Materials 2 - Section A Flashcards
Which one of the following statements is correct in relation to single crystal alloys?
a. A single crystal alloy will not undergo yielding prior to fracture because it will not contain dislocations.
b. A single crystal alloy can be assumed to be isotropic because it will not have any grain boundaries.
c. A single crystal alloy will be anisotropic.
d. A single crystal alloy cannot contain precipitates of a second phase.
e. A single crystal alloy will have poor resistance to creep because it does not contain any grain boundaries.
c. A single crystal alloy will be anisotropic.
a. Ofc it contains dislocations.
b. Has no grain boundaries but isn’t isotropic.
c. Yes anisotropic
d. Yes ofc it can
e. Has no grain boundaries which means more resistant to creep, not less.
Which one of the following statements is correct in relation to phase diagrams for binary alloys?
a. A phase diagram can reveal the rate at which a new phase will form provided that it forms through a diffusion-based process.
b. Phase diagrams can reveal whether an alloy has a ductile-to-brittle transition temperature.
c. Phase diagrams can only be used with confidence if the homologous temperature (T/Tm) is above 0.4.
d. A phase diagram can reveal both the number of phases in an alloy, and the chemical composition of each phase, when thermodynamic equilibrium is achieved.
e. The Lever Rule can be used to establish whether a material is thermodynamically stable.
d. A phase diagram can reveal both the number of phases in an alloy, and the chemical composition of each phase, when thermodynamic equilibrium is achieved.
Which one of the following statements is correct in relation to atomic diffusion?
a. Diffusion in metals only occurs when there is a thermodynamic driving force for a phase transformation.
b. Interstitial diffusion in metals and alloys is contingent on the presence of vacancies in the crystal lattice.
c. The movement of dislocations in a metal can only occur through a process that involves substitutional diffusion.
d. At a given temperature, diffusion in face-centred cubic (FCC) iron is faster than diffusion in body-centred cubic iron because FCC crystals have more close packed directions.
e. At a given temperature, diffusion in BCC iron is faster than diffusion in FCC iron because BCC iron has a lower atomic packing factor.
e. At a given temperature, diffusion in BCC iron is faster than diffusion in FCC iron because BCC iron has a lower atomic packing factor.
a. No, diffusion can occur without a driving force.
b. No, interstitial atoms neither replace nor substitute. They fit in the small spaces in-between atoms in the interstitial sites.
c. Can also happen via interstitial.
d. No, diffusion is due to concentration gradients only.
e. Yes, lower APF means less packed, so diffusion is easier.
BCC vs FCC
How many close packed planes and close packed directions?
Atomic packing factor, and its effects?
What are the transitions and properties?
BCC: No close packed planes, 4 close packed directions. Lower APF, meaning diffusion is easier as more space. A ductile to brittle transition.
FCC: 4 close packed planes and 12 close packed directions. Higher APF and is ductile.
What are the main differences between theoretical and real strength?
- Theoretical is calculated by estimating the stress required to separate planes of atoms.
- Theoretical assumes crystals are perfect.
- Real crystals contain defects (dislocations).
List and describe the 5 strengthening mechanisms in metals?
- Work hardening: Causes dislocations to move through the lattice, which means higher concentration of defects, hence hardening.
- Solid-solution strengthening: Alloyed atoms which are a different size are introduced into the lattice, causing dislocations.
- Precipitation hardening.
- Grain boundary strengthening: More boundaries means more grains meeting at different orientations. Hence harder to dislocate.
- Transformation strengthening: Heating then cooling to introduce phase transformation.
What is meant by “stable”?
Stability is defined by the thermodynamic stability in the metal. Thermodynamic stability is achieved when Gibbs Free Energy is at a minimum.
How is Thermodynamic Equilibrium achieved?
It can be achieved by either very slow cooling or heating. This gives diffusion more time to take place.
How does diffusion occur?
Diffusion occurs due to concentration gradients. I.e. high to low concentration etc.
List and describe the 2 types of diffusion?
Diffusion occurs by either vacancy or interstitial diffusion.
Vacancy means alloying elements that are a similar size to the solvent atoms in the structure, move into a vacancy nearby or replace existing, similar sized atoms.
