Merken

# Mechanical properties of steel 24: polycrystal strain hardening

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00:02

but the last part of the course it's about using them more than 1 phase 2 obtained interesting steel properties that I want to remind you of the this graph here as I was looking at it I can and in life that you really want to go back in change isn't signal you're just strain and here just us and that was it that kind of more consistent the left-hand side with the right hands and my when he wouldn't showed us is that in general and it's not only for steel many materials metals and alloys on we get this kind of stress-strain cover we have a starting the value of the strain hardening an intermediate value and a saturation value and and we know that we can understand this basically by thinking about strain hardening as a consequence of dislocation density pollution and that's when you reach saturation here you have basically a a point where the of dislocations to generate is balanced by a process called dynamic recovery means you basically annihilate the the dislocations in the microscope when you get that you cannot accumulate dislocations anymore so the strain hardening is finished and so it indeed a few plotted the strain hardening in general but it'll always go down and it always go down and that was why we so interested in this strain hardening well because of this .period here this this point here where the strain hardening is legal to distressed yes yes deployed where we I have the maximum strength and the maximum uniform along age and we want to get this point we want to increase their support we want to increase said and beyond this value of the strength of the US and we will and we want to increase it Beyond this value Of the strain hardening so we need to have ways to increase the strain hardening of it and it's very difficult Foucault's Yukon you cannot really do this by adding by adding solutes you cannot do it by adding by reducing the grain size if you act precipitates saying things in general which you actually find out is that but when you reduce the grain size we've already seen as you actually have collapse of plasticity you know so you get lots of strength that you don't have plasticity and that's not that's interesting but for for many applications then and the same had holds 4 of precipitates we have lots of precipitates in a solid you will tend to see a reduction in the form of a letter OK so and so on there is a need to go into alternative concepts and this is what

04:21

I will do good to be talking about now this has How do we go about doing this and these Mike

04:33

rastructure role please

04:37

strength and steals so

04:44

so what we do you it's up to achieve this with 1 of the things you can do is then inserted as hot 2nd phase in the MicroStrategy soul and so it seems a little bit interesting that you'd be able to improve plasticity by adding something hard and the microscope shows because you would think if I put something hard and Mike restricted can make a Have a composite affected make ran stronger but not necessarily accumulate more dislocation and get more strain hardening of you Yukon have make use of something that's really interesting in steals and that it's up to now not that much exploited in many of the situation is the fact that still goes through a phase transformations and you can use this to control the MicroStrategy became can also use it to control mechanical properties test and that's that and that's also available him and and not that many of metals and alloys which have that possibility so you can have either the matrix or part of the matrix undergoing stress or strain and use information 1 of 2 things that steal this and ferrous alloys can also do is under good information twinning produce that phenomenon and you can do it but you can make a bulk alloy that undergoes stepped phenomena yes or but you can have a dispersed face that undergoes of the deformation .period so it's very interesting and you have lots of of of possibilities with the possibilities are endless limitless however and that's been 1 of the main reasons why this this is not being exploited as much as it could be exploited is it's it's requires a very much a deeper understanding of what's going on in the MicroStrategy so and of being 1 of the last what I call the plasticity enhancing mechanism because you don't only have to control the strengthening his effects but also the transformation effects and then which will see that there is a complex interaction between the phases also that you have to take into account so

07:29

having said this multifarious material spot was actually very common this is constructional steel introduced on on Tuesday on it contains . 2 % Carbone the carbon is not present as common is present as a Carbide and that it's actually present in all its almost entirely in this black blackface here this higher magnification and if you look at it in SUM you can see this nice alternating fringe pattern and of course you all know that this is Pearline so that's where the uh the carbon is its present as Carbide and as the Mela states and so when when we add carbon stews 2 of the IRA and carbon allied with which you'll see is that the as you increase the amount of carbon right you increase the empirical from a 10th of a per cent of carbon when 4 . 5 per cent . 8 cent of carbon you gradually increase the amount of Earl of perlite constituents in the microphone so thank you can already see what the defect is stuffed adding this phase is what you need to UTS there a affair writers from around the what depend on how much it's alloyed but know 100 200 and MPH so as you add to this the the perlite and you have something like a prolific steel or a hyper-real attacked city is steel or ultra high carbon steel you see that you can achieve strength levels which go you know which are just absolutely fabulous look so that's interesting you can already see that and and you know that you know we're getting close to you know what people agree is about the theoretical strength of of steel side was of a Jedi divided by 13 G divided by 15 depends on how you approach but it's new it's extremely strong material yeah mentioning interesting about this phenomenon and on about this perlite and then in particular the fact that we have for Lamela

