Atoms in bainite, atomic mechanisms: APMS conference
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Transcript: English(auto-generated)
00:30
Okay, so the first talk is going to be given by Francisca Caballero, who is from the National Center for Metallurgical Research in Madrid, in Spain, and she's going to be entertaining
00:41
us with a talk about the distribution of atoms in nanostructure bainite. So, Francisca. Hi, good afternoon. The work I'm about to present is our collaboration between the Spanish National Center for Metallurgical Research and the Ourin National Laboratory. And in this work, what we have done is just to track carbon distribution during bainite
01:03
reaction at atomic scale using atom-proof tomography. I believe that most of you know that there has been, since the discovery of bainite, there has been much discussion on the mechanism that controls this transformation.
01:22
If you check early literature, you can find at least two very different explanations of how these reactions take place. You can read that bainite transformation is a displacive transformation. That's a displacive theory. That the state of bainite is ferrite form by share, and that the transformation is
01:44
essentially martensitic in nature. That means that individual atoms will not move less than one interatomic spacing during the reaction. In the literature for the same years, at the same time, you can read the opposite
02:00
explanation, bainitic bainite transformation is a record-structured transformation, and the transformation takes place by movement of thermal-activated atoms, and that bainitic ferrite grows by the movement of growth length on broad
02:23
faces of the interface. However, today, I believe it's generally affected that bainite transformation is a displacive transformation, and that occurs since experimental evidence on the invariant
02:45
plane strain-to-fail relief effect were provided by Professor Vadicja using atomic force microscopy. However, displacive transformation does not always imply diffusion-less transformation,
03:02
and nowadays the discussion is focused on the role of carbon during the reaction, on the role of carbon on the bainitic ferrite growth process. And if you read literature nowadays, you can find, again, two different explanations. You can read that bainitic ferrite growth supersaturated in carbon, and after that
03:27
plate of bainitic ferrite is formed, the carbon will partition into the austenite or can precipitate inside the bainitic ferrite plate at lower temperature, forming what we know lower bainite, that diffusion-less explanation.
03:43
But nowadays, you can also read that the transformation, the bainitic ferrite growth is a carbon diffusion control process, and it's completely the same transformation, the same type of transformation that we must have in ferrite. And bainitic ferrite growth is carbon diffusion control, and if we have precipitation during
04:05
the reaction, cementite will precipitate on the austenite ferrite interface at the same time that it's moving. What do we have to do to check what is the process that is taking place during bainitic
04:22
ferrite growth? What we need to do is just to study, investigate that very early moment of the transformation when we have that very first bainitic ferrite plate, and to measure the carbon content in that plate. If the carbon content corresponds to the carbon in the parent austenite, then we have
04:44
for sure a diffusion-less process. If the carbon content is much lower and corresponds to that given by the perikilibrium, then the growth for sure will be a carbon diffusion control growth. But unfortunately, and you can understand why from this very simple calculation of
05:03
carbon diffusion, for the temperatures of which bainite is 4, that can be between 400, 450 degrees C, the time needed to fully de-carbonate that very first bainitic ferrite plate will be less than a second. Then we cannot investigate from an experimental point of view that very early moment.
05:27
What have we been doing all these decades? Instead, we have looked at the carbon content in that residual austenite when the transformation had finished. And that's what we call the incomplete reaction phenomenon, and it's an indirect
05:43
validation of the diffusion-less nature of the transformation. What we do is just to measure, for instance, by x-ray analysis, the carbon content in the retained austenite. And if that carbon content, when the transformation has finished, follow the thermodynamics limit, this AE3 line, then we can state that the growth was
06:06
during the transformation carbon diffusion control. But if in the state the carbon content is much lower and follow what we call the T-O limit, then we can state that the growth was diffusion-less.
