Pop-in Behavior during Nanoindentation on Steel Alloy
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Transcript: English(auto-generated)
00:29
OK, so our next speaker is from Seoul National University. It's Professor Hyung Nam Han, and he's going to be talking to us about pop-in behavior during nanowindentation.
00:41
So, over to you. OK. Thank you, Chairman. It is my honor to have a presentation in APMS workshop in Cambridge. Thank you very much for Professor Harry Badesha and also APMS
01:04
team. The title of presentation is pop-in behavior during nanowindentation on steel alloys. So my name is Hyung Nam Han, and I'm working for Seoul National University. This work was supported by the Korean government
01:21
and the post-core steel company in Korea. So this work is collaborated with the white one, so I think, so Oak Ridge National Laboratory in the US, and the Kims in Korean government research laboratory,
01:41
and Seoul National University, and Hanba University, and post-core. So when the mechanical property, a small scale, was measured, normally we use the nanowindentation technique. So by using the AFM or SPM then, so we
02:01
can recognize the appropriate position of the material. For example, it's a special phase, or it's a special grain on steel. So we can obtain this kind of load displacement curve. Is there any pointer? This one? OK. So like this. By using this load displacement curve,
02:24
we can obtain an intrinsic property of a special phase or a special grain, the mechanical response. This is a kind of the fingerprint of the material. I think the mechanical response of the material. So by the precise analysis of this kind of the load
02:41
displacement curve, we can have the various information during the mechanical response of the material, I think. So in the nanowindentation analysis, we must consider the two special phenomena. First one is indentation size effect. And the other one is probably effect
03:01
of the dislocation nucleation or dislocation source activation. Indentation size effect indicate that increasing the nanohardens data with the decreasing of the indentation depth. It is well known that the indentation size effect is caused by the geometrically necessary
03:22
dislocation underneath the indented tip. As for the probable effect of the dislocation nucleation or dislocation source activation during the nanowindentation, as shown in this figure.
03:40
So if the indented size is smaller than the average spacing of dislocation, in this case, there is very low probability that the volume underneath the indenter contains existing dislocation. In this case, the starting of plastic deformation
04:02
is governed by the dislocation nucleation, not dislocation source activation or dislocation multiplication. But a large indenter case, normally, the plastic deformation was governed by the dislocation source activation or dislocation multiplication.
04:23
So in this presentation, I will talk on the Poppin special behavior in narrow indentation. So this is a normal loading and unloading curve during the narrow indentation. In some cases, so very sudden displacement
04:44
excursion occurs during the narrow indentation. This is called as Poppin. So this Poppin is kind of the softening process. So this Poppin is related to the geometrical softening or material softening of the material.
05:00
So I'd like to talk on that. So as you know, strange smart instinct transformation. This is one of the geometrical softening process. So in this case, large shear deformation occurs. The massive movement occurs. This can cause the geometrical softening.
05:20
And also, epsilon martensic transformation. In this case, massive partial dislocation movement occurs. This is one of the geometrical softening events. And also, I will talk on the eel drop in pearlitic steel. This is one of the large geometrical softening processes. So I'd like to talk on this nanoindentation Poppin,
05:43
the relationship between the nanoindentation Poppin and the eel drop in macroscopic tensile test. The most popular case of nanoindentation Poppin is insufficient plasticity.
06:00
So as you can see, in the nanoindentation, the plastic deformation procedure are the first dislocation nucleation, and the dislocation source activation, and dislocation multiplication. So the load for dislocation nucleation is normally larger than the dislocation source activation and the dislocation multiplication.
06:22
So after the plastic deformation, then a kind of geometrical softening event then this can cause a nanoindentation Poppin
06:41
like this. So in this presentation, I'd like to talk on the other possible source of nanoindentation Poppin in steel. As I mentioned above, I will deal with the mechanical industry and mass transformation. And the mechanical industry, pure mass inside transformation.
07:02
And the last one, so I will talk on the relationship between the eel drop and the nanoindentation Poppins. And first, I will talk on the strangest alpha-prime martensitic transformation. We used this kind of material with high manganese content.
07:26
So after the appropriate heat treatment, we can obtain this kind of the microstructure. And we carry out the combination method of EBSD technique and the nanoindentation technique.
