Solidification of Pure Metals and Alloys
ah [Music] [Music] welcome to the lecture on further education of pure metals and alloys so in the last lecture we discussed about how nucleation stars under what condition it starts and then further it grows so that there is complete
transformation from liquid to solid state now we will see its effect on that certification behavior in case of pure metals and alloys now we are already understood the fact that in case of pure metals the solidification is at a constant temperature so basically what happens when the liquid metal is poured into a cavity suppose you have a cavity and if the liquid metal is poured so the liquid metal and if the surface is at room temperature so certainly at the liquid metal passage these surfaces or all these surfaces the liquid metal experiences maximum under cooling and heat transfer from these surfaces is the maximum so there will be crystallization of solid particles solids edges now since the heat transfer rate is maximum here and we have also understood that with larger degree of under cooling the nucleation rate is maximum so basically what happens you have formation of very small nuclei at these surfaces and the skin is formed so because you have a chilling effect on this point and due to the chilling effect a skin is formed a stream is formed so that is filling star breast to mold wall and a skinnies fourth to not solid film or a screen or solid metal is started to form at this point and it further growth now thickness of this skin which is frozen varies with the square root of x so there is an expression which talks about the thickness of the skin formed and that is given as K under root P minus C so that is why it is written that the thickness of this skin as it grows with time so this thickness these thickness of the skin and K and C are constants now the the value of K try and see Dean constants they are governed by certain factors so what way they are going to be changed now magnitude of K is determined by size of casting and how fast the heat is extracted from the mold so the first constant k if magnitude will be determined by the size of casting and how fast the heat is extracted from the mold like what type of mold it is metal or sand also and magnitude RC is determined by degree of superheat so these are the two constants which come into picture in this particular correlation which talks about the thickness form of the solid field when we are going to cast a particular pure metal and if you look at this you can very well say that if you have suppose at zero time if you have such structure which is poured into now at further time the celestial formation will start from both the sides and it will look like this so basically this is the solid fill so suppose this is at 20 seconds so further at 40 seconds say the solidified sail Blair look like this and because the further education is from the two sides as well as from the bottom so that this will be the solidified self at suppose say 60 seconds and further onwards this may be completely solidified so this is how the skin thickness wears on increasing in the case of pure metals and basically it is also known as plain front solidification where you have a front which is having a plane and that basically advances towards the center of the cutting so we will also see what way the structure looks like in case of metal now in case of heterogeneous nucleation results have foreign particles altered by a liquid for interfacial energy and not to assistant assistant nucleation they are by reducing the amount of super chilling now this was so in case of heterogeneous nucleation the only difference is that due to the presence of certain foreign particles the free energy requirements change energy requirements changes and the nucleation is more preferred more favored at preferred sides so the degree of under cooling what is required in terms of homogeneous under cooling that much requirement is not there in case of heterogeneous nucleation that is the difference in case of heterogeneous nucleation and that is why more nucleation size in case of heterogeneous nucleation and whenever we try to have more nucleation points or more nucleus then sometimes it is also a common practice to provide the heterogeneous nucleating agent now what happens further that we will see in our next slide so what happens that if suppose you have a cavity of this shape and it is in a sand mold this is all sand or any molding material so what we see is this is your cavity and this is mold now once we pour the liquid metal into it what happens the lipid metal touches these walls and the nucleation starts at these walls so all the fine nucleoid starts to fall at these walls and further they grow that will discuss how they grow so they grow inwards so what we mean to say is you suppose you have a point here this is point one and you have a point this course to this point this is point two then you have a point suppose to here three and one more point is here 0.4 if we try to draw the puddin curve of these points what has happened here in this case if we try to draw the cooling curve of the point and if this is the equilibrium temperature so what we see is we see that it is quickly solidified at point one so this is 0.