06:20
And that's why, what that T-O line means, the T-O line, what it means is that when the transformation has stopped and we have that carbon content in a balance of energy, of free energy, this is free energy, and this is temperature, on a balance of free energy, that means that beyond this
06:41
point, transfer austenite to bainite ferrite of the same composition by the diffusion-less process will be forbidden according to thermodynamics, because we will increase the energy power system, and all we know that that's not possible. Then, even though we have lower amount of carbon in the retained austenite than
07:00
the equilibrium, even though we have not approached the equilibrium and we still have retained austenite, the transformation will stop. That's what we call the incomplete reaction phenomenon, and we have been invalidated the bainite ferrite growth process looking at this phenomenon. And on the right, you can have, for instance, an experimental test where we
07:22
look at the bi-x-ray analysis and the incomplete reaction phenomenon and the T-O line. And you can see, we measure the austenite carbon content with the transformation of the units after the austenite decomposition of a medium carbon high silicon steel at different temperatures. And it's quite clear that when we transfer, when we decompose austenite in
07:44
the bainite region between BS and MS, the carbon content, when the transformation has been complete, follow the T-O line. And over BS, when we transfer to with mustatin ferrite, and we still have it in austenite, and we make sure that avoid precipitation of
08:00
the mentite, it follow the peri-quillibrium value. This is a indirect verification of the diffusion-less nature of the transformation, but it still insists that, goodness, that it will be really nice to be able to see and to measure the carbon super-saturation in the
08:23
bainite ferrite during the reaction when the transformation is taking place. And we thought that a slow transformation kinetics, that it cannot be very sexy for industry, it can be really, really nice to solve this fundamental problem.
08:42
And here you have ensured that you have already heard about the development of a high-carbon, high-silicon steel that when transfer at 200 degrees C, evolve the austenite decomposition, is a nanostructure mixture of bainite ferrite and ruthenite austenite.
09:01
And the, this is still a way to, so much interesting industry because of the mechanical properties. You will hear in this conference much more about that, more, more details. But for me it was really interesting because, in my opinion, solved the fundamental problem that we have.
09:21
And you can see, talking about, as I told you, that this is a very low kinetics process. You can see here on the left, some kinetics data. We are measuring here the evolution of the different phases as a function of time for this high-carbon, high-silicon steel transforming at 200 degrees C. And the transformation will take place between two and six days.
09:45
Then, I believe that we will have time to look at how that bainite ferrite is decarburizing when the transformation is taking place. That's what we did. And the first thing we did is to use x-ray analysis.
10:00
And you see here, in green, you see the evolution of the phases. That's the, how the transformation progress. In blue, you see that carbon enrichment in the retainer, austenite. And in red, you see the carbon content in the bainite ferrite as a function of time. Again, we, we validate here that, the incomplete reaction phenomenon.
10:24
And it's quite clear that when we reach the TO value after 150 hours, beyond that point, we don't get additional bainite ferrite formation. And we don't get additional carbon enrichment. Then the transformation has stopped.
10:41
But we were not able to monitor the corresponding decarburization of that bainite ferrite. At this point, we knew that x-ray analysis is not the right technique to look at the carbon super saturation in the bainite ferrite. And that's because with x-ray analysis, always we have average values of
11:03
the carbon content in the, in phases, in the retainer, austenite, or in the bainite ferrite. And if we have some local carbon enrichment in our bainite ferrite plates, we will catch that carbon and all water measurement. Those measurements does not correspond to carbon and solid solution in our bainite ferrite.
11:23
That's how, that's how we approach atom proof tomography. We need a technique that allows us to determine the carbon content locally at the nanoscale. And a way of any possible carbon enrichment regions.
11:44
And here you have a nice example of a needle-shaped atom proof sample. With this technique we are able to reconstruct in three dimensions the position of different atoms.
12:00
Here you can see the carbon map, carbon distribution map. And every point corresponds to a carbon atom. But we can have the same information for all the substitution and solute. The big region on the right is a high carbon region, that's retinostinate. And the low carbon region in the left, that's bainite ferrite.
