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Engineering Senior Review > Materials > Flashcards

Materials Flashcards

1
Q

Define stress.

A
  • Stress, represented by the Greek letter σ, is the pressure or
    force per cross sectional area exerted on a body.
  • Units are N/m2
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2
Q

Define strain.

A

Strain, represented by the Greek letter ε, is a term used to measure the deformation or extension of a body that is subjected to a force or set of forces.

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3
Q

What is Young’s Modulus? & how is it calculated?

A

• Young’s Modulus, represented by the Greek letter, γ, is calculated by dividing strain into stress
Young’s Modulus, γ = σ ε
• Units are N/m2
• Young’s Modulus is represented by the part of the stress-strain curve that is linear.
• Young’s modulus, γ, is a measure of the ability of a material to withstand changes in length when under
lengthwise tension or compression.
• The higher the value of Young’s modulus, the stiffer the material.

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4
Q

Name the elements of a stress-strain graph.

A
  • Proportaional limit (Young’s Modulus)
  • Yield strength/point (Young’s Modulus)
    • Strain hardening
  • Ultimate Tensile Strength (UTS)
    • Necking
  • Fracture
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5
Q

What is the Proportional limit?

A
  • Also known as Plastic Limit
  • The point where the Stress-Strain graph becomes non-linear
  • The stress and strain values at this point are known as the proportional-limit stress and strain,
    respectively.
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6
Q

What is the Yield Point?

A
  • The yield point corresponds to the point where the material begins to have permanent
    (unrecoverable) deformation.
  • Although the yield and the proportional limit points are close to each other, they do not
    correspond to the same location on the stress-strain
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7
Q

What is the Ultimate Tensile Force?

A
  • The maximum tensile stress a material is capable of carrying is known as the ultimate tensile
    stress.
  • It corresponds to the highest point on the stress-strain graph.
  • The corresponding strain is known as the ultimate tensile strain
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8
Q

What material properties can be interpreted from a Stress-Strain graph? Provide a quick summary on those properties?

A

Stiffness: is an indicator of the tendency for an
element to return to its original form after being
subjected to a force.

Strength: measures how much stress can be
applied to an element before it deforms
permanently or fractures.

Hardness: measures a material’s resistance to
surface deformation. For some metals, like steel,
hardness and tensile strength are roughly
proportional

Others include: ductility, elasticity

The ratio of stress to strain is the elastic
modulus—stiffness, but the stress, and only the stress, defines the strength of the material.

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9
Q

What is Toughness?

A
  • The capacity of a material of absorb energy prior to failure. Therefore, its ability to resist a shock impact.

Toughness = Area under the curve
- Its value is equal to the entire area under the stress-strain curve up to the fracture point.

  • In order to be tough, a material must be both strong and ductile.
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10
Q

Draw a Stress-Strain Graph for;
- mild steel
- ceramics (glass, aluminimum oxide)
- polymers (brittle polymer, plastic, elastomer)

Identify where they would be used, as a result of their Stress-Strain graph.

A
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11
Q

What is a Phase?

A
  • A phase of a substance is a form of matter that is uniform throughout in chemical composition and
    physical state
  • Most substances have a solid, liquid, and gas phase
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12
Q

What is a Phase Change? Name the different types of Phase changes.

A

Is the change in the physical state of a substances.
Solid –> Liquid (Melting)
Liquid –> Gas (Vaporisation)
Gas –> Liquid (Condensation)
etc = Sublimation
Deposition
freezing

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13
Q

What is Liquid-Vapour Equilibrium?

A
  • In a closed container, molecules in the gas
    phase cannot escape, and the pressure
    exerted by the vapour in the “headspace”
    begins to increase.
  • The liquid and vapour reach a state of
    dynamic equilibrium – liquid molecules
    evaporate and vapour molecules condense
    at the same rate
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14
Q

What is a Binary-Phase diagram?

