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Mechanical properties of steel 13: defects, irradiation

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that will start without them and so we use this this is stocked with vacancies because they're really important of the yen the union among them going into this into much detail that you know that I love the very important for instance for a diffusion and the and there also the predominant .period defects and thermal equilibrium enough in any crystallize material certain steel and you can basically computer if you know the the formation of an antelope appeal both be the vacancy you can you can compute the concentration of vacancies that's the ratio of the vacant lattice places to total lattice of positions knows and it's basically this exponential function so I think you can plug in the values of the people the vacancy formation and the handful of the Boltzmann constant and for the temperature is safe for instance we choose our reheating temperature of steel In many industrial processes around 1250 and get the 15 will take you obtain into the mine itself so it's all very well so there are a lot phenomenon .period defects but in our case we were interested in in vacancies not at equilibrium I didn't know equilibrium state situation so that when you process steals who very often do things like going quenching and for or plastic deformation and when you do this you always generate we went well and dock with a mean many more vacancies than that at room temperature for instance then you anticipate that we haven't access density of vacancies and also said that France is when you when you have these loops sudanese died polls you could have interstitial Bibles and that's the way of creating interstitials aimed at your material but about the amount of vacancies you make by positive about which very much larger than right now and of course diffusion self diffusion in substitution all diffusion in steals is related to the concentration and mobility of these vacancies and so I if you have an access vacancies it will impact of Florence and diffusion but also the mechanical problem you can't is so another thing that's important is looking at a ratio of it migration to formation and told the of the point defect have an idea of its mobility so the gentle of vacancy formation BCC metals is about half the melting temperature divided by thousands you're 1 . easy migration and is related to elastic properties and and so their formulas for that uh shown here and that gives you about .period 57 you if you make the ratio of the . 57 and 1 120 get values .period 3 OK but what it turns out that that's something that's a really low value cancer and it is a good indication To tell you that vacancies are very mobile in Alpha Iris and that means that France when you you reheat you you you re crystallization annealing of stealing you you need to quench the material quenched the material that you will have a large amount of excess vacancies at room temperatures in what we call them 1 of vacancies isolated vacancies and but that doesn't savory long like that they will quickly formed complexes and they will attach themselves to substitution atoms but or interstitial atoms audit will attach the youth to each other they will form the vacancy clusters and basically little empty spaces in your crystal by clustering with about typically 100 vacancies and you can observe the stakes for instance take a steel and you are heated up I and a new quench it really rapidly and you can observe that your structure is full of these but blobs let's say and if you you orient them if you you managed to orient you crystal correctly if you you will in certain directions they these blobs will will actually look like very crystal graphic I have very crystallographic directions stated it will actually be on lying on a wonderful place what what art is is basically clusters of vacancies so if what what you're actually seeing here is the vacancies so this is not whiteness pure lactose I you fall you've got all these vacancies in the lattice that's come together yeah and but they coagulate right so when 1 of the vacancy comes here that means that in an iron atom has replaced the sensors and when all these vacancies come together on a plane in yes therefore a little platelets vacancies and so if I redraw this but now using a dislocation picture yes this is what it looks like it's as if I have an extra half plane coming this way an extra half plane coming that way the between nothing right it's as if I had to something wouldn't vacancy of an end so you form a little circle yes and then this these are the circular he's Ardys circles that you see everywhere here in this structure so did idea you know that there there there real and if you have an excess vacancies but they will they will tend to served concentrations high to form
