Research project P7/21 (Research action P7)
The general ambition of the project is to contribute to a better understanding and prediction of the multilevel interactions between interfaces present in materials and the deformation and failure mechanisms, in order to inspire the development of new materials with extreme structural performances. The field of research sits at the frontier between materials science and solids mechanics. One of the main driving forces for this project is the recognition that the significant progresses recently made in the development of nanostructured materials, especially towards ultra hard systems, must now be integrated into a more hierarchical approach of the material structure involving the control of multiple length scales. This is a necessity for the implementation of a multiproperty vision of materials, considering that several mechanical properties can only be improved if larger length scales are properly architectured. Among the features that can be used to modify and optimize the mechanical properties of materials, interfaces, such as grain, twin and phase boundaries, free or anodized surfaces, play a central role. It is indeed the combined effect of all these interfaces that interact with, trigger, or mitigate the deformation and fracture mechanisms that ultimately control the mechanical response of materials. As a matter of fact, the effect of the presence of interfaces is often larger than the effect of the chemical composition. The focus will be on mechanical properties involving the strength, ductility, fracture, creep, fatigue and wear resistance of metal based systems with a high density of interfaces organized at different scales, possibly involving organic layers. More precisely, the investigations will be performed on 3D bulk materials and on small dimensions systems in the form of thin films or nanowires with a high density of interfaces. Some key scientific issues concern the importance of rate dependent behaviour and back stress originating from the abundance of these interfaces, the stress/strain driven formation and mobility of these interfaces, the interactions with the elementary atomistic deformation mechanisms and with the cracking mechanisms, and the resulting size effects. Meeting this grand ambition will allow unravelling how interfaces can be best organized over a range of length scales in materials with hierarchical structures designed to enforce the adequate mechanisms. The practical outcomes consist in the development of predictive models and of improved processing routes as well as improved characterization methods towards the development of new materials with enhanced performances.
The specific objectives are organized in terms of categories of interfaces, setting also a natural workpackage (WP) structure for the project, in the following way.
(i) Static and dynamic interfaces, e.g. grain, twin and phase boundaries. The objective is to characterize, understand and model the interactions between internal static interfaces and elementary deformation mechanisms, and the macroscopic impact on the strength, strain hardening, ductility and creep resistance in metals with a high density of such interfaces, accounting for possible stress or strain assisted interface evolution.
(ii) Localized bands and cracks, e.g. shear bands, adiabatic shear bands, roping, interface and fatigue cracks. The objective is to characterize, understand and model the interactions between localized bands and cracks and the elementary deformation mechanisms, their formation and the macroscopic impact on the strength, ductility, fatigue and toughness balance in metals.
(iii) Graded and architectured interfaces, e.g. regions with gradual or hierarchical transitions in microstructures in terms of grain sizes, second phases, texture, hardening precipitates, dislocation density, multilayers involving oxides, and/or solute solution. The objective is to characterize, understand and model the effect of gradients of microstructure in architectured materials towards the optimization of the macroscopic strength, ductility, fatigue and wear balance.
In addition to these investigations which will deliver new fundamental knowledge, the goal will be also to go a step beyond by articulating this new knowledge and elucidate how all these interfaces can be optimally architectured at different scales to cooperate towards reaching higher performance materials. The philosophy will be to start with an expected set of requirements/properties for specific applications and determine the best structure following a “materials-by-design” approach. A specific transverse task will be dedicated to this effort.
The characterisation of the complex interactions between defects and interfaces, which take place at the atomic level and at higher scales, requires a panoply of model materials and experimental techniques. New constitutive model formulations based on a physical and micromechanical description that account for interfaces have to be formulated and improved numerical methods must be developed for properly representing the different scales and the presence of the, possibly evolving, interfaces. In order to address all these scientific challenges, the partners of the network will thus share and continue developing their expertise regarding a wide variety of advanced techniques, tools, models and codes. These “methods” will be grouped into transverse methods.
Model materials will be produced with 2D or 3D architectures. The processing of 2D systems will involve thin films and multilayers deposited by CVD, PVD, ALD, laser cladding and melting, electrochemistry, with possible anodized layers, involving precipitates, twins or solid solutions, as well as soft polymeric layers, with patterns made by FIB or lithography. The processing of 3D systems will involve ultra fine grained alloyed materials produced by large and fast deformation methods such as ECAP, dynamic torsion, friction stir processing, explosive forming, cumulative roll bonding with additional macrolithography or de-alloying techniques.
Characterization and testing will rely on the various in-situ mechanical tests coupled to SEM, TEM or synchrotron and neutron diffraction, microdiffraction or tomography, in-situ stress monitoring during film deposition, X-ray CT scan, crystallographic mapping in SEM (EBSD), TEM analysis, on-chip mechanical testing of thin films, nanoindentation and nanoscratching, nanoDMA, interface cracking and wear tests, as well as more classical static and dynamic mechanical tests.
The advanced micromechanics-based constitutive models worked out in the past by several partners and integrated into multiscale modelling strategies will have to be extended with an emphasis on properly treating size effects, back stress and thermally activated mechanisms. Crystal plasticity theories with or without gradient effects, informed by lower scale molecular dynamics or discrete dislocation dynamics, will constitute the foundations to address the mechanisms of interest. Improved numerical approaches will be needed for the implementation in finite element codes in order to achieve the scale transitions up to the macro-level and the prediction of strength, ductility, fatigue and wear as influenced by static, dynamic, graded interfaces or surfaces. Additional developments will enable dynamic interfaces to move or to break, and to model contacts for wear and scratch.
The final motivation for this research is that successful answers to all these fundamental questions can guide the processing of new materials with improved structural performances. The long-term economical development of our society is facing critical and urgent challenges in the management of energy and raw materials resources, and of the environment questions, such as those concerning solid wastes and end-of-life goods. High performance materials with better sets of properties go along with more efficient use of resources, safer and more reliable applications, and longer life of the systems made out of these materials. For instance, transportation pushes for lighter and safer materials, coatings must be more wear resistant to allow longer protection times, energy production, harvesting or recovery requires materials with versatile properties often at high temperature, microelectronic and micro-electromechanical devices must become more reliable.
The project will be carried out by a vivid and very integrated group of 5 Belgian teams and 4 international partners, representing (for the Belgian partners only) about 20 faculty members, 120 researchers and 35 technical/administrative staff directly working on the topics of the WPs, all of them entertaining long term fruitful collaborations. In order to keep the focus on scientific challenges and questions more than on techniques, the project is primarily focused on the WP activities. This vertical approach favours interactions between experimentalists in charge of processing methods, experimentalists in charge of characterization and testing, numerical experts, and theoreticians. It forces a multidisciplinary approach. As a rule of thumb, each WP involves 3 tasks, each of them carried out by typically 2 to 4 researchers from at least 3 partners. The project funding will support about 15 full time researchers, about half of the manpower required to reach the targets, the rest coming from other sources secured by the partners, e.g. payroll of the university. The tasks will all start during year 1 or 2 and generally last for 4 or 5 years. The network activities involve also graduate and post-graduate education, dissemination and visibility.
