Random Ph.D stuff

Here I put couple of stuff from my Ph.D. It may or may not be interesting to you depending on how much you understand in physics, since some pages are dedicated to my courses.

For my students in TPT: click here

Personal random programming stuff: click here

Short professional introduction

As most of structure materials, steels are subjected to static and/or dynamic ageing, which limits their lifespan. According to the theory of Cottrell and Bilby, dislocations, along with their associated stress field, interact with the carbon atoms present in the matrix in small quantities, leading to their aggregation in and around the dislocation core, known as "Cottrell atmosphere". The existence of such atmospheres leads to a global hardening and eventually embrittlement, due to the pinning force exerted by carbon atoms on the dislocations.

Despite the intensive studies of ageing of steels over years, the structure of Cottrell atmospheres, the kinetics of their creation, as well as the pinning force exerted on the dislocation are not well understood. The Ph.D thesis of R. Veiga investigated the kinetics of one carbon atom, which, however, could not deliver expected overall kinetics of formation of the Cottrell atmosphere.

In our study, we developed a simulation framework, which calculates carbon migration energies from local stress field using molecular statics, a calculation technique denominated “LinCoSS method” (Linear Combination of Stress States, article in review), which are then incorporated into a kinetic Monte Carlo simulation dealing with the migration of carbon atoms. Over the course of the creation of a Cottrell atmosphere, we update the local stress field which then enables us to update migration energies of carbon atoms on the fly. This framework makes it possible to predict successfully the kinetics of the formation of Cottrell atmosphere.

So far, we have been able to observe an aggregation of carbon atoms around the dislocation core, for which the evaluation is in progress.

For non-scientists

... back in the old days of good old Europe, namely in April 1912, there was a huge ferry called "Titanic", going from England to the Unitated States. Thanks to James Cameron, we all know what happened to this unlucky boat. However, did you know that it broke up into two pieces before it sank entirely? With all your reasonable reflections, it's not hard to figure out how it happened, though it is still an important point to discuss, whether it would have been possible to hinder it.

My research is not about how to solve this problem, since there's too much experimental physics involved (which I'm proud to not be able to understand anything about), but rather to understand, how it happens, from the macroscopic point of view.

Firstly though, it is still important to understand the difference between "how" and "why", because we know why it happens. There is a lot of stuff in the world made of steel (v.s.), which is a composite of iron and a very small quantity of carbon. The iron atoms create the well-aligned so-called bcc crystal structure. However, it doesn't matter how well you make steels, there are always defects, like dislocations. Since the iron atoms around a dislocation are frustrated, iron atoms arrive at and around the dislocation core by interstitial jumps to compensate the spaces, creating a sort of cloud, called Cottrell atmosphere.

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