Interstitial means alloying elements that are small enough to fit in between atoms in the interstitial sites. Carbon is a common interstitial alloying element.
What structure is Ferrite?
BCC
So a ferritic structure is a BCC structure,
What structure is Austenite?
FCC
So an austenitic structure is an FCC structure,
What effects the rate of diffusion?
- Number of vacancies
- Crystal structure
- Diffusing atoms
- Temperature
What does a phase diagram show?
It shows the number of phases present in an alloy at a given weight and temperature. And also the chemical composition of each phase when thermodynamic equilibrium is achieved.
Label the phase diagram?
What phases exist at points A, B, and C? And are they BCC or FCC?
Label the Stress-Strain diagram
Eutectic = x -> Solid A + Solid B
Eutectoid = y -> Solid A + Solid B
Eutectic = Liquid -> Solid A + Solid B
Eutectoid = Solid -> Solid A + Solid B
If Carbon % is: <0.8%, =0.8%, >0.8%?
< 0.8%, Hypoeutectoid
= 0.8%, Eutectoid
> 0.8%, Hypereutectoid
What is the relationship with carbon concentration and strength, ductility and pearlite?
Increased carbon concentration =
Increased strength, decreased ductility, more pearlite.
For Ferrite/Cementite and Pearlite:
Near Equilibrium?
Cooling Rate?
Diffusion?
Crystal Structure?
Near Equilibrium - Yes
Cooling Rate - Slow
Diffusion - Both substitutional and interstitial
Crystal Structure - BCC
For Bainite:
Near Equilibrium?
Cooling Rate?
Diffusion?
Crystal Structure?
Near Equilibrium - No
Cooling Rate - Medium
Diffusion - Interstitial
Crystal Structure - BCC
For Martensite:
Near Equilibrium?
Cooling Rate?
Diffusion?
Crystal Structure?
Near Equilibrium - No
Cooling Rate - Fast
Diffusion - No diffusion at all
Crystal Structure - BCT
What is Bainite and how is it formed?
Bainite is formed by shear transformation. The carbon is not trapped and can escape.
What is Martensite?
And explain why tempered Martensite increases toughness?
Martensite has a high amount of carbon trapped, and hence is hard and brittle. It has a small grain size due to high nucleation rate.
Tempered Martensite causes trapped carbon to escape the Martensite and form carbides. This decreases hardness and brittleness, but increases toughness. It dramatically changed the distribution of Cementite.
What do Continuous Cooling Transformation Diagrams show?
They show the rate of austenite transformation if the temp is dropped at an instant. They indicate which phase will form when equilibrium is not maintained.
How is Pearlite formed?
What are the differences between coarse and fine Pearlite?
Pearlite is formed due to the diffusion of carbon. It is alternating layers of Ferrite and Cementite.
Coarse Pearlite is soft and formed at high temps.
Fine Pearlite is hard and formed at low temps.
Fine Pearlite has more interfaces and as a result means it’s harder for dislocation to move, hence stronger.
What influence do alloying elements have on diffusion?
They can slow transformations down that depend on diffusion. Imagine large alloying elements that block atoms that want to diffuse.
They make it easier for Martensite to form as by slowing diffusion you can trap the carbon.
They can be added to steel to increase hardenability and strength.
Reheating Steel after Quenching causes what?
It causes a decrease in strength but toughness to be restored.
What are Stainless Steels?
Steels with >12% Chromium alloyed to prevent oxidation.
What are the effects of adding Chromium and Nickel as an alloying element?
We know the structure of the alloying elements promotes the same structure in the steel they’ve been added to. Therefore:
Chromium, which is BCC will promote Ferrite as BCC, but reduce Austenite phase as FCC.
Conversely, Nickel which is FCC will promote Austenite but reduce Ferrite.
What is an effect of alloying regarding solidification?
What influence does the structure (BCC or FCC) of Stainless Steels have on it’s properties?
Alloying can lead to a complex solidification process where more than one phase forms.
An FCC structured Stainless Steels promotes ductility and toughness.
A BCC promotes corrosion resistance.