Energetic TWIP
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Transkript: Englisch(automatisch erzeugt)
00:28
Our next presenter is David Dye from the University of Imperial College. Thank you. Our Imperial College, sorry. Cool, thanks. I'll just do that.
00:40
There we go. Right, so, yes, Imperial College, which is, ooh, I'm quite loud. So Imperial is very beautiful. We're up there in lovely London with these lovely big green spaces that are wonderful for running around. And what I've been asked to talk, I've been, like every, my title's been shortened to Energetic Twin. And this is mostly summary of Khandarkar Rahman's PhD.
01:03
Mezhan's just over in the audience there. And with some help on the TM by Vaz Van Soffer and Anthony Sankaran, he's also come to work on this project. And this is about armor steel. So this is a Pete Brown, can you do something crazy for me project. And the problem for armor, of course, you have, initially, naively, you think you have this problem,
01:25
which is that you have something like a long rod penetrator here impacting on your protective armor. And that it's going to locally impart so much energy that it's going to burrow its way through, come close to melting, if you happen it does. And you have two problems there.
01:40
You have behind armor effects, things like spore fragments coming off the back. And so even if you defeat the projectile with the armor, that might still not be so great if you're inside. And the other problem is that you may burrow through, you may sheer plug here. And that sounds like a very difficult problem to deal with. Now, that's true, up to a point, if you're dealing with these guys.
02:03
But actually, armor systems are a much more complex and interesting topic. And typically we think of an armor system having a couple of components. We have a disturbance on the front side. And that might be our ceramic, that might be the ceramics guys.
02:21
That might be our very, very hard super bainite, something like that. And the purpose of that is to twist the projectile over. It might actually be a mixture of air and steel, in fact. And its purpose is to twist the projectile over, spread out its area, and get it to a point where it can't burrow through anymore, where you can absorb the energy without melting behind the projectile,
02:43
because it has so much energy concentrated in such a small area. It's trying to burrow its way through your material. Then, you have a second task. And then you have to absorb all of that energy imparted in such a way that you, on the inside of the structure, get to survive. And that's your absorber.
03:00
And then you may or may not have a spool liner on the inside to catch any fragments that come off the back. So you have a system that looks like this. Quite often, you'll have a vehicle hull, which is providing a lot of your absorber here. You'll have a spool liner on the inside, which quite often will be a composite. And then you would have armor plate that you may or may not put on as a so-called applique, which you would vary depending on the nature of the threats.
03:24
You might transport your vehicle as an air mobile, and then transport your applique armor separately, various options. So the idea is you can do your rapid deployment with your vehicle, and then you decide, oh, I don't like the threats, I'll fly out some additional plate which I'll hang on the outside. And you may or may not have bits and bobs of composite in there,
03:43
so that if you're standing on this surface and this comes up through, you, again, don't break your legs. Now, this then makes you think, well, okay, the task we're thinking about in this, in trip steels, is absorbing the energy, that stuff we make the vehicle hull out of. And this is the stress strain curve for a trip steel.
04:01
You have a yield down here. And then you have a very linear, this is true stress, true strain, a very linear work hardening rate. And mostly these materials fail by necking. So they fail by classic, consider those criteria, what we all did as undergrads, local instability when we stop having enough hardening
04:20
to defeat a small change in geometry, some machining imperfection or whatever it is. And, but people typically don't think about the stress strain curve like this, they plot the tensile strength, by which they mean the UTS. Then it's the elongation on the dreaded banana diagram. And then you say, well, okay, if I take my strength times my elongation,
04:44
multiply the two together, that's approximately the area under the curve. And so if I put a one over X, these are all equivalent, basically. That's the way the argument goes. Now, the area under the curve isn't that number times that number. Okay? It's actually completely wrong.
05:02
Okay? This is not the right picture to think about optimizing a steel. And our twip steels are typically plotted up here. The steel I'm talking about mostly starts out here. Okay? These are high manganese steels with a stacking fault energy of about 30 or so, modules per meter squared.
05:20
There are actually quite a lot of calculations out there for stacking fault energy. But a stacking fault energy that is low enough that you get lots of nicely widely spaced stacking faults, but not so low that you get epsilon martensite on deformation at room temperature. And within that constraint, so you end up at quite high manganese content, within that constraint you want to avoid things like cracking on forming or after forming.
