 Friends, today we are going to discuss regarding binary equilibrium diagrams. I am P. P. Mitravutri, associate professor in Department of Mechanical Engineering at Vulture Stop Technology, Solaapur. At the end of this session, students will be able to analyze the solidification behavior of various compositions in the binary system and students will be able to interpret significance of various transformations taking place in the binary system. Now, in the earlier session, we have already discussed significance of cooling curve and what are cooling curves. Cooling curves are drawn for one single particular composition of an alloy system, but they will not give us any idea regarding solidification behavior of total alloy system. For that, equilibrium diagram will help us. Equilibrium diagram can give us idea regarding solidification behavior and phase transformations taking place in each and every composition of alloy system at a glance. Equilibrium diagrams are drawn considering equilibrium conditions. Equilibrium diagram is generally nothing but a plot of temperature versus composition. Now, what is a system? System is part of surrounding under study and we are going to construct equilibrium diagrams for binary systems. What is binary system? Binary system is system having two components and in metallurgy, when we consider two components, it is an alloy system of two metals A and B. Now, in this alloy system, there are many types of equilibrium diagrams, but we are considering equilibrium diagram for isomorphous system and equilibrium diagram for system exhibiting partial solid solubility. Now, isomorphous system is the system exhibiting complete solubility in solid state. For an alloy formation, it is absolutely necessary that the two metals A and B must be soluble in each other in liquid state and if they are able to retain their solubility completely in solid state, such a system is called as isomorphous system and in any isomorphous system, all the alloys will solidify as solid solution and this system is called as solid solution system also. Now, for drawing isomorphous system, let us consider a case of two metals A and B. Suppose, melting point of A is 1000 degree Celsius and melting point of B is 1200 degree Celsius. Now, with this data, we are unable to construct total equilibrium diagram because A represents pure metal, B represents another pure metal. So, we have to consider various compositions of A and B. For that, we have to determine liquidous and solidous temperatures for various compositions in the system of metals A and B and we have to note liquidous and solidous temperature of each and every composition and after noting down that, after getting that data, we are able to construct the equilibrium diagram for isomorphous system. For that, first of all, along y axis, we have to take temperature in degree Celsius and along x axis, we will write percentage B. On the extreme left, it will be 0 percent B means 100 percent A. At extreme right, it will be 100 percent B that is 0 percent A means extreme left will represent pure metal A and extreme right will represent pure metal B. So, let us write melting temperature or freezing temperature of metal A that is to be 1000 degrees. Let us mark freezing or melting temperature of metal B, pure metal B that is 1200 degree Celsius. Now, we have already noted down, we have to note down those liquidous and solidous temperature for various compositions. Suppose, we have noted them down, then for various compositions like this, we have to plot all liquidous temperatures, then all solidous temperatures too shall be plotted. Then after that, we have to join a line joining all liquidous temperatures and we have to draw the line joining all solidous temperatures. This will complete construction of equilibrium diagram for isomorphous system or equilibrium diagram exhibiting complete solid solubility. Now, uppermost line, I will name it as x, y, z, it is liquidous line and another line, I will name it as x, p, z is a solidous line. Liquidous line is the line representing liquidous temperatures of various compositions in the system and solidous line is the line representing solidous temperatures of various compositions in the system. Above liquidous line, there will obvious will be liquid phase present and below solidous line, there can be none other than solid solution present and in between liquidous line and solidous line, there will be liquid and solid present. Now, in this case, if you observe closely, for every composition in the system, liquidous temperature and solidous temperature is different. This is characteristic of solid solution and all solid solution alloys are characterized by freezing range and this freezing range will cause liquid to solid shrinkage during casting. So, these alloys are supposed to be difficult to cast alloys, but as these alloys are forming solid solution below solidous line, they can be very easily hot and cold work that is their characteristic. We can forge cold forge, hot forge, we can extrude cold extrude hot extrude, we can roll them very easily. We can in short, we can plastically deform these alloys to form the components from them and now in this case with this diagram, we can study solidification behavior of any composition. Now, let us take this particular composition. The alloy at this temperature will be in liquid state. Now, this liquid alloy cools slowly till it reaches liquidous line. Here, it has to begin its solidification. Now, solidification of this liquid will begin with crystallization of solid and crystals of solid or solid solution will grow at the expense of liquid with drop in temperature and eventually, when we reach a solidous temperature, it is observed that all the liquid has been transformed to solid and solid solution that is solid solution and it is here solidification of an alloy is complete and as a result of completion of solidification, we have single phase solid solution structure and cooling curve for each and every composition in this system will resemble like this only this liquidous and this solidous temperature and this freezing range for every composition will depart. This is all about isomorphous system. Now, another system that we are going to have is equilibrium diagram for system exhibiting partial solid solubility, but due to lack of time, we are unable to discuss this during this session. We may discuss it in future session and for further reference, I will recommend you to use the book by Dr. V. D. Kodgire material science and metallurgy and book by Mr. B. K. Agarwal that is introduction to engineering materials. Thank you, friends.