Our research is centred around structural and chemical atomic complexity, defect phases and how they relate to material properties.
These are current examples of our research in the Mg-Al-Ca system on solid solutions, intermetallic materials and their composites.
Microalloyed Mg with improved intrinsic ductility
Sandlöbes, S., Friák, M., Korte-Kerzel, S., Pei, Z., Neugebauer, J. and Raabe, D., 2017. A rare-earth free magnesium alloy with improved intrinsic ductility. Scientific reports, 7(1), pp.1-8.
In preliminary work, we demonstrated successful ab initio guided treasure hunting in terms of designing ductile dilute Mg alloys. However, in spite of the success of this approach, the underlying complex solute-defect interaction and the related physical mechanisms are not yet entirely understood as the structure, interaction and stability of defects is not clear – overcoming this gap in understanding is at the focus of the SFB1394. It was shown that, for Mg-Y and Mg-RE alloys, the increase in ductility is related to a change in the activation of slip systems being associated with a certain stacking fault energy, the so-called I1 intrinsic stacking fault energy (I1 SFE). We used these observations as a guideline for identifying also less costly solid solution alloying elements that might enable similar effects relevant to ductilisation. For this purpose, the I1 SFE of Mg-X binary and ternary solid solution alloys was applied systematically as a guiding parameter for the design of a general class of ductile Mg alloys. We identified the rare-earth-free ternary Mg-Al-Ca system as having a significant reduction in the I1 SFE. First experimental observations show that this material possesses indeed an increased activity of pyramidal <c+a> dislocation slip facilitating enhanced ductility. However, the exact mechanisms of this ductility increase remains open and will be investigated within the SFB.
Dislocation mechanisms in the intermetallic Laves phase Mg2Ca
Guénolé, J., Mouhib, F.Z., Huber, L., Grabowski, B. and Korte-Kerzel, S., 2019. Basal slip in Laves phases: the synchroshear dislocation. Scripta Materialia, 166, pp.134-138.
In fundamental work on plasticity mechanisms of intermetallics, we have studied the Mg2Ca Laves phase that is part of the model system for the first funding period. Two different mechanisms have been reported in previous ab initio studies to describe basal slip in the Laves phases: synchroshear and undulating slip. To date, no clear answer has been given on which is the energetically favourable mechanism and whether either of them could effectively propagate as a dislocation. Using classical atomistic simulations supported by ab initio calculations, we could remove the ambiguity and showed that the two mechanisms are, in fact, identical. Furthermore, synchroshear could be established as the mechanism for propagating dislocations within the basal plane in Laves phases and the work represents the first set of combined atomistic and ab initio calculations to consider the energy barrier to dislocation motion in Laves phases based on an assessment of the dynamic process (from atomistics) rather than only static calculations of theoretical intermediate configurations (by DFT).
Mg-Al-Ca metallic-intermetallic composites
Zubair, M., Sandlöbes-Haut, S., Wollenweber, M.A., Bugelnig, K., Kusche, C.F., Requena, G. and Korte-Kerzel, S., 2019. Strain heterogeneity and micro-damage nucleation under tensile stresses in an Mg–5Al–3Ca alloy with an intermetallic skeleton. Materials Science and Engineering: A, 767, p.138414.
As-cast Mg-Al-Ca alloys are among the most promising alloys for elevated temperature applications (≤200 °C) due to their superior creep properties when compared to conventional AZ or AM series Mg alloys. The microstructures of Mg-Al-Ca alloys consist of a soft α-Mg phase reinforced with hard interconnected Laves phases which facilitate the good creep resistance. The volume fraction, type and morphology of Laves phases can be controlled through the Ca/Al ratio. Consequently, the Ca/Al ratio can be used to manipulate the mechanical properties of this alloy system. We could show that a higher Ca/Al ratio results in i) higher volume fraction of intermetallic Laves phases in the microstructure, ii) improvement in the yield strength (YS), and iii) enhancement in creep resistance of the as-cast alloys. Not only the volume fraction, but also the interconnectivity of the Laves phase networks influences the mechanical response, in particular the creep resistance. This was shown by µ-DIC measurements of the local strain distribution and partitioning at the microstructural level revealing that stress localises at the α-Mg Laves phase interfaces, hence, improving strength and creep resistance. The detailed role of metallic-intermetallic phase boundaries and the co-deformation at and across these phase boundaries remain open and will be tackled within the CRC.