Final Report Summary - NANOMESO (Size Effects in Mechanical Properties) NANOMESO was a multidisciplinary team of leading European and United States (US) experts in modelling and experiments. Driven by recent striking experimental observations of size dependent plasticity demonstrating the inappropriateness of current plasticity theories, the team had among its main objectives the development of knowledge and understanding of experimentally observed size dependent plasticity by exploiting synergies between simulations and experiments. Other objectives included the development and validation of a computational tool that allows simulation of meso-scale phenomena in micron-sized objects while capturing the important atomistic aspects of dislocation nucleation at interfaces and free surfaces under loading conditions. The central goal of the NANOMESO consortium was to close the gap between experimentally observed size effects in mechanical behaviour and current theoretical knowledge by developing a computational guidance. To achieve this, the choice and structure of the NANOMESO consortium work packages attempted to integrate as best as possible computational models and experimental measurements. The simulation work undertaken by the NANOMESO consortium has lead to a number of important advancements at both the atomistic and discrete dislocation dynamics level, which will influence similar work done in the international research community. These include the computational tools for construction of atomistic nanocrystalline samples that will allow for future systematic investigations of dislocation mediated plasticity as a function of user selected microstructure and dislocation content. This overcomes one limitation of past simulations of nanocrystalline materials - the stochastic nature of dislocation nucleation during deformation that makes it difficult to predict when and where a dislocation event will take place. This aspect also precluded the simulation of multiple dislocations to study the effect of pile up and dislocation transmission within the nanocrystalline environment. With the aforementioned tools, its now becomes possible to perform a systematic analysis in order to extract the important dislocation / GB interaction physics for the development of the necessary empirical laws for use in mesoscopic DDD. The atomistic simulations of deforming nanocrystalline samples as a function of strain rate, temperature and as a function of applied shear strain have demonstrated that the plastic deformation processes seen are indeed close to the athermal limit. This demonstrates that a quantitative extrapolation of simulated stress-strain curves towards experimental strain rates is not possible. This is an important result since much published simulation work is compared too easily with experiments performed at strain rates ten orders of magnitude slower than that seen simulation. Despite this, the work emphasises once more that much knowledge can be gained by studying the fundamental atomic processes contributing to interface dominated plasticity. The new developments in DDD in which dislocations are now able to be absorbed at grain boundaries with the possibility (depending on the local stress conditions) of transmitting slip to the neighbouring grains opens up an entirely new regime of DDD simulations in which micro-structure can be realistically treated. While impenetrable grain boundaries tended to result in rather diffuse slip activity on many slip systems across the sample, the possibility of transmission gives rise to a stronger localization of plasticity - a phenomenon that is known to be fundamentally important in interface dominated materials. Documenti correlati Final Report - NANOMESO (Size Effects in Mechanical Properties)
