Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the second of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.
1.20 Analysis of Complex Phase Diagrams
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En provenance du cours de Georgia Institute of Technology
Material Processing
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À partir de la leçon
Phase Diagrams and Phase Equilibria
This course picks up with an overview of basic thermodynamics and kinetics as they pertain to the processing of crystalline materials. The first module deals with phase diagrams - charts that tell us how a material will behave given a certain set of variables such as temperature, pressure, and composition. You will learn how to interpret common and complex phase diagrams and how to extract useful information from them.
Rencontrer les enseignants
Thomas H. Sanders, Jr.Regents' Professor
School of Materials Science and Engineering
Now we're ready to begin looking at slightly more complex phase diagrams.
And what we're going to do is to look at a combination of
two very interesting looking diagrams.
On the left-hand side what we have is,
a diagram which contains two invariant reactions.
On the left-hand side at the high temperature,
what we're seeing is a Eutectic reaction.
So in that particular diagram what we're seeing is a liquid
phase that is transforming Into two solids.
An alpha solid and a beta solid.
Now, on the right-hand side of that particular diagram,
what we're seeing is another Eutectic at a lower temperature.
And that Eutectic is the single phase liquid
going into the beta phase plus the gamma phase.
Now, I want to draw your attention, because very often, the way we label these
phase fields in which we have some solubility in these various
phases that are appearing is that we begin labeling
from the left-hand side to the right- hand side going through the Greek alphabet.
So when we talk about component A,
there is a certain degree of solid solubility, of component B in that phase.
And what we do is, we call that the alpha phase.
When we go over to the right-hand side,
once again we're looking at a single phase field that we're calling gamma.
The reality of it is that that gamma has the same structure as component B.
And it has a certain amount of A which is soluble in that phase.
So consequently we're going to call that phase the gamma phase.
In the middle we have a compound that has a range of solubility and
we're going to call that single phase field the beta phase field.
Now I draw your attention to the reaction that is at the top of the page
on that left-hand side.
It shows us that what we have there is a maximum temperature and
that is a congruently melting maximum.
That is, if we are along that composition that's indicated by beta,
that dotted line.
If we have precisely that composition and we increase the temperature.
What will happen is, that material will undergo a process of
congruent melting, with a maximum temperature.
And so,
the beta phase transforms directly to the liquid phase of the same composition.
So, you can notice how those liquidous and
solidous lines come together at the congruently melting maximum.
Now, if we look at the diagram to the right, the only difference with respect to
the diagram on the left and the one on the right is that we are assuming that
the intermetallic phase that is in the middle of the diagram that we refer to
as beta that we're going to refer to a line compound.
That is, you're not allowed in this particular system any solubility
off the fixed ratio of AB.
Now suppose I pose this question, if you were going to make an alloy
of either the material of the beta phase or
the material of the AB and
you wanted to make sure that the material that you were looking at.
Was something that you could do reproduceably.
One of the big issues with respect to melting large quantities of material is to
make sure that the compositions are each timed the same value.
If we look at the composition on the right.
One of the problems we have is that if we are slightly off
the AB compound, that is either to the left or to the right, what's going to
happen is there will be a melting reaction that occurs at a much lower temperature.
So for example if we happen to have too much B as opposed to the AB, then
we're going to get the utectic reaction in which we begin to form liquid at a lower
temperature than we would if we talk about the AB compound that congruently melts.
And similarly, just to the left, where we have A plus the compound AB.
If we look to the right,
however, on the diagram to the right where we have some solubility
what that allows us to be slightly off our composition by a small amount.
But at the same time we're in a single phase field and we're not going to
experience melting at the temperatures given by two lower Eutectics.
Now one of the reasons that in bring this up is, this particular phase diagram
is very similar, to the types of diagrams where we were looking for
materials that are high melting temperature.
The types of materials that we might use, for example,
in nickel based super alloys where we like to go to higher and higher temperatures.
When you look at the diagram, what is particularly important is the fact that
the compound B melts at a much higher temperature
than either do pure component A or pure component B.