dislocation loops In addition there's something interesting effects is this is a rather high energy situation and what you would you often see it's that these vacancy loops will have to form on screw dislocations and there will be a number of the process steps that will will not talk about his process but that will end up giving you a screw dislocations that goes like this that form a helical this location and you can see them the presence is very nice 1 the couple Over the years the new form helical dislocation structures as a consequence of this FDI absorption all of many this this'll vacancies on In a screw dislocations it's almost we His right the deaths of 4 of the vacancies of 1 type of .period other types of pointy defects are interstitials use of news the interstitials we care about the hydrogen carbon nitrogen and and bore on boron is a little bit the ambiguous as point defect but let's just say a few things about height agendas of hydrogen is very small at its very best insoluble In Alpha is of and there but it's been a very mobile and hellfire actually creates a lot of problems and we'll talk hopefully about this when we talk about it hydrogen cracking in for Riddick steals them in an in-game irons very different we have a very high solubility for hydrogen and very little mobility and for this occasion that it should be you can dissolve much more hydrogen in gunmen but it doesn't defuse very quickly John it's not surprising because I know Hodgdon being such a small actor the in contrast to carbon and nitrogen which are indeed the tribunal lattice in touristy sees the hydrogen is in so-called peace I sought to track and all sorts of hydrogen lying and practical side and it has a very low activation energy for diffusion is 0 . 0 4 7 so that's that's at the activation energy for carbon is over .period 8 so it's like 20 times last rites as it's extremely rapid defusing the element in elsewhere the right so hydrogen this can be trapped at 2 this location to declare screw dislocations and that it once it's strapped into dislocations 1 of the things we know is that it's really trapped at this location so it doesn't really diffuse rapidly along the core of of the distribution to give you something that's called pipe diffusion doesn't happen with hydrogen markets that to some information about portent of indecision other important interstitial will come back to carbon that is there it was in many times in the course of but so agenda some general things the carbon is located in the tribunal Interstate sees in in both Al-Faran Demerara the solubility of the carbon in Al-Faran is extremely low and the reason is that even in that tribunal to me at all the opted he drawls sites in where you find the carbon where carbon is located it you will have considerable lattice distortion and so when when we put in a carbon atom in the opted he'll sign it pushes it this torts the uptick he dropped its expense the Oxted he drowned in busy direction and it contract sets in that deep in the plane of the doctor he right so although we had nitrogen does the SEC that although we do Anthony an introductory classes on on Steele said the people have a tendency to say Well carbon nite and that behave the same way that's not the strictly true in terms of lattice distortions in terms of diffuse 70 yes but it nitrogen in contrast to carbon will stay in solution much more easily than in carbon carbon has a very high tendency to to form Carbide's like tied nitrogen is much more difficult to get it out of solution instead and that's that's important because of you probably know from undergraduates lectures that Beijing is no traditional problem and 4 steals but it's not carbon aging that's that's the big problem it's nitrogen ages so you need to really need to stabilize the nitrogen and this more difficult at Carver the other thing is common has a really nice properties that it because it's not very soluble in the lattice it will go to grain boundary and when it's in the grain boundaries as it's all you guessed it strengthens the grain boundary cohesion so that's that's so we we don't mind a little bit of solitude carbon in the lot nitrogen doesn't do legend stays pretty much homogeneously distributed in the in the lattice so did the when carbon atom ghosts roam from 1 of 2 huge opposition to the next after he the position 1 that's buys this 1 here between these 2 items here it has to go through but Patrick neutral position and that's a very high energy position and that's what explains yet the
activation energy of . 8 electron volts about .period it the is so special properties at low temperatures it's substitution of but it defuses interstitial I'm interested in and at higher temperatures it becomes an interstitial With a very fast interstitial diffusion so if you do thermal treatments high temperature you take your you still too high temperature goes into socially and when you clincher you get lots of interstitial boron rather than substitution oral which which is the normal position of boron and at 2 temperatures because of it's kind of interesting to know but would it's so we have .period defects in would defects interact with each other and point defects also interact with dislocations so let's say a few things of of this so the steel In the atoms are not really distributed homogeneously there's a lot of point to the fact the associations going on but as the solution is very often not perfectly rendered because vacancies interstitials and substitution of items will will form complex will form pairs or the point defects complexes and so disabled interaction will have an impact because it will friends have an impact on solubility or precipitation kind brands as you did in Example if you have a pure irony carbon alloy is the carbon that in supra saturation will precipitate more rapidly then in the case of an IRA and manganese carbon alloy because in that many because the manganese when its present will form died polls so it will combine it will have various is very strong attractive interaction with carbon yes and that keeps it in that keeps the current in solution that prevents the precipitation