Mechanisms of hydrogen embrittlement in metals

The Influence of Hydrogen on Fracture Energy: A New Mechanism of Embrittlement

Researchers have long been studying the effects of hydrogen on fracture energy, and recent findings suggest that this process can lead to significant embrittlement in certain materials. The study explores how the introduction of hydrogen affects the fracture energy of a material, particularly when it comes into contact with crack tips.

In the first layer, the fracture energy decreases as the diffusion process takes hold. This is followed by the formation of a new phase, where the fracture energy reaches zero. When this layer is combined with two additional layers, the resulting plot reveals a single seed and a natural end to the diffusion process.

However, when the three layers are combined, the picture changes significantly. The fracture energy increases as it reaches a local maximum, followed by a decrease in the local diffusion region. This decrease leads to a significant drop in fracture energy, suggesting that hydrogen can cause embrittlement in certain materials.

The researchers used a combination of computational and experimental methods to study the effects of hydrogen on fracture energy. They found that the aggregation process near the crack tip is thermodynamically favorable at room temperature, allowing for a higher intensity cohesion than pure surface energy. This means that the embrittlement process is not just driven by surface energy, but also by the activation of certain planes within the material.

The study also investigated the role of hydrogen in changing the mechanical properties of materials. When hydrogen is introduced into nickel, it increases the unstable stacking fault energy, making the material more brittle and prone to cracking. This is particularly evident when the stress field around the crack tip becomes significant, leading to the formation of a precipitate that blocks the propagation of cracks.

One of the key findings of the study is that the concentration of hydrogen at the crack tip can be predicted using thermodynamic calculations. The researchers used a combination of chemical potential and entropy terms to determine the optimal concentration of hydrogen for embrittlement. This calculation showed that the concentration of hydrogen required for embrittlement can be achieved at room temperature, suggesting that this process is not limited to high-pressure conditions.

The study also highlights the importance of considering the microstructure of materials when studying the effects of hydrogen on fracture energy. The researchers found that the formation of a precipitate ahead of the crack tip was crucial in understanding the embrittlement process. This suggests that the role of hydrogen is not just limited to surface chemistry, but also involves the formation of defects within the material.

Overall, this study provides new insights into the mechanisms of embrittlement caused by hydrogen. The findings suggest that this process can lead to significant changes in the mechanical properties of materials, particularly when it comes to fracture energy. The researchers hope that their work will contribute to a better understanding of the role of hydrogen in materials science and lead to the development of new strategies for mitigating embrittlement in critical applications.

The study also has implications for the behavior of iron-nickel alloys. When the stress field around the crack tip becomes significant, it can lead to the formation of a precipitate that blocks the propagation of cracks. This suggests that these materials are prone to cracking and embrittlement under certain conditions. The researchers hope that their findings will contribute to a better understanding of the behavior of these materials and help to develop new strategies for mitigating embrittlement.

In conclusion, this study provides new insights into the mechanisms of embrittlement caused by hydrogen. The researchers found that the introduction of hydrogen can lead to significant changes in fracture energy, particularly when it comes into contact with crack tips. They also highlighted the importance of considering microstructure and defects within materials when studying the effects of hydrogen on fracture energy.

"WEBVTTKind: captionsLanguage: enand I now have the great pleasure to introduce professor William Curtin he and I have geographically intersected both the ground and quor'toth universities and he is the director of mechanical engineering and EPFL in Switzerland please my perspective on it I think we could have canceled this talking continued holistic Professor Peter Laura who provided an absolutely beautiful experiment a small scale and that's extremely helpful for me because the question of applying continual mechanically fracture mechanics down are very small scales people always wonder about it and you see all these flexor dynamics simulation switch really terribly skeptical about however we model in science or to know what we're doing but experiment is like for desert or a clearly show that a field can extend down to very small scales in crystalline materials and fracture mechanics so now fractured so we always try to start a high skills and work down and then I'll tell you a couple of off very small scale and summarize so we know about the riddle meant going back to some Birnbaum that's very well known for its experiments back thirty years ago and the important thing is that what we tend to see is a tough piles in fracture this is from experiments Rebecca lat all more recently including Rob Ritchie who is involved this week pure nickel a no hydrogen very typical fracture and you a little bit of hydrogen and then in these cases a little bit of hydrogen is about a thousand parts per million clothings one out of every thousand metal atoms there's an extra hydrogen atom these are also very low concentrations here parts per million and you see this unit a so what is a tiny amounts of hydrogen they're doing that they do from the griddle the cup of an acre to a griddle behavior now people have been thinking about this do you think that hydrogen lowers the work fractured it easier influences the dislocations and plasticity the drove the metals in the subway whether it helps bless the city earth plasticity it may change the plasticity intention Plus this is looking ability and then some materials form hydrogen's we're going to rule those out a lot of BCC materials they may be on perhaps and think about nickel as their prototype FCC material you're gonna go silent recruitment and for an entire ECC materials though in spite of these ideas and these concepts there's really no predictive theory predicted exactly when item her and what constitutes under what loading rates and part of that is because many real engineering materials are extremely complicated microstructure multiple phases and it's very hard to the study those one immediately now if I may get one of those engineering materials is recent experiments but the model can look at the data and so this is the KSR label as a freshman but it's the fracture toughness and this is a temporal cell but this is just increasing hydrogens they should read from left right to left as increasing hydrogen and this is fracture toughness so if you are over here at low inertia content these materials suppression toughness is this value or bigger it's just some value up here these are valid practice tests and when you cross some value of hydrogen content suddenly the toughness goes from this value down which is relatively brittle materials and whether or not don't worry about the line this is some theory I'm not worried about this look using this stuff and so here's the transition macroscopically in terms of measurable body the other thing you can measure is once material disfavor brittle in that repeating also gets low practice and so if you look at a crack growth rate holding steady load not not cyclical opening you see that over here