Architectured Steel
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Koseki, Toshihiko
A lecture given by Toshihiko Koseki, at the Adventures in the Physical Metallurgy of Steels (APMS) conference held in Cambridge University. Multilayered steels are described, including the theoretical framework for the design of such composites. Traditionally, physical metallurgy concerns microstructure-property correlation. In this approach, microstructure evolves as the product of interactions between composition and process parameters controlled by the thermodynamic and kinetic conditions. Attributes concerning the property are obtained as the function of volume fraction, size, shape and distribution of the constituent phases, usually described through empirical relations or even on the basis of imprecise knowledge. Hence, the approach is more evolutionary than constructive. Performance driven construction of the microstructure demands precise response and interaction of microstructural constituents under the given loading condition. An architecturally designed microstructure implies planning, design and construction of microstructure considering nature, size, morphology and distribution of the constituent phases on a suitably conceived topological framework. With the aforesaid ambition, an attempt has been proposed on construction of the ferrite-martensite microstructure, based on iso-strain architecture, aiming at maximum work hardening. In another attempt, the mechanical response of a topologically designed bimodal microstructure in single phase steel has been evaluated for maximizing the strength-ductility combination.
 Music Applause Welcome to the meeting and we are now ready for the first lecture which will be given by Professor Toshikoseki from Tokyo University of Technology. Music My colleagues here, actually Nambu is over there, who have contributed to the research that I'm going to talk about today. The title of my talk is Architectural Steel. This name was given by Harry when I gave a talk somewhere else on our multi-layer steel because I like the name. I used the name again here. So all of us here know that many different kinds of steel were developed in the 20th century and the performance and the properties of the steel significantly improved over the century. Those developments and advancement certainly thanks to theoretical basis that was also developed in the 20th century and also achieved by a low design with a different combination of rare metals and also achieved by microstructure controlled by some mechanical process and also by high purification. Also, we have used fully the strengthening mechanism to develop different steels. So it looks like to improve the steel we have done almost everything but still the demand for high performance steel is never ending. And with increasing the environment consciousness, the demand becomes stronger and stronger. For example, if we look at the automobile steels, higher strength and higher ductility demanded. So we need to meet the demand. We have to go this direction. How can we do that? We may need new alloy design and we may need a new micro and nano microstructure control that we haven't tried in the 20th century. And also we may need ultimate refinement of microstructure and grain structures. Alternatively, we may need the externally architecture steel, externally designed steel where we can get away from monolithic steels and we can part from the some dynamic restriction in the design of materials, in the design of steels. So this is my proposal here. Today I'm talking about externally designed multi-layer steels. This is an example of multi-layer steels where we combine high strength as fetched mountain site and high ductility steel. And to achieve a combination of high strength and high ductility. And although this is a 25-layer steel, the number of layers could be fewer depending on the combination of steels as I will mention later. To fabricate multi-layer steels, we stack the steel of our interest and then hot road or warm road or even cold road for bonding. And finally we heat treated to achieve a desired microstructure and to increase interfacial toughness. And for the high strength layers, we use as fetched mountain site and for high ductility layer, we use authentic steels or ferritic steels or trip steel or dual phase steels, whatever steel that has ductility. In other words, you can combine any steels of your interest and sometimes we insert cynical layer to prevent carbon diffusion between the layers. Why the layers structure? To achieve the high elongation, we need to elongate the mountain site, as fetched mountain site. In a dual phase steels, mountain site is not deformed because the stress is not partitioned to the mountain site. And there is a stress concentration in a ferrite matrix, particularly between a mountain site island which results in a voiding and eventually a fracture. In case of layer structure, stress is partitioned, both ductile layer and high strength layer. So with a plastic constraint, the mountain site should be elongated as long as the local fracture is suppressed, which is not easy. These are the data of strength and ductility of laminated metals, different laminated metals, which was summarized by Ligiere and Charby. And the strength always follows the rule of mixture, rule of average along this line, but the ductility does not