Abstract
Compositionally complex polycrystalline γ/γ′ CoNi-base superalloys, such as CoWAlloy2 (Co41-Ni32-Cr12-Al9-W5-Ti0.3-Ta0.2-Si0.4-Hf0.1-C-B-Zr) are interesting candidates for new high-temperature materials. To maximize their high-temperature strength, the γ/γ′ microstructure has to be optimized by adjusting the multi-step heat treatments. Various microstructures after different heat treatments were analyzed by scanning and transmission electron microscopy and especially in-situ small-angle neutron scattering during heat treatment experiments. The corresponding mechanical properties were determined by compression tests and hardness measurements. From this, an optimum γ′ precipitate size was determined that is adjusted mainly in the first precipitation heat treatment step. This is discussed on the basis of the theory of shearing of γ′ precipitates by weak and strong pair-couplings of dislocations. A second age hardening step leads to a further increase in the γ′ volume fraction above 70% and the formation of tertiary γ′ precipitates in the γ channels, resulting in an increased hardness and yield strength. A comparison between two different three-step heat treatments revealed an increase in strength of 75 MPa for the optimized heat treatment.
Highlights
In recent years, different compositionally complex, γ0 -strengthened CoNi-base superalloys in theCo–Al–W system were developed [1,2,3,4,5,6]
After a three-step recrystallization and aging heat treatment, the L12 ordered γ0 (Co, Ni)3 (Al, W, Ti, Ta) phase is precipitated in a multimodal distribution of particle size of large primary, smaller secondary, and ternary precipitates that are coherently embedded in the γ matrix phase
Based on the results found, a second aging heat treatment step was added to promote the growth of tertiary γ0 precipitates and to further increase the γ0 volume fraction
Summary
Different compositionally complex, γ0 -strengthened CoNi-base superalloys in the. In order to achieve the highest possible mechanical strength of the developed CoNi-base superalloys, the γ/γ0 microstructure has to be optimized by adjusting the multi-step heat treatment procedures. It is not clear whether the model of weak and strong-pair coupling can be used for CoNi-base superalloys, too, and what the optimum γ/γ0 microstructure is and how large the total γ0 volume fractions are. Different heat treatment strategies were performed to adjust the microstructure and improve the high-temperature mechanical properties of the polycrystalline γ/γ0 CoNi-base superalloy CoWAlloy. The goal was to find the optimum γ/γ0 microstructure and to check whether the existing precipitation strengthening models can be applied for these novel γ/γ0 CoNi-base superalloys
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