Scientific Events

A lot of research effort has now been dedicated to the study of complex multicomponent alloys (more commonly called High Entropy Alloys HEA). This family of materials introduced in 2004 breaks with the traditional alloying concept, since they explore the domain of concentrated solid solution(s) of +5 elements. Several studies sprovide fundamental understanding on the structure and the mechanical properties of some of these alloys, mostly fcc [1–3]. If the results are promising, as for example the incredible fracture toughness of FeCoCrMnNi at low temperatures [4], recent papers suggest that equiatomic fcc alloys with less than 5 elements, or non-equiatomic fcc concentrated alloys also display great, or even greater mechanical properties [2,5,6]. The sub-family of bcc complex multicomponent alloys has been less investigated. Therefore, a multi-scale characterization of a model bcc multicomponent alloy with composition Ti₂₀Zr₂₀Hf₂₀Nb₂₀Ta₂₀ is performed. After optimization of the microstructure, investigated by SEM (EBSD), TEM and EXAFS, the mechanical properties of the alloy are studied during both tensile/relaxations tests and shear tests. Deformation mechanisms are discussed in terms of activation volume and flow stress partitioning, interpreted with the help of microstructural investigations by transmission electron microscopy. Finally, the “HEA” concept is coupled with the chemical design based on electronic parameters Bo and Md used in Ti-alloys. This concept, first introduced by Morigana was successfully used to help predicting the structure stability, and hence the mechanical behavior – dislocation glide, twinning induced plasticity (TWIP) or transformation induced plasticity (TRIP) – of Ti-rich alloys [7,8]. The studied composition Ti₃₅Zr_27.5Hf_27.5Nb₅Ta₅ displays a large ductility of 20% and an increased work-hardening [9]. It confirms that extending the concept of “HEAs” to non-equiatomic compositions can be highly beneficial and that the design strategy developed for Ti-alloys can be used with great results in concentrated alloys. [1] F. Otto, A. Dlouhý, C. Somsen, H. Bei, G. Eggeler, E.P. George, The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy, Acta Mater. 61 (2013) 5743–5755. doi:http://dx.doi.org/10.1016/j.actamat.2013.06.018. [2] Z. Wu, H. Bei, G.M. Pharr, E.P. George, Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures, Acta Mater. 81 (2014) 428–441. doi:http://dx.doi.org/10.1016/j.actamat.2014.08.026. [3] C. Varvenne, A. Luque, W.A. Curtin, Theory of strengthening in fcc high entropy alloys, Acta Mater. 118 (2016) 164–176. doi:http://dx.doi.org/10.1016/j.actamat.2016.07.040. [4] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science. 345 (2014) 1153–1158. doi:10.1126/science.1254581. [5] Y. Deng, C.C. Tasan, K.G. Pradeep, H. Springer, A. Kostka, D. Raabe, Design of a twinning-induced plasticity high entropy alloy, Acta Mater. 94 (2015) 124–133. doi:10.1016/j.actamat.2015.04.014. [6] Z. Li, K.G. Pradeep, Y. Deng, D. Raabe, C.C. Tasan, Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off, Nature. advance online publication (2016). http://dx.doi.org/10.1038/nature17981. [7] M. Abdel-Hady, K. Hinoshita, M. Morinaga, General approach to phase stability and elastic properties of β-type Ti-alloys using electronic parameters, Scr. Mater. 55 (2006) 477–480. doi:http://dx.doi.org/10.1016/j.scriptamat.2006.04.022. [8] M. Marteleur, F. Sun, T. Gloriant, P. Vermaut, P.J. Jacques, F. Prima, On the design of new β-metastable titanium alloys with improved work hardening rate thanks to simultaneous TRIP and TWIP effects, Scr. Mater. 66 (2012) 749–752. doi:http://dx.doi.org/10.1016/j.scriptamat.2012.01.049. [9] L. Lilensten, J.-P. Couzinié, J. Bourgon, L. Perrière, G. Dirras, F. Prima, I. Guillot, Design and tensile properties of a bcc Ti-rich high-entropy alloy with transformation-induced plasticity, Mater. Res. Lett. 5 (2017) 110–116. doi:10.1080/21663831.2016.1221861. [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]

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]

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This presentation provides an overview of research that has been (and is being) carried out at The University of Manchester, with a focus on the role that phase transformations play in the development of stress in steel welds. There are several motivations for this research. Residual stresses play a significant role in affecting the long-term structural performance of safety-critical components in many power plants. They can also contribute to the driving force for crack growth and, in nuclear environments, they can activate material degradation mechanisms such as creep and stress-corrosion cracking even in the absence of operating stresses. This is significant because many safety-critical components in a nuclear plant undergo welding during manufacture, and welding is known introduce substantial levels of residual stress. Solid-state phase transformations affect the development of stresses in steels because these transformations have associated strains, which in turn affect the development of stress upon heating and cooling. Residual stresses also tend to be limited by the yield stress of the material, so the mechanical properties of transformation products will have a direct bearing on the development of stress. Some of the topics that will be covered in this presentation include the development and assessment of low-transformation-temperature filler materials for the mitigation of residual stresses, assessments of the effects of particular welding processes on the development of stresses, work towards understanding the mechanisms contributing to the development of transformation strains, and the incorporation of phase transformation effects into finite-element models for the prediction of residual stresses in steel welds. [more]