Pop-in Behavior during Nanoindentation on Steel Alloy
Han, Heung Nam
A lecture given by Heung Nam Han, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. About the nanoindentation of steel, and the pop-in effect, variant selection. Nano-indentation is an outstanding method to probe small-scale mechanical properties, which are relevant to a wide range of materials and applications. The response of a material to nano-indentation is usually presented in the form of a loadâ"displacement curve. It is known that nano-indentation pop-in is a sudden displacement excursion on the load-displacement curve during load-controlled indentation. The sources of pop-in might be basically geometrical softening behaviors. In this study, several physical events which cause pop-ins during nano-indentation of steel alloys will be discussed. First, we consider the onset of plasticity resulting from dislocation nucleation or dislocation source activation in ferritic steel, which can produce the geometrical softening in the early stage of mechanical contact during nano-indentation. The effect of strain aging on the nano-indentation pop-in is observed and compared to the well-known macro-scale yield drop in tensile test. Second, both strain-induced alpha prime and epsilon martensitic transformations of metastable austenite are investigated by nano-indentation of individual austenite grains in multi-phase steels. The pop-ins are described as resulting from the geometrical softening due to the selection of favorable variants of alpha prime martensite and partial dislocation for epsilon martensite, respectively.
 Okay, so, uh, I think we're going to use the other words, um, um, um, um, um, um, um, um, um, um, um, Okay, so our next speaker is from Seoul National University. It's Professor Hyun Nam Han and he's going to be talking to us about pop-in behavior during nanowindentation. So, good to you. Okay, thank you Chairman. It is to my honor to have a presentation in APMAS workshop in Cambridge. Thank you very much for the professor, Harry Bae-Dae-Sha and also APMAS team. Okay, the title of presentation is pop-in behavior during nanowindentation on steel alloys. So, my name is Hyun Nam Han and I'm working for Seoul National University. This work was supported by the Korean government and the post-core steel company in Korea. So, this work is collaborated with the wide one. So, I think, so, Oak Ridge National Lab, Oak Ridge National Laboratory in US and the Kims in Korean government research laboratory and Seoul National University and Hanba University and the post-core. So, when the mechanical property at small scale was measured, normally we use the nanowindentation technique. So, by using the AFM or SPM then, so we can recognize the appropriate position of the material. For example, each special phase or special grain on steel then, so we can obtain this kind of the load displacement curve. Is there any pointer? This one? Okay, so like this. By using this load displacement curve, 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 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 of the dislocation, nucleation or dislocation source activation. In indentation size effect indicates that they are increasing the nano-hardens data with the decreasing of the indentation depth. It is well-known that the indentation size effect is caused by the geometrically necessary dislocation underneath the indentative. As for the probably effect of the dislocation, nucleation or dislocation source activation during the nanowindentation, as shown in this figure, so if the indentation size is smaller than the average spacing of dislocation, in this case, there is very low probability that the volume underneath the indentation contains existing dislocation. In this case, the starting of plastic deformation is governed by the dislocation nucleation, not dislocation source activation or dislocation multiplication. But a large indent case, normally the plastic deformation was governed by the dislocation source activation or dislocation multiplication. Okay. So, in this presentation, I will talk on the popping special behavior in nanowindentation. So, this is a normal loading and unloading curve during the nanowindentation. In some cases, so very sudden displacement excursion occurs during the nanowindentation. This is called as popping. So, this popping is kind of the softening process. So, this popping is related to the geometrical softening or material softening of the material. So, I'd like to talk on that. So, as you know, strain this martensitic transformation, this is one of the geometrical softening process. So, in this case, a large shear deformation occurs. The massive atomic, massive movement occurs. This can cause the geometrical softening. And also, if you're martensitic transformation, in this case, massive partial dislocation movement occurs. This is one of the geometrical softening event. And also, I'll talk on that the yield drop in peritone steel. This is one of the large geometrical softening process. So, I'd like to talk on this nanowindentation popping, the relationship between that, between them, between the nanowindentation popping and the yield drop in macroscopic tensile test. Okay. The most popular case of nanowindentation popping is incipient plasticity. So, as you can see, in the nanowindentation, 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. So, after the plastic deformation, then a kind of the geometrical softening event occurs, then this can cause nanowindentation popping like this. Okay. So, in this presentation, I'd like to talk on the other possible source of nanowindentation popping in steel. As I mentioned above, so I will deal with the mechanical industry and mass transformation and the mechanical industry, if your mass inside transformation and the last one, so I will talk on that the relationship between the yield drop and the nanowindentation popping. Okay. And the first, I will talk on that. So, strain used to alpha-plum martensite transformation. We used this kind of material with high manganese content. So, after the appropriate heat treatment, so we can obtain this kind of the micro structure and we carry out the combination method of EBSD technique and the nanowindentation technique. Then, so we can carry out the nanowindentation on each austenite phase. Okay. Then, we obtain this kind of the loaded displacement curve like this. You can see the popping here, the other popping here, the other popping here. I'd like to know that the origin of this kind of popping. So, let's indicate the Hedgian elastic solution then. From the two curves then, the initial popping is caused by the last plastic transitions. So, from the Hedgian solution then, so we can obtain the maximum shear stress underneath the indenta. This value was calculated by the 9 gigapascal. This value corresponds to the shear modulus of over 8. This is very close to theoretical strength for dislocation nucleation. And also, under the consideration of indentation probability effect then, so normally annealed specimen, the mean distance of dislocation is 10 micrometer. But, in this case, indentative radius is just 0.2 micrometer. The popping start depth is 20 nanometer and the size of austenite grains is just 1 micrometer. So, from those data then, so we can conclude that the first popping event is likely induced by the dislocation nucleation. But, how about the second popping and third popping? So, I think this is a metastable austenite phase. So, these two poppings may be related to the martensite transformation. Okay? So, how do I check that? So, this is an