10:16

perlite took it it's really important so so let's have a look here at the the basic simple idea if I have pro-life I have basically 2 phases and what does this remind you off of a composer no it's actually very nice composite material and so you know that what what you want to understand the behavior of the composite I need to have a 1st look at alpha phase 2 far-right anteater data is not the way to the carbide is another way of saying that some entire and other world this is the this is a type of course they bear stress-strain curve of pure tied actually if you have 1 please give it to me it's it's impossible to get by and people have tried to make Simone tight and then stressed that and try to measure what is the yield .period but does it have any longer agent at room temperature so we don't really know anyway so but we can guess what it might look like if we were able to measure it is very hard and it just breaks before it heals so where we think it probably looks like something like this where the yield piled around 3 million Pascal if not higher and then we have the Führer the ferrets here there you go very soft until so we know that's that will whenever the properties will be offered of perlite it's got to be somewhere in between us and between the 2 and we also know know that we have that From theory of composites that are there will be what's called stress and strain partitioning has and that I am also a few things about this in a moment you will have very high stresses in the the hard phase in the Pearl and the Simone died very low stresses in the far but the far-right will do the defamation OK so this brings 2 the did the idea of course and if if you have material that no 1 phrase doesn't deform at all that and the other 1 can deform and so as soon as you do you apply the information the very strong phase will will probably break very you will fracture and that's what you see if you have small type particles in the Microsoft truck and when you do tensile test small Simon tied particles not her like that most of you will see that these particles break ordered the laminar anyway they're good there the cause of much trouble in terms of plasticity however if you do the same thing with Lamela perlite no problems this human tide in the land of her life can deform plastic so that is it's not only the strength of the separate phases that's important also the phase morphology in any case so let's let's have a look here of took typical Lamela perlite of course doesn't have is is a composite writers and this doesn't have the stress-strain curves it's of the Jose has stressed drinkers which you 2 of work around a thousand the maker pastor collect like this 1 here so right so now if we look at verite perlite know I have I'm not 100 per cent for life but I also have fair idea grains yes around and here in order for a Lagrange has so far have but to constituents material consisting of perlite since and far-right I can also consider this to be a composite of this would be the stress-strain curve of the the perlite and which has just said the the UTS of power close to a thousand mega Pascal and for the fair so again and the stress-strain curve I will get will depend now all these phase fractions the face fraction of perlite interface fraction of affair so if I had increased the volume fraction of polite from 22 2 43 to 65 the stress-strain curve of the steel that I get will gradually moved to the stress-strain curve of her life and always with the same of composite a fact that there is stress and strain partitioning the the hard perlite phase is under more stress yes much larger stresses that the far-right that the far-right phase undergoes more strains and the polite OK so and this in constructional steals debts to track that people use to make different rates may basically add more or less carbon carbon of course it's not the carbon that makes anything as it's actually not even the Simone tightened as the it's the Perloff effects that that good so I add more polite I get stronger constructional Steve nets and you can see I can make very soft constructional stealing only very hard work yes I have a very wide range of possibilities here and if you look at this you'd say Well boys are you know perlite steals we're already over 1 Major Pascal of strength why don't we use products to overseas it's cheap and no problems right we're happy in terms of what I've been telling you in in this course the trouble areas 1st of all the amount of strange you can get yesterday the semen tied does to form but it doesn't deformed at March 10 per cent of no less you know 10 15 per cent that's about the limits so the dissonant I'd will even the malaise will eventually break but so that's 1 thing you cannot really make panels carved panels with them but it's not the form abilities of but even then there many applications where you don't need to have pressed for the material did that the other thing is the fact that you have to add carbon and that puts the limitations on welding for instance has a that's particularly the reason why this solution is good as long as you don't have to Weld as long as you don't have to do any forming etc. so it's it's the application of possibilities of far-right provides steals or not that wide in construction areas very often of certainly in building construction not talking about like ships for instance where you do need to consider amount of welding of the variety perlite steals or not the good thing so and so 1st of all let's say let me say a few things about a few more things about this the perlite it's found a the Lamela perlite has 1 very interesting thing is that you can refine it you can into the reduced in July analysts pacing and then the material that's even stronger and there plastic deformation certainly when this plastic deformation involved compression units compression and this is quite good so what you see here and this is for the squares here for a prolific purely politics deals now and so on and this is for a hyper you take toward steel . 5 . 8 you can see that if I can make the Inter Lamela spacing smaller and I so smaller it to landless facing is in this direction I can increase I can easily so friends 400 too close to 600 and MPA in yield strength yes and from 800 to 1 thousand of maker Pascal annual strings and just just by refining the microscope and and I do