formation of copies to friends is a strong 3rd in attractive interest with nitrogen and manganese nitrogen and chrome but as the attributed the can attraction but it can also be repulsion there's this week repulsive at current carbon and chrome and a very strong repulsive interaction between carbon and spends so 1 of the more stable complexes in Alpha arrow is our carbon vacancy complexes have I been binding energy and so several carbon atoms can be associated with a single vacancy and and that's you'll the existence of these and they're very highly mobile they can move very quickly to the letters and this is this association of the carbon vacancy complexes it has been used to explain the enhancement of carbon diffuse a high temperature this can be can change dramatically when you go from Florida to to Austin attic situation so whereas carbon vacancies carbon and vacancies 4 complexes hellfire and they don't in gamma the interaction energies negative and so that's slightly repulsive so very complex things happen with point defects in in steel and it's not very much studied except for people who worry about these things and if you're in the nuclear industry has you generate a lot of point defects in your material and and you worried about what these access the point defects due to your material so this brings me to the interstitials and self interstitials in the you were when you irradiate metals irradiated are all iron alloys for alloys and Martin said Dick Ellis a using nuclear applications you are you form vacancies and you form of interstitial self interstitial atoms as this time the thing is you have to imagine that these nuclear reactors that operate for you're not for a few minutes it operate for years and years to ISO the there is this very slow but steady production of appointee facts and eventually you know it you get observable damage to do the structure of you of your material and of course everybody's very concerned about this and there's lots of work by I the steel research community in the nuclear industry bond .period defects yes it's and the neutrons that they had when they interact with the solid they give cascades of create cascades of vacancies and interstitials and that is an unstable situation you form clusters of interstitials and these clusters are surprisingly enough are extremely mobile and they're like played like clusters that can move very quickly through the latter and discusses will interact with dislocations wood grain boundaries and and and that can have it quite serious effects and we would talk about radiation damage now itself when the when we have a single self interstitial they tend to be of the same way as there interstitial carbon will form a specific crystal line structure as if it if I if I push irony atoms into interstitial position as the In radiation damage you will be able to attend to 4 1 1 0 dumbbells we know 4 1 1 1 the dumbbells on which Ospel so-called Crowder yards because the In the should only be 1 item here there 2 atoms in that space so that the crowd each other up for Kenyan you can see that these substitution all the facts costs Our very high energetic such defects
so have helped this this thing for you have your high-energy neutrons for instance did this they decreed displacement cascades us on the outside of his displacement cascade yes I will form mainly interstitials inside here I will for clusters of vacancies and this that this doesn't stay the stable universities week you will get these are excess vacancies excess interstitials will will form voids or will go to grain boundaries or will be associated with dislocations right and so as I said 1 of 2 ways to easily get rid of vacancies just simply by annealing the material right you just go to high temperature and then you re-establish thermodynamic equilibrium and you cool down slowly of course you cannot do this with a nuclear reactor units and that is that is the big challenge of course and I think we're done for this so I have never few
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list with pencil collide and I'll probably forget the next week or so and so will will be talking a little bit more about Crystal Bustos in will will try to concentrate in that chapter on really the detail the motion of dislocations in infotech steals and in Austin Tex deals and how we can model their behavior became so again
we have slipped systems in Austinite verite we have also denied 1 1 1 planes always 1 1 1 planes no other water and Burgers vector for slept they upon 2 1 1 and can social In the case of the city we can have 1 1 0 planes for 1 1 2 places you see in a moment Y I'm only interested in 1 1 old planes and I pretty much forget about 1 1 2 planes as it during the course in them you'll hear but so you have this would be such and the Burgers factors or 1 1 1 took the Burgers vector it but the In both structures is well-defined this slip plane is a little bit less defined here because we get so much so frequent across the right so the dislocations will give us microscopically the impression that the slick plane is not well defined he tell you what it yeah but at but before before I as legislatures you say something about the slip planes in Alpha to 1st of all In computer Alpha our I'm not talking about Steele's case of don't say that you know it I said something about steel yes but in Alpha are view Dick pure single crystals of Alpha against this sled plane is actually temperature T plant depend and at low temperature in single crystals