where the material were very tough there's essentially no growth and notice this is microns per second so if n minus five microns per second is entering sperm per second so this is this there's no growth here yet when you're in the hydrogen embrittlement regime you have some growth rates they're still very small and the minus one or ten minus two microns per second but definitely is so there's experiments we like understand when you go from this behavior in this page or maybe it's continuous and it's not and what what's underlying slow progress is hard to sort out what it is is important what's driving for not driving a fracture is it really a fracture problem maybe it was the deprivation problem you're changing the plasticity oil growth and it's not really a fracture problems like about the fracture and visitor corrosion problem we saw some ice for very long time - exposure 30 years where it appears of what the hydrogen does is drive the coreography precipitates the break boundaries which is a normal prosit process so there could be all different kinds of things going on that effect - Brooklyn in cloud scale and over 30 years here so we have many different phenomena and which of them are essential perhaps all public together perhaps this is a sequence of things and maybe there are different different material is a different phenomena controls what's going on we all classify the pilot for this exquisitely challenging and there are mechanistic particular theories every material is different like that and the ears because each material fails maybe rusty right now we look at models also to aspect I could infuse attractive or into some unspecified fracture process zone around the hydrogen transport that makes the problem dependent and then - plus practic specify these - these developments without these you can defeat results like today however just putting the models with these two together that doesn't imply that didn't think about the model is correct but if I if you Sun even cracked everyone have some sort of punch and failure dominant I will predict as I said the - influence plastic flow is sometimes Seattle avoids all physics happening they may not be directly connected fractured so these are the new processes there one the general public the fracture is the diffusion - is the practice because I think you must want to fit in the last as mrs. quality and it's larger because it's interstitially it's a large misfit it's attracted to the tensile field the traffic intensity around the tractor a plastic field around the craft of distress is very high so you would attract a lot either so sometimes people are inducing are using gradient plus two C of X increased stress field and the diffusion rate is also in question because you're confusing the hydrogen and there could be traps locations down 50 or 70 so we have to put there's the diffusion process but there's a you need the field and you need the diffusion rate so very generally if I have hydrostatic threat stress field as a function of 10 and again I couldn't elastic filled plastic field HRR field behind will see a stress field a chemical potential of a hydrophilic lower hydrogen atom phenol times the Misses volume the forest is the gradient there's force acting on the hydrogen atom in position and the velocity is related through the Einstein relation by fusion coefficient and so you get just looking at radio compliment of a radial force and that's attracting the hydrogen for the crack-tip thus in the fusion particle and part of the problem we can think of the standard problems and fracture you know right after you have a policeman found that allows some to fit over some cohesive stress and then you have a plasticity law as a k2 plasticity could be gradient plasticity I think though if I look at the steady-state fracture toughness divided by the brittleness or Griffith fracture toughness the cohesive zone these are different gradient ACL somewhere about them just pick one and then the cohesive strength nobody else trying to get some sort of curve like this so the macroscopic toughness is many times larger than the microscopic vertical fracture it depends on this one so when I have hydrogen to the problem I would lower the visa strength to make it easier for the material part and that would provide this here somewhere and that would be a breast awesome opening the other thing is the hydrogen could lower the intrinsic cookies of energy and just change the scale here and that would lower the toughest this is very generic fractured part of the picture and the question is can use enough islands attractive to change maybe and screw the is what the screws alright so so that the pictures was sort of the work the question is do I get enough allusion to the crack-tip to either lower the cohesive strength or lower the cohesive energy if not the lower the toughest from huge values and generally the answer is it's difficult infuse that lecture as much hydrogen would need when you think about the moral continuum it's not the best thing to do here well the chair how much okay all right so usually the diffusion models don't provide enough hydrogen to get a big effect here or a big effect here with your continual fields and these pictures were generic so to be quantitative predictive we want to understand the physical processes and the critical amounts of hydrogen and if we can do that and perhaps we could say something about why certain materials would be more resistant to certain micro structures what I want to do is is this is there's lots of complications I want to simplify it with basic fundamental problem which is the nano scale fracture mechanics and see what we can learn so let's step back to think about major modes of track sure they're brittle fracture which we saw real pictures colonel perhaps ghrelin which are governed by the fraction of is the fracture energy just a cleavage energy the way that those failed her if I start with a sharp crack I have envisioned the dislocation are and these simulation that that was the craft and once I want the correct I decrease the whole stress Tiffany a part of the fracture that happens is the rest criteria which for the moment is fine there's some stress intensity and generally this stress intensity is lower than Kaitlyn seats of this process of admission happens first instead of this then they start plunging the crack once I cut the crap I can get more plasticity out something to give more plasticity more pumping in more Pleistocene work London and select to start and get the real one to create voids so the brittle bone buffalo double mode the final fracture toughness through all displaced missing much much higher than this fracture toughness and we get large-scale plasticity I promise so we have these fundamental Tetons the fracture toughness brittle material and the omission dislocations which starts this process this is not fracture toughness or puttan it's the if it's the start of this process that enables the sharp crack become blunted and once it's blunted all kinds of other good things happen so welcome back to the experiments what do I see I don't see a line like as I see a threshold I see materials that are tough and materials that are brittle I see a threshold we're crossing over for brittle mister hydrogen saying the slow crack growth I see no crack growth I see crack growth is he a threat come in here so there's some concentration of hydrogen under certain conditions that will so my scenario is sending that without hydrogen we have this kind of picture of a fill here and with Ellucian we have the following we have plasticity in the parking doorways dislocations out there there's still plasticity it's not a brittle material like perfect silicon but the hydrogen advocates an attractive and allows the attractor locally attractive with surrounding plasticity but it never wants out and once it everyone's out they can't develop the very large plastic zone and the boiling creation is very typographer so I end up with