19:59

want to show you this very interesting phenomena about type behavior so again it's impossible to measure the properties of Simone tied the bulk of it has no blessed this city in Belgrade but if you look at and for instance this is a lot perlite Le malice here and you can see here is that there is a whole region of Miller Lemelin that has has been sheared yes and there is no fracture no the lamination anywhere and serious another very nice sharing situation no fracture no the lamination and these are no again you can see here the deformation of the the Lemelle as the breaking of the Demelis but there is no but even with the minimalist is broken medicinal and in In void formation or anything so very interesting and unique properties in terms of plastic deformation in Europe so that are there other applications and steals which are worth it talking about hot weather the Indiana in the stainless area this was a very nice representative area multi-phase deal that's the so-called duplex stainless steel which might restrict rests here shown here it's it's really might restrict you can see have alpha phase and gamma rays that has 50 per cent of its frantic 50 per cent as Boston dig at room temperature so whereas far-right per perlite steals or at the very cheap and of the spectrum yes not expensive this material is at the most expensive end of the spectrum of Steele's Nos but it contains huge amounts of chromium and nickel to achieve this MicroStrategy at room temperature you can't have and it's used mainly in forward demanding applications in the in the chemical industry where you require strength corrosion

22:24

resistance and it's could strengthen corrosion resistance basic but now How does the material like that be behave answer what was you know well let's have a look so that we can this in this example I show how this material behaves at high temperature for instance when you hear all the material deformity at high temperature so would we find out is that this the stress-strain curve at high temperature for Austinite and this is distressed Rinker for fairer the strains here are huge this is missus 400 per cent and wife can can I show you these very high amounts of stray that's because it's the results come from a portion test and when you do a torsion test you can you can have extremely high strains because there is in principle no section reduction so there is no instability and why does this this the stress-strain distress drinkers decrease here as because at high temperature you get dynamic readers .period station repent so don't worry too much about these 2 details but this is the Austinite faces in this case you can actually make the Austinite phase involved and you can make the far-right phase involved and you can measured their 2 properties in using very different the also very hard comparison to this the the fair items and resource and a duplex this has you expect somewhere in between and so on this was analyzed by 1 of my grad students in the past and this is what he found was the distribution of the strain and distress during the testimony was so if you measure if you're here for instance In the duplex steel yes actually you have this much stress and strain in the Austinite phase and this much stress and strain in the far-right so again you have this what is called stressed and strain partitioning the In recent years there's been a flurry of interest in this indecent materials and there's a lot of new ideas and this is just an example here of what would this currently already I being produced in the industry of complex space steals the peace deals will talk more about this two-day trip steals will also talk more about this today which you can multi-phase here complex pastries probably the most complex it contains variety they 98 retained all state and Martin's I altogether and we won't talk about this will talk mainly today about variety Martin signed the peace deal you will face seals and the low-carbon trips states can character so let's have a look at this as the peace deal so I have right and then Martin side this is white ireland's here or more so I have a a soft face and an extremely hard things so schematically the right here Martin side here and when I take a stress-strain curve of their duplex stainless and to touring the dual face steel this will be a point on my stress-strain so there too it is no more may know from study of composites area like true extreme cases of stress and strain partitioning will you can assume that stress is the same in both phases or you can assume that the strains are the same in both phases we you have an equal stress situation equal strange situation In theory and practice of course the situation is somewhere in between is you don't have this point or mode that point you have it .period somewhere in between here if you have to materials and you have you know they're stress-strain curves yes and you want to make a MicroStrategy containing these 2 phases and what do you do if you want to have an idea of what the composite what how the composite will behave and how .period B on the composite stress-strain curve actually corresponds to point a and point C on the the stress-strain curve of the this gives more insight and fairer well you can use them to Morris rule of mixtures and very simple and it says that this is the only important parameter it has been amount both each face press the volume fraction of these lands if I know the volume fraction of Martin side here by metal or graffiti on then I know 1 minus the volume fraction of Martin side and it's the volume fraction of the fair because in that case with this world of mixture I but I take .period B and I draw the a line through it yes line through it which give me . 8 and points see this line has a slope Q which is given by the ratio of the difference In the flow stresses of verizon Martin side and the strains a corresponding strains in the far right in the March inside there and you can see the rot breezily derived this Q slope from this equation here look so basically if if you have this stressed drinker that certain you can calculate your mixture using a rule of mixture this this rule of section of very simple and you can do it In practice runs and it's been done here indeed have misses the mark aside this is the stress-strain curve mathematical form of the stress-strain curve of the Marcos Edison mathematical form for the variety stress-strain curve and then you have measurements and you can see that indeed for every point here I have corresponding stress and strain In the not interference OK now there's a little problem and you probably can't you can see here this slope here changes and of course if the phrase fraction doesn't change so this rule of mixture it's a good empirical start to analyze the the stress and strain partitioning in multi-phase steals but it really doesn't capture fall Everything the reason why doesn't capture everything is because there is an interaction between those cases in this rule here while you just assume there's no interaction between nothing happens basically at the interface between both of content the having