follow the rule of mixture. This vertical axis is elongation of laminated metals consisting of 50% ductile and 50% brittle component. And this elongation comes down here, far below the rule of average, as the elongation of brittle component becomes low. Why do we have such low elongation? Because we have a brittle fracture during elongation in the brittle layer, as shown here, which is caused by a delamination and resulting H-shaped cracks and also so-called tunnel cracks. We have to suppress those local fractures to obtain larger elongation. In terms of delamination and H-shaped cracks, naturally increasing interfacial toughness increases elongation of multi-layer steel. And when there is a delamination, brittle layer behaves like a single component and without plastic constraint, it breaks, it fractures with low elongation like here. And if the interfacial toughness is not enough, the elongation is still below the uniform elongation predicted by a rule of mixture. And with increasing interfacial toughness, the multi-layer steels are fractured with diffused necking in a ductile manner. This boundary can be predicted by this equation, which was also developed in the research of composite. And this criterion is given by this line. Looks like this criterion works well. For the prevention of tunnel crack, there is also work about tunnel crack in the research of semi-conductors, which is given here, where the thickness of a brittle layer is limited, which is a function of a fracture toughness of brittle layers. So you need to reduce the thickness of brittle layer to avoid the tunnel crack. But this criterion was derived in an elastic situation. In case of metals, you have a plastic zone in a ductile layer in the vicinity of the tunnel crack. We have to consider that. Considering the elastic situation, we derived this criterion. Again, the thickness of brittle layer should be reduced to increase elongation, which is a function of fracture toughness of brittle layer and the use of strength of ductile layers. Certainly, the decrease of the layer thickness increases the elongation of the ascended mountain side. By decreasing the thickness of mountain side, this type 420 high carbon, mountain acidic stainless steel, can be elongated up to 20%. So without multi-layer structure, we can't elongate the ascended mountain side in this way. Also, the fracture surface changes from brittle to ductile dimpling as the thickness of mountain side layer is decreased. This is the effect of thickness of brittle layer on elongation. These two lines are from the elastic model and elastic plastic model. Many use authentic type 3.04 stainless steel for ductile layers because type 3.04 stainless steel has good work hardening. The transition from low elongation to the high elongation is close to the elastic model. When you use interstitial free ferrite as a ductile layer, because this steel does not show much work hardening. The transition behavior from low elongation to the high elongation is close to the elastic plastic model. In a multi-layer steel, the transition from low elongation to the high elongation is somewhere in between. The elastic plastic model we developed gives a lower boundary in the design of multi-layer steel. Also, the limitation of the brittle layer is the function of fracture toughness. These are confirmed here. We provided the mountain steel having a different fracture toughness and measured the transition. Increasing fracture toughness gives the thicker brittle layers. In other words, you can increase the thickness of brittle layer if you have a mountain steel with a better toughness. Or you can reduce the number of layers. By controlling the interfacial toughness and the thickness of brittle layers, we can elongate the ascension to the mountain side. If you apply a neutral diffraction, this is the result. You will measure the fully partitioning of stress. Because of the partitioning of stress, the mountain side is elongated here. As a result, we obtained steels which have high strength and high ductility. Those are plotted here. Those steels have strength more than 1200 mega Pascal and still have elongation of more than 20% percent. The product of strength and elongation is more than double of conventional monolithic steels. These mountain steel keep the elongation even under the high strain rate deformation. Here are the stress strain curves under different strain rates. The maximum is 800 per second. If the strength increases with increasing the strain rate, the elongation does not change much. Those photos are the test results of high speed buckwheat which simulate the collision of front side members of automobiles. The 1200 mega Pascal mountain steel deforms perfectly in the same way as the 590 Duelphase steels. There is still space for additional deformation because of the high strain. There is no delamination of a local truck during this high strain rate deformation. Here is another high strain rate deformation. This is an impact bending test which simulates pillars of automobiles. The bending strength is increased in the mountain layer steels. Here is the DP590 for comparison. The bending strength is increased here. In the application of multi-layer steel, we need welding. We are trying to weld the multi-layer steels using friction-stow welding. This is cross-section. The welding is successful, which has a joint efficiency