something that's possible it's cleavage life at the atomic scale with the surrounding clusters here so keep in mind we still kept barking plasticity but we're going to be looking globally interactive so a nitrogen make this transition from this behavior to this page well as professor if I think a pure material like nickel and I love occasional forget what this family boundary conditions indicate simulation start submitting dislocations omitting this location one thing the craft and as I do that I keep increasing big macroscopic entity but of course the stress attractive is being lower relatively wasn't hurt but I keep increasing has increased a bit more parking the plasticity and I'm starting to headquarter duffel failure so an Irish and modify this paper in a Kenny used this admission behavior in some way it's suppressed Commission well there's some experimental evidence look at all the experiments that be often iron three percent silicon if you don't have any hydrogen at all you see the practice practice open quite a bit disease bird Bigfoot bands which presumably and they attractive if you have a little bit of I believe in their craft becomes much chart where you see much less animation of slip and if you increase find the depression further the crack becomes very sharp and you don't see you see variants so it appears that I wouldn't because suppress the dislocation of mission from the crack-tip and enable a much sharper leaner crack these lots of lovely participating for better partners on cantilever specimens making this down to the micron scale we can analyze those more carefully we may see simply without so the ductile of brittle transition is now the question whether or not dissipations of one blunt or not it's a very vocal nanoscale question but we see the professor his loris talk with the concepts of fracture mechanics and petting fields extend them all the way down to the crack there yes remember no plastic material continuum scale we model elastic material for classical constitutive law but if I really go in and look in the material in the microscope I see atoms and dislocations then this locations give me plasticity I mean atoms are straining elastically so I really have an elastic filled with dislocations dissipation speaking elvish children and when we continue so even in the losses losses became very close so the riddle thing can happen in two ways one is I can take K 1 C which is normally higher than they would be and I can reduce it below and it reduces below pay money that question is and I reduce it enough it's not sufficient to reduce a little bit has to be reduced below this level both it's not just a phenomena that reduces the other possibility is that raise effectively can't want to make dislocation Commission harder and it goes above k1c so these are the two possibilities and let's look at this one first it's like that didn't either so here's I was praying for an HDMI connection so here's the increase of k1e this is Nick the whole here's a relatively sharp practice the black are hydrogen atoms and if I allow hydrogen diffusion which I'm not showing it ever gets around the practice is all you get this is a delivery of hydrogen let's go to the services there's a little bit inside but that's it but that's efficient at this load you would normally admit at this location but not that amount just enough permitting at this location that means stepping and load higher and if I love higher and with a stronger case field I bring in more light and I start building up hiring around the crack because of the higher okay I snuck into the dislocation so I can load it even higher now this is the microscopic penny near the crack-tip light been something different due to the surrounding plasticity this isn't attractive picture let's keep this process going keep breathing and more I lose more hydrogen and finally the only thing that it can do but the only thing you can do is there no dislocations can get in Annapolis and tear a second dislocation free zone but created by the fact that when I put hydrogen in the nickel it's very hard to admit at this location and at some point fees and because this is really relatively world material we saw a nice yesterday also about the continuum August what happens so what's happened is our suppressed at this location mission the only thing left is complete but now I have this sharp crack the surrounding plasticity but I can't get the wanted now the same thing happens in a grain boundary we remember we sought cleavage or long-range boundary and it's simply that the bread crackers are a little easier to beat than the bulk nipple but it's the same process we have to get a little hydrogen along the grain boundary but then they build up a little region and then finally it splits and it's whistle on the grain boundary visits the lowest energy and that's that's easier to do so the process happens whether in the bulk or at the grain boundary Department at the grain boundary now everyone remind you that these are these low values of penguins CDs that you see these are the practicalities that those are this down here essentially it's not this about the ceramic plasticity gives you if this value it's in these plots this toughness does not include ductile fracture processes so the measured cuff has been to be up here somewhere the local toughness is what's changing down here and ductile fracture their powers suddenly were late is activation and brittle meant two conditions great well we have kinetics fusion fracture condition you know five years and we need and what load so we go back to this problem and we look at the elastic female because around the crack locally though we have dislocations we have plastic field and we can work out how much hydrogen will come to the crack-tip in a certain amount of us are loading rate a certain amount I'm all the hydrogen in some radius used the tractor that radius but all of this come to practice so we can calculate how much hydrogen per unit length the crack comes by integrating over the cylinder that is collected hydrogen and I have some complicated messy result but that's okay it just that the total amount of hydrogen under a constant loading rate scales with the current stress intensity to eight bits comes out from the gauge field and this coefficient is this which involves the diffusion rate the misfit volume the concentration the temperature and the loading rate so this is a relationship between how much vitamin I get attractive and the current stress intensity is a function of diffusion by the concentration temperature loading rate now this parameter determines whether I think we've written or not there's a critical value for this if this if this parameter exceeds the critical value that means I have enough hydrogen or fast diffusion or slow loading I'll get rid of it before recommended dislocations if I'm below this level that means a low concentration low diffusion slow diffusion or fast loading then again another grill and because I find for fibers we found before they start wanting to crack so now this is our threshold condition are we above or below this value and we get this value from our simulations so simulations either show this buckle behavior this is the amount of hydrogen near the crack-tip this is the kuv there's the buckle better or misty brittle behavior and there's a threshold some critical value somewhere in this range for this a zero star and if I'm over here material please here all of it dissipations messes our Buffalo behavior and up a fracture if we look at this in nickel we don't have all the precise numbers when we did this with blunted cracks but let's just look at sharp products the last diffusion that was ten to the minus fourteen and with traps probably somewhere in here and clean nickel but with dislocations and other small traps and as a pump these are different hydrogen concentrations would have get incredibly both inhale a hundred experiments fall right down here they're a griddle around 500 parts per