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Materials module 2 > Metals and alloys: strengthening mechanisms > Flashcards

Metals and alloys: strengthening mechanisms Flashcards

1
Q

Why are pure metals alloyed? (2)

A

• Pure metals for load bearing applications are of little use.
In order to improve mechanical properties (and many other
properties) alloying with other elements is performed.

• Alloying is either to neutralise the effect of undesirable trace elements, or to modify and improve desired properties for specific applications.

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2
Q

What are strengthening mechanisms?

A

Any process or treatment that will restrict or prevent the movement of dislocations within the crystals or grains will result in strengthening (and hardening).

Strength is increased by making dislocation motion difficult.

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3
Q

What properties must change for the strength of metals to be increased?

A

– Decreasing grain size
- Cold work (strain hardening, work hardening)
– Heat treatment
– Solid solution alloying

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4
Q

What occurs to the grains when a metal is subject to load?

A

When these metals are subjected to loading, the dislocation motion must take place across the common boundary, from grain A to grain B

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5
Q

What happens to the grain boundary when a metal is subject to loading? (2)

A

The grain boundary acts as a barrier for dislocation, because that:
– the two grains are randomly orientated,
a dislocation passing into B needs to change its direction of motion, which becomes more difficult as the misorientation increases
– The atomic disorder within a grain boundary region will result in a discontinuity of slip planes from one grain to another

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6
Q

How does smaller grain sizes help with strengthening a material? (3)

A

Grain boundaries are barriers to slip

  • Barrier “strength“ increases with increasing angle of misorientation.
  • Smaller grain size: more barriers to slip.
  • A fine‐grained material is harder and stronger due to a greater total grain boundary area to imposed dislocation motion
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7
Q

How does grain size influence temperature and strength of a material?

A

Metals having small grains – relatively strong and tough at low temperatures

Metals having large grains–good creep resistance at relatively high temperatures

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8
Q

What is cold work?

A

Cold work is also known as strain hardening. It is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed.

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9
Q

What does cold work do in single crystals?

A

In single crystals: dislocation movements ‐> plastic deformation/slip

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10
Q

What does cold work do in polycrystalline metals?

A

In polycrystalline metals: dislocation movements occur preferentially in grains with
slip systems that is most favourably located relative to the load direction.

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11
Q

How does cold work increase strength of a metal? (3)

A
  • Rotation occurs to bring the grains into more favourable position, so as to keep the grain boundaries in contact
  • Most grains will eventually have a plane in the direction of deformation.

– A considerable amount of distortion will have occurred, and the materials will have gone straining or work hardening.

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12
Q

What are 4 types of cold work straining?

A
  • forging
  • drawing
  • rolling
  • extrusion
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13
Q

What is annealing? (4)

A

A heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled, in order to,
- Increase softness, ductility, and toughness
– Reduce hardness
– Relieve stresses
– Produce a specific microstructure.

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14
Q

What happens doing annealing of a metal after cold working?

A

allows equiaxed grains to grow, which contain few dislocations

the material falls in hardness and increases in ductility with the amount of recrystallisation.

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15
Q

What are the three steps of heat treatment?

A
  1. recovery
  2. recrystallisation
  3. grain growth
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16
Q

What is recovery in heat treatment?

A

Reduction of dislocation density by annihilation.

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17
Q

What are two ways to recover during heat treatment?

A

diffusion

vacancy diffusion

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18
Q

What is the first scenario for recovery in heat treatment? (diffusion)

A

results from diffusion where atoms diffuse to regions of tension, dislocations annihilate and form a perfect atomic plane.

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19
Q

What is the first scenario for recovery in heat treatment? (vacancy diffusion)

A
  1. dislocation blocked; cant move to the right
  2. grey atoms leave by vancancy diffusion allowing dislocations to “climb”
  3. “climbed” dislocations can now move on new slip plane
  4. opposite dislocations meet and annihilate
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20
Q

What happens in recrystallisation in heat treatment? (2)

A

New grains are formed that:

    • have low dislocation densities
    • are small in size
    • consume and replace parent cold-worked grains

All cold-worked grains are eventually consumed/replaced

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21
Q

What happens in grain growth in heat treatment? (2)

A

• At longer times, average grain size increases.