30:31

said this but using this will of Mitchell will get you far In practice you can see for instance you this is experimental measurement for you will face steel cold-rolled grades hot grades the tensile strength as a function of the amount of Martin site of volume percentage in this case of Martin and concedes it's is nice linear relation right so I if you have no better batteries a theory available you can always use to the rule of mixture it's an empirical also but if you can help you do some modeling but leaving off I too late when you look at these Microsoft truck shows what can you say about the roots of a compare ultra-low carbon steel they will face England the trips to do what we can say how much sir right to do what I had here have 100 % here typically have and so far 15 10 to 15 per cent of Martin side so at 85 per cent of variety in this case the amount of far-right is decreased to 50 % because there have been nights in the Microsoft truck the carbon abundance is very low here the carbon here is all Is that I have to have had more carbon because I need to that make the Martin side face a common is all in the Martin side face and in the case of trade I also have to add a little bit more poverty but the carbon is all in the day-night faced with something we don't see and my crystal and that's the fact that they're actually internal stresses the internal stresses in the in the Microsoft truck trip which is the result of how we make the MicroStrategy so let's have a look Bryant so if

32:40

this would be the peace deal so let's let's for instance goal here and take a sample and sample for instance and have a look at the the Microsoft drafted this

32:52

is to switch again because of this affair right brain then you have these black things surrounding it that's Martin said so you can already see a number of interesting things but it looks like being in the center of the grain here I don't have as many black lines as in the rest of the Grand Slams this year for instance also because of the in the evening these black lines of you already know dislocations that tend to be very very much around these Martin side His conceded the areas and these are what we call punched out dislocations there due to the formation of them Arkansas when you make this this business models like you go you have into critical annealing and you quench yes and the Austinite high-temperature also will transform to Martins and 2 things happen these the volume changes Bay about 4 per cent so there is plastic deformation this plastic deformation that causes these dislocations here this location in homogeneous disagreed and the other thing is of course the elastic part of the volume of strain it remains in the microscopic so around these and uh Martin site islands there is the soul of compressor stresses internal compressive stresses and all of these have an impact on your properties so what do I mean