initially austenite phase. You can see, then you can see the after non-indentation, the very clear indentation mark here. Then, by using the focal style beam then, we prepare this kind of the TM specimen then, we observe the TM microstructure like this. After that, so underneath the indenter then, so we observe the hard martensite phase. And also, we observe the gamma austenite phase remains. So, it is two phases are in the chaos of orientation relationships. That means, this alpha prime martensite was transformed to a gamma austenite phase, I think. This, so I confirm that the martensite phase after the transformation initially austenite phase. So, as you know, the martensite is a hard phase than austenite. So, this is another softening process, hardening process. Okay? Or on the point of view of just the mechanical property data. Okay? But, so this is a the bain deformation schematic diagram. You can see, the bain deformation has one compressive axis and two tensile axis. According to the compressive axis, there is three bain variants like this. So, if the applied stress is federal to the G direction like this, then bain selection occurs in this variant and not the others. Then, large pormont deformation, large, large compressive strain was developed in the material. This can cause the geometric softening. This is very simple, simple assumption. Okay? So, I think the special variant selection can cause the geometric softening. Then, this can cause of nano-identation popping. From this very loop, assume so then, so we can easily calculate the popping that's by the martensite formation from the TM microstructure then. So, we can obtain 25 nanometer. This is very close to the 20 nanometer by the indentation measurement. Okay? But, this is a very rough calculation. So, as you know, the normal martensite transformation case, we must consider the invariant shear deformation as well as the bain deformation. So, we must consider the 24kS variant or more NW variant or something like that. Okay? So, to evaluate the exact precise for nano-identation popping that's, so we must consider this kind of the deformation tensor, total deformation tensor. Consist of the bain deformation tensor and invariant shear deformation tensor on BCC crystal coordinate system. After that, though, we can calculate the transformation strain tensile like this on single austenite grain. And also, we need appropriate the variant selection model for considering the interaction energy between the applied stress and the transformation strain. Then, so we can from this numerical or the mathematical approach then, we can evaluate the popping depth precisely. Okay? Okay. I'd like to show you the another interesting data, interesting popping data. This is another austenite grain, the same material. After the nano-identation, we can obtain the multiple popping event. So, I'd like to know that the microstructure changes after the nano-identation. So, also by using the focus on beam then TM technique, then we observe this kind of the microstructure. And also, here, so by using the automatic TM mapping technique A star, then we can obtain the phase map and orientation maps like this. This is an indentation point here. You can see the remaining austenite phase. And also, you can see the various multisperient with different orientation. So, I'd like to check the origin of this kind of the multisperient. Then, so we carried out crystal-flasked FEM for single austenite grain. Then, we obtained a stress state underneath the indentation, the very complex stress field. Then, so by using the WRL theory, then, we can determine the favorable variance selection. 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 indentation. I think the first in that first martensite transformation occurs in this one. This is perfectly matches to the calculation data. But, this martensite occurs firstly and then this martensite is hard-faced and then this martensite act as a hard indentation. So, this can make a change in the stress field of this austenite grain. So, this can make a very complex martensite variant from the calculation, I think. So, this very different martensite variant can make multiple patterns during the non-identation. Okay, let's move on to the austenite-eternal martensite transformation. So, I use this kind of the material with high nitrogen. The stack-imported energy value of austenite is about 15 millijoule per meter square. For this steel with this stack-imported energy, it is known that the Y-martensite formation occurs at the initial stage of deformation. So, Y-martensite is made by the stacking fault on U1 plane every two layers. So, thermal Y-martensite has self-accommodated stacking. So, after the Y-martensite formation, there is no macroscopic strain. But, strain-induced Y-martensite has monopartial stacking then this can cause the large shear deformation. So, I think this large shear deformation can occur, can make a geometrical softening. So, these can make the small poppins during the non-identation. I'd like to check this. Okay, before the non-identation, we carry out the tensile test like this. After just 5% tensile deformation, then we obtain the austenite-orientation chip Y-martensite formation. And, just a 10% Y-martensite, 10% case, also we can observe the Y-martensite. But, 40% deformation, then so we observe the alpha-flammate site and the orientation relationships. Okay, this is the typical example of the load displacement curve after the non-identation of this grain. So, you can see the very small popping occurs at the initial stage of the formation. So, I'd like to check that. This, maybe this popping is related to the Y-martensite formation or not. So, by using the focus I and B and also Tm then, so we'd like to check the origin of this kind of the popping event. So, just underneath the indenta, we observe the alpha-flammate site. I think Tm. So, after the non-identation, just underneath the indentation part is was undertaken by the large deformation. So, I think the alpha-flammate site occurs. So, the lesion slicedly outside of the large deformation zone was found. Then, so we obtained a very small banded structure and by using the high resolution Tm then, so we confirmed that the Y-martensite formation. So, over 12 stacking fort. Okay, then, so from the analysis of the 12 balance, the balance selection of the 12 partial slit and also we can calculate the unit displacement for the one stacking fort to then. So, we obtained 0.08 nanometer the deformation along the shield, along the indentation direction then. So, we can consider that the over 12 Y-martensite and this can make a 2 or 3 nanometer non-identation popping. This is a very reasonable comparing to the experimental data. So, I think the initial stage of the popping is my related to the Y-martensite formation I think. Okay, and okay. So, this is the last part of the presentation. So, in the normal BCC steels, so from the after the nano indentation, we obtained quite large popping after the nano indentation. So, I'd like to check the other relationship between the nano indentation popping behavior and sharp yield drop of the material. As you know, the yield drop is one of the geometrical opening event, breaking a quarter of an atmosphere. Okay, so we used this kind of the phertics steel containing a carbon and nitrogen and we carry out the nano indentation like this over 100 nano indentation data. Then, before the nano indentation, we carry out the tensile test is like this. You can see the obvious yield drop here. So, just after 6% strain when reloaded after or not just right after unloading