    • Small grains shrink (and ultimately disappear)
    • Large grains continue to grow
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22
Q

What happens as annealing temperature increases?

A

As the annealing temperature is increased the annealed grain size will
get larger, given sufficient time to grow.

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23
Q

What happens if the temperature is above a certain temperature in annealing?

A

Above a certain temperature catastrophic grain growth may occur. This leads to an excessive grain size and a weak metal.

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24
Q

Why does the temperature above a certain temperature in annealing cause an excessive grain size and weak metal?

A

• It is due to the solution of precipitates in the metal at high temperature, which are no longer present to “pin” the boundaries to restrict grain size growth.

25
Q

What is hot working?

A

When deformation is achieved at a temperature above recrystallisation temperature, the process is termed hot working.

26
Q

Why is there a hardening effect due to hot working?

A

Note, two opposing effects take place at the same time as a material is hot worked. There is the hardening effect due to plastic deformation and there is the softening effect due to recrystallisation.

27
Q

What occurs during forging in hot working?

A

Forged components have outstanding grain structures in which the fibres of the metal are orientated by the forces of the process to the component shape. Forging provides the best combination of mechanical properties.

28
Q

What is the difference between hot work and cold working?

A

Hot working: deformation above TR
• Deformation temperature high enough for recrystallization
• Large deformations
• Metals remain soft and malleable
• Surface oxidation & poor final surface finish

Cold working: deformation below TR
• Deformation below recrystallization temperature 
• Small deformations
• Strain hardening occurs
• Increased strength
• Decrease in ductility
• High quality finishing surface
29
Q

What is the difference between cold working and hard working? (temperature)

A

Hot working: deformation above TR
- Deformation temperature high enough for recrystallization

Cold working: deformation below TR
- Deformation below recrystallization temperature

30
Q

What is the difference between cold working and hard working? (deformation size)

A

Hot working: Large deformations

Cold working: small deformations

31
Q

What is the difference between cold working and hard working? (properties)

A

Hot working:
- Large deformations
• Metals remain soft and malleable
• Surface oxidation & poor final surface finish

Cold working:
• Strain hardening occurs
• Increased strength
• Decrease in ductility
• High quality finishing surface
32
Q

What is an alloy?

A

An alloy is a mixture or metallic solid solution, composed of two or more elements

33
Q

How are useful alloys formed? And what are three conditions that may occur?

A

A useful alloy is only formed when the elements in question are mutually soluble in the liquid state. Upon cooling, following conditions may occur:
– Insoluble in the solid state ‐ they separate out as particles of the two pure metals.
– Complete or partial solubility in the solid state ‐ in the former case a single solid solution forms, whilst in the latter a mixture of two different solid structures results.
– An intermetallic compound, or an intermediate phase forms in the solid state.

34
Q

What is the resulted constitution of alloys? (4)

A
  • single phase solid solution
  • multiphase alloy
  • multiphase alloy with precipitates
  • highly strained metastable states
35
Q

What are the required properties for load bearing alloys?

A

combination of strength and toughness

36
Q

How can the properties for load bearing alloys be controlled?

A

These properties are controlled to a large extent by adjusting the structure of the alloy to control the behaviour of dislocations.

on a microstructural level this is done by affecting the atomic lattice configurations of the alloy

37
Q

When does a solid solution form?

A

when metals dissolve in all ratios one into the other

38
Q

What are solid state phases?

A

in which the primary element has incorporated atoms of the secondary element(s) into the primary lattice

39
Q

What are the sites for secondary element atoms? (2)

A

normal primary element sites: substitutional solid solution (SSS)
between primary element sites: interstitial solid solution (ISS)

40
Q

What do SSS and ISS atoms create?

