Thin film high entropy alloys (TF-HEAs) are an emerging class of equiatomic (or near equiatomic) multi-component metallic materials exhibiting an outstanding combination of mechanical properties including large yield strength and ductility (respectively >3 GPa and >20% for NbMoTaW) together with resistance high temperatures (> 800°C) and harsh environments [1,2]. However, their structural complexity (lattice distortion, microstructure and composition) together with the difficulty to fabricate and manipulate micrometer-scale specimens prevent the understating of the deformation mechanisms and mechanical properties, hindering the development of nanostructured films with improved performances and their scalability to industry applications. In this context, the MICRO-HEAs project aims to fabricate complex multicomponent TF-HEAs, investigating the relationship atomic structure–mechanical properties down to the (sub)micrometer scale. TF-HEAs will be produced by magnetron sputtering focusing on the AlxCoCrCuFeNi system reporting different atomic structures, crystalline (fcc, bcc) and amorphous varying the percentage of Al [3,4], whose mechanical properties are barely explored. In a second step, advanced TF-HEAs will be fabricated involving grain refinements, duplex phases (fcc+bcc, crystalline+amorphous) and Ti addition (in lieu of Al, TixCoCrCuFeNi), to further improve their mechanical properties and explore still unknown deformation behaviors. Cutting-edge techniques including optoacoustic spectroscopies (Brillouin light scattering and Picosecond laser ultrasonics) [5] and in-situ SEM compression/splitting tests of micropillars [6,7], will provide the entire elasto-plastic behavior and fracture toughness down to the (sub)micro-scale, uncovering the effect of the thickness/volume, composition and microstructure. Finally, the local micro-scale mechanical behavior of the CoCrCuFeNi HEA targets will be explored, with the aim to understand the change of mechanical properties for the bulk counterparts produced by different techniques and explore the scalability of the TF-HEA system. The project aims to expand the experimental capabilities available at the Laboratoire des Sciences des Procédés et des Matériaux (LSPM) with micro-pillar fabrication and advanced in-situ SEM techniques (micro-pillar compression/splitting, PI background [6,7]) so far not available, while enabling to study micro-scale plasticity/fracture and accessing to the live deformation mechanisms. Moreover, the project will be strengthened by the collaboration with Prof. Gerhard Dehm Max-Planck-Institut für Eisenforschung (MPIE, Germany) among the worldwide recognized scientist in the field of small-scale mechanics and already working on HEAs [8]. Overall, the MICRO-HEAs project is expected to generate significant breakthroughs for basic science together with clear benefits for France competiveness and industry applications in the field of high performance coatings, microelectronics, aerospace, defense and energy. References [1] D.B. Miracle, O.N. Senkov, Acta Mater. 122 (2017) 448-511. [2] Y. Zou et al., Nat. Commun. 6 (2015) 7748 [3] B. Braeckman et al., Scripta Mater. 139 (2017) 155-158. [4] M.-H. Tsai, J.-W. Yeh, Mater. Res. Lett. 2 (2014) 107-123. [5] T. Pham et al., Appl. Phys. Lett. 103 (2013) 041601 [6] J. Ast et al., Mater. Design 173 (2019) 107762. [7] M. Ghidelli et al., J. Am. Cream. Soc. 100 (2017) 5731–5738. [8] W. Lu et al., Advanced Materials 30 (2018) 1804727.

SUMMARY The manufacture of ball bearings is done in several steps to achieve bearing rings with very high geometric accuracy. During the various stages - turning, heat treatment, hard turning, grinding - the parts undergo metallurgical transformations giving rise to the appearance of residual stresses in the material which in turn produce geometrical distortions during manufacture. These have a strong impact on the manufacture of bearings of small sections and large dimensions. Indeed, in the case of annular bearing rings, these distortions require to multiply the finishing operations in order to obtain the desired dimensional and geometric tolerances. It may also happen that the distortions observed are too large to be removed by one or more finishing steps, which causes a significant loss of material. The residual stresses and the distortions they imply are a subject of major academic and industrial interest because of their important impact, not only on geometric tolerances, but also on the performances and life time of mechanical parts. As a result, several existing structures involving both laboratory and industry researchers are currently addressing the issue of stresses - essentially from the point of view of their experimental characterization and the analysis of raw data - to better understand their genesis and ultimately control them. Also, there are currently different methods to determine the residual stresses (destructive or not), all of which require taking into account strong hypotheses having an impact on the result, and it is therefore still difficult today to have a clear idea of the accuracy of measurement methods and their area of validity. The improvement of the quality and the productivity of the manufacturing processes of ball bearings of low section developed by ADR thus implies a better control of these stresses throughout the process. Such a control will bring a gain of competitiveness in the manufacture of ball bearings which constitutes a strategic activity of the company. ADR will then be able to respond to new, larger and more competitive markets in high-technology sectors such as optronics, SATCOM constellations, navigation of defense equipment and high-precision robotics. To do this, we propose to create the joint laboratory MACRO or STREBB (Stress Control in Ball Bearings) between the Laboratory of Process and Materials Sciences (LSPM - CNRS) and the ADR company. This LabCom aims to contribute to the improvement of the measurement and the prediction of residual stresses generated at different stages of a fabrication process of complex geometry pieces, in order to allow in the short term the optimization of the fabrication processes and in the medium term the introduction of the developed measurement and calculation methodologies to the manufacturing processes. We aim at limiting the number of finishing operations to maintain the imposed geometric tolerances, and then at extending the research to a better understanding of the links between residual stresses and service life. This project uses experimental characterization tools, but also numerical tools for predicting distortions after different operations. It will include the characterization of the metallurgical, mechanical and geometrical state of the bearing rings at various stages of a conventional manufacturing process, and then the identification of the critical points for the optimization of the process and the design of the parts.

