Physical Metallurgy of Steels - Part 12

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Physical Metallurgy of Steels - Part 12
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A series of 12 lectures on the physical metallurgy of steels by Professor H. K. D. H. Bhadeshia. The final Part 12 deals with the creation of the world's first bulk nanocrystalline metal.
Keywords Bhadeshia, Harshad Kumar Dharamshi Hansraj

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Ch OK I think we can make a start today you don't need to take on any notes everything is in your notes already the Justice enjoy yourself and what's the lecture became so my topic today is there is very exciting is the world's 1st both men and crystallize material that has been invented it for all the research and then oppressed land this the world's 1st and it has been invented using the kind of phase transformation theory that you have so let me begin by asking you a
question in supposing that you were asked to design a volcanic Priceline here and it has to be both for this about me big right you forget big and what does none of Graceland the House Small several centimeters and is 190 meters manifest like OK and it has to be very strong so what is very strongly how from how strong is very strong in your mind 1 OK and it has to be tough that means able to take and impact and absorb energy and it has to be cheap rent so we need to put some more detail on these terms because in a very strong to an aluminium person would mean the end Mike Pascoe that so we need to think a little bit more carefully and when we talk about Bob Dole wasn't looking on an amorphous materials you know team mm size Q is Bob so what do we mean by this this particular photograph joke when I was visiting the always sends mines I Canada and this is the
photograph and can there's a man standing there With Bible I mean in all 3 dimensions so I want to make a material which can be large in all 3 dimensions and immediately that should tell you that I can't make it by rapid cooling for example right what do we mean by men of Chris line well if you look at the literature people are just calling things Crestline which are 190 meters or even more inside that doesn't make sense to me if you ask :colon you know a journalist than their level of carbon nanotubes the government gives look something like this and they are of the order of 40 50 69 those in which began the we've got to make steel which has a finer scale than government use and have achieved what would you consider cheap the reports command How much would you charge call a cheap steel of achievement here OK let me let me make that bit more interesting so how much is this the cost a thousand 1 yeah that is it a thousand won 101 a case so Is it 101 thousand 1 :colon tell me I have never bought because I drink from the tap less human you know just this year 1 thousand whatever is cost you are wasting money -minus because the water in Korea is perfectly OK yeah so you are able to waste money on this subject to transported by long distance it I don't know if this is from you are not yet but what you transporting water in plastic bottles over long distances so you can afford to tour the smart money so we've got to make material cheaper rate by wait for 1 year by volume then bottled water because that's what I call a cheap material so if you've got palladium union that's no good but this so there's your bottled water we are going to make element is cheaper weight for weight or William because you know the density of water 1 Grandpa sent me to give it doesn't matter whether you consider William him already then this how do we go about this
well 1st of all I'm going to talk about very very large numbers so I want to explain to you what 1 Pasco means threat you did talk about 1 gigabyte skull and what is the meaning of 1 basket but the rate of 1 apple is 1 Newton's roughly yet the apple that fell Newton its rate is about 1 mutant Udinese's of 4 so that the unit of great but would it on 1 square meter man that's 1 Pasco so when day means I'm putting a billion apples on 1 square meter that is the incredible strength that you're asking for when you asking for 1 being is 1 million apples on 1 square meter with I will coffee with larger numbers than that so you know when you explained to your parents that you're working assuming your parents are not metallurgy assumes that you're working on a steel which is 1 the capacity strong this is the way of explain that it can support the weight of 1 billion apples for 1 square meter now Back in
1956 we were able to make iron which has a strand of Tangier bicycles with puerile nothing added it pending a possible and the reason why you can make it so strong is that as you take a material and you cut it down in size the chances of finding a defect becomes smaller therefore the material becomes stronger because as you know In metals and in many materials he said defects which determine the proper but if you cut it down sufficiently small then you will it become incredibly strong and that is why when you take a glass fiber you can do this for the rest you take that window glass you cannot do it breaks but because the probability of finding defects in a large number of glass is much more than a small object so you can see that if you try to achieve strands by making the objects smaller and smaller then the strand to collapse as you make it bigger so here for example just 15 micrometres size Crystal appliance the strength goes back to something that we recognize as 1 the best theoretically could go to 22 the apostles now that means that you're not doing your job properly because still thinking about wanting a Apasco strands but theoretically we could get to 20 to be a possibility eliminate the defects in material so I remember aim for 22 bicycles right this is