07:41
So we can carry out the nanoindentation on each austenite phase. Then we can obtain this kind of the loaded displacement curve like this. You can see the Poppin here, the other Poppin here,
08:01
the other Poppin here. I'd like to know the origin of this kind of the Poppin. So that will indicate the hydrogen elastic solution then. From the two curves then, the initial Poppin is caused by the elasto-plastic transitions. So from the hydrogen solution then,
08:22
we can obtain the maximum shear stress underneath the indenter. This value was calculated by the 9 gigapascal. These values correspond to the shear modulus over 8. This is very close to theoretical strength for dislocation nucleation. And also, under the consideration of indentations probability effect then,
08:42
so normally on the specimen, the mean distance of dislocation is 10 micrometer. But in this case, indenter t-blade is just 0.2 micrometer. The Poppin's death is 20 nanometer. And the size of austenite grain is just 1 micrometer. So from those data then, we can conclude
09:04
that the first Poppin event is likely induced by the dislocation nucleation. But how about the second Poppin and third Poppin? So I think this is a metastable austenite phase. So these two Poppins may be related to the martensic transformation.
09:22
So how do I check that? So this is an initial austenite phase, you can see. Then you can see, after non-indentation, the very clear indentation mark here. Then, by using the focus line beam then, we prepare this kind of the TM specimen then,
09:42
we observe the TM microstructure like this. After that, so underneath the indenter then, so we observe the hard martensic phase. And also, we observe the gamma austenite phase remains. So if two phases are in the K-O2 orientation relationships,
10:02
that means this alpha-prime martensite was transformed to a gamma austenite phase, I think. So I confirmed that the martensic phase, after the transformation, initial austenite phase. So as you know, the martensite is a harder phase
10:20
than austenite. So this is another softening process, hardening process. OK? Or on the point of view of just the mechanical property data. But so this is the Bayesian deformation schematic diagram. You can see the Bayesian deformation has one
10:41
compressive axis and two tensile axes. According to the compressive axis, there is three Bayesian variants like this. So if the applied stress is parallel to the G direction like this, then, then the selection occurs in this variant and the others, then largely permanent deformation, largely compressive strain was developed in the material.
11:05
This can cause the geometrical softening. This is very simple, simple assumption. So I think the special variant selection can cause the geometrical softening. Then this can cause nanoindentation popping. From this very low assumption, then, so we can easily
11:21
calculate the popping depth by the martensitic deformation from the TM microstructure then. So we can obtain 25 nanometer. This is very close to the 20 nanometer by the indentation measurement.
11:40
But this is a very rough calculation. So as you know, the normal martensitic transformation case, we must consider the imbalanced shear deformation as well as the bain deformation. So we must consider the 24ks variant or more n-w variant and something like that.
12:02
So to evaluate the exact precise amount of nanoindentation popping depth, we must consider this kind of deformation tensor. Total deformation tensor consists of the bain deformation tensor and imbalanced shear deformation tensor on PCC crystal coordinate system.
12:22
After that, we can calculate the transformation strain tensor is like this on single oscillatory grain. And also, we need appropriate the variant selection model for considering the interaction energy
12:40
between the applied stress and the transformation strain. So from this numerical or the mathematical approach, then we can evaluate the popping depth precisely. I'd like to show you another interesting popping data.
13:07
This is another oscillatory grain, the same material. After the nanoindentation, we can obtain the multiple popping event. So I'd like to know that the microstructure change after the nanoindentation.
13:22
So also, by using the focus on the TM technique, then we observe this kind of the microstructure. And also, in here, so by using the automatic TM mapping technique A*, then we can obtain the phase map and the orientation maps like this.
13:41
This is an indentation point is here. You can see the remaining austenite phase. And also, you can see the various martensic variants with different orientation. So I'd like to check the origin of this kind of the martensic variant then.
14:03
So we carried out a crystal plus FEM for single austenite grain. Then we obtained a stress state underneath the indenter, the very complex stress field then. So by using the WRL theory then, we can determine the available variant selection.
14:23
Then after that, so we found that just the four variant matches to theoretical data. But interesting is that the alpha 1 position is here. This position is just underneath the indenter. I think the first martensic transformation
14:41
occurs in this one. This is perfectly matches to the calculation data. But this martensic occurs firstly. And then this martensic is hard to phase then. This martensic has a hard indenter. So this can change the stress field of this austenite grain.