million and so the diffusion pumps and somewhere in here we would predict a little bit those bet the experiments are in Britain about a thousand and again the diffusion coefficient is somewhere around here then we would predict that these material should be in Britain at the spring rates and estheticians so there's no fitting here but we're not making exact prediction but we see that what we find for this is perfect condition where in there out the right all are the other part I have not mentioned was that after you leave the hydrogen will follow the crack-tip and you might think well that's maybe a slow process but in fact if you start with the crack here this is all the hydrogen in the three-dimensional picture if you click the movie here if you now just let hydrogen diffuse in if you see higher ed in sent me an acceptance a lot of this hydrogen will end up over here and so because there's very strong driving force that local field is pulling the hydrogen with it once I've aggregated the hydrogen then most of that hydrogen carry along so as a consequence to continue practically need to saturate new crack surface as the crack grows it's time to cover the surfaces tightly and the rest of the hydrogen is carried along plenty to diffuse in a little bit of pilot run and you can calculate then how much I wouldn't need and you can calculate crack growth rate and it's given by that assisting my practice birds very slowly with that so there's a certain predicted cracker imprint that's right up there ain't no a bear that stay calm quickly into the data here's the data and there's the prediction absolutely that there's the prediction of the crack growth rate again against the stick a little bit Korea it's not perfect but it's very close it shows the same trend as the experiment because of the threshold the threshold is when I crossed over that is here a scar and then there's a slow crack growth regime and I predicted that the more I was going to happen or the higher end a or whatever the bastard that's what we see a good agreement of magnitude we've published that I showed this a few years ago sitting on ferritic seals that won't repeat it because even injury-time I'm gonna introduce on right and we found in all these heretics feels that we can predict the enrollment in all the way second one and third one of them our condition is that this number is less than 1 or greater than 1 and we're very close to that all the rest of the be ridiculous so for most nickel and iron birth 13 systems were structured other things as a function different hydrogen concentrations different materials different loading loading rates okay so that was that was this process here where the hydrogen aggregation makes dislocation hard and basically makes locally makes empirical more like silicon and like nickel or iron the other process is the second process which is maybe I can reduce k1c enough and then a care about the dislocation issue now Jared's on responsive to first principles these quantum mechanical evolution of the surface energy of hydrogen on nickel and what you see is this is a pure this is fracture toughness that comes from that energy and for pure nickel you get fracture toughness up here and down here that's why maple emits dislocations as you saturate the the service is bad - this goes down but at 50% all coverage were hunters and fault if I separated 50% of the surfaces of the most I think that it's 50% and I'm still up here in England C and that's above k1 e so for a long time we thought this process and the curve because we're still at this point well of a 1 e and the materials mid dissipation you can't go to 100% coverage because there's no place by boat here you can only 50% coverage so this is what was driving the other model that we were always stuck here however we now realize very subtle back that I don't like it me together slightly that's why we form hydrides and if you think w hydrogen then you aggregated into three layers the hydrogens like to be slightly together and that lowers the energy of this structure it won't happen away from the practice human entropy but near the crack-tip it will happen and now when I fractured this material I have hydrogen on the surface of 50% and hydrogen in the sulfur is it a hundred percent and that fracture energy is lower than the fracture energy when I don't have this extra hydrogen here an extra contribution in the search reducing the surface energy compared to the to the original service worthless that 50% coverage and then if some of this hydrogen now infuses a surface where it really wants to be I could make the fact that fracturing this is actually less than zero the materials to spontaneously come apart so so if I have this process of the hydrogen aggregating in three layers and nickel and then it can fracture do this may be a tenement and I'll show you in a moment and then when it goes to this is never going back as its negative energy is much lower than the original bulk energy so here's these here's the surface energy showed you before which you go up to 50 percent you keep covering it but there's no way to get there we're free in with three layers the energy goes from here in here and then with that bowl diffusion now I've got enough to get 100% I can get down here this is the most I can get in the bulk but if I getting in those three layers a fracture energy or fracture energy goes down here and then I hit local diffusion and the fracture energy goes to zero now this is if I put this it oncee plot what do I get here's a one seed and can end here naturally but with the three layers I go down here and now below it 1e and then that the fusion that I'm Way down here and so this desert is now really written as a1c is lower than k1 e but I have to aggregate the three layers the pilot just one single air hydrogen do what the diffusion process will it was a combination of hydrogen if I look at assisted by the crack tip stress field as the diffusion but also the energy energetics so I've had the concentration just as attractive and have a far end concentration of C 0 then in equilibrium the chemical potential of hydrogen the bulk and the chemical potential the hydrogen of the the practive have to be equal and the crack-tip I have the entropy term I have the driving force due to the pressure around the crack field and the hydrogen hydrogen interaction this should be seen aged here and that should be equal to essentially the entropy or behaviour and far wet if you look at this best mystic volume aren't using Omega before that's the hydrogen - binding energy nickel we calculate that and that's the pressure field right here the crack there few angstroms so like what that in I don't have to make how much hydrogen is at the crack-tip as a function of temperature and you can see that I can get very close to 100% dependent on the K value and if we focus on these guys at room temperature I got 100% aggregation around the crack there absolutely temperature at okay which is below the admission and it is around the fracture head so the aggregation go back to this picture though this aggregation process near the crack-tip happens at room temperature as equilibrium at room temperature at any values where do we get the fracture at a value in the center down here in the point five range so that means that this process can happen it's thermodynamically favorable and this provides us with a real embrittlement process where it's a higher intensity cohesion but it's not just pure surface energy - the spot there has to activate not just on the surface within this slight vault region and that lowers the surface energy so I'll summarize - to be his duck with a brittle transition and go from one of these pictures one of these pictures fortunately these pictures you see before we go it was we go from London buckle fracture picture to very locally in cleavage picture attractive because hydrogen air either lock submission or really kind of an equal real cleavage fitted here due to the aggregation