05:43
You want to have a reasonably cheap steel that you can melt. So how do they work? Now, the remarkable thing is, this is a deformed twip steel, 20-odd micron grain size, fairly randomly taxed initially, that's been strained to about 22%. And you have lots of fine, you can see them here, the little black lines,
06:04
very feathery, very fine twins. And ignore the annealing twins, okay? That's not what we're looking for. We're looking for these very fine guys. And the way they work is that you put in lots and lots of these very fine twins. You can see them nicely here. You can see here's a nice twin, it has a nice load of stacking faults in it.
06:23
It's great. And you have a whole row of stacking faults there in a twin that's out of contrast. You have a whole load here. And what you're doing is what's called dynamic hall patch. So that is you put a twin in, that subdivides the grain in effect, gives you something a bit like a not very great, because remember it's a sigma three boundary,
06:41
bit of grain size strengthening, then you have a bit more deformation, those partial dislocations coalesce into another twin, it's very fine, then it gives you a bit more hardening, then you subdivide again, you get a bit more hardening, and so on and so on and so on. And so you get hardening all the way up the stress-strain curve. Classical twinning, you would have a hardening rate
07:00
that would start out very low until you have a big enough population of dislocations. Then you would come up as you had twinning. Twinning can only give you a maximum of 8% strain, so it saturates at something like 20%, then the hardening rate drops again. That's not the picture we're talking about here. Here, we twin all the way up through the stress-strain curve. All the way up. And notice how fine they are. That's the key. The key to these is that the partial dislocations are sufficiently spread out
07:25
and sufficiently confused about where they're going, simple picture, terrible anthropomorphism, et cetera, that you can condense into a whole series of different twins. And that's the magic. So, now the question is, people have shown that this works nicely in things like high-speed tensile tests, in hops and bar tests,
07:44
shown that they work for things like vehicle crashes, originally what they're really aimed at. Now, do they work in blast? Blast is actually a similar sort of strain rate, hundreds, and the answer is yes, it does. Here's a hops and bar test, nice high strain.
08:02
Here is a blast test. This is, you hang your sheet of steel over a frame, you put an explosive above it at some standoff distance of some mass, you explode your explosive, and you crack your 10mm thick RHA plate, that's sort of a World War II armour, and you take your much thinner twipped steel and it survives,
08:25
and you do a nice metal forming process. And this is it deforming around the edges of the frame. And the question is, do we see similar sorts of microstructure things going on here? This has had a biaxial test done on it, okay? So the strain state is not the same,
08:41
even if it is a similar rate. This is the EBSD for that. Again, you see some very fine twins in there, lots of annealing twins, very fine twins. Do you see the same sort of thing? And the answer is yes, we do. Here's our twin. It's actually a very dense set of stacking faults
09:02
within each of these twins here. If you go and look at this twin and look at its internal structure in high res, it's a lot nicer on my screen than it is on yours, but then you have lots of very fine, very fine twins, okay? Even finer than we saw in the uniaxial test. So it's really, really great, okay?
09:23
One thing to note that's entertaining, you would say, oh, well, you know, do I see more twinning in grains with a higher COSS that are in the right orientation for twinning? Yes, of course you do. That's true. It's also progressive as you load up. That's all fine, that's well known. If you go into one of those very fine twins that are only a few nanometers that you come into,
09:41
this guy here, that's within that same twin, and then come and look here, you can see there's actually a couple highlighted here, stacking faults. There's also some down here, and there's something funny going on over there. There are a whole load of stacking faults. Lots and lots of intrinsic stacking faults. They're so separated, and that's the point.
10:02
So it really does work, and it really does work in BLAST. So this makes people very excited, and just for fun, here's the edge of one of those twins, and this reminds me of the Andromeda Strain, if you remember the movie from the 50s. You have lots of contrast here going in two directions. Absolutely amazing. Beautiful, beautiful microstructures.
10:21
This is TM for the sake of it, but it's so beautiful that I mean, why not, while you're there? And so the question then was, can we do better? So Pete's question was, well, okay, that's very nice. But can you do better? Because his armor guys, for reasons I'll come to in a moment, said to him, well, that's nice, but actually it's not that much better
10:41
than I can do with World War II armor in application. There are some issues here. You might look a lot better in a BLAST test, but I don't think it'll be that much better for you in a real vehicle. So one thing we said was, well, okay, let's add more hardening mechanisms, see if we still get twinning to work. And I have a little bit of a thing about measuring grain sizes, I'm sorry. So this is the EBSD frequency plot
11:03
for the grain size. Put those into a cumulative distribution function, fit a distribution to that, we used a variable, there's no particular science behind that, that's just what we used. Then differentiate that and plot that again and find your mean grain size by fitting a distribution rather than doing some simple sterology.
