Lec 11 -
Phase Diagrams 1
Phase Diagrams 1
What constitutes a phase in a system, and how does it relate to homogeneity and boundary surfaces? Additionally, how do alloys vary in phase composition, and what factors influence solubility in these phases?
Any portion of a system that is homogeneous and bounded by a surface which separates it from other portions.
They can be either single- or multi-phase
Solubility is affected by different thermodynamic states.
What is a binary system?
A binary system refers to a system with 2 solid structures (hence binary) and a complex phase diagram involving 3 phases.
What is the general trend for microstructure of steels? Give the names.
Spheroidite, Coarse Pearlite, Fine Pearlite, Banite, Tempered Martensite, Martensite
Note that the reverse order is for least ductile to most ductile.
On an isothermal phase diagram for a steel, what does the letter F represent?
Depends. Generally it is Ferrite, but could be proeutectoid ferrite.
On an isomorphous phase diagram, what is the two phase region? What is the freezing range, and how does it relate to the two phase region?
How are the constant temperature phase compositions of a material determined inside the two phase region?
The two phase region is where the material exists as partially as a solid and partially as a liquid (alpha + L), and is between the liquidus and solidus lines.
The freezing range is the difference between the liquidus and solidus lines. A larger freezing range means that there is a greater two-phase region.
Using the horizontal tie line method. The leftmost point of the horizontal tie line indicates the liquid composition of the alloying element. The rightmost point of the tie line indicates the solid composition of the alloying element.
What are the important 3-phase transformations? (Hint: there are 3)
Bonus: Name the other 2
Eutectic: L -> alpha + beta (Liquid to 2 solid phases)
Eutectoid: gamma -> alpha + beta (solid phase to 2 different solid phases)
Peritectic: alpha + L -> beta (solid and liquid to different solid phase)
Monotectic: L1 -> L2 + alpha
Peritectoid: alpha + beta -> gamma
Explain the difference between Coarse Pearlite, Fine Pearlite, Bainite, and Spheroidite.
Coarse Pearlite: Formed at higher temperatures and over a greater period of time, therefore leading to more diffusion happening. Distance between ferrite and cementite is larger
Fine Pearlite: Formed at a lower temperature than Coarse Pearlite, diffusion is thus typically limited. Stronger than Coarse Pearlite but less ductile
Bainite: Formed at even lower temperatures, yet is a diffusion controlled process. Strips of long rods of cementite dispersed in ferrite. Higher strength than Fine Pearlite, yet more brittle.
Spheroidite: Spherical cementite dispersed amongst ferrite. Diffusion dependent. Very soft.
What are CCT diagrams and what are they used for?
Continuous Cooling Temperature Diagrams. They are used to reflect the nature of continuous cooling.
The lever rule is used for what?
What is the equation for the weight fraction?
What 2 rules does the level rule satisfy?
Weight fraction of phases.
(length of opposite lever arm)/(length of tie line)
1) Sum of weight fractions = 1
2) Conservation of Mass (of alloying element)
What is a eutectic alloy?
Explain the difference between hypoeutectic and hypereutectic alloys.
A eutectic alloy has a composition that lies directly on the line crossing L -> alpha + beta. It's composition has the lowest melting temperature and freezes or melts at that temperature. It has a lamellar microstructure, and its grain growth is directional. The interlamellar spacing is determined by the growth rate.
Hypoeutectic is to the left of the eutectic composition (but to the right of the leftmost end of the eutectic reaction tie line)
Hypereutectic is to the right of the eutectic composition (to the left of the rightmost end of the eutectoid reaction tie line)
On an iron-carbon phase diagram, what does the eutectoid reaction give us, and what is that material composed of? How does this material affect the mechanical properties of steel?
Discuss the difference between the hypoeutectoid phase and the hypereutectoid phase.
