noc18-mm20 Lecture 31-Method of measuring Phase diagram

[Music] [Music] okay so good morning again so we have this lecture number 31 today of phase equilibria we'll just first recap the last lecture and then what we have done is we looked at the phase diagrams of phase diagrams and corresponding micro structures microstructures in ferrous and non-ferrous alloys so we looked at in ferrous alloys we looked at mainly steel and cast iron these are two alloys from the perspective of engineering applications very important and then we non-ferrous alloys category we looked at a rhenium zinc which is brass so alpha and alpha beta brass brass as we saw that it has Auto paid many peritectic reactions because of huge differences between the melting points of - sorry copper zinc not aluminum zinc my apologies this is copper zinc and then we looked at copper tin which was bronze so both of these systems show plenty of quite a few pirattitude reactions because of melting point differences and typically we operate for example brass at compositions for relevant to alpha brass or alpha plus beta brass at knowledge of phase diagram is necessary to choose the appropriate composition for achieving desired microstructure similarly in tin based materials copper tin based materials we have phosphor bronze or we have a barium Rodgers which we haven't discussed but there are a lot of bronzes beryllium bronze is contained by helium tin bronze is contained in a phosphor bronze is contain tin and phosphorus and so on and so forth so these are again useful materials they have again microstructure which can be useful for example Delta phase intent-based bronzes provides strength so if you are around the Delta phase composition then you have this two phase mixture which provides you high strength to bronze and then we discussed we also looked at aluminum copper alloys and we looked at titanium aluminum so these are non-ferrous alloys which show by and large most of many of these systems show peritectic and also at places eutectic reactions and knowledge of these phase diagrams is necessary to basically correlate to the micro structural development that you will obtain for a given alloy and then you can tune it by further carrying out phase transformations by changing the kinetics which is not part of this course 20 to tailor them so so this phase that so basically the knowledge of phase diagrams tells you what are the equilibrium phases that you obtain what is the Akira microstructure likely to be but the morphology of microstructure will be changed by kinetics kinetics is something which will again play an important role in determining the microstructure but the phase diagram will tell you what are the equilibrium phases present what the temperatures what are the compositions they will help you in designing appropriate compositions so now we discuss in this lecture so so before we move on to how to determine the phase diagram let me just briefly touch upon they so it's sort of a summary so you can say effect of Delta H mix so when you have so for example when you have two elements which have complete solid solubility but they don't have large difference in the melting points so you start from this state let us say liquid liquid plus alpha and as you keep increasing - as you keep changing to higher enthalpy of mixing so as enthalpy of mixing keeps increasing order for systems increasing order for systems with not so exaggerated exaggerated delta T M which means melting points are not vastly different so when you increase the free energy enthalpy of mixing then the free in the phase diagram tends to split in such fashion so that you have this kind of behavior so this is liquid this is liquid plus alpha again liquid plus alpha you have alpha and then you have alpha 1 alpha 2 and in between you will have alpha 1 plus alpha 2 if you further increase the Delta H mix it changes to so this basically this region will keep growing so I did as it keep growing it starts becoming bigger and bigger and this liquidus and solidus comes closer even further this is liquid liquid plus alpha liquid plus alpha again you have alpha 1 alpha 2 alpha 1 plus alpha 2 and so as Delta basically what it means is that as Delta H mix is increasing the solid solubility of two phases two elements within solid state is decreasing and as a result they tend to phase separate and this phase separation increases as as the Delta H mix increases and so you can see that as Delta H mix is increasing this phase separated part is getting bigger and bigger in area and finally when it becomes too high then you have something like this so you have this is just a schematic okay so you cannot take this as phase it will have you know even subtle variations so melting point something like that okay so the two-phase region would have been like this but it has merged with the liquidus and solidus as a result you have liquid liquid plus alpha one liquid plus alpha 2 this is alpha one this is alpha two and then you have alpha one to that so eventually this isomorphous phase diagram goes through this kind of solid state region at lower temperatures phase separation at lower temperatures and then eventually it converts it to a eutectic phase diagram when the temperature difference is huge then this converts into you know the peritectic kind of system so when when Delta TM is large then you know you start from this kind of system so you have substantial difference between the melting points let's say okay so this is temperature this is X B this is liquid plus alpha alpha and as you increase Delta H mix this tends to again show similar behavior you have this kind of system L L plus alpha alpha 1 alpha 2 alpha 1 plus alpha 2 and Access's remain X's remain the same so I don't I'm not drawing labeling the axes and eventually it turns to something like this as Delta H mix further increases it appears something like this liquid liquid plus alpha 1 and alpha 2 and then you have alpha 1 alpha 1 plus alpha 2 and then alpha 2 and then finally when it increases Delta H mix increases even further so this is basically increase in Delta H mix and which is greater than zero okay so essentially what you have is you have this part being as if you know so what you're going to have is something like this so this is liquid this is liquid plus alpha one and then you