then yield drop disappeared. Okay, but after 30 hours strain aging then, so yield drop recovers. This is a very fundamental strain aging effect, but so I'd like to check these. If popping in ferrite is related to the yield drop, another spinamina must exist in the case of nano indentation. I'd like to check. Oh, sorry. Okay, we carry out the pre-strain nano indentation of the pre-strain material and strain-aged material at room temperature as you can see right after pre-strain that you can see the popping disappeared after 30 hours later after 3 weeks later. So, popping this reappeared and more frequently large popping appeared. That means, probability of popping increase with strain-aged time from this, the nano indentation popping is very closely related to the macro scale yield drop phenomenon. Okay, thank you very much for your time. Thank you very much. Thank you very much, Professor Han. Are there any questions from the floor at all? We've seen sometimes this popping occurred in the valve metals when you hit the austenite, you see the popping behavior. So, you always thought that it could be because of both dislocation activity and transformation. How would you differentiate this two? You had to go through this fib analysis to confirm that or how would you do that? So, we have both the phenomena, slip phenomena and also the transformation behavior. Yeah, so, you know that the normal, the plastic deformation for dislocation phenomena, so in nano indentation case, the first one is dislocation equation and next dislocation multiplication or dislocation source activation and dislocation multiplication and these two stress or two the load for this dislocation multiplication or this source activation is much smaller than dislocation nucleation. So, first initiation of the plastic deformation is related to the dislocation popping but the second popping or third popping, plastic deformation occurs. So, that means there are so many dislocations underneath the indentation. So, we must think on the different popping source. Okay. Right, right. Thank you for this interesting presentation. Did you try to correlate the values of the force displacement curves, the popscene 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 plus these demodulers. Yeah, so actually, so in my laboratory, so I did the development of the CPF and code and so the starting point, the motivation of this nano indentation work is the obtain an intrinsic property of this mechanical property but so we must consider the very complex mechanical behavior. I mean that the indentation size, that means that geometrically necessary dislocation and also the popping behavior so the very complex at this moment. So, I want to obtain that the appropriate input data for CPF PM but at this moment it's not easy to evaluate that this kind of the material parameters. Thank you. This is future work. A question from Ji Hong 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 determine the different type of transformation energy by this popping effect? Would you read again please? So, since the area of force displacement under the curve 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 valiant selection then so we must consider the mechanical interaction energy between the external pressure and the strain, the transformation strain. So, I think the 40, 20 per valiant then so I think the same chemical free energy from austenite to martensite so I don't know understand but so anyway so we are supposed to have a dinner with Ji Hong Kang then so I'll talk about that. Yeah, she got another question about recluse tragedy so she asked about do you see any recluse tragedy during Nado indentation? Recluse? Recluse tragedy so recluse tragedy is a kind of the thermal reactivase process except this dynamic recluse tragedy but so during the Nado indentation I cannot induce the dynamic recluse tragedy so it's not easy to obtain the... Okay, thank you. Thank you. I wanted to get at the relationship between discontinuous yielding intention and the pop-in in nano compression. Yeah. 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 you would measure in nano indentation 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. Yeah, so that's a good question. So I'd like to do that so I tried it but in nano indentation is just one point data but the tension test on macroscopic data so it's not easy to correlate this between the two mechanical property but the only thing is there the the relationship between them is valid but the quantitatively the 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 a weakest hardness and to relate the yield strength and of course through the nano-intenter we also try to get some data and for the nanoparticle presentation in ferrite we also try to get some information in order to to find out the deformation behavior so but in your case you have a uniform structure compared with the main nano carbide in ferrite structure so so maybe we can relate the nano-intenter hardness data to the yield strength. Do we have any idea to how to convert to the yield strength through the basicly through the mathematical calculation? Yeah but at this moment no but so so normally in advanced the high strength steels there contains a barrier structure in the various phases so by using nano indentation that 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 big difference at this moment but so we can compare the two-phase or three-phase as a mechanical property in nano scale. Okay I think we're gonna have to leave there if we could just thank our speaker again.
 Okay, so, uh,  I think we're going to use  the other words,  um,  um,  um,  um,  um,  um,  um,  um,  um,  um,  um,  Okay, so our next speaker is from Seoul National University. It's Professor  Hyun Nam Han and he's going to be talking to us about pop-in behavior during  nanowindentation. So, good to you. Okay, thank you Chairman. It is to my honor to  have a presentation in APMAS workshop in Cambridge. Thank you very much for  the professor, Harry Bae-Dae-Sha and also APMAS team. Okay, the title of  presentation is pop-in behavior during nanowindentation on steel alloys. So, my  name is Hyun Nam Han and I'm working for Seoul National University. This work was  supported by the Korean government and the post-core steel company in Korea. So,  this work is collaborated with the wide one. So, I think,  so, Oak Ridge National Lab, Oak Ridge National Laboratory in US and the Kims in  Korean government research laboratory and Seoul National University and Hanba  University and the post-core. So, when the mechanical property at small scale was  measured, normally we use the nanowindentation technique. So, by using the  AFM or SPM then, so we can recognize the appropriate position of the material.  For example, each special phase or special grain on steel then, so we can  obtain this kind of the load displacement curve. Is there any pointer? This one?  Okay, so like this. By using this load displacement curve, 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 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 of the dislocation, nucleation  or dislocation source activation. In indentation size effect indicates that  they are increasing the nano-hardens data with the decreasing of the  indentation depth. It is well-known that the indentation size effect is caused by  the geometrically necessary dislocation underneath the indentative. As for the  probably effect of the dislocation, nucleation or dislocation source  activation during the nanowindentation, as shown in this figure, so if the  indentation size is smaller than the average spacing of dislocation, in this  case, there is very low probability that the volume underneath the  indentation contains existing dislocation. In this case, the starting of plastic  deformation is governed by the dislocation nucleation, not dislocation  source activation or dislocation multiplication. But a large  indent case, normally the plastic deformation was governed by the  dislocation source activation or dislocation multiplication. Okay. So, in  this presentation, I will talk on the popping special behavior in nanowindentation.  