“Plasticity of the structurally related intermetallic phases in the Sm-Co system”

Intermetallic compounds, particularly topologically close-packed phases, often share structural motifs, which gives reason to the hypothesis that a relation in mechanical properties, and especially plastic deformation behaviour, can be inferred. This thesis elucidates the interplay of mechanical and structural properties in the Samarium-Cobalt intermetallic system, specifically exploring SmCo₂ and SmCo₅ as fundamental building blocks and their derivatives SmCo₃ and Sm₂Co₁₇.Through nanoindentation, micropillar compression and TEM, combined with DFT calculations, the mechanical properties, slip systems, stacking fault states and atomic bonding environments were characterized. In SmCo5, dislocation-mediated plasticity is revealed to be the dominant deformation mechanism as opposed to the previous hypothesis suggesting amorphous shear as the main mechanism (Publication 1). Nanomechanical testing, in form of slip trace analysis and micropillar compression, identified two primary slip systems, the second order pyramidal slip system {2 1̅ 1̅ 1̅}‹2 1̅ 1̅ 6› and the basal slip system (0 0 0 1)[2 1̅ 1̅ 0]. The presence of basal and pyramidal stacking faults was confirmed via TEM. DFT and atomistic simulations validated these stacking fault states by calculating energy barriers for rigid bodey displacement along these slip planes. It is further suggested that pyramidal stacking faults may form through a synchroshear slip mechanism (Publication 1). A broader investigation into Sm-Co phases (Publication 2) revealed that the other building block SmCo₂ shows prominent slip along (1 1 2) planes, while also showing signs for the activation of (1 1 n) slip. The structurally more complex phases SmCo₃ and Sm₂Co₁₇, that incorporate the building blocks in their structure, demonstrate deformation behaviour that cannot be solely inferred from combination of the individual building blocks due to alterations in the local bonding environment, particularly within Co Kagomé-net layers. For SmCo₃, basal slip in the SmCo₅ building block layer is identified to be the dominant mode, ascribed to an increased bonding hybridization between Co layers, which raises the energy barrier for basal slip in the SmCo₂ building block layer. In Sm₂Co₁₇, a high frequency of basal slip activation indicates an inhibition of non-basal plasticity accompanied by higher critical resolved shear stresses. This work shows that, while the hardness of the four Sm-Co phases is influenced by the cobalt content to a moderate extent, the indentation modulus increases significantly with higher cobalt content, aligning with Young’s modulus calculations from DFT. Elastic properties across the Sm-Co phases follow a rule of mixtures; however, plasticity mechanisms diverge due to complex interactions within the crystal lattice. The Sm-Co system serves as a representative model for intermetallic systems characterized by structural relationships between phases within the system or with other systems. It is crucial to perform thorough investigations into their mechanical properties and underlying mechanisms to further elucidate their origins and determine how these properties might be inhibited or facilitated. To estimate mechanical properties with regard to the material’s phases or crystallographic orientation, a correlative tool was developed, that enables the evaluation of elastic constants using EBSD, EDS, and mappings of mechanical properties with high-speed nanoindentation (Publication3). The primary objective of this tool is to advance the analysis process of mechanical properties intermetallic phases and other materials, and provide materials researchers with an accessible procedure to correlate mechanical properties with orientation or phase information across diverse material systems.