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The role of the base element in γ-strengthened cobalt/nickel-base superalloys
Two series of polycrystalline model superalloys – one without and one containing Cr – with varying Ni/Co ratios and otherwise constant element contents were produced and are characterized by a variety of methods. All alloys form a γ/γ' microstructure after a standard aging treatment at 900°C for 100 h. Upon long term aging for 1100 h, the Cr containing Co-base alloy forms undesired intermetallic phases. Liquidus and solidus temperatures are hardly influenced by the Ni/Co content, but the γ' solvus temperature is strongly decreasing with increasing Co content. Addition of Cr to the alloy series lowers liquidus and solidus, but does not influence the solvus temperature substantially. Neutron diffraction experiments conducted on the Cr containing alloys show that the misfit is negative for alloys rich in Ni, but linearly increases to positive values with increasing Co content. 3D atom probe tomography shows that W preferentially partitions to the γ phase for the Ni-base alloys, whereas it is enriched in γ' for the Co-base alloys. The partitioning of Co (γ), Cr (γ) and Al (γ') to their respective phases is much more pronounced in Ni rich alloys, i.e. all elements distribute more equally in the Co-base alloys. Nanoindentation on the Cr free alloy variants reveals that the hardness of the Co-base alloy is higher than the one for the Ni-base alloy. However when Cr is present in the alloy, the hardness of the Ni-base alloy is superior. The same behavior is observed in compressive creep experiments at 900°C. The addition of Cr strongly improves the creep resistance of the Ni-base alloy, whereas it slightly worsens the performance of the Co-base alloy.