of more than 90%. It is interesting to note that this layer structure remains not only in the heel-affected zone but also in the steel zone. Using multi-layer steel, we can look at deformation behavior of a splenched mountain site, which was difficult before because of the load activity of a splenched mountain site. We are now conducting many in-situ observations on the deformation of a splenched mountain site using an EBSP. Here are the multi-layer steels. This is monolithic mountain steel. We found that the slip is always in the plane, in the plane, and after certain strain levels. Beyond that, the slip across the last direction appears. This slip is concentrated in the region where the cement factor in that plane is high and no slip in those regions where the cement factor is low. This is a similar result using a digital image correlation using a silver nanoparticle during a tensile test. Again, the stress concentration is in a mountain site block where the cement factor along the in-last plane is high. Another part is not deformed significantly, even though the cement factor is high, which is out of the last plane. The cement factor along the in-last plane is low because this is low. Further improvement of the multi-layer steels, we need to improve the process. We are now looking at lower pressure bonding, which makes the fabrication much easier and more efficient. In terms of components, not only use the high carbon, mountain-steel steels, we are now using a HTP metal such as magnesium and titanium to achieve a lighter multi-layer steels. We are now using a steel with high impurity, like a scrap steel, so that we can use a scrap to fabricate high-performance steels. Here is an example of a magnesium steel multi-layer. We have developed a good bonding process to join the magnesium steel and we fabricate a three-layer magnesium steel multi-layer. Magnesium is commercially available lightest metal, but the problem is the ductility. Because of HCP structures, elongation is up to 20% or even less. By employing a multi-layer structure, we can increase the ductility of magnesium without any break, up to 35-40%. The strength is also increased because of the steels. This is the summary of my talk. Thank you very much for your attention. I was wondering how you deal with the differential volume contractions and expansions that you get when you quench the multi-layers. Does that set up residual stresses between the layers that might result in crack propagation being easier and delamination becoming easier? Yes, you are right. There must be some residual stress. We are now measuring how much residual stress and what the effect on the mechanical properties is. Certainly, there is some residual stress. You are saying that the decrease in martensite layer thickness is increasing the toughness. Is it not due to locally plane strain loading condition starting in only the martensitic layer? You are right. As the thickness decreases, the situation comes to that. The thickness can be thicker. If the fracture toughness of the martensite layer is maybe medium or not high. Today, I showed the martensite, which is type 420 high carbon martensitic stainless steel. This is really brittle. The fracture toughness is really low. The normal carbon steel, the martensite carbon steel, is not such low. We can increase more. The situation is not simply a plane. So far, you have made these materials by starting with the original layers and then rolling them together. Have you looked at other methods of fabricating this kind of structure? Additive layer manufacturing looks like it would be well suited to a layered structure like that with electron beam or laser. Depositing powder and then you have a lot of flexibility to pick material or thickness. The multi-layer structure is employed everywhere, like in a semiconductor. In that case, as you mentioned, they deposit layer by layer. But this is a structure still. We need volume. So the easier and the simpler fabrication is better. We sort of have many possibilities, but at the moment, I think this is the simplest way. What sort of thickness are you aiming to get? You say it's easy to get good volume material. What sort of thickness and volumes are you interested in fabricating? In our case, the thickness. Anyway, this is sheet material. So the thickness is about 0.5 to 2 mm or something like that. The final thickness. A sheet quite wide, I guess. In our research, we already made 60 mm steels, coil to coil. So when you do the hot rolling or warm rolling of martensite multi-layer. So the martensite phase actually during rolling is to change back to also the night or it's in martensite. In hot rolling and warm rolling. Could you repeat again? When you do hot rolling or warm rolling of your multi-layer steels, one layer, suppose it's a martensite. So the martensite in that temperature has actually changed back to also the night or. I understand. During fabrication, martensitic steels does not have the micro structure of martensite. It's like a mixture of ferrite and cementite or something. During hot rolling. And after that, the heat treatment is made after the alternate region and the quenched. So where the martensite is formed. Thanks for a very interesting presentation. Reminds me of the fiber metal laminate work that was done a few years ago. Introducing polymers and metal layers together. I just wondered