So, this is a normal loading and unloading curve during the  nanowindentation. In some cases, so very sudden displacement  excursion occurs during the nanowindentation. This is called as popping.  So, this popping is kind of the softening process. So, this popping is  related to the geometrical softening or material softening of the material. So,  I'd like to talk on that. So, as you know, strain this martensitic  transformation, this is one of the geometrical softening process. So, in this  case, a large shear deformation occurs. The massive  atomic, massive movement occurs. This can cause the geometrical softening. And  also, if you're martensitic transformation, in this case, massive  partial dislocation movement occurs. This is one of the geometrical softening  event. And also, I'll talk on that the yield drop in  peritone steel. This is one of the large geometrical softening process. So, I'd  like to talk on this nanowindentation popping, the relationship between  that, between them, between the nanowindentation popping and the yield  drop in macroscopic tensile test. Okay. The most popular case of nanowindentation  popping is incipient plasticity. So, as you can see, in the nanowindentation, 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. So, after the plastic  deformation, then a kind of the geometrical softening event occurs, then  this can cause nanowindentation popping like this. Okay. So, in this presentation,  I'd like to talk on the other possible source of nanowindentation popping in  steel. As I mentioned above, so I will deal with the mechanical industry and  mass transformation and the mechanical industry, if your mass inside transformation  and the last one, so I will talk on that the relationship between the yield drop  and the nanowindentation popping. Okay. And the first, I will talk on that. So,  strain used to alpha-plum martensite transformation. We used this kind of  material with high manganese content. So, after the appropriate heat  treatment, so we can obtain this kind of the micro structure and we carry out the  combination method of EBSD technique and the nanowindentation technique. Then, so  we can carry out the nanowindentation on each austenite phase. Okay. Then, we obtain  this kind of the loaded displacement curve like this. You can see the popping  here, the other popping here, the other popping here. I'd like to know that the  origin of this kind of popping. So, let's indicate the Hedgian elastic solution  then. From the two curves then, the initial popping is caused by the  last plastic transitions. So, from the Hedgian solution then, so we can obtain  the maximum shear stress underneath the indenta. This value was calculated by the  9 gigapascal. This value corresponds to the shear modulus of over 8. This is very  close to theoretical strength for dislocation nucleation. And also, under  the consideration of indentation probability effect then, so normally  annealed specimen, the mean distance of dislocation is 10 micrometer. But, in this  case, indentative radius is just 0.2 micrometer. The popping start depth is  20 nanometer and the size of austenite grains is just 1 micrometer. So, from  those data then, so we can conclude that the first popping event is likely  induced by the dislocation nucleation. But, how about the second popping and  third popping? So, I think this is a metastable austenite phase. So, these two  poppings may be related to the martensite transformation. Okay? So, how do I check that?  So, this is an initially austenite phase. You can see, then you can see the after  non-indentation, the very clear indentation mark here. Then, by using the  focal style beam then, we prepare this kind of the TM specimen then, we observe  the TM microstructure like this. After that, so underneath the indenter then, so  we observe the hard martensite phase. And also, we observe the gamma austenite  phase remains. So, it is two phases are in the chaos of orientation relationships.  That means, this alpha prime martensite was transformed to a gamma austenite phase,  I think. This, so I confirm that the martensite phase after the transformation  initially austenite phase. So, as you know, the martensite is a hard phase than  austenite. So, this is another softening process, hardening process. Okay? Or on the  point of view of just the mechanical property data. Okay? But, so this is a  the bain deformation schematic diagram. You can see, the bain deformation has one  compressive axis and two tensile axis. According to the compressive axis, there  is three bain variants like this. So, if the applied stress is federal to the G  direction like this, then bain selection occurs in this variant and not the others.  Then, large pormont deformation, large, large compressive strain was developed  in the material. This can cause the geometric softening. This is very simple,  simple assumption. Okay? So, I think the special variant selection can cause the  geometric softening. Then, this can cause of nano-identation popping. From this  very loop, assume so then, so we can easily calculate the popping that's by  the martensite formation from the TM microstructure then. So, we can obtain 25  nanometer. This is very close to the 20 nanometer by the indentation  measurement. Okay? But, this is a very rough calculation. So, as you know, the  normal martensite transformation case, we must consider the invariant shear  deformation as well as the bain deformation. So, we must consider the  24kS variant or more NW variant or something like that. Okay? So, to evaluate  the exact precise for nano-identation popping that's, so we must consider this  kind of the deformation tensor, total deformation tensor. Consist of the  bain deformation tensor and invariant shear deformation tensor on BCC crystal  coordinate system. After that, though, we can calculate the transformation  strain tensile like this on single austenite grain. And also, we need  appropriate the variant selection model for considering the interaction  energy between the applied stress and the transformation strain. Then, so we can  from this numerical or the mathematical approach then, we can evaluate the popping  depth precisely. Okay? Okay. I'd like to show you the another interesting data,  interesting popping data. This is another austenite grain, the same material.  After the nano-identation, we can obtain the multiple popping event. So, I'd like  to know that the microstructure changes after the nano-identation. So, also by  using the focus on beam then TM technique, then we observe this kind of the  microstructure. And also, here, so by using the automatic TM mapping technique  A star, then we can obtain the phase map and orientation maps like this. This is  an indentation point here. You can see the remaining austenite phase. And also, you  can see the various multisperient with different orientation. So, I'd like to  check the origin of this kind of the multisperient. Then, so we carried out  crystal-flasked FEM for single austenite grain. Then, we obtained a stress  state underneath the indentation, the very complex stress field. Then, so by using  the WRL theory, then, we can determine the favorable variance selection. 