Explain Briefly Phase Diagram in Material Science

Phase Diagram Material Science

Hi everyone, welcome back to the topic on introduction to material science and engineering. today we will discussing about something which is known as phase diagrams. Today will be just to introduce you to several concepts upon which we will build further knowledge. So the fundamentals before we begin, you need to understand that there are certain definitions that needs to be understood better before we understand what phase diagram is. 


So one of the things that you will encounter many times when discussing about phase diagrams and phases is known as component. Component in metallurgy or in material science refers to a pea or metal or compound out of which alloys are made. So it is either a pure metal or a compound. Now in order to give you idea about each of these terms. I will be using a simple example of sugar water solution. Sugar water solution using this will understand each of the terms which will define here. So in a sugar water solution, the components are sugar and water. These are the two components present in a sugar water solution. 

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Now what is a solution? Solution can be referred to as a of alloys consisting of same components. What do I mean by this?? What I mean is that you again using the sugar water example. You can have 10% sugar in water or you can have 20% or 30 or 20 files. So there are different combinations of sugar and water and amount that can be present. So this is a series of alloys we normally use in metals. But here we can say series of mixtures or series of solutions which has the same components. You see this components are sugar and water. What we are varying is, we are varying the amount of the independent components of the individual components. This series of different solutions represent a system of solutions or a system of alloys in the case of metal systems. So now we understood what component is. We understand that what a solution is. 

Solubility Limit

Now we need to understand something which is known as solubility limit. Solubility limit basically refers to the maximum amount of solute atoms that you can dissolve in a solvent to form a solid solution. so if you think about it, if you try to mix sugar in water. You take water at room temperature you put you take one cup of water. You add one spoon of sugar, you stir it. It will mix completely. You keep on adding sugar, you keep on stirring up till one level you can keep on dissolving all the sugar in the water. But we aren't a certain level the sugar will no longer dissolve in the water. You will see sugar deposited at the bottom of the water, which no longer dissolves in the water. What that means is at that moment when the sugar stops dissolving any further, you have reached the solubility limit. Which means no longer the solute that is the sugar can dissolve in the solvent that is the water. Therefore solubility limit is a hard limit of the amount of solute, that can basically dissolve in a solvent to form a solution. But the solubility limit is also a function of temperature. It depends on the temperature of the solution or of the solvent. 

How can we figure this out in the same example of mixing sugar in water. Suppose you had water at room temperature and after four spoon of sugar the sugar no longer dissolves. Try this out, you increase the temperature, you heat the water. Then what you'll observe is that, now the water is able to take a dissolve further amount of sugar. So basically, by increasing that temperature of the water. You are able to increase the solubility limit. Now more amount of solute atoms can dissolve in the solvent. What I mean, when I say that solubility limit depends on temperature in the case of solids dissolving in liquid or solids dissolving in solid. The relation is linear, that is you not linear rather increasing. With increase in temperature, the amount of solute that can dissolve increases. 

In the case of gases, it is normally the opposite. If you increase the temperature of the solution or of the solvent, then less amount of gas will be able to be retained. That piece the gas will start to evolve out on increasing that temperature. But since we will be fundamentally concerned with metals and metallic systems. Solid systems, the solubility limit will be increasing in most of the cases with increase in temperature. Now let us see we have composition. Weight percent of sugar in this axis. This is the composition over here we have 100 percent water. 100 percent water and over here, we have a hundred percent sugar. So any intermediate position here we have 50% water and 50% sugar. This axis is the y axis is the temperature axis. As you go up, you are increasing that temperature. What we observe is that over here we have something written as L. This L is referred to as the liquid. This is basically a water plus sugar solution. 

There is no independent individual sugar left over in this region. But in this region across this line. Across this blue line here we have the liquid form of water plus sugar as well as solid sugar deposited. So we have liquid plus solid and let us take any temperature. Let's say we are at 20 degree Celsius. What we see from this graph is that at 20 degree Celsius we can dissolve approximately a maximum of somewhere close to 70% sugar in water. If you add more sugar then it will deposit as solid sugar on increasing the temperature. Let us say we are at 80 degree Celsius. At 80 degree Celsius instead of the previous amount of 70 percent. Now we can dissolve around 80% of sugar in water. So this line basically separates the region. This solubility limit basically separates the region in which sugar completely dissolves in water. The region where sugar is left over and is not able to completely dissolve in water. This line basically represents the solubility limit line. This type of representation in which we have one axis devoted to composition of the components. These are the component side water and sugar. One axis is devoted to percentage of different components. The other axis over here is the polar devoted to the temperature. Over here the whole area is divided into regions having different. What will call them as phases? We will see that these are phases this type of diagram is known as phase diagram