34:42

this is just schematically what I just said so you have the you have far-right matrix in its you have Boston confirmed at high temperatures you quench this and this also denied turns into Martin signed with the let's look at what happens is 1st of all indeed uh the Austinite Wilshire the lattice Wilshire there will be a sheer transfer because all Snyder's was meant to far-right transformations markets like translucent issue share transformation of the met certain amount of sheer and then there's also volume expansion in the density atomic density in the Allston items and the market such different so I have expansion so as a consequence inside the Martin sign I get transformation dislocations and around the if the Martin site particles I've had I get dislocations which are due to the strike and on top of that there is a around every particle there is a compressive zone yes as a result of the volume strain associated with the transformation look and and and this shows you Chris 1 graphic detail to this would be origin only these are these planes here are 1 1 1 planes as these 1 1 1 planes are sheared us the new you form this this intermediate structure it's not a correct BCC structure it has to expand a little bit and then shuffle shuffle around a little bit so that the on uncle's become correct 10 this so there is a share and some expansion was on the main the formations that so we have this particle is just sitting there In the MicroStrategy and it's surrounded by mobile dislocation and it's this also compresses stress all them but there's something else now let's deformed this material has less deformed the material that's a former aide a jewel face steel which contains a variety here I'm here this very hard face Martin son of a prime focus to what happens when we before there are plenty you generate dislocations you have dislocation loops yes which shown this prestige the dislocations here With the same but opposite Burgers vectors which will move apart away from each other this hour internal sources of dislocations and let's assume now that this little square here yes the forest as easily as as the Matrix nets so as applied to share here that particle will be shared with all leadership however if this particle says I'm not performing I don't care I don't before then the dislocations you see the dislocations that the on this side In the dislocation of the on this side of the particle which should normally shares through the Martin side In Good to the other side yes they're just stuck there there so they don't believe the interface they just stuck there so hard phases also do something about this locations at the interfaces so In other words this this non deformable particle this this Martin side of but ends up with the big envelope of additional this locations around this news and as a consequence if I compare this situation With this situation and I say well what's a different 1 this case by accumulated dislocations so I Q accumulation of dislocations remember strain hardening OK but so can we

39:41

put numbers on this is the way to 2 see how this works yes we can so let's have a look at the same situation let's try to describe In a simple model the what what happens because what we're going to describe and is so this is what we call for the creation of geometrically necessary dislocations the ending "quotation mark the geometrically necessary because you see in a moment if they weren't there there be lots of holes in my constructors because we have cute should assigned it's here this is the markings signed the schools and this is 1 inside crime against and this is my fair right brain here so and I have a little cube and that the volume is fell to the 3rd and is now at the dozen before so now I'm going to strain this intent tensile stress that those tensile strength in the strain is absent for it's OK to what should happen to this Q normally it's length should become L times the strain therefore have 10 per cent of the longer Asian the should become 10 per cent longer right so it should be 1 . 1 house wants to strain is .period 1 out because it is so let's let's not do this because it doesn't deform yes so it should fill this this volume so let's assume I have have to strain on the left and have to stream on the right to here I should have an increase in volume on both sides both epsilon times divided by 2 it's not doing this so but that and the Matrix is doing so what's happening while there is a void is there Of course there is no void there is no void yes and that we would do is we basically fill this void yes wit this dislocation of the you know that if I have to dislocations that this it's like adding an extra extra material right that's the kind of this locations I put it dislocation loops the kind of fill up this board and that's basically where the name comes from geometrically necessary wicket with those that were come from what well enough the necessary because if they weren't there you know that we know as soon as you start performing but the dual Cezil and this should be full of holes but there there are and so on there are necessary loops are created to avoid but it and epsilon is the externally applied strict so we have the the the the the the the increase in length here is epsilon times L and the number of dislocation loops and I need all all of them have the weight of the equal to the burger Specter so if if I divide is by I have number of loops that I create part particle there now if I want to know the and the yes as if if I had if I if I if I have now the volume fraction of ass Of these particles of these Martin sideboard have ever been then I can calculate the number of Martin side islands per unit volume you can so and them His then times fell to the 3rd round because that's the number reunion fall you and it is this is their volume so N number of the MarketSite islands per unit volume is S divided by L 2 the 3rd but if so so now and the number of loops that I will generate per unit volume from this of the little density is equal to the number of loops per particle times the density of particles so if I do this I find epsilon times the volume fraction of Martin's I divided by B times L. L being the size of the market inside and well in this particular case there is no blank is for our way this is for come from 1 to you have to have a closed-loop Wrightsville 1 1 2 3 4 times so for Elvis waited for help comes from so the the density of geometrically necessary dislocations I multiplied as with 4 L 4 l times the number of the density and then I get this equation here yes for Epsilon SM divided by the 2 times that has better next resources of dislocations that is created in the far because of the presence Of the Martin yes so I have by adding these modified particles and enhanced the density of dislocations in the fairer and in other words I have increased the strain hardening right now the so what