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 indentation. I think the first in that first martensite transformation  occurs in this one. This is perfectly matches to the calculation data.  But, this martensite occurs firstly and then this martensite is hard-faced  and then this martensite act as a hard indentation. So, this can make a change  in the stress field of this austenite grain. So, this can make a very complex  martensite variant from the calculation, I think. So, this very different  martensite variant can make multiple patterns during the non-identation.  Okay, let's move on to the austenite-eternal martensite transformation.  So, I use this kind of the material with high nitrogen. The stack-imported energy  value of austenite is about 15 millijoule per meter square. For this steel with this  stack-imported energy, it is known that the Y-martensite formation occurs at the  initial stage of deformation. So, Y-martensite is made by the stacking  fault on U1 plane every two layers. So, thermal Y-martensite has self-accommodated  stacking. So, after the Y-martensite formation, there is no  macroscopic strain. But, strain-induced Y-martensite has monopartial stacking  then this can cause the large shear deformation. So, I think this large  shear deformation can occur, can make a geometrical softening. So, these can make  the small poppins during the non-identation. I'd like to check this.  Okay, before the non-identation, we carry out the tensile test like this. After  just 5% tensile deformation, then we obtain the austenite-orientation  chip Y-martensite formation. And, just a 10% Y-martensite, 10% case, also we can  observe the Y-martensite. But, 40% deformation, then so we observe the  alpha-flammate site and the orientation relationships. Okay, this is the typical  example of the load displacement curve after the non-identation of this grain.  So, you can see the very small popping occurs at the initial stage of the  formation. So, I'd like to check that. This, maybe this popping is related to the  Y-martensite formation or not. So, by using the focus I and B and also Tm  then, so we'd like to check the origin of this kind of the popping event. So, just  underneath the indenta, we observe the alpha-flammate site. I think Tm. So, after  the non-identation, just underneath the indentation part is was undertaken by the  large deformation. So, I think the alpha-flammate site occurs. So, the lesion  slicedly outside of the large deformation zone was found. Then, so we obtained a  very small banded structure and by using the high resolution Tm then, so we  confirmed that the Y-martensite formation. So, over 12 stacking fort. Okay, then, so  from the analysis of the 12 balance, the balance selection of the 12 partial  slit and also we can calculate the unit displacement for the one stacking fort  to then. So, we obtained 0.08 nanometer the deformation along the shield,  along the indentation direction then. So, we can consider that the over 12  Y-martensite and this can make a 2 or 3 nanometer non-identation popping. This is  a very reasonable comparing to the experimental data. So, I think the initial  stage of the popping is my related to the Y-martensite formation I think. Okay, and  okay. So, this is the last part of the presentation. So, in the normal BCC  steels, so from the after the nano indentation, we obtained quite large  popping after the nano indentation. So, I'd like to check the other  relationship between the nano indentation popping behavior and sharp yield  drop of the material. As you know, the yield drop is one of the geometrical  opening event, breaking a quarter of an atmosphere. Okay, so we used this kind of  the phertics steel containing a carbon and nitrogen and we carry out the nano  indentation like this over 100 nano indentation data. Then, before the nano  indentation, we carry out the tensile test is like this. You can see the obvious  yield drop here. So, just after 6% strain when reloaded after or not just right  after unloading then yield drop disappeared. Okay, but after 30 hours  strain aging then, so yield drop recovers. This is a very fundamental  strain aging effect, but so I'd like to check these. If popping in ferrite is  related to the yield drop, another spinamina must exist in the case of  nano indentation. I'd like to check. Oh, sorry. Okay, we carry out the  pre-strain nano indentation of the pre-strain material and strain-aged  material at room temperature as you can see right after pre-strain that you can  see the popping disappeared after 30 hours later after 3 weeks later. So, popping  this reappeared and more frequently large popping appeared. That means,  probability of popping increase with strain-aged time from this, the nano  indentation popping is very closely related to the  macro scale yield drop phenomenon. Okay, thank you very much for your  time. Thank you very much. Thank you very much, Professor Han. Are there any  questions from the floor at all? We've seen sometimes this popping occurred  in the valve metals when you hit the austenite, you see the popping behavior.  So, you always thought that it could be because of both dislocation activity and  transformation. How would you differentiate this two? You had to go  through this fib analysis to confirm that or how would you do that? So, we have  both the phenomena, slip phenomena and also the transformation behavior. Yeah, so,  you know that the normal, the plastic deformation for dislocation phenomena, so  in nano indentation case, the first one is dislocation equation and next  dislocation multiplication or dislocation source activation and  dislocation multiplication and these two stress or two the load for this  dislocation multiplication or this source activation is much smaller than  dislocation nucleation. So, first initiation of the plastic deformation is  related to the dislocation popping but the second popping or third popping,  plastic deformation occurs. So, that means there are so many dislocations  underneath the indentation. So, we must think on the different popping  source. Okay. Right, right. Thank you for this interesting presentation. Did you try  to correlate the values of the force displacement curves, the popscene 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 plus these demodulers. Yeah,  so actually, so in my laboratory, so I did the development of the CPF and code  and so the starting point, the motivation of this nano indentation work  is the obtain an intrinsic property of this mechanical property but so we must  consider the very complex mechanical behavior. I mean that the indentation  size, that means that geometrically necessary dislocation and also the popping  behavior so the very complex at this moment. So, I want to obtain that the  appropriate input data for CPF PM but at this moment it's not easy to evaluate  that this kind of the material parameters. Thank you. This is future work. A question  from Ji Hong 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  determine the different type of transformation energy by this popping  effect? Would you read again please? So, since the area of force displacement under  the curve 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 valiant selection then so we must consider the mechanical interaction  energy between the external pressure and the strain, the transformation strain.  