Scientific Events

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The new alloying concept, known as high-entropy alloys (HEAs) or multi-principal elements alloys (MPEAs) are a new emerging class of perspective materials that possess a wide range of unique properties. Since the appearance of the first studies of HEAs, more than 1000 scientific works were published. It was investigated relationship between microstructure of new alloys, which can include SS (with BCC, FCC and HCP structures), IM and even amorphous state and their physical properties. It was shown that the HEAs possess different outstanding functional properties, like superconductivity with transition temperature Tc = 7.3 K, high level electrical resistivity, high saturated magnetization, high corrosion resistance, good hydrogen storage properties, as a template for graphene production. For achievement superior mechanical behavior and thermal stability it was designed and produced protective coatings based on HEAs. However, the research of nitride coatings based on HEA are still very limited. Clearly, the understanding of features of microstructure such non-homogeneous complex systems is essential in order to move in the improvement of the physical properties of high-entropy thin films with different intrinsic architecture. [more]

The size dependent mechanical response of materials has attracted strong attention during the past decade. While past research focused mainly on single crystalline behavior, today´s investigations target the mechanical response and underlying deformation mechanisms of heterogeneous microstructures. The summer school is aimed at providing a comprehensive overview on experimental nano- and micromechanical testing methods. Focus thereby is put on material properties which can be reliably extracted from in situ micromechanical experiments. - Which properties can we experimentally explore? - Where are the limits and pitfalls of our methods? - Where do we need support of simulation techniques? - What are future challenges in the field? The school will deal with nanoindentation as well as methods to explore the plastic and fracture properties of materials and interfaces, frequently used characterization techniques with in situ capabilities and, finally, simulation techniques. [more]

Intermetallic fascinated high temperature materials community for the last five decades. Starting with gamma TiAl, both Ti based and Ni based single phase intermetallics have been subject of extensive investigation. It took five decades for actual application in latest generation GE engine. However, very little attention has been given to multiphase multicomponent intermetallics. These, in particular eutectics, are abundant in the central regions of phase diagrams of ternary and higher components. With a hypothesis that they represent exciting opportunity, this talk will present the outline of our fairly extensive efforts in developing high temperature intermetallic composites based on a novel design of materials through microstructural engineering of intermetallics at nano scale. We shall concentrate on the Ni-Al-Zr system and show that unique complex multiphase microstructures could be developed containing intermetallics of Ni3Al, Ni5Zr, Ni7Zr2 and NiAl. The microstructures contain single or multiple coupled eutectics that are distributed seamlessly along the entire samples. For example, for an alloy Ni-12At%Al-11at%Zr, two intermetallic phases (Ni₃Al and Ni₅Zr) are seamlessly distributed along the entire sample with two different length scales and morphologies. Often these microstructures can be visualised by a 3D analysis that shows variations of connectivity among phases. Many of these alloys show strength in excess of 2GPa This architecture exhibits excellent high temperature microstructural stability, exceptional high strength with reasonable tensile ductility at high temperature. We show that this can be derived from an approach designed to exploit eutectic reactions that combine Intermetallics in a microstructural scale that restricts slip lengths to obtain both strength and ductility. Some of these alloys also have exceptional oxidation resistance that is retained up to a temperature of 973K. Finally we shall present some results of creep strength of these alloy that hints at the stress induced transformation. [more]

The atomic and micro-scale structures of most materials are 3D, but a lack of tools for experimental 3D investigation of materials has limited most published research, including simulation and modelling, to 2D datasets. In the 21^st century this situation has changed significantly. New 3D characterization and modelling methods are generating powerful insights into materials properties and microstructure formation on all length scales. [more]

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Size effects are a key ingredient to control and improve the mechanical behaviour of metallic microstructures and miniaturized components. The analysis of size effects in metals has received continuous attention in the past two decades, both experimentally and numerically. This lecture focuses on the role of grain and phase boundaries in restricting dislocation motion, giving rise to size effects. Some essential features of a thermodynamically consistent model for a grain boundary are presented, which accounts for the grain boundary energy and defect structure and evolution. The role of a phase boundary is investigated with a dislocation transport driven crystal plasticity model, revealing the explicit role of the plastic phase contrast and phase boundary resistance. Interesting size effects are thereby recovered. Size effects can also be eliminated or inhibited by other microstructural mechanisms. Two cases are addressed to illustrate this. The first case reveals the role of dislocation climb and its effectiveness in dissolving dislocation pile-ups. The second case concerns a very thin austenitic film in martensite, whereby the particular structure of the phase and its interface give rise to preferential sliding mechanisms that circumvent the common dislocation driven size effects.This lecture addresses the strengthening role of internal boundaries, constituting a major con- tribution to size effects in metals. It is shown that besides dislocation pile-ups, other mechanisms may be essential. For grain boundaries, the defect absorption and redistribution matters. For phase boundaries, phase contrast in dislocation transport alone already contributes to size effects. Moreover, dislocation-pile ups can be dissolved through climb at higher temperatures or circum- vented by other particular micromechanisms. This analysis effectively illustrates that predicting size effects in metals quantitatively remains a major challenge. References [1] van Beers P.R.M., Kouznetsova V.G., Geers M.G.D.: Defect redistribution within a continuum grain boundary plasticity model. J. Mech. Phys. Solids 83:243-262, 2015.[2] Dogge M.M.W., Peerlings R.H.J., Geers M.G.D.: Interface modeling in continuum dislocation transport. Me- chanics of Materials. 88:30-43, 2015.[3] Geers M.G.D., Cottura M., Appolaire B., Busso E.P., Forest S.,Villani A.: Coupled glide-climb diffusion- enhanced crystal plasticity. J. Mech. Phys. Solids. 70:136-153, 2014.[4] Maresca F., Kouznetsova V.G., Geers M.G.D.: Subgrain lath martensite mechanics: a numerical-experimental analysis. J. Mech. Phys. Solids. 73:69-83, 2014.[5] Maresca F., Kouznetsova V.G., Geers M.G.D.: Deformation behaviour of lath martensite in multi-phase steels. Scripta Materialia 110:74-77, 2016.[6] Maresca F., Kouznetsova V.G., Geers M.G.D.: Predictive modeling of interfacial damage in substructured steels: application to martensitic microstructures. Mod. Sim. Mat. Sc. Engng. 24(2):025006, 2016.[7] Du C., Hoefnagels J.P.M, Vaes R., Geers M.G.D.: Block and sub-block boundary strengthening in lath marten- site, Scripta Materialia,116:117-121, 2016.[8] Du C., Hoefnagels J.P.M, Vaes R., Geers M.G.D.: Plasticity of lath martensite by sliding of substructure boundaries, Scripta Materialia 120:37-40, 2016. [more]