A

X axis – % composition Y axis – temperature

Binary phase diagram shows the :
- phases present
- composition of phases
- relative amount of phases
• Lines of equilibrium or phase boundaries refer to the lines that demarcate where phase transitions occur
• To make these diagrams, cooling curves (or heating curves) of various % of the alloying material is
constructed from data obtained in experiments e.g. cooling various % combinations of alloying material

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15
Q

What is an Isomorphous Binary-Phase diagram?

A

Isomorphous systems contain metals which are
completely soluble in each other and have a single
type of crystal structure.

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16
Q

What does a Binary-Phase Diagram show?

A

The correlation between microstructure and mechanical properties, and development of
microstructure can be understood from binary phase diagrams.

Binary phase diagrams can also be used to obtain information about melting, casting, crystallization, etc.

  • It is a graphical method of showing the phases (solid and liquid) present in an alloy system at different
    temperatures and different compositions
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17
Q

What is the liquidus line?

A

Liquidus line is a line above which the substance is
stable in a liquid state – it separates the liquid phase from solid or solid + liquid phases.

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18
Q

What is the solidus line?

A

Solidus line is a line below which the substance is
stable in the solid state – it separates solid phase
from liquid + solid phase

19
Q

How do you write out the compsotion for an alloy?

A

A - x%B

Example
Copper-Alumimimum Phase-Diagram
Cu - 20%Al (80% copper and 20% aluminium)

20
Q

What is a Binary Eutectic Phase Diagram?

A

A eutectic or eutectic mixture is a mixture at such proportions that the melting point is as low as possible, and that furthermore all the constituents crystallize simultaneously at this temperature from molten liquid
solution.

Such a simultaneous crystallization of a eutectic mixture is known as a eutectic reaction, the temperature at which it takes place is the eutectic temperature, and the composition and temperature at which it takes place is called the eutectic point.

21
Q

What are the 3 types of Solid Solubility?

A

Unlimited Solid Solubility
• For two substances to have unlimited solubility, any amount of either substance must be able to dissolve completely into any amount of the other substance
• Solute and solvent are mutually soluble at all concentrations, e.g., Cu-Ni system
• Result is a “single phase alloy”

No Solid Solubility
• The two components, A & B, are completely insoluble in each other in the solid state, and no solid solution
occurs.
• A mixture of A & B exists at all temperatures below solidus (the eutectic isotherm)
• Eutectic Liquid → Solid A + Solid B
• Every alloy completely solidified must be a mixture of the two pure metals.

Limited or Partial Solid Solubility
• “hybrid” of the complete solid and liquid solution diagram and the eutectic
diagram (the metals are completely soluble in liquid state and entirely insoluble in solid state)

22
Q

What is the Eutectic point in a Binary Eutectic Phase Diagram?

A

liquid and two solid phases co-exist in equilibrium at the eutectic composition
CE and the eutectic temperature TE.

23
Q

What and where is the Eutectic Isotherm on a Binary Eutectic Phase Diagram?

A

Eutectic isotherm – the horizontal solidus line

24
Q

What is a Eutectic Reaction?

A

The transition between liquid and mixture of two solid phases, α + β at eutectic
concentration CE

25
Q

Draw the microstructures for a lead-tin phase diagram. Include the liquid, liquid + a, liquid + b, a + b, solid (a), solid (b) and at the Eutectic point.

A
  • Liquid = blank with (L or Liquid) annotation
  • Liquid + a/b= part liquid, part alpha depending on the composition (represent alpha and beta with different sketching styles
  • a + b = a mix between a and b depending
  • Eutectic Point At: TE all the liquid transforms to α and β phases (eutectic reaction).
26
Q

What are some important characteristics of the Eutectics?

A

• A eutectic is a mechanical mixture of two solid phases
• A eutectic has a constant composition, but it is not a chemical compound.
• A eutectic freezes and melts at a constant temperature.
• A eutectic has the lowest freezing point of all alloys belonging to the system.
• A eutectic has the highest strength of the whole system.
• The eutectic point is always nearer to the metal of lower melting point.