So, I think the 40, 20 per valiant then so I think the same chemical  free energy from austenite to martensite so I don't know understand but so anyway  so we are supposed to have a dinner with Ji Hong Kang then so I'll talk about that.  Yeah, she got another question about recluse tragedy so she asked about do you  see any recluse tragedy during Nado indentation? Recluse? Recluse tragedy so  recluse tragedy is a kind of the thermal reactivase process except this  dynamic recluse tragedy but so during the Nado indentation I cannot induce the  dynamic recluse tragedy so it's not easy to obtain the... Okay, thank you. Thank you.  I wanted to get at the relationship between discontinuous yielding  intention and the pop-in in nano compression. Yeah. 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 you would  measure in nano indentation 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. Yeah, so that's a good question. So I'd like to do that so I tried it but in  nano indentation is just one point data but the tension test on  macroscopic data so it's not easy to correlate this between the two  mechanical property but the only thing is there the the relationship between them  is valid but the quantitatively the 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 a weakest hardness and to relate the yield  strength and of course through the nano-intenter we also try to get some  data and for the nanoparticle presentation in ferrite we also try to  get some information in order to to find out the deformation behavior so but in  your case you have a uniform structure compared with the main nano carbide  in ferrite structure so so maybe we can relate the nano-intenter hardness data  to the yield strength. Do we have any idea to how to convert to the yield strength  through the basicly through the mathematical calculation? Yeah but at  this moment no but so so normally in advanced the high strength steels  there contains a barrier structure in the various phases so by using  nano indentation that 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  big difference at this moment but so we can compare the two-phase or three-phase  as a mechanical property in nano scale. Okay I think we're gonna have to leave  there if we could just thank our speaker again.
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Okay, so, uh, I think we're going to use the other words, um, um, um, um, um, um, um, um, um, um, um, Okay, so our next speaker is from Seoul National University. It's Professor Hyun Nam Han and he's going to be talking to us about pop-in behavior during nanowindentation. So, good to you. Okay, thank you Chairman. It is to my honor to have a presentation in APMAS workshop in Cambridge. Thank you very much for the professor, Harry Bae-Dae-Sha and also APMAS team. Okay, the title of presentation is pop-in behavior during nanowindentation on steel alloys. So, my name is Hyun Nam Han and I'm working for Seoul National University. This work was supported by the Korean government and the post-core steel company in Korea. So, this work is collaborated with the wide one. So, I think, so, Oak Ridge National Lab, Oak Ridge National Laboratory in US and the Kims in Korean government research laboratory and Seoul National University and Hanba University and the post-core. So, when the mechanical property at small scale was measured, normally we use the nanowindentation technique. So, by using the AFM or SPM then, so we can recognize the appropriate position of the material. For example, each special phase or special grain on steel then, so we can obtain this kind of the load displacement curve. Is there any pointer? This one? Okay, so like this. By using this load displacement curve, 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 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 of the dislocation, nucleation or dislocation source activation. In indentation size effect indicates that they are increasing the nano-hardens data with the decreasing of the indentation depth. It is well-known that the indentation size effect is caused by the geometrically necessary dislocation underneath the indentative. As for the probably effect of the dislocation, nucleation or dislocation source activation during the nanowindentation, as shown in this figure, so if the indentation size is smaller than the average spacing of dislocation, in this case, there is very low probability that the volume underneath the indentation contains existing dislocation. In this case, the starting of plastic deformation is governed by the dislocation nucleation, not dislocation source activation or dislocation multiplication. But a large indent case, normally the plastic deformation was governed by the dislocation source activation or dislocation multiplication. Okay. So, in this presentation, I will talk on the popping special behavior in nanowindentation. So, this is a normal loading and unloading curve during the nanowindentation. In some cases, so very sudden displacement excursion occurs during the nanowindentation. This is called as popping. So, this popping is kind of the softening process. So, this popping is related to the geometrical softening or material softening of the material. So, I'd like to talk on that. So, as you know, strain this martensitic transformation, this is one of the geometrical softening process. So, in this case, a large shear deformation occurs. The massive atomic, massive movement occurs. This can cause the geometrical softening. And also, if you're martensitic transformation, in this case, massive partial dislocation movement occurs. This is one of the geometrical softening event. And also, I'll talk on that the yield drop in peritone steel. This is one of the large geometrical softening process. So, I'd like to talk on this nanowindentation popping, the relationship between that, between them, between the nanowindentation popping and the yield drop in macroscopic tensile test. Okay. The most popular case of nanowindentation popping is incipient plasticity. So, as you can see, in the nanowindentation, 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. So, after the plastic deformation, then a kind of the geometrical softening event occurs, then this can cause nanowindentation popping like this. Okay. So, in this presentation, I'd like to talk on the other possible source of nanowindentation popping in steel. As I mentioned above, so I will deal with the mechanical industry and mass transformation and the mechanical industry, if your mass inside transformation and the last one, so I will talk on that the relationship between the yield drop and the nanowindentation popping. Okay. And the first, I will talk on that. So, strain used to alpha-plum martensite transformation. We used this kind of material with high manganese content. So, after the appropriate heat treatment, so we can obtain this kind of the micro structure and we carry out the combination method of EBSD technique and the nanowindentation technique. Then, so we can carry out the nanowindentation on each austenite phase. Okay. Then, we obtain this kind of the loaded displacement curve like this. You can see the popping here, the other popping here, the other popping here. I'd like to know that the origin of this kind of popping. So, let's indicate the Hedgian elastic solution then. From the two curves then, the initial popping is caused by the last plastic transitions. So, from the Hedgian solution then, so we can obtain the maximum shear stress underneath the indenta. This value was calculated by the 9 gigapascal. This value corresponds to the shear modulus of over 8. This is very close to theoretical strength for dislocation nucleation. And also, under