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Tymofii Khvan - MSc thesis at SCK-CEN

My name is Tymofii Khvan. I am 23 years old, studying in Kharkiv, Ukraine. Because I have been already working at SCK-CEN performing my bachelor thesis and internship after, I decided to write my master thesis there. We discussed all of the details with my mentor –Dr.Dmitry Terentyev and afterwards I arrived to Belgium.

It was cool to come back here and see my friends, which I have not seen for a long time, again. Moreover, cause I have been already working with my mentor, it was much easier to start the researching process. By the way, because of him I became really interested in nuclear materials science (previously, I was more interested in getting a research experience abroad, than in some certain sphere of physics). He provides a high level of mutual understanding with his students and sense of freedom in your researching process, as if you are not just a student under supervising; you are like a high-grade scientist.

Our goal was to examine strength properties of wires, made out of pure tungsten and tungsten doped with a small amount of potassium (60 –75 ppm). The wires are potentially applicable in fusion plasma-facing components for reinforcing the composite material, which is also made out of tungsten, to prevent a crack emerging. Previously, I have been already working with these wires, but the task was more consisted of computational modelling –I was working on the way of describing the behavior of the wire during tensile test and retrieving some information from that (true stress –true strain). We cannot obtain the TS –TS curves in a common way because of the dimensions of the wires (150 μm in diameter) and extremely short necking stage (almost all of the wires have been broken immediately after ultimate stress).

Before I came here, a set of the specimens with different annealing temperatures (to simulate the real fusion conditions –neutron irradiation and long-term operation under the high temperatures) were made and tested under 100, 300 and 500 °C. Therefore, I had manyrawcurves with several comparison parameters (pureness (W and W-K), annealing temperature and test temperature). I have processed them and built two sets of plots for more comfortable comparison of parameters mentioned above. I have also made plots with common parameters fortensile testing,which werereceived straight out from the curves–ultimate stress and uniform elongation. Some examples are shown below.

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Furthermore, no less important was to see the shape of each specimen (or at least of each type of the specimens), so I have learnt how to operate a scanning electron microscope. Using it, I made photos of the specimens for analyzing their condition after annealing and tensile tests. I have also used optical microscope to see the difference between specimens’ necks and maybe to use the photos obtained to recalculate engineering curves into true by methodology invented previously. Moreover, we had an idea to make an EBSD map of any specimen, but we postponed it until better times after several unsuccessful attempts to fix and polish the wire. Probably, we will return to this thought later. I give some examples below. 

My work in SCK-CEN has provided to me a huge scientific experience, which is for sure will be useful in my further career. I met many interesting people from all over the world and what is really makes me excited – they are all from the scientific community. I express my gratitude to my mentor – Dmitry Terentyev, comentor – Anastasia Bakaeva and colleague Alexander Zinoviev for great advices, supervising and patient filling in the gaps in my knowledge, to my colleagues – Van Eyken Jelle and Van Renterghem Wouter for providing and entrusting me a lab stuff and such expensive and complicated equipment like scanning electron microscope, and of course to FuseNet organization for their support and giving a good opportunity to obtain invaluable scientific experience for students.

- Tymofii Khvan