NCCR Marvel Distinguished Lecture
We are honoured to announce that one of our speakers will give the third NCCR Marvel Distinguished Lecture at EPFL.
On the Mesoscale Science Frontier in Materials Theory and Simulations
Prof. Sidney Yip
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, USA
Monday 31st August, 16:15, Room MXF-1
The response of many materials to external loads is not directly determined by their bulk mechanical properties, but is instead mediated by chemical and structural transformations, often nucleating at defects or interfaces, and involving reactions with environmental chemical species. As a result, a specimen’s macroscopic mechanical response will in fact depend on the properties of the transformed material, the structure of which is typically unknown and very often difficult to characterize. However, the members of the scientific community interested in modelling mechanical properties are often not experts in modelling the materials’ bulk and surface chemistry, and its coupling with externally imposed compression and tension.
This workshop aims to bring together experts from all the relevant modelling fields to discuss materials science and engineering aspects of tribology, fracture mechanics, and plasticity that involve chemical and structural modifications induced by mechanical loading. Our underlying assumption is that multiscale modelling of processes originating within the “process zone” of (point-like) asperity contacts, (1D) crack tips, or (2D) alloy interfaces is necessary to fully understand wear, stress corrosion, and creep/plasticity, and could suggest new engineering approaches for their control.
There are many examples of this. In specimens subject to tribological load, the material right beneath the sliding surfaces often undergoes microstructural transformations [1,2]. The extreme pressures reached at asperity-asperity contacts can also lead to intermixing of chemical elements from the contacting bodies , which may cause the formation of tribomaterials with entirely different stoichiometry and properties. Tribological loads may furthermore induce reactions with air or lubricant molecules, leading to surface terminations , or hundred-nanometer-thick glassy films  whose formation chemistry is not yet understood.
In fracture, gas- and liquid-phase molecules interact with crack tips, inducing bond breaking  or even diffusing into the near tip region, thus drastically modifying the material’s fracture toughness. Stress-corrosion cracking in silica glass is e.g., thought to involve coupled nucleation of voids, rearrangement of the amorphous structure and diffusion of water ahead of a crack tip .
Finally, plastic deformation can also involve transformations that strongly affect the material’s properties. For instance, glasses deform (and fail) in shear bands that carry a clear structural signature of deformation  such as changes in density. Moreover, large-scale migration of the interfaces between γ- γ’-phases in Ni superalloys occur in response to temperature  and mechanical load cycles  as a result of the coupling between local chemistry (via point-defect diffusion) and long-range stress.
Modelling the structural and chemical modifications of the “base” material in realistic operating conditions is essential for all the processes above. However, doing this accurately is at the very limit of what is currently achievable, and invariably requires a multidisciplinary approach, and contributions/expertise from different but neighbouring fields, such as surface chemistry, high-pressure physics and amorphous solid mechanics.
The workshop will focus on the three subfields identified above: tribochemistry and related microstructural changes, crack-tip chemistry, and plasticity in glassy and metallic materials. An improved understanding of all these processes is needed and has the potential for significant technological and, down the line, societal/economic impact.
Special focus will be given to identifying the most relevant open problems, particularly those which can be attacked by combining existing techniques from the three subfields. Ample time will be allocated to discussing how to combine the different available modelling techniques, and integrate them with high-precision experimental techniques. This is expected to naturally lead to new links/collaborations. The specific case studies up for discussion are listed below.
(1) Chemistry at sliding surfaces.
- Surface chemistry  and atom-by-atom processes under realistic tribological conditions. Making progress here will require coupling chemical accuracy to long-range stress fields.
- Subsurface material damage under severe load. This typically includes grain structure changes in crystalline materials  and local short-range order in glasses.
- Slow processes such as frictional ageing  or grain refinement under small-amplitude, low-frequency load, requiring time-acceleration techniques and/or mesoscale models.
(2) Chemistry of fracture.
- Stress corrosion cracking under the influence of hydrogen, oxygen or water: this requires methods to couple the long-ranged atomistic field to the quantum chemistry at the crack tip (so far limited to crystalline covalent materials [6,12]).
- Crack growth in amorphous materials. This requires faithful force-fields for network glasses that describe both small-scale ductility and large-scale brittleness.
- Combined action of structural transformations and diffusion of ambient species (e.g. water) at crack tips during stress corrosion cracking, also directly relevant for hydromechanical rock-fracturing (“fracking”). This combines the previous two points.
(3) Plasticity in glassy and metallic materials.
- Plastic deformation of glasses. A description of shear banding requires faithful force-fields and the identification of order parameters describing the local structural character of shear bands, to distinguish them from the surrounding bulk .
- Plastic deformation in complex alloys. The plasticity response in Ni-based superalloys is strongly coupled to dynamically transforming matrix/precipitate interfaces .
The interaction with a chemically complex external environment is common to all three topics: this is self-evident in stress corrosion cracking, but also very important for tribochemical processes and deformation of glasses. Moreover, feedback from (3) to (1) and (2) is inevitable due to the plastic deformations that occur during, and influence the evolution of, fracture and tribological processes.
Accurately modelling these “chemomechanical” processes is very challenging, as it typically requires coupling QM-based local chemical descriptions with large model systems to capture stress relaxation, which in realistic (e.g., three-dimensional, amorphous) systems requires petascale computational resources and non-uniform precision (e.g., QM/MM) techniques.
The three topics are also connected by the simulation techniques used to study them, each with specific strengths and limitations. The workshop will specifically explore how the classical interatomic potentials - or density functional theory-based methods - used to model tribology and plasticity could benefit the study of fracture, and by converse how the techniques combining these already used in fracture-modelling could be fed back to plasticity and tribology studies.
There is, in particular, considerable scope for exchanging ideas to address questions such as:
- How can the system size restrictions of current high-precision modelling be reconciled with the size- and time-complexity of the problems at hand, and can big-data and/or machine learning approaches help?
- How can we optimise the information flow between the different time- and length scales involved -particularly, when is hierarchical (parameter passing) multiscale modelling sufficient, or when is coupled (e.g., QM/MM) multiscale modelling instead necessary?
- Where should our method development effort focus in the next 2-3 years? Classes of options include:
- Improving hybrid embedding techniques, make them e.g., more user friendly, easier to validate, and more modular/inter-operable
- Coarse graining of atomistic simulations to construct mesoscale models
- Rare events and free energy exploration schemes - possibly importing knowledge from the biochemical community to study slow processes and extrapolate to long time scales
- What is the best use of existing methods to avoid duplication/wasted effort? e.g. parameterising new classical potentials for each new problem.
Travel Support for US-based researchers
U.S.-based participants can apply for travel funding through the travel award program of the Materials Computation Center of the University of Illinois at Urbana-Champaign (NB: this scheme does not support invited speakers).