have liquid plus alpha 2 here so this is alpha 1 plus alpha 2 alpha 1 alpha 2 this is liquid plus alpha 2 so this is when you have large differences between the melting points of two elements and when Delta H mix increases to greater than zero from being Delta H mix lower than zero when the vast difference when the Delta TM is very large so these are the two cases again this is increasing Delta H mix so obviously for this case Delta H mix is is negative and then it increases for that to becoming positive so this is the evolution of phase diagram that happens and it's sort of it gives you a better understanding of how do phase diagrams evolve or as to what is the effect of various parameters on phase diagrams what what is expected when Delta H mix is larger and delta T M is large so this is this completes basically the discussion of binary phase diagrams what kind of phase dies so what we had learnt is basically thermodynamic basis or phase diagrams regular solutions ideal solutions and you know real solutions and then we looked at the effect of Delta H of Delta V and the change in the enthalpy of mixing and change in entropy of mixing and then change is your mixing the sum of that the balance of these balance of Delta H and Delta s gives you certain stable free energy like free energy composition diagram and from those you can determine the phase diagrams and we looked at variety of phase diagrams like eutectic item offers protecting and so on and so forth and we also looked at various alloy systems now what we want to determine is so the first thing is how to determine these phase diagrams so what we are going to see here is how to experimentally determine the the phase diagrams alright so we have various methods so the first method very commonly used this thermal analysis so basically this means doing time versus temperature measurements and experiments second technique which is commonly used is microscopic examination and microscope examination of microstructure is basically microscopic examination means examination of the microstructure using appropriate techniques such as optical microscopy scanning electron microscopy and this is also associated with the compositional analysis of course using techniques such as energy dispersive spectroscopy or energy electron probe microanalysis these two techniques can give you two very clearly good picture of composition local compositions and then we have third technique which is related to structure of the phases is x-ray diffraction while SEM and optical and scanning electron microscope can tell you call it qualitatively about the differences in the phases that the exact difference is between the structure of the phases can be found using x-ray diffraction so x-ray diffraction can be carried out both at room temperature as well as temperature dependent to understand phase evolution and then correlate it to the time temperature measurements and microscopic examination and then finally there are some other measurements other methods which are also useful especially in solid-state so these are for example dilatometer II which basically utilize and change in the volume or dimensions and then change in electrical resistivity and then change in magnetic properties so these are three methods which we generally use to analyze that phase generally used these are a few methods that we use to analyze the evolution of phase diagrams how the phase diagram can be determined so let's first look at how we determine determination of liquidus and solidus okay so basically we are looking at a phase diagram like an isomorphous phase diagram so basically what it uses is something called as use of cooling curves that is you thermally analyze a material and cooling curves are nothing but time versus time on x-axis versus temperature y-axis plots so when you cool a solid or when you heat a saw when you cool a liquid or solid and when you heat a solid to liquid as temperatures you money to the time temperature as a function of time and these curves are called as cooling curves and these are essentially based on the principle is based on evolution the change in the shape change in the profile of these curves is based on evolution or absorption of heat so you basically latent heat of melting or you know heat change related to phase changes or in the roller state so basically absorption or evolution of heat during phase change so this change this this heat could be evolved or absorbed depending upon liquid solid state transformation during heating or cooling or solid to solid transformation if there is a large heat change involved and so on and so forth and what we basically analyzes analyze the change in shape of the curve shape means slope of T versus T plot at the start and end of phase change so when the phase change happens you notice the change in the shape which is basically the slope of time versus temperature plot at the beginning or at the end so it depends if you if you're cooling or heating you look at corresponding points of so if you're cooling for example liquid solid you look at the start of solidification end of solidification and if you are heating it then you're looking at start of lick melting and end of melting as the phase change so there are various curves that you may encounter during this plot so for example for pure metals for pure metals you might have a plot like this so this is time versus template so there's the same time on x-axis temperature on Y axis and for pure metals a plot like this is obtained when you don't have any so this flat region basically is the signature of solidification so in this state so at high temperature it is in liquid state at low temperature it is in solid state and at the melting point it is this temperature versus time plot is flat and what this means is that basically whatever the latent heat that evolved was equal to heat that was lost during cooling to the atmosphere to the ambient so this latent heat evolution is equal to heat of heat that is lost and that is what maintains this temperature as constant in some cases in case of pure metals you might observe that you have this plot with certain degree of under cooling before it shows so this is what as you can say is the melting point and this is what is degree of under cooling under cooling is basically this delta T okay so not this