How Does Hydrogen Affect the Mechanical Behavior of Metals and Alloys?
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Transcript: English(auto-generated)
00:04
Our main goal is to understand how hydrogen interacts with metals and alloys and how these interactions will affect their mechanical behaviour. The fact that hydrogen affects iron and steels is well known since the 19th century. But what makes these changes so remarkable?
00:23
And why are we still interested in such an old problem? So didn't we solve it already? The answer to this is rather complicated Because there have been many studies solving some of these issues, but with new technologies that are emerging, also we have new questions and new problems.
00:42
The main challenge about hydrogen is that it is very small, it is everywhere and it is in constant motion. It will attack the materials slowly and silently until enough hydrogen is accumulated, causing a catastrophic failure of the material structure. If we think about the material structure like a pomegranate, a fruit,
01:05
so we can see that in the outer shell it is shiny and smooth. However, if we open it, inside we will find pockets that contain small grains and the grains will have also small hard seats. In the case of the metals or alloys, the structure is quite similar.
01:23
So if we look closer and closer, we will find grains, precipitates, interfaces and a lot more. When hydrogen enters into a metal, it has many many places where to go. It can feel more comfortable inside some grains or around some precipitates or some other elements.
01:44
It for example likes to stay in the border where two grains meet or if there are mechanical stresses around, it will go to these highly stressed regions. All these sites that can catch hydrogen and keep it there strongly, these are called heart trapping sites.
02:02
However, it may happen that hydrogen doesn't like to stay in this particular lattice and it will wander around in the form of movable or diffusible hydrogen. So many studies dealing with materials design, they consider heart trapping sites because if we can capture hydrogen in there,
02:21
so it doesn't have any more freedom to move around and cause troubles. We can test the properties of a material with heart trapping sites by ex situ experiments. This means first we charge or load the material with hydrogen and afterwards we do the mechanical testing. But what happens with diffusible hydrogen?
02:43
So if diffusible hydrogen is in constant motion, it is a lot more challenging to track its effects. So this is where lies the main question of our research. We want to understand the effects not only of trapped hydrogen but also of diffusible hydrogen in the changes of the material's mechanical behavior.
03:06
What I do and what we aim to do in my group is to develop new technologies that target these very specific issues, meaning dealing with trapped hydrogen but more specifically with diffusible hydrogen.
03:21
And for this we need to do mechanical testing at a very small scale and also we need to provide continuously hydrogen. But why do we need to do this? Well, if we consider a material as a whole, as a complete structure and we test its mechanical strength, we will have many interactions of all these specific features that I already mentioned,
03:45
like grains, grain boundaries, precipitates, etc. And we do not know exactly what was the main feature that caused failure in the material. So we need to separate the hydrogen interactions with all these features and for that we need to study the system at the relevant length scales.
04:05
If you consider, for example, a human hair and you divide its diameter by 100, that's the relevant length scale. One of the techniques that allows us to do mechanical testing at this level is nano-indentation.
04:21
We take a very sharp indenter tip of a few hundred nanometers and we push it inside of a material. At the same time we track the force that we use during this process and we measure the distance that it penetrates. This allows us to get information, for example, on how hard is the material,
04:42
how easy or difficult is to deform it, or if there are cracks evolving. But what about hydrogen? So for that we develop our own in-house in-situ hydrogen charging cell. This is basically an electrochemical cell that is placed beneath our sample
05:01
and we produce hydrogen in there by using the hydrogen evolution reaction and the atomic hydrogen will further move towards the upper testing surface by nano-indentation. This method, this particular method that we are developing, will allow us first to track particular changes on the mechanical behavior
05:24
that are related only to the presence of hydrogen and this is because no other effects are expected on the sample surface. Second, we can track independent features, for example, what happens inside the grain, what happens at the grain boundaries
05:41
and what happens when we change the composition or at precipitates etc. And third, it allows us very specifically track diffusible hydrogen as we are providing continuously hydrogen into the material. One additional feature is that we can do also time-dependent measurements,
06:02
meaning we can track, for example, how long it takes for a material to fail for very specific hydrogen charging conditions. Since hydrogen is very important in steels, one of the first materials that we decided to test was iron
06:20
and iron with a small content of chromium. Now, if we look at the atomic arrangement of these elements in the crystal structure, we can imagine, for example, a box, a cubic box, and we put one atom in each one of the corners and one more in the center and then we expand this periodically.
06:42
This is what it is known as a body-centered cubic structure. Hydrogen particularly doesn't like to stay very long in this structure and is moving there very very fast, meaning we have diffusible hydrogen. This makes this kind of material ideal for our in-situ studies,
07:00
since if we would plan, for example, ex-situ studies, this would be very challenging as it would be expected that hydrogen it will also leave very fast the material. Our findings can be divided into parts. First, let's consider the material per se. What we find is that hydrogen indeed has a very strong influence
07:22
on the mechanical behavior of iron and iron alloys. This means, at first, it reduces the energy that is necessary to permanently deform the material. However, once the material is being deformed, it makes it harder. So this gives us a hint of an embrittling process.
07:43
If we consider chromium, chromium makes this effect even much stronger. Now, about the technique. So we conducted also other experiments using already established approaches. In these kind of approaches, the surface that is being tested by nanoindentation, for example,
08:05
is the same surface that is in contact with the electrolyte that is used to produce hydrogen. This means that we have there a competition of effects because this generates unavoidably some corrosion processes. So the mechanical data that we obtain can be misinterpreted or confusing,
08:29
and this is because we have there corrosion plus hydrogen effects. Thanks to the technique that we developed, we can track the effects related only to hydrogen.
08:43
The key relevance of our findings lies in the unique opportunity to test the mechanical behavior as a response not only to the trapped hydrogen, which is very well known in some cases, but also related to diffusible hydrogen. Our findings show that we can track in a very reliable and reproducible way
09:05
the effect of this diffusible hydrogen as a function of time and as a function of hydrogen content. If we now combine our findings on the mechanical effects together with the deformation structure, for example, by electron microscopy
09:24
and the amount of hydrogen that is located at the specific features by collaborating with other groups in our institute, we can then have a holistic view of the material behavior and we can provide also guidelines for simulations. So if we understand in a very specific way what changes when hydrogen enters into the material
09:50
and interacts with each one of these specific parts that I mentioned, we can design materials that will have a more resistant behavior against hydrogen.
10:02
So if we understand the role of the specific elements, we can then plan compositions which will contain them inside. If we understand, for example, the role of interfaces or precipitates, we can include more or less of them in the materials structure.
10:21
So our contribution provides a realistic view on hydrogen combining diffusible and trapped hydrogen, which is key for materials design. Knowing the weak points is just the starting line to understand the material's behavior as a whole.
10:42
Overall, what we aim is to create materials that are better, that are stronger, that limit the diffusion of hydrogen or, if not, we can make them more resistant to hydrogen and its effects. And this is not only for iron and steel but also for other materials that are relevant for different industries.
11:03
For example, the construction, the transport, the energy, where new technologies are continuously being developed. And these developments require also very specific materials. It is also expected that hydrogen will play a key role into the energy transformation.
11:21
And there, it is relevant to have materials that will be used, for example, for hydrogen storage or hydrogen transport. And this is one of the parts where our research becomes highly relevant.