Colloidal Metallurgy – COMET

Most solid materials in everyday life are crystals, whose constituents (atoms, molecules') are regularly positioned in space, as, e.g., in metals or ceramics. Over the past century, scientists have reached a good understanding of the electric, thermal, and mechanical properties of a defect-free crystal. However, virtually all real-life crystalline materials have defects, which strongly affect their properties. Most metals and ceramics are aggregates of crystalline grains. The crystalline lattice of each grain has a different orientation and a polycrystal is thus characterized by a distribution of orientations, its texture. The texture plays a role in almost every industry, in issues as diverse as the weight of cans (via the minimum achievable thickness of the metal sheet), the prospect of high-temperature superconducting cables, or the operating life of a material. Grain-boundaries (GBs) are two-dimensional lattice defects that separate the different grains of a crystal. GBs control bulk mechanical properties of polycrystalline materials: GB sliding and migrating play important roles in plastic (i.e. irreversible) deformation and fracture at high temperature. This is illustrated by two examples: (i) based on mechanical measurements and on post-mortem sample observation, superplasticity (the ability of some materials to exhibit extensive plastic deformation in materials with grains smaller than 10 micrometers) is believed to be due to GBs sliding. However, in-situ observations upon deformation that would constitute a direct proof of this mechanism are lacking. (ii) nanocrystalline materials possess a variety of unique physical and mechanical properties due to their fine grain sizes (smaller than 100 nm). GBs are also believed to play a role in the high yield stress of nanocrystalline materials. However, due to experimental limitations, the mechanisms at play are not totally clear. On one hand, molecular dynamic (MD) simulations suggest a change in the underlying mechanism with the size of the crystallites, from dislocation mediated plasticity to GB sliding. These results may be questionable due to the unrealistic high strain rate imposed to the samples in MD. On the other hand, current advanced techniques for polycrystals visualization preclude any information about the dynamics, because of the extremely long required measurement time. Measurements of the motion of grains under deformation are thus restricted to a few grains. In conclusion, the microscopic origin of the plasticity of polycrystalline materials is still unknown, partly because of the limitations of available experimental tools to image and record, during deformation, the dynamics of the process with a nanometer resolution. To overcome these limitations, we propose to use a close colloidal analogue of atomic polycrystals. Our project on COLLOIDAL METALLURGY takes advantage of the much larger characteristic length and time scales, much softer elasticity, and the optical transparency of a colloidal polycrystal to obtain unprecedented space- and time-resolved information on the deformation of a polycrystal under load. We will use an amphiphilic copolymer, which, under appropriate conditions, self-assembles into spherical micelles in water on a cubic face centered lattice. Nanoparticles added to the colloidal crystal segregate into the GBs, thus enabling their visualization by light or confocal microscopy. By varying the crystallization rate, we are moreover able to control the size of the crystallites, which may reach tens of micrometers. By combining mechanical measurements and the visualization of the 3D arrangement of the GBs, we will be able to collect unique time- and space-resolved quantitative information on the plasticity and structural rearrangements of the GB network upon application of a shear stress or strain, and on the solid-fluid transition that eventually occurs at large strain. The size of the crystallites will be systematically varied in order to make a closer connection to atomic polycrystals. The two partners involved in this project are complementary in terms of both experimental techniques and expertise: (A) we will combine confocal microscopy (in Montpellier) and a novel time- and space-resolved correlation light scattering set-up (STRC) coupled to a rheometer (Rheo-STRC), that we will build in Grenoble in the framework of this project. (B) Montpellier has a strong expertise in the design of original colloidal systems and in their structural and dynamical characterization. The Grenoble team is highly skilled in the mechanics and rheology of materials. In short, we propose to use a unique colloidal analog of atomic polycrystals and original, advanced measurement techniques in order to elucidate the different mechanisms at play, at a microscopic level, in the plasticity of polycrystals.