11:21
We could do better now, essentially. And that's what we used to measure a grain size quite nicely. And then this is cold rolling and flash annealing. So one minute, two minute, the as-received, 24-hour and 96-hour anneals, and what our grain size PDFs look like. And we come out with a series of grain sizes that are something like submicron, micron, 20,
11:42
lots, loads, roughly speaking. And you develop a bit of a texture associated with cold rolling, sure. And this is what finding the grain size does to you. So this is the coarsest, the 84 micron, this is the relatively untextured, as-received,
12:01
hot rolled sheet, 10 micron or so grain size, and this is the very fine 0.7. You see you've got a little bit of a requirement for some stress before you pick up twinning. The little blips there are where we took the strain gauge off at a few percent strain, and then you carry on. And you see you can get the strength to go from 450 odd to 750 odd,
12:20
and your hardening rate is retained, hooray. So grain size you can do, you can add a strengthening mechanism. Nice, right. So that's lovely. How have you affected the twinning? Is it the same? Well, what we see, this is the finest and this is the coarsest. What we see is in the finest,
12:40
the twins are very much thinner. Here, look at this guy. This guy, okay, my scale bar has dropped off the bottom for some reason, but this guy is something like 40 or so nanometers thick. Here, you go and look at these guys. They're down at five nanometers thick. So in refining the grain size, one of the things you do is you refine the twins.
13:01
So you have more opportunity to put lots of twins in. And similarly, you see lots of stacking faults here that are left in the big rock guy. They've done more condensing into twins in the fine scale guy. And this is, just as a triumph of microscopy, these are the twins in that fine grain material
13:21
down at very, very thin sizes. Really, really amazing. So the question would be, what do I want now? I've said I don't like the banana diagram. What do I really want? Well, if I take the area under this curve and if I say, well, okay, it's linear hardening
13:40
because it is for the trip steels. It's linear hardening, pretty much. Okay, I can say, okay, well, the hardening rate M is just going to be the UTS. Great. So I can rearrange and find the failure strain. And I can integrate up and I get a elastic energy that is going to jangle around when I finally achieve failure.
14:02
Or even before I achieve failure, I've got an elastic energy that's at max and a plastic energy I can absorb. And so there are two parameters that matter in a stress-strain curve, the hardening rate or the UTS and the yield stress. Yeah? Your ductility UTS yield, only two of those are independent.
14:21
And therefore you have a two-factor problem. This is the coarse-grained, as-received and fine-grained material, three different yield stresses, hardening rates that actually go up a little bit because the twins are getting finer. My strain at failure is going down. Here's what happens to my total energy absorbed. It's going up. I've improved it by a quarter, pretty much.
14:41
Okay? And this is quite well known. Now, then people go, well, yes, but Dave, it's deforming a long way. If you're sitting inside the vehicle, you're going to be crushed by the incoming vehicle. Not all of that ductility is useful. Okay, so you then have to say, well, okay, if I pick a maximum strain I'm going to allow in my design
15:03
and I'm not going to let you use it beyond that, then not all of that ductility is useful and I can come up with an energy absorbed to 30% strain. So here are our three materials. At that coarse-grained, as-received and fine-grained, that's what the energy absorbed to 30% strain does for you. If I take something like RMX 440,
15:22
yield stress of 1,300, hardening rate about 1,500, I end up with a U30 of about 0.2. And actually a lot of the steels, I've been sitting in some of the talks earlier, a lot of the high strength 2GPA, 10% ductility sort of steels end up at about 0.2-ish or so. TI-64 ends up at about 0.13-ish.
15:43
The problem with TI-64 is it's got bugger-all hardening rate, okay? But when you density normalize, that brings TI-64 up because it's 4.5 out of 8 of the density of steel. So when you density normalize, that makes you a little bit better than your RMX 440
16:01
if you've got infinite money. And what Pete would like us to do is he would like a twipped steel that has a much higher strength, similar sort of hardening rate, and therefore we'll trade ductility, but our energy absorbed will go up and we'll be over twice as good at absorbing energy as RMX 440.
16:22
And that's the question then is can that be done, right? We've shown that we can get a good chunk of the way there actually. We can double. We only need another 50%. You know, it would be easy, right? We just need to take the strength from 750 to 1,300 MPA, so easy, right, without losing the hardening rate.
16:43
How hard can that be? And so one option would be, well, nitrogen and stainless steels are really strong. We can add more interstitial hardening. That at smaller contents won't change the stacking file energy very much. I won't do anything to change the microstructure very much.