if you'd done any corrosion studies introducing dissimilar metals, often problematic. That's also a good question. We have done the corrosion test. There is a possibility that the corrosion resistant is decreased if you combine like a magnesium. Did you measure the Young's modulus of the martenite materials? Yes. But at the moment we combine steel to steel. So Young's modulus is the same. Yes, it means just follow the mixture. Yes, yes, yes. This is well known. I was wondering if you did fatigue testing and stress localization testing for formability. We are conducting a fatigue test too. At the moment there is nothing I can say about that. Certainly the martensite still affects the fatigue behavior. Did you make stress localization experiments to understand the viscoplastic behavior under forming or for forming? At the moment we have done much. Thank you. In a 3D deformation. Thank you for a nice presentation. Could you explain me in a few words what is the difference between your idea of composite material on the base of steel with, for example, the Mahagana steel or Damascus steel? Yes, maybe the origin is the same. In the past many people studied the Mahagana steel metals, even the Damascus too. But in the past it looks like the less attention was paid to the ductility. People try to increase the strength, but the research on ductility is limited as far as we investigate it. Thank you. Do you need heat treatment? Do you study how important is the diffusion of carbon between the same layers of high-glow carbon steel? Yes, it is very important to suppress the diffusion of carbon. And when we combine the different steels, we always think about the activity of carbon during the heat treatment so that maybe alloy design is needed to suppress the diffusion of carbon for brittle layer and the ductile layer. Toshie, how do you measure the interfacial strength? We use a pure test, but we can't measure if the interfacial strength is really high. Just only a brittle interface can be measured. And how do you control it? Namu, can you explain that? In the case of the interfacial toughness, the mohology of the test is the tensile direction is just 180 degrees. So the mohology and the stress direction is almost constant so we can evaluate the difference. But supposing that the interfacial strength is too low, how can I change it? Of course, in the case of very, very weak interfacial toughness, it is very difficult to evaluate. But in the case of the R2, for example, from 500 degrees C or 600 degrees C, we can evaluate the interfacial toughness. So the interfacial toughness is increased by a bonding process and heat treatment. You can increase the interfacial toughness. And I'd like to note that interfacial toughness does not be really high unless you give a pure test. Based on that, would it be possible if you did have quite a high stress at the interface to design the ductile phase so that it underwent slight plastic relaxation at the interface, which would provide a bit of work hardening and would that be beneficial in a way to reduce any stresses that you might get generated in the alloy? Would that be feasible or would the work hardening limit your layer thickness and the further work hardening capability of it? Would that then have an adverse effect on ductility, do you think? So with the residual stresses that are set up in the interface, if they were reasonably high, would it be feasible to design your ductile layer to have a yield strength such that it undergoes slight plastic deformation to relieve the residual stresses? And would that work hardening then have an adverse effect on the ductility as a result? Well, up to now we haven't found any adverse effect on that. Yes, let me be a bit adventurous here and about a little bit speculative as well. Perhaps it could decrease the carbon diffusion between layers and perhaps improve the cohesion effect. If you dealt with, let's say, nano structure steel, so you would get a strength out of a nano structure steel layer and keep your soft layer as well. This is highly speculative, you see. So it seems adequate in all of these tools to do that. What are your ideas about that? You mean the diffusion across the interface. We are now researching that and we want to minimize the diffusion layer to increase the strength and ductility for the brittle layer and the ductile layer. The diffusion and carbon decrease the strength and ductility both. So that's why I'm saying low temperature bonding is necessary to improve the performance of monthly steel. We have a feeling that the bonding of the interface is possible at medium or lower temperatures and we don't need a big diffusion to increase the interfacial strength. Did you look at interfaces at higher magnification and in general how important is the quality of interfaces? Do you have rough interfaces or more kind of intermix? We are looking at interfaces using a transmission electron microscope. Of course there is some small void and some discontinuity and some part continues. The details of the development of interfaces need to be researched more. I have a more macroscopic question. You are reducing the thickness of the martensitic layer to increase elongation. This is traditional bi-phase philosophy. You are also saying that strength is a rule of mixtures. Would you not then expect strength to drop? So you sacrifice strength to increase elongation. Which is exactly what happens in any bi-phase microscope. This is a combination. We can't go beyond the rule of mixtures. There is some compensation but we can use higher strength of steel, like a very high carbon steel, so that we can increase the strength. The elongation is always... If we make an old effort, we can achieve the rule of mixtures, even for the elongation. So you can increase... Let me explain more. Anyway, this is the mixture of high strength layers and low strength layers, and the mixture of high ductility and low ductility. We can go this way more by using higher strength steel. Thank you very much indeed. It has been an excellent talk and an excellent discussion.