27
Q

What is steel?

A

Steel is an alloy of iron and carbon containing less than 2% carbon and 1% manganese and small amounts of silicon, phosphorus, sulfur and oxygen.

28
Q

List the types of steel with carbon %. Do this in ascending order.

A

Low-carbon steel: 0.07% - 0.15%
Mild-Carbon steel: 0.015% - 0.30%
Medium-Carbon steel: 0.30% - 0.60%
High-Carbon steel: 0.60% - 1.25%
Ultra high-carbon steel: 1.25% - 2%

29
Q

What is a low carbon steel (properties, carbon percentage, and uses)

A

(0.07% to 0.15% C)
• Most common type
• Properties:
- soft and ductile
- cannot be modified by heat treatment
- can easily be machined and cold worked for strength
- High toughness
- low tensile strength
- low MP
- relatively cheap to produce

• Uses
- automobile body parts and wire products
- rolled sheeting and pipes
- engineering applications are restricted to non-critical components and general paneling and
fabrication work

30
Q

What is a mild carbon steel (properties, carbon percentage, and uses)

A

Mild carbon steel (0.15% to 0.30% C)
• Most common type
• Properties:
- cheap to produce
- easily formed, machined, welded, and fabricated
- good combination of strength and ductility and may be hardened and carburized
- High toughness, machinability and weldability
- Low hardness
- Low tensile strength
- High ductility and malleability
- low MP
- cannot be effectively heat treated
• Uses
- automobile bodies
- ships and domestic appliances
- I–Beams
- rolled sheeting and pipes
- channels and angle iron
- structural plates and sections, stampings, forgings, seamless tubes, and boilerplate
- shafts, spindles, pins, rods, sprocket assemblies, and an incredibly wide variety of component
parts
- cold formed fasteners and bolt

31
Q

What is a medium carbon steel (properties, carbon percentage, and uses)

A

(0.30% to 0.60% C)
• Properties:
- strong (high tensile strength)
- high hardness
- less ductile
- hardenable by heat treatment, quench, and tempering
- wear resistant
• Uses
- gears, camshafts and other machine parts requiring high stress
- forgings and automotive components, including shafts, axles, gears, and crankshafts
- medium carbon steels in the 0.40% to 0.60% carbon range are used for train rails, wheels and
axles
- forging

32
Q

What is a high carbon steel (properties, carbon percentage, and uses)

A

(0.60% to 1.25% C)
• Properties
- highest hardness and toughness of the carbon steels and the lowest ductility
- very hard (7.5-8 Moh’s scale)
- very strong
- very wear resistant
- brittle
- almost always hardened and tempered.

• Uses
- used for applications in which high strength, hardness and wear resistance are necessary, such as
wear parts, knives, saw blades, springs, gear wheels, chains, brackets, etc. cold chisels, wrenches, jaws for vices, pneumatic drill bits, wheels for railways service, wire for structural work, shear
blades, hacksaws
- high-strength spring materials and wires
- cutting tools
- punches
- dies
- industrial knives

33
Q

What is an ultra-high carbon steel (properties, carbon percentage, and uses)

A

(1.25% to 2.00% C)
• Properties
- very hard (7.5 -8)
- wear resistant
- brittle
- require other alloys to prevent excessive brittleness.

• Uses
- highly tempered non-industrial-purpose knives, punches, and axles

34
Q

Name some other steel alloys and their properties.

A

Other steel alloys
• stainless steel – at least 11% chromium added to make corrosive resistant (cutlery, instruments)
• nickel steel – increases toughness, magnetic and lower expansion (clocks, measuring instruments and
magnets)
• manganese – hardens and lowers critical temp. for annealing
• molybdenum – when tempered at high temperature inhibits brittleness
• tungsten - very hard (cutting tools)

35
Q

Name the different solid phases of the iron-carbon phase diagram.