commercially available still it's made by Kobe Steel in Japan which is the strength of 5 and a half the oppressed during the 5 and a half billion apples on 1 square meter and it is so that you can see there's necking here and you can die and not with it now you cannot die and not with carbon fiber which is fast-closing strength because what you 0 that but with iron you know we have a really good properties and he went at the strength of 5 and a half ago fast boats you can enormous debt so can we make a bridge from this In the cables of Britain's friends well you know in order to get this trend you have to deform its severely so that monitor strain that you put in the material it if I take 56 grams of iron and stretch it out into 2 kilometers that's the amount of strain you have to put in but when I stretch of this 56 crammed into 2 kilometers going to become very very thin so it's incredibly expensive to make a bridge from this material because it with so many of these together that it's just too expensive but the advantage of getting stand by introducing defects that this is the opposite of the previous life here we're introducing defects the advantage is that it
becomes insensitive decides this is just to show you an atom problem age .period represents an active and you can see the huge intensity of defects that have been introduced in the material adopt regions of basically dislocation cell cells but it's insensitive
decisiveness in the previous grafted actually of the single crystals registrant collapses because we need perfection to achieve strengths here there sighs of the strand is insensitive to and the commercial name for this insightful which means scientific iron the Cypher but the problem is that whenever the producer material by severe deformation it's going to be I cross-section or is a very thin sheets and so on to just illustrate do you know the size of Cypher yeah do you know the units by which define thread for making your clothes what is the common units a year by stockings such None of you know it's a really crazy unit of French unit the French are strange people so here we are
this is the unit by which you buy clothes then yeah so as as you buy smaller and smaller Danielle your clothes get finer and finer and women's stockings tendon and that means that is awaiting grams of 9 kilometers of fiber multiplied by 10 and men's socks up 50 Daniel so we are making I which is finer than the threatened the stockings that is not going to be able to make bridges from it but you can use it for example for cutting semiconductors you don't want to waste materials materially thing OK let's say carry on so far we have
unsuccessful introducing them while Continental Chris Lang steel now this is a carbon nanotubes and ,comma nanotubes are catalyzed to grow on iron particles and this some years ago this caused enormous excitement and huge amounts of money was spent because uh properties of this which were being reported were incredible so for example
a strength of 130 Giguere 130 billion apples on 1 square meter and the modulus along the land it is 1 . 2 Deripaska's means a thousand media right that this is about 6 times that of steel so people went crazy and started proposing to build an elevated to space this is an old Russian idea because you have to have a role which is 200 thousand kilometers long and therefore most Mattias cannot support their own weight but if it is amended to which you can make 20 thousand kilometers long then principle You could make it if this trend is 130 year rascals know what is wrong with this idea this is a you this trend has been measured to be 130 media passed both foreign and the trust me and even Masai invested 17 million dollars what is wrong with this as soon as you make it big the Strand collapses we knew that four-iron in 1956 yeah so it's very simple theory which you we have covered already in your
undergraduate course that the free energy of any introduced defects into material it increases the fringes and is the number of defects and that she is the and will be open but as soon as you also put it fact you increase the number of ways in which you can arrange Adams sentiment here that means increase the entropy and that favors the formation of a defect so there's a balance between these 2 and you get an equilibrium number of defects here which depends on the energy of forming the defect but also on the number of you have a new material you make it larger than the probability of finding that defect you change so isn't that you can make a better manufacturing process you will always have defects as it's a thermodynamic equilibrium number of defects that's why you're able to defuse inside the solid because you haven't equilibrium number of vacancies that may consider the effect so there is no there's no
carbon nanotubes role which is bigger than 2 mm that has probably is better than steel and I wrote a paper about this and I sent it to Nasser people have not forgotten about creating a space elevator from carbon nanotubes it so if we aim for perfection the
