Scientific Events

Deformation in metallic glasses occurs by initiation and propagation of multiple thin shear bands. This mode is rather difficult to analyse since generally, a single band soon propagates to a large extent in the specimen leading to a catastrophic failure. Exceptions are for example in creep tests under very low stress and moderate temperature or in confined deformation tests. We used instrumented nano-indentations to perform series of independent experiments at room temperature on a Mg65Cu12.5Ni12.5(Ce75La25)10 metallic glass. Loading part of the curves shows serrations which size and duration were measured using an automatic procedure. To make analyses consistent, data were considered only in the domain with similar strain rates, in the range of 1 to 0.3 s-1. Times between successive serrations follow a normal distribution characterizing a random occurrence of deformation burst in the glass. It was then conjectured, first that serration occurs through activation of appropriate zone in the glass that should naturally scale with a multiple of an elementary domain size characterizing the deformation mechanism. Second, as activated zones leading to serration are very few in the glass, the model should be described by the Poisson statistics. Data analyses reveals that serration size are well fitted by a Poisson distribution. The model predict an elementary size which scale with that of the activation volume of 3 atoms, measured from creep test at constant load in the same series of experiments. Eventually, energy dissipated during serration is analyzed as to define shear bands dynamics characteristics.Depending on time, I shall present the use of nano-indention for investigating dynamics of nanoporous metallic materials deformation. N. Thurieau, L. Perriere, M. Laurent-Brocq, Y. Champion, J. Appl. Phys., 118 (2015) 204302. [more]

The aim of modern material modeling is the realistic prediction of the behavior of materials and construction parts by numerical simulation. Experimental investigations prove that the microstructure and thus the mechanical properties may vary under loads. It is thus essential to describe the load-dependent microstructure in these cases by material models to close the system of fundamental physical equations. One elegant way for the derivation of such material models is given by the Hamilton principle which belongs to the class of variational, energy-based modeling strategies. The talk starts with fundamental investigations for modeling the simple harmonic oscillator. Afterwards, the presented modeling concept is generalized to the Hamilton principle which is also applicable to deformable solids with evolving microstructure. As first example for such materials, phase transformations in solids are modeled. The numerical results are compared to experimental observations and an industrially relevant application is presented. In the last part of the talk, the universal character of the Hamilton principle is demonstrated by solving the inverse problem of topology optimization. To this end, a growth approach as observed in biological processes is presented which computes component structures with minimal weight at maximum stiffness. [more]

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]

Although magnesium is the lightest structural metal and has a great potential to be utilized in lightweight constructions, e.g. in automotive engineering, the use of wrought magnesium alloys is limited due to, inter alia, a high mechanical anisotropy and poor room temperature formability. Against this background, understanding the underlying physical mechanisms and microstructural changes in the material during processing is crucial in order to overcome the difficulties associated with the limited ductility by innovative processing, microstructure and alloy design. In order to isolate and access specific mechanisms of plasticity, model experiments on single crystal provide an invaluable tool, as they permit a much clearer and forthright analysis compared to conventional polycrystal studies. Specifically oriented single crystals of various orientations were subjected to channel-die plane strain compression at room and elevated temperatures. The microstructure and texture evolution were characterized experimentally with respect to the deformation behavior. Pure Mg crystals of ‘hard’ orientations that were compressed along the c-axis displayed limited room temperature ductility, although pyramidal 〈c+a〉 slip was readily activated, and fractured along crystallographic {112 ̅4} planes as a result of highly localized shear. A two stage work hardening behavior was observed in ‘soft’ Mg crystals aligned for single or coplanar basal slip. The higher work hardening in the second stage was correlated with the occurrence of anomalous extension twinning that formed as a result of deformation heterogeneity and constituted obstacles for dislocation glide. The interaction between slip and twinning was further investigated by performing in-situ simple shear experiments on Mg bicrystals. It was shown that slip transmission takes place across different twin boundaries with basal slip being readily transmitted through a whole twin, which contradicts a classical Hall-Petch type hardening. The amount of twinning shear for {101 ̅2} twins in Mg was measured experimentally and discussed in terms of the shear-coupled grain boundary migration by considering the formal dislocation content of the respective twin boundaries. The coupling factor that equals the amount of twinning shear was found to result from a combination of two elementary coupling modes, i.e. the correct formal description of the twin boundary comprises two arrays of dislocations with 〈101 ̅0〉 and [0001] type Burgers vectors. [more]

High-throughput screening is a well-established method for scientific experimentation in chemistry and biology. Examples are heterogeneous catalysts, drug developments and nanoparticle toxicology. These methods involve the synthesis of small sample volumes often in form of particles that are quickly tested. These tests are designed to quickly obtain easily accessible data (called descriptors) that are related with a predictor function to the desired properties. The descriptor-predictor-relation is found through mathematical modelling and calibration. One particle based high-throughput concept for the evaluation of potential toxicological hazards will be presented in more detail. Furthermore, a new concept is presented which transfers high-throughput screening to the exploration of new structural metals. The method comprises the synthesis of many small alloy samples in form of particles. These samples obtain a defined microstructure by fast or parallel thermal and mechanical treatments and are subsequently subjected to novel fast descriptor tests while a mathematical algorithm develops the predictor function. The method presented here is a collaborative approach among many researchers and also involves sample routing and automation considerations as well as process modelling. [more]

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