Investigation of the γ′-Strengthened Quaternary Co-Based Alloys Co-Al-W-Ta

The alloying system Co-Al-W-Ta is comprehensively investigated in the vicinity of the compositional point Co-9Al-10W-2Ta, at. pct. These investigations provided a large amount of quantitative information, which can be used for alloy development, namely, the compositional dependences of the γ′-solvus, solidus, and liquidus temperatures; fraction of the extrinsic phases after casting; the compositional dependence of the γ/γ′-lattice misfit; the element partitioning between γ- and γ′-phases; and the two phase compositional area γ/γ′ in the Co-rich part of the Co-Al-W-Ta phase diagram at 900 °C. It is shown that additions of Ta elevate the γ′-solvus temperature and increase the γ/γ′-lattice misfit, but adding more than about 3 at. pct Ta results in a large amount of undissolvable extrinsic phases. Additionally, two Co-Al-W-Ta alloys with lower content of W were developed and solidified as [001] single crystals for mechanical testing in a temperature range between 20 and 1200 °C. These tests included measurement of the Young modulus, tensile tests with constant strain rate, and stress rupture tests. It was found that at temperatures up to about 750 °C the ultimate tensile strength of Co-Al-Ta-W alloys can be at the same level or even higher than of Ni-based superalloys.