aim for perfection then you will not be able to scale-up because thermodynamics tell you you will have an equilibrium number of defects and defects reduces stress unless you put in a very large concentration of defects which then again raises the strength but compared with the idea of strength defect reduces stress and if you try to produce trend by the formation then you will limit the form of the material but they said in a thin wire or a sheet except that you will limit the form those who want to make something that's big in all 3 dimensions right but this has so far been unsuccessful now this is an
incredible story we reach happened before you were actually born sometime in 1960 where the nature of Steelers changed dramatically again led to an enormous improvement in the quality of life and that's micro it because you know when we reduced the grain size we get better properties we get better strand and we get better toughness and what you do is you add small concentrations of Nairobi for example a new material so that during the formation the grains opinion by the copyrights and therefore you get much better properties suggests a very very small edition of elements like neither you and the properties of steel changed dramatically there are now about 24 billion tons of my prelates In service throughout the world without people realizing because it's such a good technology it's not like your operating system of your computer that year to update all the time and pressures and so on it is only when things like that happen you need to worry about the technology so the best technology Israel nobody notices it and this 24 billion tons of stuff serving the world right now grain size refinement is the best way you increased what strength and toughness so I want to ask the question what is the smallest grain size that I could achieve by thermal mechanical processing and lessee again in the steel industry is performing well well when refined the grains you're creating self-restraint and this is the amount of surface but volume and that scares with 1 the grain size so as your brain size becomes smaller the amount of surface by unit volume increases by multiply that by the end facial energy then that's the cost of creating interfaces yeah there unhappy with that so I'll have to do is balance that cast against the driving force for transformation From Austinite prefer and I can theoretically calculate the minimum grain size possible
can so there will be a day is simply taking the driving force for the transformation from last nite firm which you can calculate to easily and balancing it against the cost of creating interfaces In the process we destroy Austinite Austin bond with you so you end up with a relationship that the minimum fired grain size that you achieved but the inversely proportional to the driving force for transformation the fight just make them equal then I can find the minimum grain size and here is a plot the this
represents the under cooling below the equilibrium temperature and you can see that in principle they could go to . 0 1 micrometres by filmic and processing not if you take data from many many publications and steel company records you and you plug them on this we are actually start at about 1 mile from the so again we year 2 orders of magnitude out In performances from looking at if if at all possible before the case is that will look at the processing has a hard time reaching 1 micrometres there is theoretically we could get to point or 1 of my committee that there's a lot of potential for improvement and problem the reason why we have that discrepancy is something called recon lessons in Riga lessons means that when transformation happened it releases heat so the steel heats up again In other words you move 2 words this point the director transformer at a large on the cooling and the heat of transmission to heats up this to by itself so you don't achieve the large and cooling so when we take account of reconnaissance which which is very easy to calculate this again Tokyo transmission and is the heat capacity you can demonstrate that you're are not going to get much below 1 micrometres if you're trying to make large amounts of steel thick steel because you need a mechanism of removing heat yeah the ruling uniformly from U.S. Steel or you need a mechanism of reducing debt H B and Tokyo transmission that so we are unlikely do you get a medal steel by tho mechanical processing because of the heat of transformation so
tell mechanical processing is limited by recon lessons you don't worry about the terminology is basically the heat of transmission warming up to steal again so we've got to find a mechanism for storing that heat inside the material because to release it from tech materials is difficult OK so what we
need is a mechanism of storing the heat of transformation we can also reduce the rate of transmission because that means that the heated to release more gently and at this time to despair and of course transforming at a large and the cooling is important because that leads us to a smaller grain size right again
now there's another problem which was highlighted by all the work which was going on on and Priceline and that is that as you reduce the grain size the ductibility vanishes yeah the doctor later drops you very rapidly get a plastic instability so here we have a grain size of . 2 of my and you can see that immediately get some plaster stated there an instability yeah and that is no good so you get your increment in Stratton but you lose your and the reason why this happens and that as you
