Place: Intermetallics 2015, Bad Staffelstein, Germany
Alvin Noe Ladines
Topologically close-packed (TCP) phases play an important role in many modern alloys and steels. While particular TCP phases are desirable in precipitate-hardened steels, the precipitation of TCP phases in single-crystal superalloys has a detrimental effect on the mechanical properties. In order to gain a microscopic understanding of TCP phase formation, we investigate the factors that drive their structural stability. In particular, we employ electronic-structure calculations at different levels of coarse-graining, ranging from density-functional theory (DFT) to tight-binding to analytic bond-order potentials (BOPs). We demonstrate that the role of electron count on the trend of structural stability of TCP phases across the transition-metal (TM) series can be identified on all levels of coarse-graining. The interplay of electron count and size-mismatch as dominating factors for the stability of TCP phases is analysed by comparison of DFT calculations for various binary and ternary TM alloys to an empirical structure map. This structure map is further applied in a concerted effort with casting experiments and microstructure analysis in order to provide guidance for the analysis of TCP phase precipitates. In particular we show that the combination of the structure map with experimentally determined local chemical composition enables to identify TCP phase precipitates in Co-based and Ni-based superalloys.