type and both of those mechanisms seem to operated nipple the first one probably can operate in most materials the second one who may have been much more month on thermodynamics but an operational which is what in what we see experimental II so what I've done is shown you operative mechanisms brittle mechanism totally controlled by the hydrogen and this best as we can compare experiments on material and iron but and that as far as we can go everything just it's quite good enough purpose that was given unto very interesting conditions and I could you know relevant to the sub I couldn't catch up the mechanism of hydrogen so if we just take the rice right here in itself when you have hydrogen into nickel you increase the unstable stacking fault energy you make it and nickel hydride is really a very brittle material nickel hydride is not easily excited using them as for dissertations so as the hydrogen increases around the crack there it's harder to slip those planes and the k1 ee goes up so we calculated these things that's the mechanism although there are hybrids and iron rates so iron has double ACP my pride but here keep in mind that the only reason we have a hydride here or something like the hydride is because we're very close to the crack-tip and the stresses are very high so there is no hydride have in the bulk or very far away from normal hydranoid so people look at they say look at high gravity or the crack that we can see I Drive an iron nickel but this is very this is on the scale of just 2 nanometers were rounding out there and it's almost not observable experimental II so so it's not it it's not a normal highway but both parents are migrants yeah so it's like hard part of the hydride is likenesses is a precipitate that's formed ahead of the crack-tip how how much hydrogen is necessary in this session well it becomes like a precipitate minute blocks in this location and because it's where the precipitate forms forever about Paris apparently so I was just wondering this meeting is - format understanding these new mechanisms are two ways you influence what others like Portugal - all confusion but anything yes there's a slightly higherand I now have the great pleasure to introduce professor William Curtin he and I have geographically intersected both the ground and quor'toth universities and he is the director of mechanical engineering and EPFL in Switzerland please my perspective on it I think we could have canceled this talking continued holistic Professor Peter Laura who provided an absolutely beautiful experiment a small scale and that's extremely helpful for me because the question of applying continual mechanically fracture mechanics down are very small scales people always wonder about it and you see all these flexor dynamics simulation switch really terribly skeptical about however we model in science or to know what we're doing but experiment is like for desert or a clearly show that a field can extend down to very small scales in crystalline materials and fracture mechanics so now fractured so we always try to start a high skills and work down and then I'll tell you a couple of off very small scale and summarize so we know about the riddle meant going back to some Birnbaum that's very well known for its experiments back thirty years ago and the important thing is that what we tend to see is a tough piles in fracture this is from experiments Rebecca lat all more recently including Rob Ritchie who is involved this week pure nickel a no hydrogen very typical fracture and you a little bit of hydrogen and then in these cases a little bit of hydrogen is about a thousand parts per million clothings one out of every thousand metal atoms there's an extra hydrogen atom these are also very low concentrations here parts per million and you see this unit a so what is a tiny amounts of hydrogen they're doing that they do from the griddle the cup of an acre to a griddle behavior now people have been thinking about this do you think that hydrogen lowers the work fractured it easier influences the dislocations and plasticity the drove the metals in the subway whether it helps bless the city earth plasticity it may change the plasticity intention Plus this is looking ability and then some materials form hydrogen's we're going to rule those out a lot of BCC materials they may be on perhaps and think about nickel as their prototype FCC material you're gonna go silent recruitment and for an entire ECC materials though in spite of these ideas and these concepts there's really no predictive theory predicted exactly when item her and what constitutes under what loading rates and part of that is because many real engineering materials are extremely complicated microstructure multiple phases and it's very hard to the study those one immediately now if I may get one of those engineering materials is recent experiments but the model can look at the data and so this is the KSR label as a freshman but it's the fracture toughness and this is a temporal cell but this is just increasing hydrogens they should read from left right to left as increasing hydrogen and this is fracture toughness so if you are over here at low inertia content these materials suppression toughness is this value or bigger it's just some value up here these are valid practice tests and when you cross some value of hydrogen content suddenly the toughness goes from this value down which is relatively brittle materials and whether or not don't worry about the line this is some theory I'm not worried about this look using this stuff and so here's the transition macroscopically in terms of measurable body the other thing you can measure is once material disfavor brittle in that repeating also gets low practice and so if you look at a crack growth rate holding steady load not not cyclical opening you see that over here where the material were very tough there's essentially no growth and notice this is microns per second so if n minus five microns per second is entering sperm per second so this is this there's no growth here yet when you're in the hydrogen embrittlement regime you have some growth rates they're still very small and the minus one or ten minus two microns per second but definitely is so there's experiments we like understand when you go from this behavior in this page or maybe it's continuous and it's not and what what's underlying slow progress is hard to sort out what it is is important what's driving for not driving a fracture is it really a fracture problem maybe it was the deprivation problem you're changing the plasticity oil growth and it's not really a fracture problems like about the fracture and visitor corrosion problem we saw some ice for very long time - exposure 30 years where it appears of what the hydrogen does is drive the coreography precipitates the break boundaries which is a normal prosit process so there could be all different kinds of things going on that effect - Brooklyn in cloud scale and over 30 years here so we have many different phenomena and which of them are essential perhaps all public together perhaps this is a sequence of things and maybe there are different different material is a different phenomena controls what's going on we all classify the pilot for this exquisitely challenging and there are mechanistic particular theories every material is different like that and the ears because each material fails maybe rusty right now we look at models also to aspect I could infuse attractive or into some unspecified fracture process zone around the hydrogen transport that makes the problem dependent and then - plus practic specify these - these developments without these you can defeat results like today however just putting the models with these two together that doesn't imply that didn't think about the model is correct but if I if you Sun even cracked everyone have some sort of punch and failure dominant I will predict as I said the - influence plastic flow is sometimes Seattle avoids all physics happening they