Selective etching of pre-stressed multi-layered structures enables to release intrinsic stresses creating flexible macroscopic shapes (rolls, spirals, tubes…). By combining stress-engineering and photonic concepts, PHOLDING project propose to develop a numerical and experimental methodology to obtain complex three-dimensional photonic structures, by folding a two-dimensional photonic crystal membrane, in controlling material composition, mask design and etching process. This project targets specifically the generation of new families of 3D hollow optical micro-resonators, with the peculiar properties of enabling a strong trapping and enhancement of light in a low index media. Light confinement is achieved by enclosing a small region of space with the folded photonic membrane designed to present a broad-band and high reflectivity in the wavelength range of interest. This leads to the achievement of very open resonators where strong light-matter interaction can be exploited in optical devices comprising an active material embedded in a low index matrix like polymer, liquid or even gas.

A general continuum modeling framework to describe the kinetics of reconstructive martensitic phase transformation (MT) coupled with crystal plasticity (CP) at the scale of dislocations is still lacking. In this project, we propose to use the geometrically nonlinear elasticity theory as a single unified framework to model reconstructive MT and CP together. The theory is capable of distinguishing the behavior of different crystal symmetries and dealing with nucleation, and propagation of martensitic variants and their interaction with dislocations without ad-hoc assumptions. Nonlinear elasticity theory can be used to model crystal plasticity and martensitic phase transformations if the global invariance of the elastic energy in the space of finite strain tensors is taken into account. In this approach, continuum elasticity takes the form of Landau theory with an infinite number of equivalent energy wells whose configuration is controlled by the symmetry group GL(n, Z). To regularize such a highly degenerate model we use lattice-based discretization which brings a finite cut off length representing a Ginzburg- like characteristic superatomic scale. The model is mesoscopic in nature, in that it is formulated in terms of mesoscopic quantities such as stresses and strains, and at the same time fully incorporates the underlying symmetry of the crystal lattice. Our model shows that crystal plasticity together with phase transitions naturally arises from nonlinear elasticity if the symmetry of the crystal lattice is properly accounted for. It correctly describes plastic slip and displacive martensitic transformations at the atomic scale and long-range interactions between dislocations and different phases; the dislocations cores and phase boundaries are regularized and blurred on the scale of the unit cell. In order to perform quantitative simulations, we will calculate the strain energy by making use of the Cauchy-Born hypothesis, which is capable of bridging information from the atomistic scale to macro-scale and it consists of coupling of the continuum with molecular theories. More precisely, we will deform a homogeneous lattice formed by atoms interacting via an atomistic potential in order to obtain the homogenous strain energy density in the undeformed configuration as a sum of the interactions of the atoms for a given macroscopic deformation gradient. We will apply the model to study the microstructural evolution during reconstructive martensitic transformations observed in materials such as titanium, zirconium and their alloys. They are of substantial interest for several applications in the nuclear, aeronautic and bio-medical fields.

The isotopic anomalies present in the Solar system, i.e. variations at variance with predictions from the laws of isotopic fractionation, are considered due to nucleosynthetic processes in the galaxy. However, it was shown 40 years ago that a chemical reaction, the formation of ozone from molecular oxygen could yield large "isotopic anomalies" via mass-independent isotopic fractionations (MIF). Our recent experiments show the same effect, for the first time, for a refractory element, titanium. The goal of this project is to explore MIFs for key elements (Ti, Mo, Cr, Ni, O, …) by coupling theory and experiments in plasma physics to simulate the protosolar nebula, with analyses and models in cosmochemistry and astrophysics. If MIFs are found ubiquitous in such plasmas, this will open a new window to study the early Solar system.