A

create strain field within crystal lattice, which act to resist dislocation movement.

41
Q

What are 5 solid solution structures?

A 
interstitial 
substitutional 
random
clustered
ordered
42
Q

What are 3 factors that control ranges of substitutional solid solubility in alloy systems?

A

– Crystal‐structure factor ‐ as indicated above complete solid solubility of two elements is never attained unless the elements have the same type of crystal lattice structure.

– Relative size factor ‐ the size factor is favourable for solid solution formation when the difference in atomic size is less than about 15%.

– Chemical‐affinity factor ‐ the greater the chemical affinity of two elements, the more restricted is their solid solubility. Generally, the further apart the elements are in the periodic table, the greater is their chemical affinity.

43
Q

How does the crystal-structure factor control ranges of substitutional solid solubility in alloy systems?

A

complete solid solubility of two elements is never attained unless the elements have the same type of crystal lattice structure.

44
Q

How does relative size factor control ranges of substitutional solid solubility in alloy systems?

A

the size factor is favourable for solid solution formation when the difference in atomic size is less than about 15%

45
Q

How does chemical-affinity factor control ranges of substitutional solid solubility in alloy systems?

A

the greater the chemical affinity of two elements, the more restricted is their solid solubility. Generally, the further apart the elements are in the periodic table, the greater is their chemical affinity.

46
Q

How are interstitial solid solutions formed?

A

when atoms of small atomic size fit into the spaces of the lattice structure of larger atom elements

47
Q

What is the best known interstitial solid solution?

A

The best known and most important to engineers is the interstitial solution of carbon in iron. The more carbon atoms present the stronger the alloy, due to the distortion, which occurs interfering with the movement of dislocations on the slip planes of the alloy.

48
Q

What does limited solid solubility result to?

A

Limited solid solubility results from elements of differing atomic radii. When this limit is exceeded, the excess solute element is rejected into a second phase.

49
Q

What is the eutectic reaction?

A

process that transform a liquid phase into two solid phases upon cooling of a component with a eutectic composition

50
Q

What is a lamellar structure?

A

A lamellar structure of alternating layers of the two phases will usually result.

51
Q

What is the eutectoid transformation?

A

In some systems, particularly the iron carbon system, a solid solution stable at a higher temperature decomposes into a two‐phase structure on cooling below a critical temperature. This is known as a eutectoid transformation.

52
Q

How do solid solutions form?

A

Impurity atoms distort the lattice & generate lattice strains.
These strains can act as barriers to dislocation motion.

53
Q

What happens when smaller substitutional impurity occurs in forming solid solutions?

A

Impurity generates local stress at A and B that opposes dislocation motion to the right.

54
Q

What happens when larger substitutional impurity occurs in forming solid solutions?

A

Impurity generates local stress at C and D that opposes dislocation motion to the right.

55
Q

What are lattice strains around dislocations?

A

some atomic lattice distortion exists around the dislocation line due the presence of the half‐plane.There are regions in which compressive, tensile, and shear lattice strain is imposed on the neighbouring atoms

56
Q

Why are alloys stronger than pure metals? (3)

A
  • Impurity atoms imposing lattice strains on the surrounding host atoms
  • Lattice strain field interactions between dislocations and the impurity atoms result
  • Dislocation movement is restricted and therefore strength is increased
57
Q

How does solid solution alloying occur? (2)

A
  • (a) Tensile lattice strains imposed on host atoms by smaller substitutional impurity atom
  • (b) possible locations relative to an edge dislocation, leading to partial cancellation of dislocation compressive strains and impurity atom tensile strains
58
Q

What happens when large impurities are in solid solution alloying? (2)

A

Large impurities tend to concentrate at dislocations (regions of tensile strains)
• (a). Compressive strains imposed on host atoms
• (b). Possible locations of larger impurity atoms, leading to partial cancellation of impurity‐dislocation lattice stra

Materials module 2 (10 decks)

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