45:57

I did here was for a tensile situation there is some the original work by Ashby who introduces concept of geometrically necessary this location and new arrived at Formula Ford this year the situation and instead of having a factor of 4 here for Epsilon he has

46:23

8 times Gamma Gamma being the sheer strength OK but it's basically looks the same and so you did the big day important parameter is of course f over the years the density of these particles divided by their size and here the same density divided by the side of him this if I take this parameter to

46:52

be a certain value 1 the reciprocal microns for instance and I calculate the dislocation density geometrically necessary dislocation and say I find a very very rapid increase at small strength yes and so the 8 the impact of small particles hot particles in the Microsoft rupture and is 2 increase the strain hardening at lower strengths it so do will face deals have been studied in it radically in detail and you have there a formalist for the strain hardening yes the strain hardening 4 of the peace deals yes and no so all of these have agreed basically with this this theory that geometrically necessary dislocation must be present because and the strain hardening clearly shows that dependence on this square Road where this is where we come from because the you get to the square root of the dislocation density and if you calculate the string harder than that strengthening effect of dislocations so instead of having passed over the indeed for well you get square root Of this parameter as the strengthening factor but this idea of having a heart phase was developed why why wasn't developed while the In early dual face steel or any Mike restored the contains a very hot particle the increase in the strain hardening happens in the early stages you so it's the impact all at highest ratings it's not so important so what do you do if you want to have if you want to benefit From this hard 2nd phase at later stages you basically want to have a market side at the end not that to begin at the beginning you want you don't really want Martin side because you have high have already high strain hardening but you'd like to have it the end and that's what happens in trips to entrenched steals we gradually generate marked we slowly generated and we pushed back it's but With the increase we develop an increase in the volume fraction of Martin's as we strain material so basically this with Hampton trip still in the trip steal you have Austinite right just like in the case of Martin site you had most before you made Martin and now we're going to strain the material and let the Allston that gradually transform too Martin said yes and every time that happens you get the big amount this locally you have relatively soft Austinite transforms to really hard Martin site has and it expects so you get what you get you get this volume change gives you this location the fact that you have Martin side there you get geometrically necessary dislocations suddenly and then and and um and thirdly you replace the soft market side would very hard but as the 2nd phase it's very hard so you get the harder phase and then an expansion to any time there is they locally b tendency to enact yes it's stops the tendency to enact worlds will be automatically stopped and the late because you get we surprise snacking using this market To the