 Music  Applause  Welcome to the meeting and we are now ready for the first lecture which will be given by Professor Toshikoseki from Tokyo University of Technology.  Music  My colleagues here, actually Nambu is over there, who have contributed to the research that I'm going to talk about today.  The title of my talk is Architectural Steel. This name was given by Harry when I gave a talk somewhere else on our multi-layer steel because I like the name.  I used the name again here. So all of us here know that many different kinds of steel were developed in the 20th century and the performance and the properties of the steel significantly improved over the century.  Those developments and advancement certainly thanks to theoretical basis that was also developed in the 20th century and also achieved by a low design with a different combination of rare metals and also achieved by microstructure controlled by some mechanical process and also by high purification.  Also, we have used fully the strengthening mechanism to develop different steels. So it looks like to improve the steel we have done almost everything but still the demand for high performance steel is never ending.  And with increasing the environment consciousness, the demand becomes stronger and stronger. For example, if we look at the automobile steels, higher strength and higher ductility demanded.  So we need to meet the demand. We have to go this direction. How can we do that? We may need new alloy design and we may need a new micro and nano microstructure control that we haven't tried in the 20th century.  And also we may need ultimate refinement of microstructure and grain structures. Alternatively, we may need the externally architecture steel, externally designed steel where we can get away from monolithic steels and we can part from the some dynamic restriction in the design of materials, in the design of steels.  So this is my proposal here. Today I'm talking about externally designed multi-layer steels. This is an example of multi-layer steels where we combine high strength as fetched mountain site and high ductility steel.  And to achieve a combination of high strength and high ductility. And although this is a 25-layer steel, the number of layers could be fewer depending on the combination of steels as I will mention later.  To fabricate multi-layer steels, we stack the steel of our interest and then hot road or warm road or even cold road for bonding. And finally we heat treated to achieve a desired microstructure and to increase interfacial toughness.  And for the high strength layers, we use as fetched mountain site and for high ductility layer, we use authentic steels or ferritic steels or trip steel or dual phase steels, whatever steel that has ductility.  In other words, you can combine any steels of your interest and sometimes we insert cynical layer to prevent carbon diffusion between the layers.  Why the layers structure? To achieve the high elongation, we need to elongate the mountain site, as fetched mountain site.  In a dual phase steels, mountain site is not deformed because the stress is not partitioned to the mountain site.  And there is a stress concentration in a ferrite matrix, particularly between a mountain site island which results in a voiding and eventually a fracture.  In case of layer structure, stress is partitioned, both ductile layer and high strength layer.  So with a plastic constraint, the mountain site should be elongated as long as the local fracture is suppressed, which is not easy.  These are the data of strength and ductility of laminated metals, different laminated metals, which was summarized by Ligiere and Charby.  And the strength always follows the rule of mixture, rule of average along this line, but the ductility does not follow the rule of mixture.  This vertical axis is elongation of laminated metals consisting of 50% ductile and 50% brittle component.  And this elongation comes down here, far below the rule of average, as the elongation of brittle component becomes low.  Why do we have such low elongation? Because we have a brittle fracture during elongation in the brittle layer, as shown here, which is caused by a delamination and resulting H-shaped cracks and also so-called tunnel cracks.  We have to suppress those local fractures to obtain larger elongation.  In terms of delamination and H-shaped cracks, naturally increasing interfacial toughness increases elongation of multi-layer steel.  And when there is a delamination, brittle layer behaves like a single component and without plastic constraint, it breaks, it fractures with low elongation like here.  