12:23
And here, here in this example we already have those carbon enriched regions in the bainite ferrite close to interface. This is a austenite bainite ferrite interface. What is interesting is that we also can quantify
12:41
the level of carbon in solid solution in the different phases. And for this particular case that correspond to a high carbon, high silicon steel, transferred at 200 degrees C for 10 days. That's after completion of transformation. The level of carbon in the retinostinate is comparable to that given by x-ray analysis and the T-O value. And the level of carbon, a way of those carbon enriched
13:03
regions in the bainite ferrite is lower than that given by x-ray analysis, but still higher than that given by the parecilibrium. Then with this technique, we were able to see that carbon super saturation in the bainite ferrite.
13:21
Here you see the results as a function of time. And this is atom proof tomography results. You see again the evolution of the phases with time. The blue points again correspond to the carbon enrichment of the retinostinate. It's quite clear here that we have a very wide aero bars.
13:40
And we will come later, what is the reason for that? Aero bars for any time correspond to different atom proof samples that have been analyzed. And it seems that we have a huge dispersion of data for a given treatment. But, and it is more clear on the right where I change the scale on the y-axis, that it's quite clear
14:04
that this time, with this technique, we can see the decarbonization of the bainite ferrite. And the carbon super saturation in the bainite ferrite is evident during the whole bainite reaction. Of course, we were not able to see and validate
14:22
that very early moment with a fully carbon super saturation in the bainite ferrite, with the carbon content of the parenostenite, because we are at 200 degrees C where carbon is still can move. But we are aware that there's low kinetics for bainite.
14:43
It has been always a traditional argument for that diffusion explanation and theory. And it makes sense, because we cannot explain how the carbon can be trapped inside the bainite ferrites.
15:00
The interface is moving so slowly that it's hard to understand. For this reason, what we did is to perform the same carbon content determinations for three very different steels. A medium carbon, low silicon steel that transferred to upper bainite with intralas cementite precipitation
15:24
during bainite reaction, transferred to lower bainite with inter and intralas precipitation at the same time that the bainite reaction is taking place. Medium carbon, high silicon steel that are higher temperatures transferred to carbide bainite, and the lower temperatures, we have
15:41
inter and intralas precipitation during the bainite reaction, and an additional temperature for the high carbon, high silicon steel. The three steels have very different kinetics, and we are able to track that carbon super saturation in the bainite ferrite, a wide range of transformation temperatures.
16:00
And you can see here the results. Those results correspond for the end of the transformation. And it was quite evident that for transformation temperatures below 375 or 350 degrees C, even when the reaction has finished, we still can detect and observe the carbon super saturation in the bainite ferrite.
16:24
The carbon content in the bainite ferrite for higher temperatures approach already the equilibrium. But what is interesting is that the tendency for the carbon super saturation the bainite ferrite function of temperature is quite similar for higher and lower temperatures,
16:41
it's a continuous behavior. And it's independent of the if we have or not precipitation during that bainite reaction at the same time. In my opinion, what we have here is enough experimental evidence that bainite ferrite growth super saturated in carbon.
17:01
But when we transform this still at higher transformation temperatures, all the secondary processes that are controlled by carbon diffusion are activated. And what processes are they? We have to investigate those processes at the atomic scale as well. First of all, is that carbon partitioning
17:22
from the bainite ferrite into the retained austenite. And to, I can investigate that, we were able to detect that in bainite microstructure, and these are samples correspond to the high carbon, high silicon steel,
17:40
transfer a very low temperature, but it's not exclusive of this very sophisticated steel. It happened also for soot micron carbide free bainite that different sides of retained austenite trap very different amount of carbon content. And blocky austenite, they have much lower carbon content
18:01
that's micron or nanoscale fields of retained austenite. And that's really beautiful for over microstructure because we know that with carbon, we can mechanically stabilize over austenite. And that's a way with a wide range of size of austenite. And a wide range of carbon content,
18:21
we are able to control and to have a progressive trip effect that allow us to enhance the utility and toughness in these type of steels. But look to this transmission electron micrographs because it's quite clear that close to the ferrite
18:41
austenite interface in bainite structure, we have a high dislocation density. And when the carbon is moving from the center of the bainite ferrite into the retained austenite, you will find this free space. And it's not a strain what we found by the corresponding atom-proof tomography.
