Description du sujet de thèse

Since the Bronze Age, it has been known that adding elements to a metal makes it harder: bronze is an alloy of copper and tin. The mechanical behaviour of this type of alloy is controlled by nanometric defects, called dislocations, which are the vectors of plastic deformation in metal alloys. In such alloys, the solute atoms (tin in bronze) form a random solid solution that impedes the movement of the dislocations, leading to an increase in the elastic limit of the alloy without compromising its ductility.
Today, these solid solution hardening alloys are widely used in a large number of applications in the energy (austenitic steels, FeNi alloys, etc.) and transport (aluminium and magnesium alloys) sectors, and are therefore at the heart of the energy transition. The solid solution hardening mechanism has been particularly exploited in the recent development of high-entropy alloys, which are made up of several elements in comparable quantities (e.g. Fe20Cr20Ni20Co20Mn20 alloy). Despite the thousand-year-old origin of solid-hardening alloys, the interactions between dislocations and the random solid solution take place at the nanometric scale and remain poorly understood to this day.
The main objective of this project is to use internal friction to better characterise these mechanisms.
This experimental technique involves measuring the energy dissipation of a metal sample subjected to cyclic loading.In the alloys studied here, this energy dissipation is directly linked to the movement of dislocations in the solid solution: for low amplitude deformations, the energy dissipated is linked to isolated rearrangements of the dislocation line, whereas at higher amplitudes, the dislocation line moves collectively, resulting in greater energy dissipation. To better analyse these two regimes, this project involves developing a model of dislocation/solute interaction under cyclic loading.
Following a literature review in the first few months of the thesis, the student will carefully select certain grades of Al-Mg and Fe-Ni-Cr alloys to be studied in the thesis.
The next step will be to develop various samples in the Mateis laboratory (INSA-Lyon) and to characterise their microstructure. These samples will first be tested using the torsion pendulum at the Mateis laboratory.During the second year of the thesis, a long stay in Japan will lead to the use of the internal friction machine at IMR (Tohoku University), which will enable high-frequency measurements to be obtained, complementary to those of the torsion pendulum. In parallel with this experimental work, a numerical model of dislocation/solute interactions will be developed in order to better interpret the experimental measurements and thus gain a better understanding of the solid solution hardening mechanisms. The results of this work will be promoted through the publication of articles in peer-reviewed journals and participation in national and international conferences.

Contexte de travail

-During this joint PhD (INSA / Tohoku University), the candidate will divide his/her time between the MATEIS laboratory at INSA Lyon (METAL team) and the ElyTMaX laboratory in Sendai, with long-term stays at both institutions to interact with the researchers involved in the project and carry out the experiments and numerical models required for the project. The cotutelle thesis programme between these institutions has led to more than 20 theses being defended in 10 years. The cost of the stay in Japan will be covered by the laboratory.