may not be directly connected fractured so these are the new processes there one the general public the fracture is the diffusion - is the practice because I think you must want to fit in the last as mrs. quality and it's larger because it's interstitially it's a large misfit it's attracted to the tensile field the traffic intensity around the tractor a plastic field around the craft of distress is very high so you would attract a lot either so sometimes people are inducing are using gradient plus two C of X increased stress field and the diffusion rate is also in question because you're confusing the hydrogen and there could be traps locations down 50 or 70 so we have to put there's the diffusion process but there's a you need the field and you need the diffusion rate so very generally if I have hydrostatic threat stress field as a function of 10 and again I couldn't elastic filled plastic field HRR field behind will see a stress field a chemical potential of a hydrophilic lower hydrogen atom phenol times the Misses volume the forest is the gradient there's force acting on the hydrogen atom in position and the velocity is related through the Einstein relation by fusion coefficient and so you get just looking at radio compliment of a radial force and that's attracting the hydrogen for the crack-tip thus in the fusion particle and part of the problem we can think of the standard problems and fracture you know right after you have a policeman found that allows some to fit over some cohesive stress and then you have a plasticity law as a k2 plasticity could be gradient plasticity I think though if I look at the steady-state fracture toughness divided by the brittleness or Griffith fracture toughness the cohesive zone these are different gradient ACL somewhere about them just pick one and then the cohesive strength nobody else trying to get some sort of curve like this so the macroscopic toughness is many times larger than the microscopic vertical fracture it depends on this one so when I have hydrogen to the problem I would lower the visa strength to make it easier for the material part and that would provide this here somewhere and that would be a breast awesome opening the other thing is the hydrogen could lower the intrinsic cookies of energy and just change the scale here and that would lower the toughest this is very generic fractured part of the picture and the question is can use enough islands attractive to change maybe and screw the is what the screws alright so so that the pictures was sort of the work the question is do I get enough allusion to the crack-tip to either lower the cohesive strength or lower the cohesive energy if not the lower the toughest from huge values and generally the answer is it's difficult infuse that lecture as much hydrogen would need when you think about the moral continuum it's not the best thing to do here well the chair how much okay all right so usually the diffusion models don't provide enough hydrogen to get a big effect here or a big effect here with your continual fields and these pictures were generic so to be quantitative predictive we want to understand the physical processes and the critical amounts of hydrogen and if we can do that and perhaps we could say something about why certain materials would be more resistant to certain micro structures what I want to do is is this is there's lots of complications I want to simplify it with basic fundamental problem which is the nano scale fracture mechanics and see what we can learn so let's step back to think about major modes of track sure they're brittle fracture which we saw real pictures colonel perhaps ghrelin which are governed by the fraction of is the fracture energy just a cleavage energy the way that those failed her if I start with a sharp crack I have envisioned the dislocation are and these simulation that that was the craft and once I want the correct I decrease the whole stress Tiffany a part of the fracture that happens is the rest criteria which for the moment is fine there's some stress intensity and generally this stress intensity is lower than Kaitlyn seats of this process of admission happens first instead of this then they start plunging the crack once I cut the crap I can get more plasticity out something to give more plasticity more pumping in more Pleistocene work London and select to start and get the real one to create voids so the brittle bone buffalo double mode the final fracture toughness through all displaced missing much much higher than this fracture toughness and we get large-scale plasticity I promise so we have these fundamental Tetons the fracture toughness brittle material and the omission dislocations which starts this process this is not fracture toughness or puttan it's the if it's the start of this process that enables the sharp crack become blunted and once it's blunted all kinds of other good things happen so welcome back to the experiments what do I see I don't see a line like as I see a threshold I see materials that are tough and materials that are brittle I see a threshold we're crossing over for brittle mister hydrogen saying the slow crack growth I see no crack growth I see crack growth is he a threat come in here so there's some concentration of hydrogen under certain conditions that will so my scenario is sending that without hydrogen we have this kind of picture of a fill here and with Ellucian we have the following we have plasticity in the parking doorways dislocations out there there's still plasticity it's not a brittle material like perfect silicon but the hydrogen advocates an attractive and allows the attractor locally attractive with surrounding plasticity but it never wants out and once it everyone's out they can't develop the very large plastic zone and the boiling creation is very typographer so I end up with something that's possible it's cleavage life at the atomic scale with the surrounding clusters here so keep in mind we still kept barking plasticity but we're going to be looking globally interactive so a nitrogen make this transition from this behavior to this page well as professor if I think a pure material like nickel and I love occasional forget what this family boundary conditions indicate simulation start submitting dislocations omitting this location one thing the craft and as I do that I keep increasing big macroscopic entity but of course the stress attractive is being lower relatively wasn't hurt but I keep increasing has increased a bit more parking the plasticity and I'm starting to headquarter duffel failure so an Irish and modify this paper in a Kenny used this admission behavior in some way it's suppressed Commission well there's some experimental evidence look at all the experiments that be often iron three percent silicon if you don't have any hydrogen at all you see the practice practice open quite a bit disease bird Bigfoot bands which presumably and they attractive if you have a little bit of I believe in their craft becomes much chart where you see much less animation of slip and if you increase find the depression further the crack becomes very sharp and you don't see you see variants so it appears that I wouldn't because suppress the dislocation of mission from the crack-tip and enable a much sharper leaner crack these lots of lovely participating for better partners on cantilever specimens making this down to the micron scale we can analyze those more carefully we may see simply without so the ductile of brittle transition is now the question whether or not dissipations of one blunt or not it's a very vocal nanoscale question but we see the professor his loris talk with the concepts of fracture mechanics and petting fields extend them all the way down to the crack there yes remember no plastic material continuum scale we model elastic material for classical constitutive law but if I really go in and look in the material in the microscope I see atoms and dislocations then this locations give me plasticity I mean atoms are