And if the interfacial toughness is not enough, the elongation is still below the uniform elongation predicted by a rule of mixture.  And with increasing interfacial toughness, the multi-layer steels are fractured with diffused necking in a ductile manner.  This boundary can be predicted by this equation, which was also developed in the research of composite.  And this criterion is given by this line. Looks like this criterion works well.  For the prevention of tunnel crack, there is also work about tunnel crack in the research of semi-conductors, which is given here, where the thickness of a brittle layer is limited, which is a function of a fracture toughness of brittle layers.  So you need to reduce the thickness of brittle layer to avoid the tunnel crack.  But this criterion was derived in an elastic situation.  In case of metals, you have a plastic zone in a ductile layer in the vicinity of the tunnel crack.  We have to consider that.  Considering the elastic situation, we derived this criterion. Again, the thickness of brittle layer should be reduced to increase elongation, which is a function of fracture toughness of brittle layer and the use of strength of ductile layers.  Certainly, the decrease of the layer thickness increases the elongation of the ascended mountain side.  By decreasing the thickness of mountain side, this type 420 high carbon, mountain acidic stainless steel, can be elongated up to 20%.  So without multi-layer structure, we can't elongate the ascended mountain side in this way.  Also, the fracture surface changes from brittle to ductile dimpling as the thickness of mountain side layer is decreased.  This is the effect of thickness of brittle layer on elongation.  These two lines are from the elastic model and elastic plastic model. Many use authentic type 3.04 stainless steel for ductile layers because type 3.04 stainless steel has good work hardening.  The transition from low elongation to the high elongation is close to the elastic model.  When you use interstitial free ferrite as a ductile layer, because this steel does not show much work hardening.  The transition behavior from low elongation to the high elongation is close to the elastic plastic model.  In a multi-layer steel, the transition from low elongation to the high elongation is somewhere in between. The elastic plastic model we developed gives a lower boundary in the design of multi-layer steel.  Also, the limitation of the brittle layer is the function of fracture toughness. These are confirmed here. We provided the mountain steel having a different fracture toughness and measured the transition.  Increasing fracture toughness gives the thicker brittle layers. In other words, you can increase the thickness of brittle layer if you have a mountain steel with a better toughness.  Or you can reduce the number of layers.  By controlling the interfacial toughness and the thickness of brittle layers, we can elongate the ascension to the mountain side.  If you apply a neutral diffraction, this is the result. You will measure the fully partitioning of stress.  Because of the partitioning of stress, the mountain side is elongated here.  As a result, we obtained steels which have high strength and high ductility.  Those are plotted here. Those steels have strength more than 1200 mega Pascal and still have elongation of more than 20% percent.  The product of strength and elongation is more than double of conventional monolithic steels.  These mountain steel keep the elongation even under the high strain rate deformation.  Here are the stress strain curves under different strain rates. The maximum is 800 per second.  If the strength increases with increasing the strain rate, the elongation does not change much.  Those photos are the test results of high speed buckwheat which simulate the collision of front side members of automobiles.  The 1200 mega Pascal mountain steel deforms perfectly in the same way as the 590 Duelphase steels.  There is still space for additional deformation because of the high strain.  There is no delamination of a local truck during this high strain rate deformation.  Here is another high strain rate deformation. This is an impact bending test which simulates pillars of automobiles.  The bending strength is increased in the mountain layer steels.  Here is the DP590 for comparison. The bending strength is increased here.  In the application of multi-layer steel, we need welding.  We are trying to weld the multi-layer steels using friction-stow welding. This is cross-section.  The welding is successful, which has a joint efficiency of more than 90%.  It is interesting to note that this layer structure remains not only in the heel-affected zone but also in the steel zone.  Using multi-layer steel, we can look at deformation behavior of a splenched mountain site, which was difficult before because of the load activity of a splenched mountain site.  We are now conducting many in-situ observations on the deformation of a splenched mountain site using an EBSP.  Here are the multi-layer steels. This is monolithic mountain steel.  We found that the slip is always in the plane, in the plane, and after certain strain levels.  Beyond that, the slip across the last direction appears.  This slip is concentrated in the region where the cement factor in that plane is high and no slip in those regions where the cement factor is low.  This is a similar result using a digital image correlation using a silver nanoparticle during a tensile test.  