So there would be an advantage of looking at a particular system of the beta phase.
All right, so we're going to look at the diagram and
we're going to determine first of all, all of the phase fields.
Then what we want to do is to identify all the invariant reactions.
So, when we do that, what we find is, let's go back to this figure.
What we can see is, on the left-hand side of the diagram,
we're going to call that single phase field over there alpha.
As we cross going from pure A over toward pure component B,
we see another single-phase field and we're going to call that beta.
The one that is in toward the center of the phase diagram
we're going to call the gamma phase.
And the one all the way over to the right is the delta phase.
So if we now take a look at the reactions, then what we see is alpha.
There's this two phase field of alpha + beta.
Then we go in to the single phase beta region.
And on either side of that single phase of the two phase,
beta + gamma, what we have is those two single phases.
And then when we look at the gamma + delta two faced field,
it is sandwiched between two single phased fields.
So this is a description, then that we would have for this particular diagram.
Now, to answer the question regarding the invariant reactions.
There is a low temperature reaction, and
that reaction is, liquid going into alpha + beta.
When we move to a slightly higher temperature and
we´ŕe working our way over to the right.
What we see is, the liquid phase and
the gamma phase, going down into a single phase, namely the beta phase.
So here we have a liquid and a solid going into a solid so that´s a perictetctoid.
At the top we have a congruently melting phase.
We move over to the right and what we see is another Eutectic.
And what we have is a liquid transforming into gamma + delta.
So there we are.
Our Eutectic, our Peritectic, our congruently melting phase at the top.
In the Eutectic phase.
So, we have Eutectic, which is liquid going to alpha + beta.
The Peritectic, which written in a symbolic form is
liquid + gamma going to beta, which is two phases going to one.
And then finally, the single phase liquid going into two more solids,
the + the delta for the Eutectic reaction.
Now that's what our picture looks like in terms of our phase diagrams.
Now what I would like to do is to have you focus your attention and
looking at the horizontal lines.
Those are the lines that represent the invariant reactions.
So when you look a complicated phase diagram what you want to do is to identify
where the invariant lines are and identify those lines according to.
The type of reaction that's occurring here.
So that helps you with the analysis of a complex diagram,
like the one I've illustrated here.
Let's turn our attention now to a slighty different diagram.
This time what we have our line compounds that go all the way across the diagram.
We have essentially no solubility in component A of component B.
And again, no solubility of A into component B.
So, when we label the phase diagram,
we see our low temperature reaction that occurs first on the A side of the diagram.
That is liquid transforming into A + A to B.
So that's a Eutectic reaction.
Remember it involves a liquid plus two solids.
Now we go to the next reaction, which is a slightly higher temperature.
And that reaction is a Peritectic.
That is, we have a liquid + a solid transforming into a solid.
Of a third composition so that reaction is referred to as a Peritectic.
We then have the line compound that melts congruently at a maximum temperature and
in addition to that we go over to the right
then we have the high temperature Eutectic.
In which what we have is the liquid phase transforming into two solids.
Solid B, out of component B, plus the inner metallic compound AB.
So you can begin to see how we can start analyzing these diagrams
As we focus on specific composition, moving across the phase diagram.
Before we leave these complex systems,
let's take a look at the diagram that I've illustrated up on the screen.
It is a diagram of the aluminum nickel system.
And this is a particularly important system, because this system Is utilized
in the high temperature alloys, that are often referred to as super alloys.
The diagram is plotted from left to right, aluminum ultimately going to nickel.
And when you go through, and you look at these different reactions, what you see is
we have, at temperature 640, what we're going to see is the Eutectic.
As the temperature goes up, we see a Peritectic, and at 1133,
we see another Peritectic reaction.
And then what we see at the highest temperature, we have, again,
a congruently melting maximum.
Now if we take a look at what's happening in that circular region in there,
I've expanded it, and what we actually have is
a Peritectic reaction going into a Eutectic reaction.
So this then helps us go through and take a material like an aluminum
nickel alloy and begin to think about how we take apart the particular
phase diagram and how we can go through our analysis of the diagram.
Thank you.
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