straining elastically so I really have an elastic filled with dislocations dissipation speaking elvish children and when we continue so even in the losses losses became very close so the riddle thing can happen in two ways one is I can take K 1 C which is normally higher than they would be and I can reduce it below and it reduces below pay money that question is and I reduce it enough it's not sufficient to reduce a little bit has to be reduced below this level both it's not just a phenomena that reduces the other possibility is that raise effectively can't want to make dislocation Commission harder and it goes above k1c so these are the two possibilities and let's look at this one first it's like that didn't either so here's I was praying for an HDMI connection so here's the increase of k1e this is Nick the whole here's a relatively sharp practice the black are hydrogen atoms and if I allow hydrogen diffusion which I'm not showing it ever gets around the practice is all you get this is a delivery of hydrogen let's go to the services there's a little bit inside but that's it but that's efficient at this load you would normally admit at this location but not that amount just enough permitting at this location that means stepping and load higher and if I love higher and with a stronger case field I bring in more light and I start building up hiring around the crack because of the higher okay I snuck into the dislocation so I can load it even higher now this is the microscopic penny near the crack-tip light been something different due to the surrounding plasticity this isn't attractive picture let's keep this process going keep breathing and more I lose more hydrogen and finally the only thing that it can do but the only thing you can do is there no dislocations can get in Annapolis and tear a second dislocation free zone but created by the fact that when I put hydrogen in the nickel it's very hard to admit at this location and at some point fees and because this is really relatively world material we saw a nice yesterday also about the continuum August what happens so what's happened is our suppressed at this location mission the only thing left is complete but now I have this sharp crack the surrounding plasticity but I can't get the wanted now the same thing happens in a grain boundary we remember we sought cleavage or long-range boundary and it's simply that the bread crackers are a little easier to beat than the bulk nipple but it's the same process we have to get a little hydrogen along the grain boundary but then they build up a little region and then finally it splits and it's whistle on the grain boundary visits the lowest energy and that's that's easier to do so the process happens whether in the bulk or at the grain boundary Department at the grain boundary now everyone remind you that these are these low values of penguins CDs that you see these are the practicalities that those are this down here essentially it's not this about the ceramic plasticity gives you if this value it's in these plots this toughness does not include ductile fracture processes so the measured cuff has been to be up here somewhere the local toughness is what's changing down here and ductile fracture their powers suddenly were late is activation and brittle meant two conditions great well we have kinetics fusion fracture condition you know five years and we need and what load so we go back to this problem and we look at the elastic female because around the crack locally though we have dislocations we have plastic field and we can work out how much hydrogen will come to the crack-tip in a certain amount of us are loading rate a certain amount I'm all the hydrogen in some radius used the tractor that radius but all of this come to practice so we can calculate how much hydrogen per unit length the crack comes by integrating over the cylinder that is collected hydrogen and I have some complicated messy result but that's okay it just that the total amount of hydrogen under a constant loading rate scales with the current stress intensity to eight bits comes out from the gauge field and this coefficient is this which involves the diffusion rate the misfit volume the concentration the temperature and the loading rate so this is a relationship between how much vitamin I get attractive and the current stress intensity is a function of diffusion by the concentration temperature loading rate now this parameter determines whether I think we've written or not there's a critical value for this if this if this parameter exceeds the critical value that means I have enough hydrogen or fast diffusion or slow loading I'll get rid of it before recommended dislocations if I'm below this level that means a low concentration low diffusion slow diffusion or fast loading then again another grill and because I find for fibers we found before they start wanting to crack so now this is our threshold condition are we above or below this value and we get this value from our simulations so simulations either show this buckle behavior this is the amount of hydrogen near the crack-tip this is the kuv there's the buckle better or misty brittle behavior and there's a threshold some critical value somewhere in this range for this a zero star and if I'm over here material please here all of it dissipations messes our Buffalo behavior and up a fracture if we look at this in nickel we don't have all the precise numbers when we did this with blunted cracks but let's just look at sharp products the last diffusion that was ten to the minus fourteen and with traps probably somewhere in here and clean nickel but with dislocations and other small traps and as a pump these are different hydrogen concentrations would have get incredibly both inhale a hundred experiments fall right down here they're a griddle around 500 parts per million and so the diffusion pumps and somewhere in here we would predict a little bit those bet the experiments are in Britain about a thousand and again the diffusion coefficient is somewhere around here then we would predict that these material should be in Britain at the spring rates and estheticians so there's no fitting here but we're not making exact prediction but we see that what we find for this is perfect condition where in there out the right all are the other part I have not mentioned was that after you leave the hydrogen will follow the crack-tip and you might think well that's maybe a slow process but in fact if you start with the crack here this is all the hydrogen in the three-dimensional picture if you click the movie here if you now just let hydrogen diffuse in if you see higher ed in sent me an acceptance a lot of this hydrogen will end up over here and so because there's very strong driving force that local field is pulling the hydrogen with it once I've aggregated the hydrogen then most of that hydrogen carry along so as a consequence to continue practically need to saturate new crack surface as the crack grows it's time to cover the surfaces tightly and the rest of the hydrogen is carried along plenty to diffuse in a little bit of pilot run and you can calculate then how much I wouldn't need and you can calculate crack growth rate and it's given by that assisting my practice birds very slowly with that so there's a certain predicted cracker imprint that's right up there ain't no a bear that stay calm quickly into the data here's the data and there's the prediction absolutely that there's the prediction of the crack growth rate again against the stick a little bit Korea it's not perfect but it's very close it shows the same trend as the experiment because of the threshold the threshold is when I crossed over that is here a scar and then there's a slow crack growth regime and I predicted that the more I was going to happen or the higher end a or whatever the bastard that's what we see a good agreement of magnitude we've published that I showed this a few years ago sitting on ferritic seals that won't repeat it because even