51:58

string induced by and now you you come into this very Kumble complex the additional the physical mental achieved in the steals is that right how do you manage this and in this transformation argue engineers steel that does this and that's politically the challenge if you want to buy use this phenomenon took to 1st of all you are all familiar little it's a temperature scale here temperatures it and you are all familiar with TMS temperature of Austin right and you think that's the only temperature Martin site transformation temperature therein well it isn't what you have hamas you have MS Sigma you have can you even have what's called and the 30 here somewhere and you have no less than read or 4 temperatures that characterized the transformation behavior of Austin the DMS temperature you familiar with that's the 1 where you create thermal Martin site if you quench the this temperature between MSN Sigma you conform stressed assisted Martin side and you stressed that the Austinite as it is no plastic deformation of the Austin distress is high enough the Martin cycle for In the Austin once you go Over MS signal yes you get strain induced Martin side that means you deformed the material you pass the yields .period and the Martin site only forms after yielding that's white weak said straining used because the Austinite isn't the form while it transports and if you increase the temperature what happens at high temperature stacking fault energy increases very much and you need stacking falls to generate Martin has and above a certain temperature there will be no Martin site for and this matter how much you strain you always get this location so if you want to have so hold of understand and control this phenomenon of this trip a fact you need to know what is this to rein the penance Of the Martin site following fracture and what is the temperature dependence of this phenomena because it's no good having a trip steel at 100 deg C when this the materials can be used at room temperature 4 people and so you need to look at the McCain kinetics so here you have a percentage of transformed damage In both grants but on the left we look at an Austin epic stainless steel there are stainless-steel which undergo this transformation and their homogeneous yes and you have no carbon uh trips deals which are multi-phase deal in and where this phenomenon takes place in the indeed the United States the nite contains some retained Austin campus but the phenomenon is the same This is the transformation range at minus 40 degrees C of the the 301 you get very fast transformation at 0 degree the kinetics unless and at 80 degrees you can see it's already a very low and the same with 2 strips at 20 deg C you have 92 per cent transforms at 100 degrees C transformation saturates at around 45 you can selectively look for instance at 301 and you you look at the maximum percentage of gamma that's years you see that at around 80 degrees yes but there is no more transformation so this this allows me to the term ending the MD tentatively means beyond this temperature you'll never be able to 4 the Martin side by deformation In the definition of altered the way you determined the Andes 30 temperature His by looking at the transformation kinetics the Austinite at different temperatures different temperatures and then determined at which 1 at which temperature you get 50 per cent of transformation for a strain of 30 % that temperature at which that occurs is called the MD 30 temperature and it's technologically the very important parameters look at now let's look at what this what effect does this transformation has but it will look at this true stressed to restrain an curve here of this matter stable Boston attic steel and what you see is that as we string the material and we measure the amount of Martin site we mountain models site increasing we see that at the same time this slope of this line it's not very clear but the slope of this line hasn't upward curvature instead of having a slogan goes like this it has a slope has upward curvature so in other words if you measure the end value near the end value as a function of strange instead of being a maximum value of about 2 . 2 as you have for frantic steals or . 4 as you have for Austin antics steals it continues to increase and goes up to . 6 so yes trip fact gets me a very large increases in the In both strength and plasticity the reason is because of the strain hardening industry strain hardening itself comes from and that by this multi-phase Mike restricted we've added more the capacity of the material to store dislocated right and just seeing it in a couple of minutes overtime but if you have a class to go to your welcome to go otherwise I'd like to stay here forever around and just go to the rest of the material Eleven of what should be given to