1 one Supporter is the wall of the solid due to under cooling quickly it comes and then it has a perilous period which is somewhat horizontal and then it goes on like that if we take the point here here basically the temperature has just come and if this is the suppose time at which casting is sectioned so at this point this is yet to solidify so this temperature is still there coming and after that it is supposed to cross this point temperature so for this reason still has not taken place but it is it will come in certain time now at this point if you look at the cooling discours of temperature so temperature curves will be like this similarly on 4 it will be like this so basically these are the ISO temperature lines where so where the temperature is more so for this it will be further you have these lines and you expect that in future they will sometimes be crossing over this equilibrium temperature line when the nucleation will take place now let us see further how there is the growth and nucleation in case of pure metal so what is the happening in case of pure metals that once this is the wall once the liquid metal is touching to this wall there is formation of very small grains and these are there is a large number of nucleation because of large under cooling that is experienced when the liquid metal is touching this wall and this grains are known as equiaxed grains so this is fine equiaxed grains there is formation of fine eQuest grains at the mold wall now what happens that from this mold wall the heat is obstructed he distracted in this direction now because of as we have already discussed that because of larger degree of under cooling experienced at the mold wall the nucleation rate is maximum at the mold wall and you get a fine equiaxed grain structure in on the mold wall now heat is extracted in the opposite direction further what happens that the heat which is extracted because of solidification because of the dependence of the surface but as the solidification is going on there is also release of latent heat so heat extracted heat extracted in negative x-direction here you have negative direction the heat is extracted from the mold wall now what happens when this crystal is getting solidified it needed is latent heat so the crystal moving latent lead upon solidification increases the temperature in nearby region nearby John what is happening is that suppose this crystal has solidified it has lost certain of its latent heat this latent heat release will flow in all directions it will flow in this direction as well as in this direction once it goes into this direction it is basically immediately increasing that temperature of the adjacent liquid now once the temperature of the adjacent liquid is increased that degree of under cooling experienced for this liquid path is becoming less and that is why nucleation is disturbed the nucleation which was expect the rate of nip later which was there because of large under cooling at that zone close to the wall has been depleted because of the increase in temperature in a region that near this solid cell fault and because of that the nucleation series and because that there is including temperature in that vertically increase the temperature zone growth becomes predominant this is the growth also is there what occurred great occurred in a direction opposite to that of heat flow so what happens since were heat flow direction is in the negative direction so it will go into positives as direction one is that there will be growth nucleation will be sealed nucleation in the door left to Equis act fine eQuest grains will cease because of increasing temperature or decrease in degree of under cooling now since this growth becomes predominant here from here there will be the breadth of these particular grains which are found in the direction towards the center of the cutting growth is also in a preferred crystallographic directions so basically growth will certainly like to take place in certain preferred historical directions as well as working also in the direction opposite to the direction of heat flow so what happens after that the growth starts taking place this is center of the casting center line of casting so what happens after this you have the formation of columnar grains these grains which are the form because of the growth of these grains because the laser has stopped because of the conditions which we have discussed earlier because of the decreased value of under cooling then there is formation of the drains and the group of grains like this and they are known as columnar grains so these grains are known as columnar grains and from this side also similar thing happens so this is also wall so similar thing will happen this side also and ultimately ultimately you will have the structure becomes like this now what we see is you have wall on both the sides you have foundation of fine Italy as grains on the mold wall and these columnar grains extended up to the center of the casting and if there is a wall on the bottom also there will be formation of the required join at the bottom of the wall also as well as from the side also so what we see this is a typical characteristic of the type of grains which are formed in case of pure metals in case of pure metals that there is plain France for education this front advances uniformly and goes till the center of the carton and this is basically a typical characteristic of your nutters so what we see is next to the final crest very from the wall a gern of columnar grains is formed now what we see is in this we may think of having this structure what happens that the grip they try to grow in all the directions but being this as the predominant de-reference mostly the growth in other directions is peeled off they are basically utilized and that the dominant direction of the growth of the crystal is in this direction from here that is positive x direction from here it is negative x direction okay so this way you see that how these grains are there in case of pure metals now what happens in case of alloys in case of alloys the group of columnar grains are interrupted by equiaxed grain growth that we are going to discuss when we talk about the freedom of alloys now we will discuss about the freedom of alloys now what happens in case of a noise as in case of alloys the reason is over a range of temperature freezing does not happen at a particular temperature so there is no plane and solidification in case of metals the certification is at a particular temperature