15:04
So this can make a very complex martensic variant from the calculation, I think. So this very different martensic variant can make multiple patterns during the non-indentation.
15:25
Let's move on to the austenite eternal martensic transformation. I use this kind of the material with high nitrogen. The stacking fault energy value of austenite is about 15 millijoule per meter square.
15:43
For this steel with this stacking fault energy, it is known that the epsilon martensic formation occurs at the initial stage of deformation. So epsilon martensic is made by the stacking fault on O1 plane every two layers.
16:05
So thermal epsilon martensic has self-accommodated stacking. So after the epsilon martensic formation, there is no macroscopic strain. But strain-induced epsilon martensic has a monopartial stacking. Then this can cause the large shear deformation.
16:20
So I think this large shear deformation can make a geometric softening. So this can make small puffings during the non-indentation. I'd like to check this. OK. Before the non-indentation, we carry out the tensile test like this.
16:40
So after just 5% tensile deformation, then we obtain the same orientation relationship, epsilon martensic formation. And just 10% epsilon martensic, in 10% case, also we can observe the epsilon martensic. 40% deformation then, so we observe the epsilon martensite
17:00
and double orientation relationships. OK. This is the typical example of the load displacement curve after the non-indentation of this grain. So you can see the very small puffing occurs at the initial stage of deformation. So I'd like to check that. Maybe this puffing is related to the epsilon martensic
17:23
formation or not. So by using the focus RMB and also TM then, so we'd like to check the origin of this kind of the puffing event. So just underneath the indenter, we observe the alpha plumb martensite, I think.
17:46
So after the non-indentation, just underneath the indentation part was undertaken by the large deformation. So I think the alpha plumb martensite offered. So the lesion, slightly outside of the large deformation zone,
18:07
was found. So we obtained a very small banded structure. And by using the high resolution TM then,
18:23
so we confirmed that the epsilon martensite formation. So over 12 stacking fault. OK? Then, so from the analysis of the 12 balance selection
18:42
of the 12 partial slit, and also we can calculate the unit displacement for the one stacking fault to dance. So we obtained 0.08 nanometer deformation along the shear, along the indentation direction then. So we can consider that the over 12 epsilon martensite,
19:06
then this can make a two and three nanometer non-indentation puffing. This is very reasonable to comparing to the experimental data. So I think the initial stage of the puffing is might be related to the epsilon martensite formation,
19:22
I think. And OK. So this is the last part of the presentation. So in the normal BCC steel, so after the non-indentation, we obtained quite large puffing, of course,
19:42
after the non-indentation. So I'd like to check the relationship between the non-indentation puffing behavior and the sharp yield drop of the material. As you know, the yield drop is one of the geometrical softening event, breaking a quadratic atmosphere. OK, so we used this kind of the feltic steel
20:03
containing carbon and nitrogen. And we carry out the non-indentation like this, over 100 non-indentation data. Then before the non-indentation, we carry out the tensile test like this. You can see the obvious yield drop here.
20:21
So just after 6% strain, when reloaded after, or not just right after unloading then, yield drop disappeared. But after 30 hours strain aging then, so yield drop recovers. This is a very fundamental strain aging effect. So I'd like to check this.
20:41
If puffing in ferrite is related to the yield drop, analogous phenomena must exist in the case of non-indentation. I'd like to check. Oh, sorry. OK. OK. We carry out the free-strained non-indentation of the free-strained material and the strain
21:01
aging material at room temperature. As you can see, right after free-strain, as you can see, the puffing disappeared. After 30 hours later, after three weeks later, so puffing just reappeared. And more frequently, larger puffing appeared. That means probability of puffing increased with strain aging time. From this, the non-indentation puffing
21:20
is very closely related to the macroscale yield drop phenomena. OK. Thank you very much for your attention. Thank you very much, Professor Han. Are there any questions from the floor at all?