of of you get 1 1 1 0 slipped when you increase the temperature yes it's 1 1 Tuesday and not 1 1 0 1 1 2 In elsewhere however and of course there there is a domain where both 1 1 0 0 and 1 1 to slip cars and then you so you can get frequent cross on all these plates however aim alloys this in alloys In forensics steals In the low temperature slipped on 1 1 0 planes is extended so we get slipped on 1 1 0 planes in now the fact that the you get so much frequent cross led In the case of BCC means that when you look at it deformed steals look at the graves you never sleeps slipped 1 universities slipped 1 and the reason it is not because you don't have slipped specific well-defined slipped late it simply because of its location cross slip so often so and this process is described as pencil glide and you know what I brought this with me so what what I have here is pencils there now I'm going to I'm going to glider yes glider record so far from your perspective and all the pens have gone this way yeah very very clearly this way didn't go a little bit of editors went like this all of them however if I now make you look in this direction yes and ask you what West Islip well you say well and you know it depends and I would have to know on this here and there is so you'd have a Crockett's like when you would actually look at the stuff like that on a macroscopic level where you're sitting but if I now give you the pants you will see that they're all hexagonal yes and that actually they all got their world gliding on a similar type of 1 1 0 play except not it's not a single 1 right so this happens in Fertik's steals in the C C R & as in gamma are it doesn't happen like this but and the reason why it's it can do this in indecency is because the dislocations can move up and down the change quite like this In there is no cross and this is what happens this location stays also it's a plane and so at the microscopic level we should all be but press kopeck level the slip plane it's very visible but even where you stand there you can see a life right which which had been seen just a moment ago that you can play around with us as a given around the which feeling about the difference but if you take if you know if you ever get a chance to for instance if you
take a the hardness test very simple hardness test on the frantic steel and on most mythic steel the you will see exactly that it is you will be hard pressed to find straight lines In the grains on of next to the hardest indentation when it's a frantic steel however the Austin antics steel will be full of very sharp very sharp and very long lines thank you what basically is a very clear on the expression of the fact that if you have been dissociated dislocations as in most Austin experience you have limited cross slip and you have playing glide rather than wait the Glide which is the case for sir and and and most foreign extremists will give you this again this this is this picture so it could be the Sept direction in these issues very well defined as a 1 1 1 direction in this slip planes are also very well-defined there 1 1 0 direction 1 1 2 1 1 0 planes however this location can cross so often that macroscopic clean it you know the specific blame appears not to be well the fact so it's a case that but my then to its the appearance
OK because in the past people would also on the slick is known not crystal cracked right which which is really a very unfortunate choice of words parent OK so what
what we will do I'm going to stop you produced in the last minute to what will do that now will try to understand you know how much force it takes us to remove dislocations and and and as we go will try we will see that there is no there thermal effects in the motion of dislocations thermal effects and that there had been a far-right and and so important for Riddick steals this terminal offensive very pronounced very very pronounced consequence is that the In the Riddick steals as we decrease the temperature stress needed to make dislocations move increase very strongly very strong and that has impacted on the on many mechanical properties of steel the interesting thing for us is that this the start of this initiation of the increase in interest in the stressed needed to move dislocations in in fair I haven't actually at around room temperature right the many applications which are impacted by this thermal dependence of the year and the year this trust needed to move dislocations in the office and fatigues appear well thank you for your patience thank you also for coming Friday afternoon and Johnson next week I'll be out of town until Wednesday and with the court the class will be on Thursday as usual that would be it
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Metadaten

Formale Metadaten

Titel Mechanical properties of steel 13: defects, irradiation
Serientitel Mechanical properties of steel
Teil 13
Anzahl der Teile 24
Autor Cooman, Bruno C. de
Lizenz CC-Namensnennung 3.0 Unported:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen.
DOI 10.5446/18318
Herausgeber University of Cambridge
Erscheinungsjahr 2013
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Technik
Abstract The 13th in a series of lectures given by Professor Bruno de Cooman of the Graduate Institute of Ferrous Technology, POSTECH, South Korea. Deals with point defects, dislocation loops, irradiation, crystal plasticity.
Schlagwörter The Graduate Institute of Ferrous Technology (GIFT)

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