the consideration of indentation probability effect then, so normally annealed specimen, the mean distance of dislocation is 10 micrometer. But, in this case, indentative radius is just 0.2 micrometer. The popping start depth is 20 nanometer and the size of austenite grains is just 1 micrometer. So, from those data then, so we can conclude that the first popping event is likely induced by the dislocation nucleation. But, how about the second popping and third popping? So, I think this is a metastable austenite phase. So, these two poppings may be related to the martensite transformation. Okay? So, how do I check that? So, this is an initially austenite phase. You can see, then you can see the after non-indentation, the very clear indentation mark here. Then, by using the focal style beam then, we prepare this kind of the TM specimen then, we observe the TM microstructure like this. After that, so underneath the indenter then, so we observe the hard martensite phase. And also, we observe the gamma austenite phase remains. So, it is two phases are in the chaos of orientation relationships. That means, this alpha prime martensite was transformed to a gamma austenite phase, I think. This, so I confirm that the martensite phase after the transformation initially austenite phase. So, as you know, the martensite is a hard phase than austenite. So, this is another softening process, hardening process. Okay? Or on the point of view of just the mechanical property data. Okay? But, so this is a the bain deformation schematic diagram. You can see, the bain deformation has one compressive axis and two tensile axis. According to the compressive axis, there is three bain variants like this. So, if the applied stress is federal to the G direction like this, then bain selection occurs in this variant and not the others. Then, large pormont deformation, large, large compressive strain was developed in the material. This can cause the geometric softening. This is very simple, simple assumption. Okay? So, I think the special variant selection can cause the geometric softening. Then, this can cause of nano-identation popping. From this very loop, assume so then, so we can easily calculate the popping that's by the martensite formation from the TM microstructure then. So, we can obtain 25 nanometer. This is very close to the 20 nanometer by the indentation measurement. Okay? But, this is a very rough calculation. So, as you know, the normal martensite transformation case, we must consider the invariant shear deformation as well as the bain deformation. So, we must consider the 24kS variant or more NW variant or something like that. Okay? So, to evaluate the exact precise for nano-identation popping that's, so we must consider this kind of the deformation tensor, total deformation tensor. Consist of the bain deformation tensor and invariant shear deformation tensor on BCC crystal coordinate system. After that, though, we can calculate the transformation strain tensile like this on single austenite grain. And also, we need appropriate the variant selection model for considering the interaction energy between the applied stress and the transformation strain. Then, so we can from this numerical or the mathematical approach then, we can evaluate the popping depth precisely. Okay? Okay. I'd like to show you the another interesting data, interesting popping data. This is another austenite grain, the same material. After the nano-identation, we can obtain the multiple popping event. So, I'd like to know that the microstructure changes after the nano-identation. So, also by using the focus on beam then TM technique, then we observe this kind of the microstructure. And also, here, so by using the automatic TM mapping technique A star, then we can obtain the phase map and orientation maps like this. This is an indentation point here. You can see the remaining austenite phase. And also, you can see the various multisperient with different orientation. So, I'd like to check the origin of this kind of the multisperient. Then, so we carried out crystal-flasked FEM for single austenite grain. Then, we obtained a stress state underneath the indentation, the very complex stress field. Then, so by using the WRL theory, then, we can determine the favorable variance selection. 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 indentation. I think the first in that first martensite transformation occurs in this one. This is perfectly matches to the calculation data. But, this martensite occurs firstly and then this martensite is hard-faced and then this martensite act as a hard indentation. So, this can make a change in the stress field of this austenite grain. So, this can make a very complex martensite variant from the calculation, I think. So, this very different martensite variant can make multiple patterns during the non-identation. Okay, let's move on to the austenite-eternal martensite transformation. So, I use this kind of the material with high nitrogen. The stack-imported energy value of austenite is about 15 millijoule per meter square. For this steel with this stack-imported energy, it is known that the Y-martensite formation occurs at the initial stage of deformation. So, Y-martensite is made by the stacking fault on U1 plane every two layers. So, thermal Y-martensite has self-accommodated stacking. So, after the Y-martensite formation, there is no macroscopic strain. But, strain-induced Y-martensite has monopartial stacking then this can cause the large shear deformation. So, I think this large shear deformation can occur, can make a geometrical softening. So, these can make the small poppins during the non-identation. I'd like to check this. Okay, before the non-identation, we carry out the tensile test like this. After just 5% tensile deformation, then we obtain the austenite-orientation chip Y-martensite formation. And, just a 10% Y-martensite, 10% case, also we can observe the Y-martensite. But, 40% deformation, then so we observe the alpha-flammate site and the orientation relationships. Okay, this is the typical example of the load displacement curve after the non-identation of this grain. So, you can see the very small popping occurs at the initial stage of the formation. So, I'd like to check that. This, maybe this popping is related to the Y-martensite formation or not. So, by using the focus I and B and also Tm then, so we'd like to check the origin of this kind of the popping event. So, just underneath the indenta, we observe the alpha-flammate site. I think Tm. So, after the non-identation, just underneath the indentation part is was undertaken by the large deformation. So, I think the alpha-flammate site occurs. So, the lesion slicedly outside of the large deformation zone was found. Then, so we obtained a very small banded structure and by using the high resolution Tm then, so we confirmed that the Y-martensite formation. So, over 12 stacking fort. Okay, then, so from the analysis of the 12 balance, the balance selection of the 12 partial slit and also we can calculate the unit displacement for the one stacking fort to then. So, we obtained 0.08 nanometer the deformation along the shield, along the indentation direction then. So, we can consider that the over 12 Y-martensite and this can make a 2 or 3 nanometer non-identation popping. This is a very reasonable comparing to the experimental data. So, I think the initial stage of the popping is my related to the Y-martensite formation I think. Okay, and okay. So, this is the last part of the presentation. So, in the normal BCC steels, so from the after the nano indentation, we obtained