get to smaller and smaller grain size is the amount of material which is inside the bond increases inside the bonding of news backing so what happens is that dislocation simply go into the boundary and I lost left with keen grains without dislocations now why is that important well in order to get work hardening you need to have dislocation and directions from if you lost earlier dislocations annual grain boundaries you do not get look at me if you do not get work hardening you get plastic instability right so we've got to do so 1 more thing and that
is to introduce a look hardening mechanism inside of material any ideas we talked about this in the last election How can we increase the work hardening capacity this should have been said the tip of your tongue that means media How can increase the work capacity the transformation in history that 3 need to introduce some retained Austinite into of material OK let's see how to do this well that we are the only way that I can think of storing heat inside the material is due exploitation the shape
deformation data transmission yet the shape deformation stores a lot of strain energy in Mikhail here we talked about 600 700 Jews from all right so we need to use displays of transmission that produces the and change and transformation so the agony of
familiar with it if we use the reaction then the cupboard is rejected from the plate shortly after the growth and that :colon enriches the Austinite and we can cut the reaction here before you get precipitation by adding what we can stop the reaction here by adding water to over here Silicon Aluminium yet so that's no problem we can introduce Austinite and innovate cheap way because we are stabilizing the US nite by problems but we need to do we need to make the
scale final that this is 1 micrometres that Nowhere in the plates are about a quarter-mile from that's not non-crystalline Frank so I want to produce this day at a low temperature what is the lowest temperature at which I can produce banal so yeah is limited by the modern site again but but I can also depressed amassed so what is the lowest temperature do you think at which I can produce Spain just take a Gaston worried what's a reasonable temperatures rejected Tuesday nite about room temperature no within 200 degrees centigrade OK let's let's imagine it's in the hundreds of degrees centigrade but we don't know right but we haven't asked this question before but we have all the theory to calculate the transformation temperatures so here are some
example calculations again you can just download a computer program from our website and you can do all these calculations so obviously we're limited by a massive we've got to depress both Hamas and the brain I start temperature and that's not always possible with all allies have to select you Alamance properly again this computer programs to help you do this but the theory we covered in elections but this is calendar so this dashed line is room temperature not according to this I could produce nite at room temperature if I make my steel with 1 . 4 weight per cent carbon I could make the nite at room temperature rise this is just 1 more calculation I need to show you which is how fast it should be
so it would take about 100 years To produced a nite at room temperature now if you in the wine industry yeah you would make wine and you give it for 100 years and sell it for a expensive cost so we could make no large amount of steel and story and then sell at a very big price after a hundred years you have to use your imagination yeah yet so Apple Computer's sell very well because of the heavy imagination is no reason why we can't make such material and keep it for 100 years and then selling but at the moment we want a little bit faster again so let's go all about 108 % we should take a few days to transform at about 200 degrees centigrade so even a few days you know is bizarre to people from industry but if you produce a material which is justified note that can get excellent properties it's no problem In the trading for a few days OK so he has a
designed ally which has won by 8 % governed the silicon is there to stop seem intent prospection we have some elements for ability because we want stop alike and so forth and molybdenum here is important because when we make commercial steers you will have impurities and things like phosphorus a very important when you go to extremely strong jails because the segregated the OS 9 grain boundaries and you get written as an effect of molybdenum used to kill that so you don't get for used in Britain so it's a very simple ally and the heat
treatment is very simple as well you Austin prices again and then you transform at a low temperature 4 hours over 4 months let's see now the escaped that
slide but the mike structures very simple is a mixture of the far-right and common industry data last nite depending on what temperature you form the structure you get a different fraction of us tonight and this
is what it looks like in an article microgram the white regions there are little regions of Austin and this is your brain on now there is nothing special in this image it's really beautiful to look at yes depending on what you call beauty yeah I find it's beautiful but it's not unusual you see this kind of a structure but the next micrograph assure you might shock you but that they could be breath so I'm going to show you a transmission electron micrograph but the important thing do not from the article Mike represent this is isotropic it is no alignment here but when we go to a very fine scale and we're looking at just a very tiny region it looks like everything is pointing in 1 direction so it's important to do what optical and high-resolution microscopic look
