The discovery of an ordered L1₂ precipitate (Co₃(Al, W)) phase in the ternary Co-Al-W system in 2006 which is stable up to temperatures of 950 °C attracts significant research on microstructure design for a new class of load-bearing Co-base high-temperature alloys. The low mismatch between the fcc γ-Co solid-solution phase and the L1₂ γ’-Co₃(Al, W) phase helps to establish a microstructure with coherent cuboidal γ’ precipitates embedded in a continuous γ matrix, analogous to Ni-base superalloys. These new Co-base superalloys have the potential to exhibit excellent high-temperature mechanical properties and are considered to be ideal turbine blade materials.

    In the ternary Co-Al-W system, it is reported that the L1₂ phase is in equilibrium with the B₂-CoAl and D0₁₉-Co₃W phases and transforms into those after extended annealing^[1]. In order to increase the temperature capability and stabilize the γ’-L1₂ phase, additional alloying elements are added to the Co-Al-W system. With additions of Cr, Mo, Ni, Re, Ta and V, the solidus and liquidus temperatures of Co-Al-W alloys are 100-150 °C higher than those of advanced Ni-base single-crystal alloys strengthened by the L1₂ phase^[2]. It is speculated in literature that Mo, V, Nb, Ta and Ti increase the γ’ solvus temperature of Co-Al-W superalloys because they occupy B-sites in the A₃B ordered γ’ phase and thereby increase its volume fraction, while Fe, Mn and Cr tend to distribute to the γ phase and decrease the amount of γ’ phase^[3]. Because the distribution of additional elements will influence the morphology and amount of the γ’ phase, it is important to investigate their partitioning behavior and atomic site occupation in the Co-Al-W system. Therefore the high-temperature strength and thermal stability of Co-base superalloys are optimized.

    In this study, alloy samples with nominal compositions Co-9Al-9W-2X (X=Ti, Nb, V, Ta, Cr and Mo, at.%) were produced. The partitioning behavior of the alloying elements between γ and γ’ phases were investigated by energy-dispersive X-ray spectroscopy (EDS) in the transmission electron microscope (TEM). In order to analyze the element occupation at atomic sites, atom location by channeling enhanced microanalysis (ALCHEMI) technique was applied.

    The details about the partitioning behavior and the atomic site occupation of alloying elements will be discussed in the presentation.

References

[1] S. Kobayashi, Tsukamoto, Y., Takasugi, T., Chinen, H., Omori, T., Ishida, K., Zaefferer, S., Intermetallics, 17 (2009) 1085-1089.

[2] T.M. Pollock, Dibbern, J., Tsunekane, M., Zhu, J., Suzuki, A., JOM, 62 (2010) 58-63.

[3] S. Meher, Yan, H. Y., Nag, S., Dye, D., Banerjee, R., Scripta Mater, 67 (2012) 850-853.

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EMC Abstracts - https://emc-proceedings.com/abstract/element-partitioning-and-atom-location-of-alloying-elements-in-co%e2%80%90base-superalloys/