Final Report Summary - STOCHPLAST (Stochastic and statistical properties of dislocation plasticity)
The dominant mechanism for producing large irreversible (plastic) strain in atomic crystals is the motion of interacting dislocations, which are line defects in the crystalline lattice. Their collective dynamics plays a dominant role in plastic yielding, strain bursts and size-effects observed at the micron-scale. The project was motivated by the apparent technological need for developing a profound physically-based understanding of these phenomena. State-of-the-art experiments and well-established dislocation simulations were applied in order to:
(i) Investigate the stochastic properties of micron-scale plasticity and dislocation avalanches;
(ii) Explore the nature of the plastic flow transition;
(iii) Develop a continuum plasticity model that accounts for boundaries and fluctuations;
By applying elements of non-equilibrium statistical mechanics higher scale models of these dislocation mediated phenomena were developed. The project did not only to lead to top-level scientific results on the stochastic properties of collective dislocation dynamics but also provided tools that are promising candidates for further technological applications.
The most significant advancements made in the directions listed above are as follows:
(i) Deformation properties of micron-scale metals are unpredictable, because of the random strain burst caused by the intermittent motion of dislocations. The statistical properties of this stochastic plastic response were analysed with a novel approach: instead of studying the mechanical properties of individual samples, experiments on a large number (more than 40) of identically prepared microscopic cylindrical specimens were performed. The analysis of pillar strengths lead to a precise definition of the yield stress at this scale, and unveiled an underlying weakest link phenomenon governing plasticity. These results will have potential applications in those areas of nanotechnology, where plastic deformation takes place.
(ii) Plastic deformation accumulates in strain bursts resembling earthquakes. It has been proposed earlier that yielding is in fact a continuous phase transition like demagnetization of ferromagnets at a certain temperature. In this project this analogy was further refined. It was found that the average strain burst size is always dependent on the specimen size. This means that dislocation activity during a burst is not localized but may span the entire specimen.
(iii) A new model for crystal plasticity has been proposed that is based on continuous dislocation density fields and is derived from the equation of motion of individual dislocations using the classical toolbox of statistical physics. The model was found to properly account for internal boundaries in a crystalline specimen (like grain or precipitate boundaries) and predicts the formation of dislocation patterns equivalent to those obtained by lower scale discrete dislocation models. This multi-scale model has potential technological applications because it provides higher accuracy predictions for the plastic response of crystalline materials.
(iv) A novel experimental method was established by combining acoustic emission and micro-testing experiments. This means that not only the stress-strain curves of the micron-scale objects (micro-pillars) can be obtained but in parallel the acoustic signals emitted during strain bursts can be recorded that are related to the elastic energy release during individual plastic events. This additional information opens new perspectives in studying strain bursts and testing statistical physics models of plasticity that give predictions on the distribution of both strain drops and released energy.
(v) A new theory was proposed to describe the stochastic features of plasticity in both crystalline and amorphous materials, based on the assumption that plastic events initiate at the weakest sites of the heterogeneous microstructure. Its predictions on the plastic strain accumulation in the microscopic regime as well as the stress fluctuations observed for different specimens were confirmed by lower scale numerical simulations and micropillar compression experiments, too. Since the model gives an in-depth statistical characterization of stochastic features of plasticity it bears high practical relevance.
The EU contribution helped the fellow to establish himself as an individual researcher after his mobility period by providing funding to visit international conferences, to maintain and develop collaborations with international partners, and to build his own research group. The fellow holds a tenured assistant professor position at the host institution, so the prospects of his permanent integration are auspicious.
(i) Investigate the stochastic properties of micron-scale plasticity and dislocation avalanches;
(ii) Explore the nature of the plastic flow transition;
(iii) Develop a continuum plasticity model that accounts for boundaries and fluctuations;
By applying elements of non-equilibrium statistical mechanics higher scale models of these dislocation mediated phenomena were developed. The project did not only to lead to top-level scientific results on the stochastic properties of collective dislocation dynamics but also provided tools that are promising candidates for further technological applications.
The most significant advancements made in the directions listed above are as follows:
(i) Deformation properties of micron-scale metals are unpredictable, because of the random strain burst caused by the intermittent motion of dislocations. The statistical properties of this stochastic plastic response were analysed with a novel approach: instead of studying the mechanical properties of individual samples, experiments on a large number (more than 40) of identically prepared microscopic cylindrical specimens were performed. The analysis of pillar strengths lead to a precise definition of the yield stress at this scale, and unveiled an underlying weakest link phenomenon governing plasticity. These results will have potential applications in those areas of nanotechnology, where plastic deformation takes place.
(ii) Plastic deformation accumulates in strain bursts resembling earthquakes. It has been proposed earlier that yielding is in fact a continuous phase transition like demagnetization of ferromagnets at a certain temperature. In this project this analogy was further refined. It was found that the average strain burst size is always dependent on the specimen size. This means that dislocation activity during a burst is not localized but may span the entire specimen.
(iii) A new model for crystal plasticity has been proposed that is based on continuous dislocation density fields and is derived from the equation of motion of individual dislocations using the classical toolbox of statistical physics. The model was found to properly account for internal boundaries in a crystalline specimen (like grain or precipitate boundaries) and predicts the formation of dislocation patterns equivalent to those obtained by lower scale discrete dislocation models. This multi-scale model has potential technological applications because it provides higher accuracy predictions for the plastic response of crystalline materials.
(iv) A novel experimental method was established by combining acoustic emission and micro-testing experiments. This means that not only the stress-strain curves of the micron-scale objects (micro-pillars) can be obtained but in parallel the acoustic signals emitted during strain bursts can be recorded that are related to the elastic energy release during individual plastic events. This additional information opens new perspectives in studying strain bursts and testing statistical physics models of plasticity that give predictions on the distribution of both strain drops and released energy.
(v) A new theory was proposed to describe the stochastic features of plasticity in both crystalline and amorphous materials, based on the assumption that plastic events initiate at the weakest sites of the heterogeneous microstructure. Its predictions on the plastic strain accumulation in the microscopic regime as well as the stress fluctuations observed for different specimens were confirmed by lower scale numerical simulations and micropillar compression experiments, too. Since the model gives an in-depth statistical characterization of stochastic features of plasticity it bears high practical relevance.
The EU contribution helped the fellow to establish himself as an individual researcher after his mobility period by providing funding to visit international conferences, to maintain and develop collaborations with international partners, and to build his own research group. The fellow holds a tenured assistant professor position at the host institution, so the prospects of his permanent integration are auspicious.