Intermetallic phases exist in thousands, as they consist of two or more metallic elements, allowing a variety of element combinations. They exhibit a high strength up to high temperatures but are brittle at room temperature, which severely limits their application up to now. However, they are often found as, sometimes unwanted, precipitates in superalloys, where their mechanical behaviour and influence on the alloy properties remain poorly understood. Due to their complex crystal structure and the resulting brittleness below the brittle-to-ductile transition temperature, the knowledge on their deformation mechanisms is still very limited. However, a deeper understanding of how crystal structure and chemical composition influence the mechanical properties and plasticity would help tailor these properties, to optimise and increase the application of intermetallics as promising structural and functional materials. This thesis focuses on the Ta-Fe (-Al) system, which includes both a C14 Laves phase and a μ-phase in its binary, Ta-Fe, and ternary, Ta-Fe-Al, form. Since the Laves phase structure constitutes a “building block” within the μ-phase crystal structure, and both intermetallic phases exhibit wide homogeneity ranges, a systematic investigation is feasible. For this work, bulk intermetallics with sufficient homogeneity and grain size were prepared to enable nanomechanical testing, as its small scale allows the examination of plastic deformation of macroscopically brittle materials. Nanoindentation tests were conducted to determine the hardness and indentation modulus. Furthermore, analysing the slip traces that formed around the indents and correlating their positions with the grain orientation enabled a statistical determination of activated slip planes. Micropillar compression tests as well as subsequent transmission electron microscopy investigations of plastically deformed regions were performed to verify these statistics. During the preparation of bulk intermetallics, several key factors were identified as critical. These include the high purity of the input materials, the adjustment of the sample weight based on the amount of input materials and their physical properties as well as the synthesis route, particularly the order of element addition. By considering these factors, 13 samples of the Ta-Fe (-Al) system were prepared. Seven binary samples, including three Laves phase and four μ-phase compositions, and six ternary samples with three distinct compositions per phase were synthesised. For the binary samples, the Ta content was varied, while for the ternary samples, different Fe: Al ratios were used, with the Ta content remaining constant for each phase. Nanoindentation tests revealed that the binary Laves phase exhibits a slightly higher hardness and a higher indentation modulus than the binary μ-phase, with the indentation modulus decreasing as the Ta content increases. The ternary Laves and μ-phase exhibit similar mechanical properties to their binary counterparts, with the Fe: Al ratio having little effect. When studying the plasticity of these phases by slip trace analysis, it was found that the Laves phase predominantly deforms via non-basal slip, whereas the μ-phase primarily exhibits basal slip. Increasing the Ta content in the binary system led to an increase in basal slip for the Laves phase but a decrease for the μ-phase. Alloying with Al resulted in a higher proportion of non-basal slip in both phases. Despite the commonly known slip systems for hexagonal structures, including the (0 0 0 1) 〈1 1 2̅ 0〉 basal, 1 0 1̅ 0 〈1 1 2̅ 0〉 1^st order prismatic, 1 1 2̅ 0 〈1 1̅ 0 0〉 2^nd order prismatic, 1 0 1̅ 1 〈1 1 2̅ 3〉 1^st order pyramidal and 1 1 2̅ 2 〈1 1 2̅ 3〉 2^nd order pyramidal slip system, two new pyramidal slip planes could be identified in the μ-phase. The 1 1̅ 0 5 〈5̅ 5 0 2〉 slip system was found both by micropillar compression tests and within in the plastic zone underneath an indent, where the 1 1̅ 0 26 A-B-A triple layer plane was also observed to accommodate slip.
Christina Gasper (Wed,) studied this question.