19:01
That is that carbon will segregate on those dislocation. And think about that. We have an extra strengthening in over microstructure through cultural atmospheres. And finally, depending of the transformation temperature and the precipitation kinetics,
19:21
maybe Fahrenheit can fall before that carbon will escape from that bainite ferrite. In that case, we will have interlock precipitation in the ferrite and what we have, what we all know by lower bainite. I believe that at this moment,
19:41
we have plenty of experimental evidence that we can state that bainite formation is displacive and diffusionless in nature.
20:03
Thank you very much, Francisco. Very, very entertaining talk as well. Thank you very much. Throw it open to the floor, as we have done so far today. Is there any questions? Yep. You talked about dislocations accumulating
20:21
in the retained austenite. I wanted to ask, do you know, is there any work being done where the slip can transmit from the austenite back into the ferrite? Are those interfaces opaque or transparent to glide? Yeah, those dislocations are generated by the plastic accommodation of a retained austenite
20:40
during bainite reaction. But if we think in the crystallography, a match between overbainite ferrite and a retained austenite, we have some possible planes, matches that if it can give us the idea
21:01
that those dislocation can be heritage and transferred from the retained austenite to the bainite ferrite. Francisco, you showed a very nice graph
21:21
with the carbon in ferrite going very low, continuously going up to all the way to C bar, probably, finally, when it reaches the martensitic transformation. So there was a work long time back then on the coupled diffusion and displacive growth. Can we develop a model rather than calling bainite ferrite and martensite as a continuous flux going from C to B?
21:41
Okay, this is a chemical analysis, a carbon in a bainite ferrite. We don't have crystallographic information in atom-proof tomography, but nowadays there are investigations on what is that crystallography of that ferrite that is trapping so much carbon. That's one approach to the question
22:02
that you said why so much carbon. And another approach that I think that we couldn't forget is that when we see the homogeneous distribution of carbon in the bainite ferrite by atom-proof tomography, it looks like it's in solid solution. And I don't doubt that, but we still have also vacancies
22:21
as in a martensitic still. And if I have a vacancy in a bainite region, I ensure that carbon will be comfortable as well, but I'm not going to notice. When it is in these locations, we can notice. But when it is in the vacancy, it's not. Then I'm thinking on different systems,
22:42
BCT, FCC, but why not BCT, FCT, vacancy system to explain that level of carbon in nowhere structure. There is no tetragonal D. There is no what? Tetragonal D. Harry has been investigating that and it's not evident so far.
23:01
Calculations can be an explanation, but the experimental evidence are not conclusive yet. First of all, your original question. So there's no evidence for continuity. Otherwise, there's nothing to stop the reaction from proceeding to the para-equilibrium curve and the evidence for tetragonality is inscriptor,
23:22
experimental evidence. You were not very clear in your conclusion.
23:48
Enrichment of a carbon atoms at dislocations. Does it involve diffusion, diffusion, diffusion of carbon? But your conclusion is that we have diffusion.
24:01
Yeah, yeah, you were right. That since we have dislocation at the interface, carbon partitioning into a retained austenite, into the residual austenite is not as high as where we can expect. And it's like how much carbon can transfer?
24:21
It will be lower. So enrichment of carbon, it needs a diffusion. And another question. Oh yeah, yeah, but that's secondary process. I understand your point. And another question is formation of a nucleus of carbide.
24:42
It's also need a jumper of carbon atoms. It's impossible. Okay, we have seen in the bainitic structures, if we have seen the carbon super-saturated in the bainitic ferrite, that enriched retained austenite at dislocations with the carbon.