17:01
I'm just making the matrix stronger, right? And that would be fine, except there aren't any pressurized ESR facilities in the UK. They're made by a company in Austria. And that's sort of annoying given that we developed this technology, but, you know, that's the country we're in now. So there you go. But we can probably do some things along those lines
17:22
that are feasible for a normal steel processing. But there will be lots of problems with nitrides, right? We have aluminum in these steels. So how much can we get there? That's one option. Another one that's been explored in the literature is to put some micro alloying in. Surprisingly, adding some very fine carbides
17:43
doesn't kill you for the twinning mechanism. The twinning mechanism is just at so much a finer scale. We're talking about, you know, your 10-nanometer type size carbides. So you're going to have to do some, play some tricks in processing to get them to stay in solution and then age them in at a relatively low temperature.
18:00
It'll also give you some grain size control. But if you get it right, you can get a few hundred MPA that way. You get a few hundred MPA from nitrogen, maybe it'll work out. And if it did work out, then that would be very, very nice. And so we're going through trying to do that at the moment. So micro alloy,
18:22
austen size, hot roll, cold roll, heat treat to get the right grain size, try and get the right carbide distribution at the same time, having bubble through your nitrogen, and where do you end up? We haven't got the nitrogen sorted out yet. Doing the micro alloying is, of course, fairly easy. Here it is, and this is the hot roll condition,
18:40
nice fine grain size. We haven't gone and developed the heat treatment yet to get the carbide distribution. If I had, I'd be very happy. I'm about three months behind where I'd like to be, to be honest. But we are getting to the stage of being able to do that and answer the question of, can we do this? So, conclusions. The twin mechanism does operate in the blast regime.
19:01
It looks attractive as a way of absorbing energy. Not all of the ductility is useful, so we want to trade some for some strength. And therefore, we'd like higher strength trip alloys that retain the hardening rate associated with twinning. And I've given you some thoughts on how I hope to go about doing that, and we'll see.
19:22
Thank you. Thank you, David. Question? Suresh. Is it possible to do a sandwich? On the top, put a tip. In the middle, you put advanced high strength with a lot of strength, so that you can take both,
19:40
have the both of them? There are lots of options in producing our systems, yes. So, you could have a sandwich, which you might glue together. You might say, well, okay, can I do something fun with a case hardening sort of process, where I have something very, very, very hard, then my tip on the inside,
20:03
and so on. Lots of, there are lots of possibilities. For the intrusion question, I always think, well, at the moment, a lot of the, sort of, armored vehicles with the V-shaped hulls, the crew are hanging off the ceiling, their seats are hanging off the ceiling, so that the intrusion doesn't break their legs. Okay?
20:21
So, there's a central empirical for blast injury studies that is worrying about how to rehabilitate people with these very complex fractures when they're not hanging off, but hanging off the ceiling. But hanging off the ceiling when you're bouncing around is kind of dull, right? You know, you don't like that. So, I think there's a lot of options for the spool liner to be a big, thick aluminum foam, for instance, and then you can stand on the floor.
20:42
So, there are lots of things to think about in designing an armor system. Lucy? Hi, so that was really, really exciting. Apologies if you may have mentioned this, but I was taking photos all the time. So, I mean, it's exciting that this does operate
21:00
as well at high strain rates as it does at low strain rates, but is there kind of like a strain rate dependency or any kind of strain rate effects that you do see? There is a strain rate dependency in that, so I haven't put up the going across the strain rate decades. You don't lose a lot of strength and you don't lose the hardening rate. So, the hardening rate does still seem to be there.
21:21
You don't lose a lot of the ductility. So, and you don't lose a lot of strength. Either, but it's interesting, right? We think of twinning as being a sheer process that operates in zero time, right? And if there's two things I hope to have persuaded you of, actually, really, it's not a sheer process
21:40
that operates in zero time. It's a dislocation process. Dislocations in motion are actually moving quite fast and we don't understand dislocation dynamics very well, okay, if we're honest. We can engineer with them with our brains and some insight, but the dislocation dynamics is quite immature.
22:01
And the other point that I was hoping to make is there's more to life than a UTS ductility curve. I have two questions. The first one is the third point of the conclusion. Let's look at that.
