Claire Maurice

The properties of polycrystalline materials are significantly influenced by annealing treatments. This article introduces a laser-annealing method that facilitates the investigation of high-temperature transformations, with a specific focus on tungsten restoration. The aim of this research is to establish a controlled temperature gradient in the examined sample to expedite the study of restoration at high temperatures by reducing the number of samples. To achieve this, simulations are employed to design the desired temperature profile, and a laser-based setup is adapted to generate and regulate this profile. Furthermore, uncertainties and errors associated with temperature measurements in the experimental setup are quantified. The proposed laser-annealing method enables precise temperature control during the annealing process. By heating one side of a tungsten rod using the laser system, a steady-state temperature gradient is induced. The annealing process consists of two steps: the initial heating phase to reach the desired temperature profile, followed by the maintenance of constant temperatures at specific positions along the rod for a defined duration. The study investigates the impact of absorbed power by the sample on the temperature profile and assesses the softening of tungsten after annealing using hardness measurements. Overall, the proposed laser-annealing method offers a promising avenue for advancing material science research. Its ability to precisely control temperature gradients and observe material behaviors at high temperatures opens up new opportunities to optimize the properties of polycrystalline materials beyond tungsten, thus providing broader applications in material engineering and manufacturing.

A laser-irradiated surface is the paradigm of a self-organizing system, as coherent, aligned, chaotic, and complex patterns emerge at the microscale and even the nanoscale. A spectacular manifestation of dissipative structures consists of different types of randomly and periodically distributed nanostructures that arise from a homogeneous metal surface. The noninstantaneous response of the material reorganizes local surface topography down to tens of nanometers scale modifying long-range surface morphology on the impact scale. Under ultrafast laser irradiation with a regulated energy dose, the formation of nanopeaks, nanobumps, nanohumps and nanocavities patterns with 20–80 nm transverse size unit and up to 100 nm height are reported. We show that the use of crossed-polarized double laser pulse adds an extra dimension to the nanostructuring process as laser energy dose and multi-pulse feedback tune the energy gradient distribution, crossing critical values for surface self-organization regimes. The tiny dimensions of complex patterns are defined by the competition between the evolution of transient liquid structures generated in a cavitation process and the rapid resolidification of the surface region. Strongly influencing the light coupling, we reveal that initial surface roughness and type of roughness both play a crucial role in controlling the transient emergence of nanostructures during laser irradiation.

Measurement of recovery and recrystallization kinetics of tungsten at high temperature is a key issue for many applications, such as plasma facing units in the framework of thermonuclear fusion. These kinetics are mostly derived from Vickers hardness and EBSD measurements, which can lead to some inaccuracies due to the competition between recovery and recrystallization mechanisms. A complementary/alternative approach based on statistical grid nanoindentation is proposed in this paper. The basic idea is to assume that the fraction recrystallized can be deduced using the hardness probability density function measured on a fully recrystallized sample. The hardness probability density function of the set of non-recrystallized grains can then be analyzed. The methodology was applied to rolled tungsten samples annealed at high temperature. It was clearly observed that recovery and recrystallization overlapped in terms of softening fraction in the investigated time–temperature range. Activation energy of the static recovery mechanism is in the correct order of magnitude compared to bulk self-diffusion in tungsten. High-throughput nanoindentation analysis appears as a promising way to investigate recrystallization/recovery mechanisms in metals.

Multiscale models involving crystal plasticity are essential to predict the elastoplastic behavior of structural materials with respect to their microstructure. However, those models are often limited by a poor knowledge of the local constitutive behavior. This article reports a method to measure the mechanical behavior directly, at the micrometre scale, in bulk crystalline materials. Local strain and stress states were evaluated at the surface of a bent stainless steel crystal by combining total strain measurements – performed with the digital image correlation technique on optical images – with elastic strain measurements obtained by Laue microdiffraction. A local constitutive relation was measured, in an efficient nondestructive way, without the need for full-field simulations. The method was validated by a comparison between the measured local behavior and the macroscopic behavior of the single crystal.