 Hello friends, I am P.P. Mitrabodri, Associate Professor from Mechanical Engineering Department of Vulture and Stop Technology, S.O. Lapur. Today, I am going to discuss about cooling curves for pure metals and alloys. And after studying this, students shall be able to identify phase transformations and they shall be able to analyze phase diagrams and phase rule. So in this session, we are going to discuss what are cooling curves, significance of cooling curves, then cooling curve for pure metals, cooling curve for solid solution alloy, as well as cooling curve for alloy having eutectic composition and cooling curve for binary of eutectic alloy. So we will begin our discussion with definition of cooling curve. Cooling curves are graphical representation of phase transformation occurring during liquid to solid phase. This cooling curves are plot of temperature versus time, temperature on y axis time on x axis. These cooling curves are very much significant in study of metals and alloys because cooling curves will give us an idea regarding transformations occurring during cooling. Secondly, cooling curves of alloys will give us idea regarding freezing range of an alloy and due to getting of an idea regarding freezing range, we can understand boundary properties of that alloy. Thirdly, cooling curves will help us in constructing equilibrium diagrams and phase diagrams. Now we will begin our discussion with cooling curve for pure metal. Cooling curve from liquid to solid state will have three stages of cooling. First stage will be liquid to liquid stage, second stage will be liquid to solid state and third stage will be solid to solid cooling. And cooling curve for pure metal will appear like this. 1 to 2 will tell us regarding liquid to liquid transformation. 2 to 3 there will be liquid to solid transformation and then from 3 to 4 solid will cool from higher temperature to room temperature. Now when we are drawing cooling curve or when we are studying cooling curve, liquid metal at superheated temperature is considered. Now when this liquid metal at superheated temperature is poured in a mold cavity, first of all it will lose its superheat while cooling from 1 to 2. When it reaches temperature corresponding to 0.2, this liquid metal has reached its freezing point and pure metals solidify isothermally. So during this isothermal transformation from 0.2 to 0.3, liquid will get transformed to solid as time passes. If we are near to 0.2, we have more amount of liquid and less amount of solid and when we are nearing to 0.3, we find that amount of solid is increasing and amount of liquid is decreasing and when we reach 0.3, we find that all the liquid has been transformed to solid and here from solid to solid transformation begins. And this cooling curve for pure metal can be regarded as a unary equilibrium diagram for pure metal. It can be regarded as a single component system. Now next cooling curve that we are going to discuss is of cooling curve for solid solution alloy. Now cooling curve for solid solution alloy will differ from cooling curve for pure metal because pure metals have fixed melting point and freezing point while solid solution alloys freeze over a range of temperature. So their cooling curve will appear slightly different from cooling curve for pure metal. 1 to 2 liquid to liquid cooling, 2 to 3 liquid to solid transformation or solid solution we can say and 3 to 4 will be solid solution. Now this liquid to solid transformation in this case is taking place over a range of temperature. Temperature corresponding to 0.2 at temperature corresponding to 0.2 liquid will begin its transformation to solid and when we reach temperature corresponding to 0.3 we find that all the liquid has been transformed to solid or solid solution and temperature difference between 0.2 and 0.3 is regarded as a freezing range. And this freezing range is an important entity in determining liquid to solid contraction taking place and boundary properties of solid solution alloy will depend upon their freezing range. Higher the freezing range greater the freezing range greater will be liquid to solid contraction and greater amount of risering needs to be provided to the alloy. And in this case we get solid solution as a result of completion of solidification. Now third type of cooling curve that we are going to consider is cooling curve for an alloy having eutectic composition. Such type of alloy will exist in non-solid solution system. There will be one fixed composition at which liquidus temperature and solidus temperature will be same and such alloy will also solidify like a pure metal that is isothermally. From 0.1 to 2 superheated liquid metal will cool and at 0.2 which happens to be liquidus temperature which also happens to be the solidus temperature solidification of the alloy will begin and along with liquid eutectic mixture will begin to form and as we go away from 0.2 towards 0.3 we find that amount of eutectic mixture will increase and amount of liquid will decrease and when we reach 0.2, 0.3 we find that all the liquid has been transformed to eutectic mixture and below 0.3 up to 0.4 eutectic mixture will cool slowly and at the end of the solidification as a result of solidification we will get eutectic mixture at room temperature. Then we will have next cooling curve which is cooling curve for binary of eutectic alloy which can either be hypo eutectic or hyper eutectic. Hypo eutectic alloy is one which is having composition lesser than eutectic composition while hyper eutectic alloys are alloys which are having composition greater than eutectic composition. So in this case there is precipitation or there is formation of pro eutectic constituent and then eutectic constituent forms. So this curve differs from earlier curve. This curve is bit complex one and in this curve we will find that from 0.1 to 0.2 we will have superheated liquid cooling to liquidus temperature. 0.2 will represent liquidus temperature. Now from 0.2 to 0.3 that is 3 will represent solidus temperature. Liquid will give away pro eutectic constituent from it in such amount that remaining liquid will get transformed to eutectic composition. So when we reach 0.3 that is eutectic temperature or solidus temperature we find that we have pro eutectic solid along with eutectic liquid. Now from 0.3 to 0.4 pro eutectic will remain as it is while liquid having eutectic composition will transform to eutectic mixture. So while cooling from 0.3 to 0.4 which is isothermal cooling transformation of liquid to eutectic mixture will be in progress and as we go towards 0.4 amount of eutectic mixture that is solid will go on increasing and amount of liquid will go on decreasing and when we reach 0.4 we will have eutectic mixture and from 0.4 to 0.5 eutectic mixture and pro eutectic solid will cool slowly to room temperature and in the microstructure of alloy we will have eutectic mixture and pro eutectic solid. Pro eutectic solid present in hypo and hyper eutectic alloy will be different because in hypo eutectic alloy is we have liquid reaching liquidus temperature is containing lesser amount of A or B while in hyper eutectic system hyper eutectic alloy is liquid is containing more amount of B than eutectic. So pro eutectic solid given out from the liquid in hypo eutectic alloy will be reach in A while pro eutectic solid formed in hyper eutectic alloy will be reach in B and these cooling curves are basis for construction of equilibrium diagram basis for understanding phase rule and cooling curve for every composition in an alloy system will be different will be similar but temperatures at which phase transformation begins and temperature at which phase transformation is completed is different now from these cooling curves we can define some terms that is liquidus temperature and solidus temperature liquidus temperature is the temperature at which solidification begins during cooling and liquefaction is completed during heating and above liquidus temperature there will always be liquid phase present while solidus temperature is the temperature at which solidification is completed during cooling while liquefaction begins during heating. So I think we have learnt these things properly and for further references I will suggest my student friends to read a book by material book on material science and metallurgy by Dr. Kodgire Vidi and introduction to engineering materials by B.K. Agarwal. Thank you very much.