59:34

people so just a few words about the strain induced Martin side so when that's the that's the range that the temperature range at which the the trip effect a curse between MS Sigma and Andy and there's uh the reason why this the uh temperature range is most important is because in that temperature range you generate a lot of nuclei for Arkansas additional nuclei from Arkansas and they come for instance from intersection of slid bonds or intersections of epsilon Martin side plates for special dislocation configurations as an example here where you have at the intersection of slip bands epsilon Martin you you form in addition to the new of Martin side completion this was found this year that this was a structure for a homogeneous Boston Steal This is the structure of many nights in a low-carbon trip steel and so yet this year France's typically day-night the white stuff that you see here that's retained Boston and that's where the trip effect comes from the small retained Austinite islands here the 3 of them in this 1 and this 1 has but when you strain the material they will transform to side and that gives you the trip effect and the trip this fact it is a function of temperature and the amount of Martin site you make as a function of is a function of temperature high temperature you suppress that was and is also a function of the thermodynamics both the the Austinite face this be the thermodynamic stability of the Austin I think so the composition of the Austin III has an influence is considered different composition years here as silicon is partially replaced by aluminum here it's fully replaced by aluminum when you consider as I replaced the silicon by aluminum in this MicroStrategy like suppressed the transformation 2 and again a tramp steel a low-carbon trip still is being composite material consisting of a nite and far-right so that we can make these phases separately and analyze them because the debate 930 strong phase the far-right is again a soft matrix ways the trip Steel has a stress-strain curve in between both I guess answer and again we debt load transfer stress and strain partitioning etc. In recent years I just wanted to finish with this there's been a lot of interest in a defamation 20 if you the increase manganese content in the Austinite phase you can get a steal to undergo defamation 20 minutes training material and it forms To and and there we get a the hardening effect which is a bit different and we usually refer to it as a dynamic all patch effect but it basically does in that phenomena is sigh well but if you strain material and you have a mechanism whereby you gradually reduce the grain size that will have an impact all b the strength with the gradual In fact you don't have to start with tiny graves you will end with tiny grain so give lots of strength and you don't get the negative impact of small grain sizes on the plasticity why are twins interesting is because that they function as Greenbaum very strong grain boundaries so at the beginning of a few wins and mine this locations have large mean free paths and I'm sorry increase the density of wins the mean free path of the dislocation become smaller and smaller in the extremely small and so I'll get a dynamic hold patches the crystallography of the winning is very similar art-lovers this has some relation To the crystallography of Martin site formation of waterside transformation so in these systems alloy systems with manganese you you can observe 20 behavior and also tripped behavior depending on the composition of this is a typical trip still has 22 manganese . 6 Carver stacking fault energy of 20 military per square millimeter is needed to 2 get this behavior and you can model this very nicely do you remember that so be because we have these interfaces that we can accumulate dislocations the nicely and we use this dislocation evolution equation that we know that we saw in a previously and we basically change it a little bit we add terms yes that is very similar to weigh the the term that we need between useful or grain size and but in this case it's the size of the twins and that this is a dynamic as this will change would strain so if I do it can integrate this equation I can simply put it take the square root of this and then I get this basically my stress-strain curve ball so this so hot this is the equation that the describing the the effect of the twins tends to be in the distance between 2 twins this a 20 this is a small win here is another small twin here in the equivalent to the grain size of this twinned MicroStrategy is indeed 20 it's the distance between 2 twins the twins themselves around fence he has never thickness so far and that the relation between the date that the kinetics are described by the distance between 2 twins is 2 times the thickness times this factor here 1 minus after divided by half Epsilon and f Absalon gives me the amount of twins as a function of string again this very room relatively simple to to integrate this equation taking into account the change in density of twins and if you do this this is what you get to the system this if this is the right curves shows you what happens if you have no twins you know 20 drinkers shows you that what happens if the the MicroStrategy fully 20 there's you can see here very nice upward curvature and strain hardening in which you have would have in reality is this rule curve ordered black from being model in Blacker being data has where are you see that only partial the volume fraction of of the volume of the material doesn't fully transformed too doesn't fully you go through defamation 20 for certain reasons but what is very important and that's like closing thing here is that the the kind Maddox of twinning is important right so I can hear it in the simulations I can reduce this is the twinning fraction as a function of strain I can reduce the twinning kind antics and when I do this I can't say I see here that I can also to reduce the the strain hardening very nicely and this is the stress strain did the strain hardening as a function of strains to normally this curve should go like this right because of this the defamation twinning you see that and this upward the behavior of the stress-strain curve you can see this here you get to facts you get harder material hardest astrologer and also you delay the the onset of necking and basically you and announced the plasticity all right I think this is less of a concern and so we come to the end of the course it was the 1st time his new cost and I thought that this semester I hope that my idea and a teaching this course was a gift people who were and still research number of maybe better all 4 of you idea of you know where strength comes from and you know what is that this will be the dislocations and stress-strain curves how it how do they relate in practice and does so that was basically my in my home that I've achieved some of and and and that you will not perhaps use this type of approach In the end the research work you do OK thank you very much for your attention and your patience .period

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### Metadaten

#### Formale Metadaten

Titel | Mechanical properties of steel 24: polycrystal strain hardening |

Serientitel | Mechanical properties of steel |

Teil | 24 |

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 | 10.5446/18294 |

Herausgeber | University of Cambridge |

Erscheinungsjahr | 2013 |

Sprache | Englisch |

#### Inhaltliche Metadaten

Fachgebiet | Technik |

Abstract | The 24th in a series of lectures given by Professor Bruno de Cooman of the Graduate Institute of Ferrous Technology, POSTECH, South Korea. Deals with the strain hardening of polycrystalline steel. |

Schlagwörter | The Graduate Institute of Ferrous Technology (GIFT) |