injury-time I'm gonna introduce on right and we found in all these heretics feels that we can predict the enrollment in all the way second one and third one of them our condition is that this number is less than 1 or greater than 1 and we're very close to that all the rest of the be ridiculous so for most nickel and iron birth 13 systems were structured other things as a function different hydrogen concentrations different materials different loading loading rates okay so that was that was this process here where the hydrogen aggregation makes dislocation hard and basically makes locally makes empirical more like silicon and like nickel or iron the other process is the second process which is maybe I can reduce k1c enough and then a care about the dislocation issue now Jared's on responsive to first principles these quantum mechanical evolution of the surface energy of hydrogen on nickel and what you see is this is a pure this is fracture toughness that comes from that energy and for pure nickel you get fracture toughness up here and down here that's why maple emits dislocations as you saturate the the service is bad - this goes down but at 50% all coverage were hunters and fault if I separated 50% of the surfaces of the most I think that it's 50% and I'm still up here in England C and that's above k1 e so for a long time we thought this process and the curve because we're still at this point well of a 1 e and the materials mid dissipation you can't go to 100% coverage because there's no place by boat here you can only 50% coverage so this is what was driving the other model that we were always stuck here however we now realize very subtle back that I don't like it me together slightly that's why we form hydrides and if you think w hydrogen then you aggregated into three layers the hydrogens like to be slightly together and that lowers the energy of this structure it won't happen away from the practice human entropy but near the crack-tip it will happen and now when I fractured this material I have hydrogen on the surface of 50% and hydrogen in the sulfur is it a hundred percent and that fracture energy is lower than the fracture energy when I don't have this extra hydrogen here an extra contribution in the search reducing the surface energy compared to the to the original service worthless that 50% coverage and then if some of this hydrogen now infuses a surface where it really wants to be I could make the fact that fracturing this is actually less than zero the materials to spontaneously come apart so so if I have this process of the hydrogen aggregating in three layers and nickel and then it can fracture do this may be a tenement and I'll show you in a moment and then when it goes to this is never going back as its negative energy is much lower than the original bulk energy so here's these here's the surface energy showed you before which you go up to 50 percent you keep covering it but there's no way to get there we're free in with three layers the energy goes from here in here and then with that bowl diffusion now I've got enough to get 100% I can get down here this is the most I can get in the bulk but if I getting in those three layers a fracture energy or fracture energy goes down here and then I hit local diffusion and the fracture energy goes to zero now this is if I put this it oncee plot what do I get here's a one seed and can end here naturally but with the three layers I go down here and now below it 1e and then that the fusion that I'm Way down here and so this desert is now really written as a1c is lower than k1 e but I have to aggregate the three layers the pilot just one single air hydrogen do what the diffusion process will it was a combination of hydrogen if I look at assisted by the crack tip stress field as the diffusion but also the energy energetics so I've had the concentration just as attractive and have a far end concentration of C 0 then in equilibrium the chemical potential of hydrogen the bulk and the chemical potential the hydrogen of the the practive have to be equal and the crack-tip I have the entropy term I have the driving force due to the pressure around the crack field and the hydrogen hydrogen interaction this should be seen aged here and that should be equal to essentially the entropy or behaviour and far wet if you look at this best mystic volume aren't using Omega before that's the hydrogen - binding energy nickel we calculate that and that's the pressure field right here the crack there few angstroms so like what that in I don't have to make how much hydrogen is at the crack-tip as a function of temperature and you can see that I can get very close to 100% dependent on the K value and if we focus on these guys at room temperature I got 100% aggregation around the crack there absolutely temperature at okay which is below the admission and it is around the fracture head so the aggregation go back to this picture though this aggregation process near the crack-tip happens at room temperature as equilibrium at room temperature at any values where do we get the fracture at a value in the center down here in the point five range so that means that this process can happen it's thermodynamically favorable and this provides us with a real embrittlement process where it's a higher intensity cohesion but it's not just pure surface energy - the spot there has to activate not just on the surface within this slight vault region and that lowers the surface energy so I'll summarize - to be his duck with a brittle transition and go from one of these pictures one of these pictures fortunately these pictures you see before we go it was we go from London buckle fracture picture to very locally in cleavage picture attractive because hydrogen air either lock submission or really kind of an equal real cleavage fitted here due to the aggregation type and both of those mechanisms seem to operated nipple the first one probably can operate in most materials the second one who may have been much more month on thermodynamics but an operational which is what in what we see experimental II so what I've done is shown you operative mechanisms brittle mechanism totally controlled by the hydrogen and this best as we can compare experiments on material and iron but and that as far as we can go everything just it's quite good enough purpose that was given unto very interesting conditions and I could you know relevant to the sub I couldn't catch up the mechanism of hydrogen so if we just take the rice right here in itself when you have hydrogen into nickel you increase the unstable stacking fault energy you make it and nickel hydride is really a very brittle material nickel hydride is not easily excited using them as for dissertations so as the hydrogen increases around the crack there it's harder to slip those planes and the k1 ee goes up so we calculated these things that's the mechanism although there are hybrids and iron rates so iron has double ACP my pride but here keep in mind that the only reason we have a hydride here or something like the hydride is because we're very close to the crack-tip and the stresses are very high so there is no hydride have in the bulk or very far away from normal hydranoid so people look at they say look at high gravity or the crack that we can see I Drive an iron nickel but this is very this is on the scale of just 2 nanometers were rounding out there and it's almost not observable experimental II so so it's not it it's not a normal highway but both parents are migrants yeah so it's like hard part of the hydride is likenesses is a precipitate that's formed ahead of the crack-tip how how much hydrogen is necessary in this session well it becomes like a precipitate minute blocks in this location and because it's where the precipitate forms forever about Paris apparently so I was just wondering this meeting is - format understanding these new mechanisms are two ways you influence what others like Portugal - all confusion but anything yes there's a slightly higher\n"