Deformation mechanisms and their microstructural indicators in the compaction of crushed salt as a geotechnical barrier

Following 30 years of research, it is common sense that crushed salt is the most suitable geotechnical material for encapsulating radioactive waste in a rock salt repository (e.g., Chaikowski et al., 2020). After emplacement, it provides sufficient permeability to allow outflow of unwanted canister-corrosion gases. In the long term, however, when it becomes compacted by converging cavity walls, it safely hinders any fluid flow from and to the waste. Hence, it is essential to know the evolution of (1) the material's key parameters during compaction, such as porosity and permeability, backfill resistance and viscosity; (2) the material's response to environmental controls, such as temperature, humidity, and stress; and (3) the material's long-term rheology. Here we align microstructural deformation indicators with physical processes that underlie compaction (Mills et al., 2018a). We strive to identify and – where feasible – to quantify the dominant deformation mechanisms (Blenkinsop, 2002; Jackson and Hudec, 2017). As a preliminary result, we show that the abundancy of deformation indicators increases with increasing compaction state. In early compaction, we observe more brittle mechanisms, such as grain fracturing (Fig. 1a) and cataclastic flow. At later stages, especially in the presence of moisture, plastic deformation overtakes. Therein, we observe an increased presence of indicators for pressure solution precipitation (grain boundary seams) and dislocation creep (subgrain formation, Fig. 1b), with progressing deformation. In our upcoming work, we aim at linking the observed indicators to environmental controls, such as moisture content, temperature, and strain rate by applying our approach to larger suits of samples compacted under best-known controlled conditions. Final goal is the joint interpretation with findings from in situ-compacted material (Mills et al., 2018b). Do lab tests mimic in situ processes of crushed salt compaction adequately? Can we learn how to do better by means of microstructural investigations?

Vibration-Assisted Ball Burnishing

Vibration-Assisted Ball Burnishing is a finishing processed based on plastic deformation by means of a preloaded ball on a certain surface that rolls over it following a certain trajectory previously programmed while vibrating vertically. The dynamics of the process are based on the activation of the acoustoplastic effect on the material by means of the vibratory signal transmitted through the material lattice as a consequence of the mentioned oscillation of the ball. Materials processed by VABB show a modified surface in terms of topology distribution and scale, superior if compared to the results of the non-assisted process. Subgrain formation one of the main drivers that explain the change in hardness and residual stress resulting from the process.

Flow Stress Modelling and Microstructure Development during Deformation of Metallic Materials

A constant strain hardening rate is characteristic for large strain deformation at low temperatures and often observed during wire drawing. This stage of deformation, in the following referred to as stage IV, is determined by the microstructural evolution of dislocation cells. At elevated temperatures, rapid stress saturation is typically reached and no stage IV behavior is observed. This behavior is modelled in the present work, following the concept of state-parameter based plasticity, evolving dislocation density and subgrain formation as functions of strain rate, strain and temperature. It is demonstrated that the temperature dependence of state parameters at different deformation stages is closely related. The present model is compared to a series of compression tests carried out on a Gleeble 1500 thermo-mechanical simulator. EBSD micrographs of the same material reveal the microstructural evolution during plastic deformation. It is shown experimentally that the transition from cell forming behavior to subgrain formation correlates well with the disappearance of stage IV and the overall change in the dominant mechanism for overcoming obstacles. In combination with thermally activated yield stress prediction, this model, recently implemented in the software package MatCalc, offers a powerful tool for flow-curve simulation.