22:20
The third? The third point. Oh, yeah, sorry. Yeah, yeah. So, in fact, we can add vanadium carbide to improve the strength. We already did the experiment. And there's a really nice paper by Scott and co-authors here in what used to be giant shift,
22:41
which goes through this in great detail, actually. Yeah, so there's a number of groups I think you have, but yeah, congratulations. I'm not claiming novelty there. Okay, and my second question is you have shown cost-grant trip and fine-grant trip, and I found the elongation difference. And so I would like to know the mechanism through the twin interaction
23:05
and also dislocation and twin interaction. So, simplistically, I'm changing the hardening rate a little bit here. Okay. But I'm changing the yield strength more.
23:22
So, what I'm doing is I'm dropping. If I take those two things and calculate the ductility, the ductilities I'm measuring are a bit different, but then my test isn't very well aligned and it's a thin sample. I actually believe these more. This is a ductility just from considering necking. So, if you make it fine-grained and you increase the strength,
23:40
you do change the hardening rate a bit, and that will give you a drop in ductility. So, I'm making sort of a simple argument. There is probably more to the story, for sure, but the first part of the story is you'll lose quite a bit of the ductility just from a necking argument. Okay. And therefore, I wouldn't expect, and you can't beat necking, yeah,
24:04
unless you can suddenly add hardening later in life, but you've already used up a lot of the subdividing you can do. So, to a first approximation, that would be my view on what's going on there, but it is a first approximation. I have a question. When you measure the grain size, do you consider an in-between?
24:26
No. No, we don't. So, we ignore them. You could have a debate, but the number density of them is not very large. So, it's a sort of a, yeah, yeah, you could.
24:43
I don't think it would make a large difference. Harry. Why is there a half in front of the M? I can see the half in front of the sigma squared M. Well, to a first approximation, you have an area here and a triangle.
25:05
I'm integrating under this curve, so I'm taking that sigma Y times the ductility, which is one minus sigma over M, plus a half the total amount of hardening times that strain again.
25:21
So, the half, I think, comes out from that. If you go through the calculation and add up all the elastic terms, you then throw away the elastic terms, and it comes out on you. I can go through it with you. It's correct, I'm sure. It's such a simple calculation, and I've done it about three times, including on the train at three in the morning, and I always got the same answer,
25:40
because I keep asking myself where the half comes from. We've got one question from the internet. Hi Wen Luo asks what the composition of the trip steel that you've used is. Is it a manganese carbon? Oh, you're not allowed to tell us. It will come out in publication to within some hour bar.
26:00
It's a high manganese aluminium silicon-containing trip steel that's not amazingly different to anybody else's. Okay, thank you. But I'm formally not allowed to say, because it's not my steel. As long as twinning is correlated to the crystallographic region,
26:24
the crystallographic direction, did you check if your hardening rate is as beautiful in non-one-dimensional deformation? In, sorry, biaxial, for instance. For instance. Okay, so that's one of the interesting things about the blast test is that it gives you a good,
26:40
and you can go and look at the side wall as well, and you can go and look at the bend around the corner, and you get a very similar story. One of the things is, because we're in a hot roll recrystallized material, we have almost no starting texture, so you don't have to worry too much about texture effect. But we have gone and worried about texture effect a bit, and they decided we were being silly.
27:01
Yeah, the background behind the question is that you will reach necking point much faster than in a uniaxial. Right, yeah, yeah. If you think about it from a metal forming, if you're an auto guy, yeah. Yes, yes, you will, yes. Oh, Russian. Thank you. I guess you're trying to meet two objectives in one material.
27:24
As you said in the beginning, you want, as a projectile is coming in, you want a harder phase to blunt it, and then you want material to absorb the energy. Is this anything you can tailor the microstructure very carefully so that you can get a harder phase?
27:40
And if you can do that, then you can even, you know, meet even faster. In making it harder, you could, and you could try carbo, carbo guys in case hardening sort of approaches, and they will be quite effective if you want to. But I think in current UK,
28:02
I wouldn't go as far as to say that, but the way I'm thinking about it currently, you would probably have a very hard applique armor of some sort on top that would probably be bolt on. Because remember, you know, people shoot at these vehicles, and you don't really want to bring them back to the UK to re-engineer them,
28:21
so you want to be able to take off the applique armor and put on a replacement. The other thing is it's easy to do field repair, right? It's about a three, it can be a three-month turnaround from some of these locations to bring it back, work on it, and send it back out again. So you end up with a significant portion of your vehicle fleet being out of service because it's in transit.
28:40
Even the ones that are so mid, for example, and the surface, so mid-type. Right, there are lots of things you can do with Suramats. Yes, absolutely. You can have good fun. Thank you, David. Thanks, everyone. We will be back at half eleven.