Ultrafast X-ray diffraction of microstructured materials subjected to a laser pulse. Application to laser shot peening (M/F)

Job Description

Thesis Subject

Context

Laser shot peening (LSP) is a process designed to introduce compressive residual stresses into the surface of a metallic material in order to improve its fatigue properties and crack resistance, and ultimately to reduce the environmental impact of emissions from transport (aeronautics, automotive). This high value-added process is already used primarily in the aerospace industry. Indeed, LSP increases the durability of materials, whether in a mechanical component or a structure, thereby reducing the frequency of replacement and the use of materials. The process involves focusing a high-energy laser (power density larger than GW/cm²), with a diameter of a few millimetres, for around ten nanoseconds to produce a shock wave within the material. If the amplitude of this shock is sufficient, it induces a heterogeneous field of plastic deformations, ultimately leading to residual stresses once static equilibrium is reached. Although this process has been in industrial use for several years and a great deal of research has been carried out, fundamental scientific questions remain unanswered. Among these, the impact of crystal orientation and microstructure on shock wave propagation and stress relaxation has been scarcely addressed. Yet this determines the local generation of stresses that can weaken the material, as recent numerical simulations at these scales have shown. Until now, no experiment has been able to demonstrate this. Indeed, such studies require well-controlled textured materials and time resolved (smaller than ns) dynamic measurements at the grain scale (tens of micrometres). The recent advent of in situ measurements on large scientific instruments (LULI and HERA lasers, synchrotrons, and XFELs) is revolutionising this fundamental field of research by providing experimental capabilities that are absolutely unique in the world. These are the ambitious challenges set out in this fascinating thesis, carried out in a stimulating international environment that is among the most advanced in the field.

Aim of the thesis

The aim is to track both the in-situ propagation of a laser shock and the post-shock stress fields in specially fabricated textured materials. These measurements will be carried out using time-resolved (picosecond resolution) or spatially resolved (micron resolution) X-ray diffraction, made possible by synchrotron and X-FEL sources, coupled with Doppler velocimetry. In practical terms, the candidate will be required to prepare, carry out and analyse experimental campaigns at the European X-FEL in Hamburg (first experimental campaign scheduled for December 2026) and the Japanese X-FEL in Osaka, at synchrotrons (ESRF in Grenoble or Diamond in the UK) and on the HERA laser at the École Polytechnique in Palaiseau. The extraction of stress fields under well-controlled loading conditions requires the implementation of inverse methods, and will be compared with new numerical models to be developed within the ANR-DFG L-SPIRITS project, of which this PhD forms part. The ultimate aim will be to optimise the LSP process by adjusting the process parameters (laser spot size, intensity, pulse duration and overlap ratio).

Required skills

The candidate must hold a Master's degree (M2) in one of the following fields: materials science, condensed matter physics, mechanics of materials, or large-scale instrumentation.
The following skills are required: curiosity and an interest in interdisciplinary work, rigour and the ability to synthesise information, organisational and communication skills, a willingness to work in a team, and the ability to work independently.
The following would be an advantage: knowledge of impact mechanics or physics, familiarity with one or more analytical techniques (X-ray diffraction, electron microscopy, etc.), proficiency in a programming language (e.g. Python), and a good level of written and spoken English.

Your Work Environment

The successful candidate will join the LASER and COMET teams at PIMM. This project is part of the Franco-German ANR-DFG L-SPIRITS project (Principal Investigator: Laurent Berthe), led by the PIMM and I2M laboratories on the French side. Teaching duties is possible (up to 64 hours of teaching per year within ENSAM's courses)

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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