h1

%0 Thesis
%A Freund, Martina Sophia Dominique
%T Dislocations in complex intermetallic phases – deformation mechanisms determination by nano-mechanical testing
%I Rheinisch-Westfälische Technische Hochschule Aachen
%V Dissertation
%C Aachen
%M RWTH-2025-00002
%P 1 Online-Ressource : Illustrationen, Diagramme
%D 2024
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2025
%Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2024, Kumulative Dissertation
%X Laves phases represent a class of intermetallic compounds with unique structures and properties. Exploring these phases can lead to the discovery of novel materials with desirable characteristics for various applications, such as in the aerospace, automotive, and electronics industries. Interesting are these for multi-component alloy systems like the Mg-Al-Ca, to strengthen the main magnesium (Mg) matrix. By studying their formation and properties, researchers can optimise alloy compositions for specific purposes, such as enhancing corrosion resistance, improving mechanical properties, or reducing material costs. Especially to be able to investigate this system and the interaction of the main matrix with the Laves phases, the properties of Laves phases must be unravelled to get more knowledge in terms of plasticity and mechanical properties, to have further the ability to understand the interaction. Investigating different compositions of Laves phases contributes to our understanding of phase stabilityand the influence of chemistry on the mechanical properties. This knowledge is crucial for predicting the behaviour of materials under different conditions. The overarching goal of this thesis is to get more insights into the dislocation behaviour of the formed Laves phases of the Mg-Al-Ca system, namely the hexagonal C14 CaMg2 and the cubic C15 CaAl2 Laves phase. Therefore, this thesis was subdivided into sub-goals: i) which slip systems can be activated, ii) the influence of the temperature and the local chemistry on the dislocation behaviour, and iii) the attempt to answer the question if the findings of mechanical properties and dislocation behaviour and their changes with temperature and local chemistry can be transferred to similar stacked phases and furthermore be generalisable for Laves phases at all. The possible slip systems were analysed by a correlative investigation of the surface traces around indentation marks. Therefore, the grain orientations (using electron backscatter diffraction(EBSD)) were taken and aligned with secondary electron (SE) images of forming slip traces around an indentation mark. Transmission electron microscopy (TEM) analyses have identified new slip planes 112, 113, 114, 115, 116, and 1 1 11, summarized as the 11n planes, with a Burgers vector 𝑏 of 1/2<110>. Statistical analysis showed the highest activation on 11n planes in the cubic C15 Laves phase, with dislocation motion on 111planes being up to three times lower. Micropillar compression tests revealed that the critical resolved shear stress (CRSS) of 111 and 112 planes are similar. However, the 11n planes have three times more independent slip systems than 111 planes. Dislocation cross-slip was detected between 111 and 112 planes, and frequently between 114 and 115 planes. Atomistic simulations confirmed the new slip systems, showing low energy barriers and crossslip ability. Temperature influenced the dislocation structure, with dissociated dislocations observed after reaching the BDTT. The local chemistry had negligible influence on slip systems in Ca33Al67, Ca33Al61Mg6, and Ca36Al53Mg11, all showing main plasticity on 11n planes. However, the addition of Mg and Ca decreased hardness and indentation modulus, indicating softening behaviour with the deviation from the stoichiometric compound. Atomistic simulations showed a decreasing energy barrier with increasing Mg and Ca to the CaAl2 Laves phase, and increased anisotropy related to stiffness and surface morphology. The hexagonal C14 MgCa2 Laves phase was investigated in the range of 50 – 250 °C revealing at lower temperatures dislocation pinning which decreases with an increase in temperature, facilitating dislocation motion. Contrary to this observation, the visible surface traces were strongly influenced by the orientation and temperature, and at 250°C none were visible. Over the investigated orientations the pyramidal planes were the most detected followed by the 1st order prismatic and then the 2nd order prismatic with the basal plane. However, the mechanical properties like hardness and indentation modulus remained at one level throughout the investigated temperatures. With the combination of the study of the C15 Laves phase prototype, namely the MgCu2, which presented the same trend for the influence of the temperature regarding mechanical properties as the C15 CaAl2 phase the question of a generalisation can be assumed. By comparing both mechanical properties and their resulting plasticity (here just for room temperature (RT)) and coming up to similar results, although the radius ratio rA/rB deviates around 0.13, the first steps in terms of generalisation can be made. Moreover, isostructural similar planes can be found in the C14 and C15 Laves phase, for example the Laves phase typical triple layer, the basal plane and 111 plane, and prismatic 1st order with the newly found 112 planes. The last two planes have the calculated lowest CRSS at RT for the CaMg2 and CaAl2 Laves phase. With the high amount of Laves phases which are listed up to now, the structurally equivalent planes can be useful to take predictions how changes can influence mechanical properties and plastic behaviour based on already known research. Aiming to have mechanism- and plasticity maps to facilitate sample synthesis and only specific samples have been measured. The findings acquired during the analysis provided important insights into the dislocation behaviour of Laves phases, especially the new found slip planes for the cubic Laves phase and the ability for dislocations to cross-slip, showing that the introducible plasticity introduced is greater than expected. The influence of the local chemistry seems to have a critical composition to influence the mechanical properties, like the hardness or already the reported brittle ductile transition temperature (BDTT).
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2025-00002
%U https://publications.rwth-aachen.de/record/999906