25:01
If we have a cementite precipitation, inter-lactite precipitation, that is evident by TEN or atom-proof tomography. But in terms of nanoclusters or carbon cluster, what we have seen is that with longer agents, much later than the transformation has complete, or during a subsequent temporary,
25:22
those carbon enriched dislocation evolve in carbon clusters. And those carbon clusters, we believe are the perfect nucleation plate for epsilon carbide. Thank you very much. Very nice presentation and clear.
25:40
I have just a question. You show in the analysis that for thinner austenite grains, you have more carbon than for thicker. Do you have any comment about that? Yeah, it's a question that how far the retained austenite is on the progression of the reaction.
26:00
Then those two micron nanoscale fields are between bainitic ferrite plates and blocks are between sieves and can be observed by light optical micrographs. But we have a maximum volume fraction of bainite that can be formed. According to the incomplete reaction phenomena
26:22
and those block would be that remainder austenite. Yeah, lovely work. The idea of high silicon is to suppress cementite formation. And you showed a nice image of a very small cementite particle. Is it surrounded by an enriched area of silicon?
26:44
Did the silicon actually have to diffuse out of the way at 200 degrees C to allow the cementite to form? Okay, I believe that this particular case, you can see that the silicon, we can consider is homogeneously distributed and is trapped inside the cementite particle.
27:01
It's a parakilibrium cementite particle. In the last precipitate. We have some question from the guy from internet. Sujai Shaka asked, what is your thought for low carbon steel where the bainite transformation
27:22
happened at higher temperature, especially for upper bainite? Do you still believe that upper bainite is a displacement in nature? Okay, I agree that the carbon super saturation study, we didn't go lower in carbon content
27:42
at 0.3 weight percent. And I believe it can be much more complex because we have a higher variety of morphologies and everything. But I believe what Professor Jam saw us today, granular bainite, it can be applied like bainite
28:01
and it can be formed by the same mechanism. I agree that we don't have experimental evidence now on the table. But I believe we have a wide range of temperatures and we can think that we will have the bainite ferrite growth by the fusionless process
28:22
and other secondary processes will be anticipated even earlier and earlier the higher the transformation temperature. I believe that there is no different types of bainite. It's just bainite. Thank you. Another question.
28:40
Your work is based on mixture of bainitic ferrite and austenite. What about classic bainite? This mean a mixture of ferrite bainite and carbide. Do you observe the same enrichment of ferrite? Yeah, that's the reason why we test the medium carbon low silicon steel
29:01
which is a conventional bainitic steel. That was formed between 375 degrees C and 525 degrees C. And you have in this slide, I don't know if we can point out what is light. We have two examples of transformation for that steel, 500 and 375.
29:23
500 is a conventional upper bainite and 375 is a conventional lower bainite. And carbon super saturation was not detected because this cement type precipitation was carbon super saturation correspond to those blue points but it's still 13 carbon super saturation for 375.
29:45
But of course for higher transformation temperatures all these precipitation processes take the carbon away of the carbon bainite ferrite. And we cannot detect because it's already precipitated.
30:00
Okay, we just about have time for one more quick question. So. Yeah, thanks. So at the end of the presentation you mentioned about the crystallography of the ferrite. So this is rated with plasticity or it's related with carbon segregation to its interface. Because if it's a carbon segregation that's playing a role then maybe you should take a look at the interface.
30:22
And then the question is, how are you going to do it by APT? Okay, the carbon in bainite ferrite I think that we have evidence by that tomography that is homogeneously distributed. There is not carbon segregation as a interface as a carbon peak as a interface.
30:41
What we have for the interface and that can make very difficult that crystallography analysis is that we have as a interface a high dislocation density. And for high resolution TN it can be really hard to determine the actual structure.
31:02
A part of the carbon super saturation we create also distortion in that bainite ferrite. Just meaning plasticity, what you consider. But I don't know what you mean by plasticity. You mean the dislocation activity. Oh, dislocation of that interface, okay, yeah, yeah. Okay ladies and gentlemen I think we'll have to leave it there and move on.
31:21
So if we could all show our appreciation to our fantastic guests.