this is carbon nanotubes so magnification and these are the plates all they need to ferret this is 29 200 Arabs the 29 leaders take there and these other regions of Austinite and this is present in a large John or steel yeah literally unwilling my hands like this ever show your images but this is truly a bald man line steel now let me just go over limited further are the transformation to 10 days at 200 degrees centigrade so what temperature data cockpit yeah it's approximately 220 degrees centigrade yet so it's not expensive just make a big beats the steely their a slow-moving Bell fall 10 days
and the properties are very good eye you can get stand up to 2 and ago past those and the toughness up to something like 40 megabytes code would meters you here is
the hardness this is harder than many might addictive the 7 and 10 because and the scale of the structures 22 14 readers remember that when you have a complaint the mean free slipped distances twice the thickness in other words we are not concerned with the length of the plate but with the thickness of the blade OK so it's a it's a wonderful material and if
I show you what happens To the elongation and they're very my William fraction of Austin you can see the donation can go from 7 per cent to something like 27 percent with an almost all of it is uniform Musonda slides that the slides at the rushed past that you know this is
almost flat trade that means most of the longest in his but they're
not my question is why are we getting such a large variation in England vision as a function of the Austinite content remember that transformation plasticity did in the last lecture makes a very minor contribution to the longest we calculated something of the order of end so we cannot explain these results by transformation plasticity by itself so
here is a plot all the strain failure that means the longer against a fraction of Austinite and you scientists like straight line to the throne of state line through there and there's no other reason checking if I extrapolate the straight line he doesn't go to 0 right so that indicates to me that there is a critical element of Austin I don't need after which the material fractures so if I start 30 % than it a take longer strain before each reach a critical value and then the material break so why do we have this critical value so this is
another way of looking at the same thing but we start with this amount of tonight and as podium on divorce decreasing and now a lot on this the points at which fracture of and you can see approximately 10 per cent of Austinite all the samples break and this is a
case where we are monitoring the Austinite as people the samples using neutron diffraction and again you can see that . 1 sent of . 1 % retained last nite the sample breaks so I can control my elongation by controlling the starting amount of arsenic but I need some kind of explanation for this critical value now this
is a diagram in which he imagined that the blue region is the tonight and the white region is far and he had said the amount of Austinite is beyond what we call the percolation threshold that means I can draw a line through the blue region continuously disorder forest and I started a fire at a new low corner it would easily spread through all the trees because they're touching each other so this is about a book election threshold and in
contrast here I cannot grow continues line to the red regions so if a fire started at 1 of those streets it would not spread similarly as I transformed my Austinite and I'll use continuity that means that the cracks can simply probably 8 to the modern site regions which have formed from the Austin so as soon as we lose the population To the Austinite which is a relatively ductile face him then the material fractures so it's very interesting which Austinite controls deductibility of material I need of course through this year and the 30 for population
is very well-established because you know 1 of 1 of the users of this series to control forest fires now you've got to put breaks in and so on so all objects are shaped like plates another with spheroids of some sort of late approval it's that steroids and the theory allows you to predict the the population threshold and this is 1 of 1 of the book collection thresholds of what 10 percent of Austinite corresponds to this point here and it predicts 40 aspect ratios of plates that read observe that the population Treasuries at about 10 per cent of Austin so it's consistent and that is what is controlling the fracture of his very fine now when new
strain rate sensitivity the the standard encourages dogmatically to stand he got Moscoso a deviation from the straight line indicates past history so here effectively we are getting yielding at 10 gigabytes so at high strain rates the spending increases dramatically so what can I make out of this given that it's very hard to get and if you wanted even half it had a much faster rate then it gets stronger what can I make so yeah I mean I don't know how many of you have done military service His most of you so you would be In a protected hopefully report with deals which resist ballistic impact yet this is perfect for that this is a
picture I received today or fracture production of the material you and this is made into armor now I cannot