Physical and Chemical Properties

We will see this in much more details. So here it is what is a phase, a phase is basically a homogeneous portion of a system, which has uniform physical and chemical properties. Uniform physical and chemical properties that is the most important criteria for our given phase. So here what we saw is in the sugar water solution case, here this liquid or this solution basically which had sugar and water completely dissolved had the same physical and chemical property. Throughout whether we took it over here anywhere. So this was our phase whereas in this region we had two phases. One was water and sugar solution and the other was solid sugar which was deposited. Because it could not further be dissolved. So we had two phases here we had single phase here, whereas for both the cases we had two components water and sugar. 

Now that we have an idea what phase is and this is what I already discussed that in the sugar water syrup case. If complete sugar is dissolved then we have single phase. If complete sugar is not dissolved then we have two phases. When the sugar water set up and the other is the solid sugar. Now how do you distinguish phases, we already know the basic definition of phase. But a phase the phases are basically separated by distinct boundaries. Two phases will be separated by distinct boundary and what is the property across the boundary. Across the boundary you will have either different physical properties or different chemical properties or both of them. Different both physical and chemical properties different. So two phases need to have at least some physical property. Different or some chemical property, different or both physical and chemical properties different. You cannot have two different phase having identical physical and chemical properties. 

Just to give you an example in the case of water. You can have ice, you can have liquid water and you can have vapor. But these three are different phases though they are same component. They have the same chemical formula. These are different phases they will have same chemical properties that is all will be h2o. But they will have different physical properties. So this will be in solid phase, this will in the liquid phase, this will be Decius. They have different physical manifestations. Now finally in case there is a single phase present. In case you have a system where the single phase present then we call it homogeneous system. In case there are multi phases present since there is more than one phase it is no longer homogeneous. What we call it now is either heterogeneous or it is termed as a mixture. So depending on the number of phases present whether it is singular or plural. It will be termed as homogeneous or heterogeneous. 

Phase Diagram of Water

Now let us discuss see the phase diagram of water. As I already said that water is a single component. Water is single component h2o but it has ice water and steam three phases present now. What phase will be present at a particular temperature and particular pressure will be defined by the combination of the temperature and pressure. Here in this phase diagram we have one axis for temperature, the other axis for pressure. There is no access for component percentage because it is single component. It is always hundred percent water. So we do not need a component percentage axis. We will have a temperature axis or pressure axis and depending on the combination of temperature and pressure. We will get whether will have the solid state the liquid state or the gaseous state.

Atmospheric Pressure

Let us take an example let us take atmospheric pressure. This is atmospheric pressure that is 180 m and let's say we are at minus 50 degree Celsius. Let's assume this is minus 50 degree Celsius and we are at one atmospheres pressure. We end up in this region and this region as we can see is the region for eyes by our experience. We know that at minus 50 degree Celsius, we are supposed to have eyes and nothing else given that. The pressure is atmospheric pressure. Now suppose that you take this you keep the temperature at minus 50 degree Celsius itself but you start reducing the pressure you reduce the pressure so low that you enter this pink zone. Suppose you are over here at this pressure. Then what will happen what is basically happening is that the ice is getting transferred or converted into steam. Why so basically vapor exists when the vapor pressure is able to overcome the atmospheric pressure and any solid has some vapor pressure associated with it. So I also have some vapor pressure associated with it because there are atoms on the surface which may come off and provide some amount of pressure. Similarly liquid water has also some vapor pressure associated with it. Therefore what is happening over here that the atmosphere pressure is coming so low that the vapor pressure of the atoms from the I is exceeding the atmospheric pressure. There were the solid ice is converting into vapor steam. So here you can see the different phenomena actually happening. 

How you can tweak the pressure and temperature combinations to get a different phase altogether. Let's jump into the water region if you keep one atmospheric pressure but you go to zero degree Celsius. We know that at zero degree Celsius we have water as well as ice present. So if water as well as ice is present at zero degree Celsius that is exactly what is happening here. Zero degree and one atmospheric pressure is falling at the interface of water ice therefore over here we will have both water and ice increase the temperature. Further you reach hundred percent water peak that temperature 200 degree Celsius. Now you are at the equilibrium of water and steam. So you will have attended degree Celsius and 1 atmospheric pressure both water and steam increase the temperature further and we get complete steam or complete water vapor. So this is how increase of temperature effects. The phase changes and that can be deduced from this phase diagram. 

