

A

Low temp : alpha iron (ferrite)
Medium (900-1400) temp: gamma iron (austenite)
High temp: delta - ferrite
then liquid at 1538C

20
Q

What is developed when you slowly call through the eutectoid temperature?

A

The microstructure consists of alternating layers of the alpha ferrite and Fe3c (cementite) phase. This is called pearlite which is not a phase.

21
Q

What happens if you slowly cool a Hypoeutectoid and a hypereutectoid steel

A 
Hypoeutectoid steel (before) would get some alpha ferrite in the microstructure
hypereutectoid steel (after) would get some Fe3C (cementite) in the microstructure.
22
Q

What phases are created when rapid cooling occurs?

A

Fast cooling = Bainite

Very fast = Martensite (alpha’ )

23
Q

Describe a Time-Temperature-Transormation diagram.

A

TTT diagrams graphically represent the nucleation of growth kinetics of transformation products associated with the decomposition of austinite. (gamma)

24
Q

How do you create coarse and fine pearlite?

Which is stronger?

A

Cool Austenite to around 680 C and hold the temperature. (Coarse)
Cool Austenite to around 560C and hold the temperature. (Coarse)

Fine pearlite.

25
Q

what would happen if you rapidly cooled austenite to 500 C or room temperature and held it there?

A

Bainite would form. (500C)

Martensite would form. (room temperature).

26
Q

Whats is Martensite?

A

A phase that forms as a result of a diffusionless solid-state transformation due to rapid cooling. (quenching). non-equilibrium phase transformation.
BCT Body centred tetragonal.

27
Q

What are duel phase steels?

A

They have microstructure with islands of martensite (hard and brittle) in a matrix of ferrite (alpha). Low yeild strength but very high UTS

28
Q

What is tempering and why is it done?

A

Its the process of heating a martensitic steel to some temperature below the A1 temp (727 C).
Its done to make the martensite softer and more ductile making it more useful.

29
Q

What is the microstructure of a tempered martensite?

A

It consists of extremely small and uniformly dispersed particles of the cementite phase embedded within a continuous alpha-ferrite matrix. The size of the cementite particles control the mechanical properties of tempered martensite.

30
Q

What two types are metals alloys sorted into?

A

Ferrous: Fe is the principal constituent.

Nonferrous: all alloys that aren’t iron-based.

31
Q

Describe Low-Carbon Steels:

A

Produced in the greatest quantity
Contain generally less than 0.25 wt% carbon
Unresponsive to heat treatment but strengthened with cold working. (Hitting it with a hammer)
Low yield strength but high toughness and ductility.
Used in car frames and I beam

32
Q

Describe High-strength, low alloy steels:

A

Contain other alloying elements such as Cu, Vanadium, Ni, Molidium as high as 10 wt%
Can be strengthened by heat treatment and 300.550n MPa yield strength.
More resistant to corrosion

33
Q

Describe Medium-Carbon steels:

A

Between 0.25 and 0.6 wt% carbon
Can be heat treated by “austenising”, quenching and tempering.
For plain medium-Carbon steels this can only be achieved in very thin sections and rapid cooling.
Additions of Cr, Ni and Mo sometimes added, improve their “hardenability”.
yield strength 400-600 MPa yield strength. But sacrifice ductility and toughness.
Used in railway tacks and gears.

34
Q

Describe high carbon steels:

A

Between 0.6 and 1.4 wt% Carbon
Almost always heat treated by “austenising”, quenching and tempering. (250 C so doesn’t lose too much strength.
Sometimes contain extra alloying elements including, C, Cr, W and Mo.
Hardest and strongest (500-1000 MPa yield strength).
Least ductile
USed for cutting tools and springs.

35
Q

Describe Stainless steels:

A

Corrosion-resistant (not always)
Main alloying element is Chromium (typically > 11 wt%). Enhanced by Ni and Mo additions.
3 microstructure classes = Ferrite, Martensite or austenite.
Yield strength 200 - 1600 MPa
Martensite: rifle barrels and cutlery
Ferrite: higher yield strength, less expensive, good formability and low work hard rate. used for High temp valves and combustion chamber.
Austenite: low yield strength, expensive, excellent formability and high work hard rate. used for chemical and food processing equipment.

36
Q

Difference between ductile and brittle failure.

A

ductile: slow failure, High energy mode of failure and results in the dimpled fracture surface

Brittle: Rapid failure with no warning, Low energy mode of failure and results in a cleavage fracture surface

37
Q

What 3 conditions encourage plastic deformation?

A
  1. Deformation at low temperatures
  2. High strain-rates
  3. Tri-axial stress rate. e.g. through the presence of a notch.
38
Q

Describe the Charpy test:

A

Measures impact energy.
The specimen is a bar with a notch in it.
A load is applied as an impact blow from a weighted pendulum hammer.
the height measured after the specimen is broken relates to the absorbed energy.
Semi-quantitative but mainly used as qualitative.

39
Q

Where is the “ductile-to-brittle transition” temperature?

A

Very hard to define such temperature but some people guess at this. Not all engineering materials show a ductile to brittle temp. Mainly Low-strength steels have this.

40
Q

The “ductile-to-brittle transition” is sensitive to?

A
  1. alloy composition

2. Microstructure.

Materials (3 decks)

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