comment on what kind of object with fire at this to do the testing but it is a big enough gun that you have to put it behind a Jeep right and you can see that it resists the ballistic impact of this is this is commercially available now right and manufactured on you know thousands of tons None this application does not require any rally I cannot well this material is got about 1 vapors and covered in red and you know that as you increase the :colon you get might incite well and that's a bad thing this high :colon so we cannot make things reach large structures which require joining by welding yet so that's a limitation what other things could you make which don't require well what in your car doesn't require welding forget the body here because it has to be rather than what he knew about doesn't require reading How many of you have but almost all of what components don't require welding but carrying stresses I what inside engineer right "quotation mark shop about gears bearings so let me show you look at this is
just an apple but dimension from this particular material as battle what we call a ballistic mass efficiency that means the ability to defeat the projector compared with ordinary armor taking account of density so it outperforms and you may not take any malaise and the conventional or remember taking account of density if and you Anakin not take more than 1 shot this can multiple shop Standard much EPA right now this is
a small aircraft relatively speaking but it is this this was a picture I took 2 thousand and when was 9 11 2002 I and when I was taking this picture of someone came to me and said Why are you taking the picture that had so I explained look you know I want to show the size of the engine look at the lorry here and look at the size of the engine aircraft engines and the current engines are even bigger if you look at the at 380 is bigger could have 2 people standing on top of each other inside against it and the reason for that it is the
local "quotation mark fan blades and the 2nd error and most of their goes from outside it doesn't go to the engine but is providing the trust and that is which travels outside the engine also shields the noise From the engine so for civil aircraft it's important to share the noise but if you make a military jet it would fit inside this engine military jets of very small and therefore they're very noisy OK but the real point is that this needs to get bigger and bigger and therefore the the shaft here which is made from steel has to be able to tolerate much dork and at the same time 1 of these blades breaks it's a huge momentum right on my website you can find a video showing you what happens when 1 of these blades break it's like the momentum of 3 cars so the shaft has to bend in plastic to accommodate the imbalance but that's quite important that you cannot have just material which you fracture it has to be able to ban plastic me so that you can shut the engine down safely right and this sort a nite has blessed history and is strong so the the shot
France and there he treated
1st Boston and then you put it into sold 5 at
200 degrees centigrade but I need to keep it there for 10 days and is too expensive so you take it out from there and stick it into an oven at 200 degrees centigrade but these are large objects that's truly broke "quotation mark metallurgy Colonel Chris Lehane material and you get a
uniform structure 2 because it's a slow transformation big advantage of a slow transformation is uniform by the time the transmission starts which takes about a day or 2 the material is at a homogeneous temperature so you don't have the quenching stresses for example which is very important in components like this in years bearings and charts you do not really want quenching stresses to be locked into your material that so just to summarize area this is the material and it's
very strong up to 2 and a half ago Posco 700 because hardness it has uniformed Dr. and it does
not require any defamation to produce a structure it's a phase transformation so we don't need rapid cooling we saw that promote big shot from my heart for an innocent man is transported and put into a solid pop and then into an hour or so we don't lock in a system of residual stresses
and it's very cheap and uniform large sections soviet gone are in the lab. We're going to 80 mm but that is the shops that he saw a much bigger than 80 mm so I don't actually know when we would have a problem with it Fred this
salty slide that assured you earlier where I explained they would take 100 years do yet they if I increase the :colon concentration camp so we made such an ally and we made it in 2004
here it is and if you have a go at the London you go to the Science Museum you've ever seen the steel so polished it completely flat so what would happen if transformation happens would the sample change it's very nice try so you would get displacements which you can see so you don't want to verify because of the experiment started in 2000 and 4 and it will be finished in 2 thousand 104 if it happens before then I'll be upset because it would prove that the wrong yeah but what you have to do you see is I won't be around at that time so you have maybe maybe I hope you are but you have to spread the story you tell your children that go to London and observed this a check whether the transmission of happened but it's the best homework we regret it and I thank you very much that the end of schools the trick


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