It is now believed that the mechanical behaviour of many amorphous materials (metallic, colloidal, oxide glasses, foams, etc) is controlled by similar physical mechanisms related to their disordered structure. A fundamental understanding of these mechanisms is thus pivotal for technological progress in many advanced applications, which depend on being able to formulate new materials with designed thermodynamic and mechanical properties. Scientific progress in this key area relies on a currently fragmented community of researchers with diverse domains of expertise (mechanical engineers, physicists, rheologists, …). This Action will gather leading experts to form a core network that reaches across, and breaks down, traditional community boundaries. It will promote the development of a coherent scientific effort in Europe on this topic, via the communication of theoretical concepts and analysis techniques; the education of early-stage researchers; and the dissemination of major ideas.

BACKGROUND, PROBLEMS

This application follows a COST Strategic Workshop entitled “Physics of Amorphous Solids: Mechanical Properties and Plasticity”, which received the enthusiastic participation of early-stage and senior researchers with very diverse backgrounds: theoretical and experimental physicists, material scientists, mechanical engineers, metallurgists, rheologists etc. Held in March 2010, the workshop revealed the need and importance to promote collaboration between scientific communities in Europe on the topic of amorphous solids. It motivated a first application to form a collaborative Action network, which was strongly appreciated by DC, made it to full proposal, was again very much appreciated by EEP, but just missed the DC hearing: we resubmit because we know we can respond to the few comments made by EEP and make our application even stronger.

Amorphous materials are present at all levels of industry; they range from pastes or foams, through polymeric glasses (e.g. the material of DVD disks), to optical or window glasses, and most recently metallic glasses. Many high-tech developments, such as low energy-loss power transformers (metallic glasses), highly-transparent fibres for communications, or the production of low-CO2-impact cements, rely on being able to formulate new amorphous materials with designed mechanical and thermodynamic properties. Being able to do this, however, requires a precise understanding of the links between macroscopic properties of these materials and their microstructure.

This Action will gather a community of researchers who are trying to reach a fundamental understanding of the mechanical and thermodynamic properties of amorphous (glassy) materials. It will promote studies of materials under external strain, which include small and large deformations, homogeneous and inhomogeneous flows and the transition to failure via strain localisation. Research on these topics directly impacts technological progress in material processing and forming, in the control of strength and durability and in the end in the design of new materials. The potential benefits of this Action are thus enormous.

Fundamental progress on amorphous materials must rely on strong collective effort involving diverse domains of expertise. Indeed, materials as different as colloidal pastes and metallic glasses present an array of similar macroscopic properties such as aging (the slow evolution of their properties with time) or non-Newtonian flow behaviour. These similarities, it is now agreed, reflect the fact that structural disorder plays a leading role in determining mechanical and thermodynamic properties of glasses: metallurgists and rheologists face the same fundamental problem.

In glasses, the absence of periodic structure raises major difficulties in experiments, analytical theories and simulations. As structural disorder limits the available means of investigation in any given material, progress must rely on meaningful comparisons between experimental systems of very different compositions. In the past, prominent steps have thus come from daring analogies, e.g. between the flow of foams and metallic glasses.

The field is now evolving rapidly. New experimental methods have been invented while computing power has reached a level where meaningful simulations can be envisaged. Fundamental progress on amorphous materials is therefore now within reach, but must involve cross-interactions between several communities (mechanical engineers, metallurgists, rheologists, physicists, …) that have evolved separately for decades.

This fragmentation of the community working on amorphous materials is particularly severe in Europe, as opposed to the US, where interdisciplinary programs have been very active for decades. Indeed it was striking at the COST Strategic Workshop that all the attending US professors had expertise in multiple fields (in e.g. colloidal/metallic or polymeric/colloidal glasses) while the European communities working on colloidal, polymeric and metallic glasses were very separated and had difficulties in finding a common language. As the phenomena observed and some physical mechanisms are identical, this results in a tremendous lack of efficiency due to duplication of effort.

Finally, while the scientific study (both basic research and industrial research) of amorphous solids is relatively well advanced in France, UK and Germany, this very important subject is much less developed in the rest of the EU. This is all the more critical as the subject is evolving quite fast. There is an important educational need here, both for young and senior researchers in related fields.