quite large popping after the nano indentation. So, I'd like to check the other relationship between the nano indentation popping behavior and sharp yield drop of the material. As you know, the yield drop is one of the geometrical opening event, breaking a quarter of an atmosphere. Okay, so we used this kind of the phertics steel containing a carbon and nitrogen and we carry out the nano indentation like this over 100 nano indentation data. Then, before the nano indentation, we carry out the tensile test is like this. You can see the obvious yield drop here. So, just after 6% strain when reloaded after or not just right after unloading then yield drop disappeared. Okay, but after 30 hours strain aging then, so yield drop recovers. This is a very fundamental strain aging effect, but so I'd like to check these. If popping in ferrite is related to the yield drop, another spinamina must exist in the case of nano indentation. I'd like to check. Oh, sorry. Okay, we carry out the pre-strain nano indentation of the pre-strain material and strain-aged material at room temperature as you can see right after pre-strain that you can see the popping disappeared after 30 hours later after 3 weeks later. So, popping this reappeared and more frequently large popping appeared. That means, probability of popping increase with strain-aged time from this, the nano indentation popping is very closely related to the macro scale yield drop phenomenon. Okay, thank you very much for your time. Thank you very much. Thank you very much, Professor Han. Are there any questions from the floor at all? We've seen sometimes this popping occurred in the valve metals when you hit the austenite, you see the popping behavior. So, you always thought that it could be because of both dislocation activity and transformation. How would you differentiate this two? You had to go through this fib analysis to confirm that or how would you do that? So, we have both the phenomena, slip phenomena and also the transformation behavior. Yeah, so, you know that the normal, the plastic deformation for dislocation phenomena, so in nano indentation case, the first one is dislocation equation and next dislocation multiplication or dislocation source activation and dislocation multiplication and these two stress or two the load for this dislocation multiplication or this source activation is much smaller than dislocation nucleation. So, first initiation of the plastic deformation is related to the dislocation popping but the second popping or third popping, plastic deformation occurs. So, that means there are so many dislocations underneath the indentation. So, we must think on the different popping source. Okay. Right, right. Thank you for this interesting presentation. Did you try to correlate the values of the force displacement curves, the popscene 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 plus these demodulers. Yeah, so actually, so in my laboratory, so I did the development of the CPF and code and so the starting point, the motivation of this nano indentation work is the obtain an intrinsic property of this mechanical property but so we must consider the very complex mechanical behavior. I mean that the indentation size, that means that geometrically necessary dislocation and also the popping behavior so the very complex at this moment. So, I want to obtain that the appropriate input data for CPF PM but at this moment it's not easy to evaluate that this kind of the material parameters. Thank you. This is future work. A question from Ji Hong 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 determine the different type of transformation energy by this popping effect? Would you read again please? So, since the area of force displacement under the curve 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 valiant selection then so we must consider the mechanical interaction energy between the external pressure and the strain, the transformation strain. So, I think the 40, 20 per valiant then so I think the same chemical free energy from austenite to martensite so I don't know understand but so anyway so we are supposed to have a dinner with Ji Hong Kang then so I'll talk about that. Yeah, she got another question about recluse tragedy so she asked about do you see any recluse tragedy during Nado indentation? Recluse? Recluse tragedy so recluse tragedy is a kind of the thermal reactivase process except this dynamic recluse tragedy but so during the Nado indentation I cannot induce the dynamic recluse tragedy so it's not easy to obtain the... Okay, thank you. Thank you. I wanted to get at the relationship between discontinuous yielding intention and the pop-in in nano compression. Yeah. 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 you would measure in nano indentation 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. Yeah, so that's a good question. So I'd like to do that so I tried it but in nano indentation is just one point data but the tension test on macroscopic data so it's not easy to correlate this between the two mechanical property but the only thing is there the the relationship between them is valid but the quantitatively the 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 a weakest hardness and to relate the yield strength and of course through the nano-intenter we also try to get some data and for the nanoparticle presentation in ferrite we also try to get some information in order to to find out the deformation behavior so but in your case you have a uniform structure compared with the main nano carbide in ferrite structure so so maybe we can relate the nano-intenter hardness data to the yield strength. Do we have any idea to how to convert to the yield strength through the basicly through the mathematical calculation? Yeah but at this moment no but so so normally in advanced the high strength steels there contains a barrier structure in the various phases so by using nano indentation that 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 big difference at this moment but so we can compare the two-phase or three-phase as a mechanical property in nano scale. Okay I think we're gonna have to leave there if we could just thank our speaker again.
National University. It's Professor Hyun Nam Han Cambridge. Thank you very much for the professor, nanowindentation on steel alloys. So, my name is mechanical response of the material. So, by the phenomena. First one is indentation size effect dislocation, nucleation or dislocation source is well-known that the indentation size effect is effect of the dislocation, nucleation or dislocation source activation during the and unloading curve during the nanowindentation. In some cases, so very sudden displacement excursion occurs during the nanowindentation. is related to the geometrical softening or martensitic transformation, this is one of the partial dislocation movement occurs. This is one I'll talk on that the yield drop in peritone plasticity. So, as you can see, in the nanometer and the size of austenite grains is prime martensite was transformed to a gamma the special variant selection can cause the maps like this. This is an indentation point energy, it is known that the Y-martensite formation occurs at the initial stage of strain-induced Y-martensite has monopartial from the after the nano indentation, we obtained indentation, we carry out the tensile test is is a very fundamental strain aging effect, but so the events that you mentioned like nucleation, you think that it will be possible to consider the mechanical interaction energy intention and the pop-in in nano compression.