Again, the stress concentration is in a mountain site block where the cement factor along the in-last plane is high.  Another part is not deformed significantly, even though the cement factor is high, which is out of the last plane.  The cement factor along the in-last plane is low because this is low.  Further improvement of the multi-layer steels, we need to improve the process.  We are now looking at lower pressure bonding, which makes the fabrication much easier and more efficient.  In terms of components, not only use the high carbon, mountain-steel steels, we are now using a HTP metal such as magnesium and titanium to achieve a lighter multi-layer steels.  We are now using a steel with high impurity, like a scrap steel, so that we can use a scrap to fabricate high-performance steels.  Here is an example of a magnesium steel multi-layer.  We have developed a good bonding process to join the magnesium steel and we fabricate a three-layer magnesium steel multi-layer.  Magnesium is commercially available lightest metal, but the problem is the ductility. Because of HCP structures, elongation is up to 20% or even less.  By employing a multi-layer structure, we can increase the ductility of magnesium without any break, up to 35-40%. The strength is also increased because of the steels.  This is the summary of my talk. Thank you very much for your attention.  I was wondering how you deal with the differential volume contractions and expansions that you get when you quench the multi-layers. Does that set up residual stresses between the layers that might result in crack propagation being easier and delamination becoming easier?  Yes, you are right. There must be some residual stress. We are now measuring how much residual stress and what the effect on the mechanical properties is. Certainly, there is some residual stress.  You are saying that the decrease in martensite layer thickness is increasing the toughness. Is it not due to locally plane strain loading condition starting in only the martensitic layer?  You are right. As the thickness decreases, the situation comes to that. The thickness can be thicker. If the fracture toughness of the martensite layer is maybe medium or not high.  Today, I showed the martensite, which is type 420 high carbon martensitic stainless steel. This is really brittle. The fracture toughness is really low.  The normal carbon steel, the martensite carbon steel, is not such low. We can increase more. The situation is not simply a plane.  So far, you have made these materials by starting with the original layers and then rolling them together. Have you looked at other methods of fabricating this kind of structure?  Additive layer manufacturing looks like it would be well suited to a layered structure like that with electron beam or laser. Depositing powder and then you have a lot of flexibility to pick material or thickness.  The multi-layer structure is employed everywhere, like in a semiconductor. In that case, as you mentioned, they deposit layer by layer. But this is a structure still. We need volume.  So the easier and the simpler fabrication is better. We sort of have many possibilities, but at the moment, I think this is the simplest way.  What sort of thickness are you aiming to get? You say it's easy to get good volume material. What sort of thickness and volumes are you interested in fabricating?  In our case, the thickness. Anyway, this is sheet material. So the thickness is about 0.5 to 2 mm or something like that. The final thickness.  A sheet quite wide, I guess. In our research, we already made 60 mm steels, coil to coil.  So when you do the hot rolling or warm rolling of martensite multi-layer. So the martensite phase actually during rolling is to change back to also the night or it's in martensite. In hot rolling and warm rolling.  Could you repeat again?  When you do hot rolling or warm rolling of your multi-layer steels, one layer, suppose it's a martensite. So the martensite in that temperature has actually changed back to also the night or.  I understand. During fabrication, martensitic steels does not have the micro structure of martensite. It's like a mixture of ferrite and cementite or something.  During hot rolling. And after that, the heat treatment is made after the alternate region and the quenched. So where the martensite is formed.  Thanks for a very interesting presentation. Reminds me of the fiber metal laminate work that was done a few years ago. Introducing polymers and metal layers together. I just wondered if you'd done any corrosion studies introducing dissimilar metals, often problematic.  That's also a good question. We have done the corrosion test. There is a possibility that the corrosion resistant is decreased if you combine like a magnesium.  Did you measure the Young's modulus of the martenite materials?  Yes. But at the moment we combine steel to steel. So Young's modulus is the same.  Yes, it means just follow the mixture.  Yes, yes, yes. This is well known.  I was wondering if you did fatigue testing and stress localization testing for formability.  We are conducting a fatigue test too. At the moment there is nothing I can say about that. Certainly the martensite still affects the fatigue behavior.  Did you make stress localization experiments to understand the viscoplastic behavior under forming or for forming?  