A

Ferrite (a)
Austenite (b)
Cementite / Iron Carbide (Fe3C)
Ferrite
Pealite

36
Q

What is Ferrite (a) and describe its physical properties?

A

• solid solution of carbon in α iron
• at 0% °C, is pure iron, with a BCC crystal structure
• maximum solubility of carbon in iron is 0.022% at 723°C and falls to 0.008% at 0°C
• transforms to γ-austenite at 912°C
• carbon atoms are located in the crystal interstices
• soft and ductile – imparts these properties to the steel
• exists at room temperature

37
Q

What is Austenite (γ) and describe its physical properties?

A

• solid solution of carbon in γ iron
• is important in that it is the structure from which other structures are formed when the material
cools from elevated temperatures.
• on further heating, converts into δ-ferrite at 1395°C.
• is unstable at temperatures below eutectic temperature (727°C) unless cooled rapidly.
• solubility reaches a maximum of 2.14% at 1148o
C and decreases to 0.8% at 723o
C
• carbon atoms are dissolved interstitially
• difference in solubility between the austenite and α Ferrite is the basis for the hardening of steels
• does not exist below about 723o
C

38
Q

What is Cementite (Fe3C) and describe its physical properties?

A

• an intermetallic compound that contains 6.67% C and 93.3% Fe
• has a fixed composition of Fe3C
• decomposes extremely slowly at room temperature into iron and carbon (graphite) – but this
decomposition time is long and it will take much longer than the service life of the application at
room temperature.
• hard and brittle – its presence in steels causes an increase in hardness and a reduction in ductility
and toughness

39
Q

What is Ferrite and describe its physical properties?

A

• a solid solution of carbon in iron and has a BCC crystal structure
• maximum solubility or C in Fe is 0.09% at 1495o
C
• has a similar structure as that of α-ferrite but exists only at high temperatures
• has a melting point of 1538°C
• has no real practical significance in engineering

40
Q

What is Pearlite and describe its physical properties?

A

• A laminated structure formed of alternate layers of ferrite and cementite
• The average carbon content in pearlite is 0.76%
• Combines the hardness and strength of cementite with the ductility of ferrite and is the key to the
wide range of the properties of steel
• The laminar structure also acts as a barrier to crack movement as in composites – giving toughness

41
Q

What happens during a EutecTOID reaction in an Iron-Carbon phase diagram?

A

Austinite becomes Pearlite (Ferrite + Cementite)

Austenite precipitates Fe3C at the Eutectoid transformation temperature (727°C)

• In eutectoid reaction, austenite transforms into a phase
a mixture of ferrite (containing 0.76% C) and cementite –
this phase mixture is known as pearlite.
• When cooled slowly, forms pearlite, a lamellar or
the layered structure of two phases: α-ferrite and
cementite (Fe3C)and looks like Mother of Pearl
• Mechanically, pearlite has properties intermediate to
soft, ductile ferrite and hard, brittle cementite.

42
Q

What is a + Fe3C?

A

Pearlite

43
Q

Draw a microstructure for Pearlite.

A

a laminar structure with a mixture of Ferrite and Cementite.

44
Q

What are the effects of Carbon in iron?

A

In steels, none of the carbon is present as free carbon – it is all dissolved in the iron

increasing the carbon content decreases the amount of ferrite and increases the proportion of pearlite in
the structure, leading to an increase in strength and hardness and a reduction in ductility

This continues until there is 0.8% carbon at which point the structure is 100% pearlite – eutectic structure

As carbon content increases beyond 0.8%, no more pearlite can be formed. The excess carbon forms cementite which is deposited in between the pearlite grains. This increases the hardness but slightly reduces the strength. The ductility of all plain carbon steels over 0.8% carbon is very low

Engineering Senior Review (4 decks)

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