5 Werkstofftechnik
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A nickel-based eutectic alloy with a γ/γ'-NbC structure was directionally solidified with a planar front. The specimens were tested for creep under sawtooth thermal cycling in the temperature range from 600 to 1100°C. It has been established that the lifetime under the conditions of thermal cycling is about five times shorter than it is predicted by the linear damage accumulation rule on the basis of results of isothermal creep tests. Faster creep under thermal cycling is caused by the rapid coarsening of the γ/γ' microstructure due to the periodic partial dissolution and reprecipitation of the γ' phase in heating and cooling half-cycles.
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 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.