and it's smelly this front moving towards the center of the cutting whereas in case of alloys in case of corrosion if you look at the freezing is over a range of temperature so suppose you have you have a binary alloy we are deceived our solute concentration so and this is the temperature now what happens when the temperature comes down when it comes to this temperature for where at this temperature the concentration of the solid will be this and concentration of the liquid will be this so basically if it is a composition decision or solid this is a and this is concentration of the liquid now what happens in this case that when the solid will start precipitating from the liquid the solute which is going away the solid which is going away it has lower solute concentration whereas the liquid which is away it has higher solute concentration so basically because of that a concentration gradient is developed and if you look at the so in such systems the precipitate in solid differs in composition from liquid due to which a concentrated concentration gradient is set up in the liquid so what you see is here you have left CS at this point which is yes and this is CL you see the point a now what happens this type of culture now what happens that concentration are the solute in the solid is less and that is why in the immediate vicinity there will be large concentration of the solute and further in the jawed it will basically neutralize so there will be a concentration gradient set up we see the set up of the concentration gradient now because of this what happens so you have so what we see is now because of this concentration gradient you will have the liquidus line behaving like this this is the point so this much portion is poly defied this is the portion which is for unified and the or liquidus line behaves like so earlier it was like this but because of the concentration gradient this lipid up line has shown to behave like this now once your the other Davidic line changes your nucleation will occur according to this line now what happens is suppose this is you are now new liquidus line which is generated because of the changing composition now for this if suppose you have a cooling rate if you name is followed using line one if suppose your quality is maintained like this if you are providing a cooling rate which is represented by line a1 in that case your plane solidity plane front solidification will occur but what happens getting this rate of solidification rate of cooling is not always possible so if suppose you are providing a cooling like this so if Moline is followed by nine – now if willingly followed by line – then what will happen you can experience a throng which is underpinned although this bone is liquid this is a liquid zone this is a solid zone but in the liquid zone itself away from the solid you have a zone which is already under cooled and once you have under control there will be nucleation started in a person which is in the liquid itself basically in ideal case what happens in case of metals your nucleation only has to start at this point I try to go from here but in this case what happens because of this concentration gradient and because of the existing winning mechanism where certain area becomes under cooled and that will result into formation of nucleus or formation of solid phases in the zone from where so what happens so suppose some path – I decide that in that zone from where something has formed now this will try to since it is an isolated mass in the liquid there is no preferred direction of heat flow at that particular point so it will grow in all the directions having certain preferred that a Sun and normally it looks like a dendritic structure so it is a tree like structure or dendrite so Bay will basically known as be known as Ben right now what happens be dendrites are basically formed at a place which is still liquid and if these dendrites are formed all along the points in that case there may be certain joints which are trapped some food or liquids or may be trapped inside the branches or region rights or in the arms of the dendrites and there that soul may be prone to having shrinkage defects so in this case you get a dendritic type of structure so as we see in that case you have a question followed by columnar structure you have a columnar dendritic structure in this case it will go further in this direction but you have a directed type of structure in this piece we have an analytic structure and further what we can see is further if your nucleation suppose so if the nucleus heating rate is like this so when under cooling is furthermore in the case of 3 the under cooling needs further I mean it is quite low enough to show that promotes random nucleation and it we have trains at the center so basically typically in the case of alloys what happens to the near doors you have it the columnar dendritic structure but in the middle in the middle person where the moving road is rated is still true you will have again equate dendritic type of structures because of the random mutilation so what we can see is if we try to five now if the pudding is followed by line two you have formation of dendrite dendritic structure if coulis is followed by line three under cooling is there enough to promote random location and it we asked grace so what will be the resultant microstructure that we can see further so what we see is in case of alloys you have you have usually you have formation of ETH grades on the mold walls and further you have the growth of columnar grains and in the middle portion you have further requests Browns and you have columnar grains from the both sides so so what we see is in case of alloys you you get these are the corner dendritic structures you have requests grains and did you not regress genetic structure typically this is the macro structure for an alloy whereas you get reviews the heterogeneous nucleating agents use of heterogeneous nucleating agent use of heterogeneous nucleating agent will all give you well acquiesced structure whole equiaxed grains so because the hydrogen is remaining agents will further as nucleation centers at different points and if the nucleus or centrioles are more they will inhibit the growth of individual grains so you will have more and more grains and there will be giving you the acquiesced type of grains so this is how you get the different type of bearings in case of metals and alloys thank you [Music] you [Music].
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