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]

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]