COST is ideal for the proposed Action because the key need is not to provide more funding to existing research groups, but to link up existing activities so that all synergies are fully exploited and new ideas generated.

BENEFITS

The scientific benefits of this Action would be:

• progress in fundamental understanding of thermodynamic and mechanical properties of amorphous solids
• facilitating communication among a fragmented community, via transversal propagation of theoretical concepts and data analysis techniques
• providing opportunities for the education of young researchers, and dissemination of knowledge produced by a quickly evolving research field
• broader impact on other driven systems, such as crystalline systems or magnetic walls.

The potential technological benefits of this Action are enormous. The experimental groups involved in the network are actively working on the formulation of new materials in relation with major industries. Participants in the network have active contacts with major industrial groups such as: Lafarge, Rhodia, Saint-Gobain or Unilever.

OBJECTIVES, DELIVERABLES AND EXPECTED SCIENTIFIC IMPACT

Much of the classical work on elasto-plastic dynamics is not firmly based on microscopic observations; rather it is based on some constitutive relations based primarily on symmetry, yet with too many fit parameters. This approach works for well-known, tabulated materials, but is not predictive, hence cannot guide advanced applications. Every new material must be tested again before any reliable description can be constructed to predict its behaviour.

The primary objective of this Action is to establish clear links between microstructure and macroscopic behaviour. This process involves addressing several sub-issues which can be decomposed as follows:

• Understand how elementary mechanisms of deformation give rise to constitutive equations (these are input for material engineers), starting from the steady, homogeneous flows
• Understand the conditions of brittle failure, which is what determines the working conditions of most materials
• Relate studies of deformation in glassy materials with studies of structural relaxation in the absence of deformation. Clarify what are the similarities in elementary mechanisms, and what is specific to strained systems
• Identify what is specific to e.g. metallic glasses versus colloidal glasses, and how the specific form of interaction impacts material behaviour
• Stimulate other, still hardly touched upon, topics like magnetic and electric properties, which are crucial for the development of the subject including industrial applications

SCIENTIFIC PROGRAMME AND INNOVATION

This Action reaches across, and aims to break down, traditional boundaries between Physics and Engineering. It rests on a collaborative, cross-disciplinary effort, with at the core the transversal communication of theoretical concepts and data analysis techniques. Routes for exploration include:

• Systematic comparison between existing theoretical approaches to construct experimental or simulation tests that can discriminate between the various scenarios
• Identify limiting cases (hard/soft particles, slow/fast deformation etc) where existing concepts can be shown to work accurately or conversely to break down
• Construct scenarios that interpolate between disparate domains, e.g. going from hard (colloidal) to soft (metallic) interactions to clarify commonalities and differences
• Help experimental groups become more aware of recent, often technically demanding, theoretical developments; conversely, encourage theorists to adapt models to reflect details of new materials for advanced technology being pursued by experimentalist participants.

ORGANISATION

The Action will be overseen by a managing committee (MC) that will meet around twice a year. The MC will comprise a core group of leading scientists with expertise in the topics of the action, and

• organise general Action conferences (roughly 1/year) intended to bring together most of the participants, with topics depending on which of the Action objectives require further progress
• put out calls (every 6 months) for applications for smaller workshops proposed by Action participants; prioritise projects which link previously disparate areas
• put out calls for applications for short visits, with priority given to visits that strengthen connections between groups in different areas (theory/simulation/experiment; colloids vs metallic glasses; etc)
• ensure that all Action meetings contain an educational component, e.g. via the incorporation of initial “bridging” tutorial sessions (where experimentalists explain the basis of their techniques to simulators, etc)
• for oversubscribed meetings, monitor the balance of researchers of different genders, levels of seniority, and nationalities, so as to give preference to more junior researchers and those from underrepresented groups where appropriate.

The Action will be open throughout, and welcome the participation of researchers from related fields. A dedicated web site will be set up to help disseminate Action results beyond publication in the open literature, and to facilitate submission and assessment of workshop and visit applications.