At the moment we have done much.  Thank you.  In a 3D deformation.  Thank you for a nice presentation. Could you explain me in a few words what is the difference between your idea of composite material on the base of steel with, for example, the Mahagana steel or Damascus steel?  Yes, maybe the origin is the same. In the past many people studied the Mahagana steel metals, even the Damascus too.  But in the past it looks like the less attention was paid to the ductility. People try to increase the strength, but the research on ductility is limited as far as we investigate it.  Thank you.  Do you need heat treatment? Do you study how important is the diffusion of carbon between the same layers of high-glow carbon steel?  Yes, it is very important to suppress the diffusion of carbon.  And when we combine the different steels, we always think about the activity of carbon during the heat treatment so that maybe alloy design is needed to suppress the diffusion of carbon for brittle layer and the ductile layer.  Toshie, how do you measure the interfacial strength?  We use a pure test, but we can't measure if the interfacial strength is really high. Just only a brittle interface can be measured.  And how do you control it?  Namu, can you explain that?  In the case of the interfacial toughness, the mohology of the test is the tensile direction is just 180 degrees.  So the mohology and the stress direction is almost constant so we can evaluate the difference.  But supposing that the interfacial strength is too low, how can I change it?  Of course, in the case of very, very weak interfacial toughness, it is very difficult to evaluate.  But in the case of the R2, for example, from 500 degrees C or 600 degrees C, we can evaluate the interfacial toughness.  So the interfacial toughness is increased by a bonding process and heat treatment. You can increase the interfacial toughness.  And I'd like to note that interfacial toughness does not be really high unless you give a pure test.  Based on that, would it be possible if you did have quite a high stress at the interface to design the ductile phase  so that it underwent slight plastic relaxation at the interface, which would provide a bit of work hardening  and would that be beneficial in a way to reduce any stresses that you might get generated in the alloy?  Would that be feasible or would the work hardening limit your layer thickness and the further work hardening capability of it?  Would that then have an adverse effect on ductility, do you think?  So with the residual stresses that are set up in the interface, if they were reasonably high,  would it be feasible to design your ductile layer to have a yield strength such that it undergoes slight plastic deformation  to relieve the residual stresses? And would that work hardening then have an adverse effect on the ductility as a result?  Well, up to now we haven't found any adverse effect on that.  Yes, let me be a bit adventurous here and about a little bit speculative as well.  Perhaps it could decrease the carbon diffusion between layers and perhaps improve the cohesion effect.  If you dealt with, let's say, nano structure steel, so you would get a strength out of a nano structure steel layer  and keep your soft layer as well. This is highly speculative, you see.  So it seems adequate in all of these tools to do that. What are your ideas about that?  You mean the diffusion across the interface. We are now researching that and we want to minimize the diffusion layer  to increase the strength and ductility for the brittle layer and the ductile layer.  The diffusion and carbon decrease the strength and ductility both.  So that's why I'm saying low temperature bonding is necessary to improve the performance of monthly steel.  We have a feeling that the bonding of the interface is possible at medium or lower temperatures  and we don't need a big diffusion to increase the interfacial strength.  Did you look at interfaces at higher magnification and in general how important is the quality of interfaces?  Do you have rough interfaces or more kind of intermix?  We are looking at interfaces using a transmission electron microscope.  Of course there is some small void and some discontinuity and some part continues.  The details of the development of interfaces need to be researched more.  I have a more macroscopic question. You are reducing the thickness of the martensitic layer to increase elongation.  This is traditional bi-phase philosophy. You are also saying that strength is a rule of mixtures.  Would you not then expect strength to drop? So you sacrifice strength to increase elongation.  Which is exactly what happens in any bi-phase microscope.  This is a combination. We can't go beyond the rule of mixtures.  There is some compensation but we can use higher strength of steel, like a very high carbon steel,  so that we can increase the strength. The elongation is always... If we make an old effort, we can achieve the rule of mixtures, even for the elongation.  So you can increase... Let me explain more.  Anyway, this is the mixture of high strength layers and low strength layers, and the mixture of high ductility and low ductility.  We can go this way more by using higher strength steel.  Thank you very much indeed. It has been an excellent talk and an excellent discussion.
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