Triple Point or Invariant Point

I would like to draw your attention to a few important details here one is what is known as triple point. Triple point or it's also called invariant point. You will encounter invariant point several times when discussing phase diagrams further. But for water it is called triple point. Triple point is the combination of temperature and pressure at which all the three phases coexist. So at zero point zero six atmospheric pressure and 0.01 degree Celsius temperature. We have ice water and steam all of them three coexisting that is same here because all the three lines meet at this point. Then we have this point that is 218 atmospheric pressure very very high pressure and 374 degree Celsius quite high temperature. Over here beyond that you will have something which is known as supercritical fluid.

Supercritical fluid

Supercritical fluid is basically something which has properties of both a liquid and vapor. Phase will not go into details of that. You can study up more about supercritical fluid if you want But we do not need for our understanding any further. The only take-home supercritical fluid has properties of both liquid and saw a vapor state coexisting and that temperature and pressure combination is known as critical point. Beyond which we have supercritical fluid. So this gives you an idea about phase diagram fundamentals here. Again what we see is that we have distributed that temperature pressure area into different regimes. Having different phases existing and we have boundaries along which phases coexist. with this back background. 


Let us understand what is microstructure. This gives you a brief understanding about phase diagram. Now let us see microstructure, microstructure is one of the most important things that you need to understand while understanding metallurgy and material science. Microstructure is basically as the name suggests Micro that is small structure. It is the structure as observed under a microscope. What is the importance of a microstructure? Microstructure by studying it we can predict what is the property of the material at hand. The property is influenced drastically by the nature of the microstructure that exists. Now what are the different components of a microstructure when you see. Let's say a steel sample under a microscope, you will be able to see the phases different phases present. You will be able to identify what is the grain size of the different phases present. 

There will be also able to observe what are the proportion of the different phases present. How much is each phase present and how are they distributed? How are the different phases distributed. What is the shape of the different phases? is it needle-like, is it equi-axed it, is it elongated everything together. All these properties all these observations together lends to the ultimate property that the material will have. But what are the things that decides this parameters. What is it that decides what will the phases present the grain size? How much amount of differences will represent that is kind of dictated by several things? One is what are the alloying elements present like. If you have a steel whether the disk Romey imprison tor not whether there is boron present or not. What are the alloying elements present that will affect the arrangement of the phases and the phases what is a concentration of the alloys that you add? Are you adding two percent from i am. Are you adding ten percent chromium to it? 

That will affect the microstructure and last but not the least and one of the most important thing is the heat treatment schedule. That is you are undergoing you take a steel. You take it to 800 degree Celsius and you cool it rapidly to in ice cold water alternatively. You take a steel you heat it to 800 degree Celsius and you let it fall in the school that is it cools very very slowly. These two are two completely different kind of heat treatment schedule and this difference in heat treatment schedules will lead to completely different micro structure. Thereby completely different properties. So the take home here is that micro structure dictates property and the micro structure itself is dictated by several factors. Including the alloying element, their concentration and the heat treatment schedule. That you follow among which the heat treatment setting is one of the most because you are able to form the heat treatment schedule as per your requirement. 

You cannot change the alloying elements as per your requirement always. Now these are two examples I have put up just to show you how micro structures look, how are different microstructures look? Thus different microstructures look this is copper to give you an idea this is 50 micron. This is about the thickness of a hair single here. So this is quite a magnified view this is copper single cells. You can see these are different range. This is one grain of copper, this is a different grain of copper. So what we see is that not all veins are equally sized. Some are quite small means some are relatively large grains. But the average size will tend to be close by the medium size will not vary a lot from the smallest or the largest and these things how they are. What is the average median size? How are the grains arranged? This will affect the property, this is another example of microstructure in which you have two phases present. You can see a dark phase and you can see a light phase. So this was single phase this is a two phase microstructure. Two phases need not exist only like this. Two phases can also exist like this, in which some of them are one phase and some of them are different phase like this may be one phase. This may be other phase and intermixed together. Here they are basically lap in a lamellar shape. When we'll study further, we will see that this kind of structure is known as lamellar structure obviously and this is obtained by eutectoid transformation dip toil'd transformation. 

So just to give you a brief glimpse about what we study today. We began by the basic definitions component system, solubility limit phase and different kind of phases that can exist. We saw the phase diagram for either rather water and we understood how to read the phase diagram for hit from here and what are the details that we can extract from a phase diagram. Then we discuss what is the microstructure. We discuss the details that can be understood from a microstructure. What is the relevance of microstructure.

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