The eutectoid reaction is from Austenite to ferrite and Cementite (gamma -> alpha + Fe3C). The product of this phase change is Pearlite, which is a lamellar structure of ferrite and cementite. It is these microconstituents that provide dispersion strengthening (hinderance towards dislocation movement, thus increasing strength).
The hypoeutectoid phase is constituted by proeutectoid ferrite and pearlite. The hypereutectoid phase is constituted by proeutectoid cementite and pearlite.
For a CCT diagram, what is the term for the minimum quench rate that will produce a martensitic structure?
How is this minimum quench rate related to hardenability?
Critical quenching rate.
Alloys with a lower critical quench rate have better hardenability.
Describe the microstructure solidification process in a solid solution alloy at equilibrium.
As the material cools, the composition of the solid alloying element overtakes the liquid composition of the alloying element.
Remember that it starts off as a high solid composition due to the tie line having a higher Cs.
Describe what a congruent phase transformation is.
When one phase changes directly into another phase without any alteration in composition during the transition. (i.e. L -> gamma)
By controlling the austenite grain size, what occurs to the microstructure of steel? What happens if we control the cooling rate also?
Pearlite grains/colonies grow at grain boundaries of austenite, thus reducing grain size/increasing number of austenite grains can increase pearlite colonies. More/smaller pearlite grains increases strength.
If we increase cooling rate, the diffusion also reduces, thus making finer lamellae. In turn, finer pearlite results in increased strength (but less ductility).
What would be considered a "successful" heat treatment of steel? What does this depend on?
How does hardenability factor into this?
A successful heat treatment would be comprised of 100% martensite (followed by tempering) with a uniform microstructure. This depends on the composition of the alloy, the type of quenching medium, and the size and shape of the components.
Hardenability is the influence an alloying metal's composition has on the ability for steel alloys to transform to martensite for a particular quenching medium.
Explain microsegregation and how you can reverse it.
Explain macrosegregation.
Microsegregation: It is the result of non-equilibrium solidification, which occurs due to fact that cooling a material too quickly does not allow for diffusion, which is necessary for equilibrium. Microsegregation means that the composition of the alloying element varies over short distances between dendrite arms.
Heat treatment below the (non-equilibrium) solidus temp. This allows time for diffusion to eliminate composition differences (homogenization).
Macrosegregation: May result from too quick of cooling (i.e. extreme non-equilibrium). CANNOT be eliminated by heat treatment because the diffusion distance is too great. Can be reduced by hot working.
Why does increasing the amount of eutectic microconstituents increase the strength of the alloy?
Increasing the amount of eutectic microconstituents can increase alloy strength due to the presence of fine, dispersed particles within the microstructure. These particles act as obstacles to dislocation movement, hindering their motion and impeding the propagation of plastic deformation. As a result, the alloy exhibits higher strength because it requires more energy to initiate and propagate dislocations through the material. This strengthening mechanism is known as dispersion strengthening.
Discuss how martensite is formed and what effect it has in steel.
What does tempering martensite do?
It is formed by diffusioless solid-state transformation. Athermal - does not depend on heat or temperature.
In steels with <0.2 wt% C, quenching causes FCC austenite to transform into BCC martensite. In high carbon steels, austenite FCC transforms into BCT martensite. This transformation increases the potential carbon atom sites, thus less slip planes (less dislocation movement).
Tempering martensite precipitates out the ferrite and cementite phases, thus increasing ductility but reducing strength.
For eutectoid steel, describe the process you would need to turn it into tempered martensite, then to pearlite, then to bainite + pearlite.
(All processes follow one another)
From eutectoid temp. -> quench to martensite -> raise temp to below eutectoid temp -> tempered martensite
TM -> raise above eutectoid temp -> bring to 600 degrees and hold for at least 100 sec -> quench -> Pearlite
P -> raise above eutectoid temp -> bring down to 400 degrees and hold for approx. 50 sec -> bring up to 600 degrees and hold for 100 sec -> Banite + Pearlite