21:41
We've seen sometimes this puffing occur at weld metals. When you hit the austenite, you see the puffing behavior. So you always thought that it could be because of both dislocation activity and transmission. How would you differentiate this two? You had to go through this fib analysis to confirm that, or how would you do that? So if you have both phenomena, slip phenomena and also
22:02
the transmission behavior. Yeah. So you know that the normal, the plastic deformation for dislocation phenomena. So in non-indentation case, the first one is dislocation nucleation. And next, dislocation multiplication, or dislocation source activation
22:20
and dislocation multiplication. And these two stress, or two, the load for this dislocation multiplication or dislocation source activation is much smaller than dislocation nucleation. So first initiation of the plastic deformation is related to the dislocation puffing. But the second puffing or third puffing,
22:41
plastic deformation occurs. So that means there are so many dislocations underneath the indenter. So we must think on the different puffing source. Thank you for this interesting presentation.
23:03
Did you try to correlate the values of the force displacement curves, the puffing, and the actual energies of the events that you mentioned, like nucleation, glide, and so on? Because this could be very useful input for crystal plasticity modelers.
23:22
Yeah. Actually, in my laboratory, I did the development of the CPFM code. And so the starting point, the motivation of this nanoindentation work is to obtain an intrinsic property of this mechanical property. So we must consider the very complex mechanical behavior.
23:46
I mean that the indentation size, that means geometrically necessary dislocation, and also the puffing behavior, so very complex at this moment. So I want to obtain the appropriate input data for CPFM.
24:00
But at this moment, it's not easy to evaluate this kind of material parameters. Thank you. This is the future work. A question from Jihon Kang. Since the area of force displacement curve can be considered as a work or energy, do you think it will be possible to quantitatively
24:24
determine the different type of transformation energy by this popping effect? Would you lead again, please? So since the area of force displacement under the curve
24:41
can be considered as a work or energy, do you think that it will be possible to quantitatively determine the different type of transformation energy of this popping effect? I don't know exactly the transformation energy. So anyway, for the variant selection,
25:02
then we must consider the mechanical interaction energy between the external air pressure and the strain. The transformation strain. So I think for the training for variant, so I think the same chemical free energy from austenite
25:26
to martensite. So I don't understand. But so anyway, so we are supposed to have a dinner with Jihon Kang, then, so I'll talk. Yeah, she got another question about recrystallization. So she asked about, do you see any recrystallization
25:43
during non-indentation? Recrystallization. So recrystallization is a kind of the thermally activated process, except this dynamic recrystallization. So during the non-indentation,
26:00
I cannot induce the dynamic recrystallization, so it's not easy to obtain the. OK. Thank you. Thank you. Thank you. I wanted to get at the relationship between discontinuous yielding and tension and the pop-in in nanocompression.
26:24
So how good is, so we have steels that have continuous yielding and discontinuous yielding and then discontinuous yielding with variations in the amount in a macroscopic tension test. How good is the correspondence between what
26:44
you would measure in nanoindentation to what you would predict in tension when you look at that spectrum of yield point elongations? Could you take a very small specimen of a macroscopic material and predict and understand its yield point elongation?
27:00
Yeah, so that's a good question. So I would like to do that. So I tried it, but the non-indentation is just one point data. But the tensile tests are macroscopic data, so it's not easy to correlate this between the two mechanical properties. But the only thing is the relationship between them
27:24
is valid, but quantitatively it matches. It's not easy, I think, at this moment. I'm sorry. Final question. In fact, I have exactly the same question, because we always try to use Vickers hardness
27:41
and to relate the yield strength. And of course, through the nanointender, we also try to get some data. And for the nanoparticle presentation in fluoride, we also try to get some information in order to find out the deformation behaviour. So but in your case, you have an uniform structure
28:05
compared with the nano carbide in fluoride structure. So maybe we can relate the nanoindentered hardness data to the yield strength. Do you have any idea how to convert to the yield strength
28:25
through the basically through the mathematical calculation? Yeah, but at this moment, no. But so normally, in the high strength steels, they contain the various structures
28:40
and the various phases. So by using the nano indentation, we can measure the intrinsic property for each phase. And we can compare two properties. So it's reasonable, I think. But macroscopic value between the macroscopic value and nano value is quite a big difference at this moment.
29:01
So we can compare the two-page or three-page as a mechanical property in nanoscale. OK, I think we're going to have to leave there. If we could just thank our speaker again.