this one so before it again becomes flat again and this degree of under cooling is required because as you will see in phase transformations you need to form a stable nuclei and that formation of a stable nuclei requires some energy and that energy comes through external cooling as a result you require to cool the material a little bit lower than its melting point before you come back to its melting point doing solidification so again this these are the two kinds of plot that you observe in case of pure metals similar plot is also observed in case of eutectics for example because eutectics also have fixed melting point as a result so any reaction which undergoes a fixed at a constant temperature you are expected to obtain similar sort of curve and then for two phase regions so you can say two-phase region liquid plus solid region so if you have solid solution for instance okay so if you have in case of this is in case of solid solutions so again time into our system picture so what you obtain Jen typically is you have this liquid which cools and at down so at the onset of solidification it changes shape before it goes back to a sharper slope again so this this change this the point of the point where the slope change begins is the TS that is start of solidification or you can say lick TL you can say this is the basically start of solidification means liquidus and the point at which the solidification ends is basically according to phase diagrams the soliders so this is the solidus point and this is for a fixed composition for a fixed composition so you take many you take a you take many compositions of an alloy you take let's say 0 to 10 percent you take 20 compositions every every five percent interval you obtain such cooling curves starting from your a to pure be at every 5 percent of B and then you stitch them together to obtain a phase diagram so you will have to conduct many such experiments for various kinds of compositions and if you have a lower F with liquid plus solid region with solidification culminating into eutectic followed by eutectic kind of behavior then you will have a mixture of these two in the beginning you will have liquid cooling then liquid plus solid and then again you will have a eutectic reaction followed by so this would be your eutectic temperature te this would be the start of solidification that is TL and so this is liquid this is liquid plus solid this is the eutectic solidification and this is solid in this case this would be liquid this would be liquid plus solid and this would be solid so these are the these are the few different kinds of curves that you are expected to obtain as you stitch them as you as you as you obtain various cooling curves so now let's say you have a system for which you want to draw the cooling curves and so what you do is that basically you have let's say so this is a and B let's say we have a isomorphous system okay and this is temperature so I obtained cooling curves for different compositions so let us say we start from pure a which has a melting point a and this has a melting point TM B so for TM a when for the four for the pure metal the cooling curve is going to be something like this so this is going to be the first point that I obtain in terms of where the qualification happens so this is the TM now as you go to as you go to another composition let us say I go to this composition for this composition let us say the curve is going to be you have curve like this and then this so you have one of this point another at this point then you have another one let's say for this composition you start with we have to be really careful so that we don't end up going to down so we have so if you just join them together so it's probably it's gone little too if you join these points together you just use a different color I think if you join these points together for example this with this and if you keep doing it you will obtain something like you know it doesn't look very neat but this is what you're going to obtain so similarly if you draw a cooling curve it will look like this for example if you draw for this composition it will look like this so for this composition it will look like this so this is how you have cooling curves so basically if I redraw it a little better you have you have this phase diagram and cooling curves are going to look like this is for pure metal on both sides and for these you are going to have something like so this is for composition let's say you are going to have different compositions so one two three four five six and when you stitch them together you get a nice phase diagram okay so this is how you you you make a phase diagram by looking at the cooling curves of various systems and and now what we are going to do is that we are going to so this way what you can do is that if you if you do experiments in this way you can construct liquidus and solidus okay construction of solid s is a little tricky because what happens is that when you reach the solid often because of so this is somewhat somewhat difficult because of compositional variances that occur as a result of non equilibrium cooling so non equilibrium cooling which I explained that it creates composition differences this live gives right lowering of solid us so as a result measurement of solid s is somewhat tricky so what you do is that basically to avoid it or to get better results you do a during heating so heating measurements because what you do is that when you do heating so before you before you do heating you homogenized alloy so homogenize asian followed by so you have homogenous asian homogenization will remove the pouring effects and then you do the heating treatment so heating analysis and that will give you the better estimation of the solidus it will be slightly higher than what you observe during the cooling cycle so what we have done in this lecture is we have looked at how the phase diagram is measured analyzed using thermal analysis by measuring cooling curves of alloys for both pure metals so you have different type of poly curves of pure metals and eutectic sand and different curves for liquid and solid regions what we will do is that we will now look at some more aspects of this measurement of phase diagrams before we move on to the next topic of ternary system thank you very much [Music] [Music]