Generally, Cobalt-based superalloys lack a coherent strengthening phase. Although the strength is low at medium temperature (only 50-75% of nickel-based alloys), they have higher strength, good thermal fatigue resistance and thermal corrosion resistance at temperatures higher than 980℃ And abrasion resistance, and has better weldability.
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Carbide strengthening phase. The most important carbides in cobalt-based superalloys are MC, M23C6 and M6C. In cast cobalt-based alloys, M23C6 is precipitated between grain boundaries and dendrites during slow cooling. In some alloys, the fine M23C6 can form a eutectic with the matrix γ. MC carbide particles are too large to directly have a significant effect on dislocations, so the strengthening effect on the alloy is not obvious, while finely dispersed carbides have a good strengthening effect.
The carbides located on the grain boundary (mainly M23C6) can prevent the grain boundary slip, thereby improving the endurance strength. The microstructure of the cobalt-based superalloy HA-31(X-40) is a dispersed strengthening phase (CoCrW)6 C-type carbide. The topological close packed phases that appear in some cobalt-based alloys, such as sigma and Laves, are harmful and make the alloy brittle. Cobalt-based alloy rarely use intermetallic compounds for strengthening, because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but in recent years, cobalt-based alloys that use intermetallic compounds for strengthening have also been developed. The thermal stability of carbides in cobalt-based alloys is better. When the temperature rises, the growth rate of carbide accumulation is slower than the growth rate of the γ phase in the nickel-based alloy, and the temperature of re-dissolving into the matrix is also higher (up to 1100℃).
Therefore, when the temperature rises, the cobalt-based alloy The strength of the alloy generally decreases slowly. Cobalt-based alloys have good thermal corrosion resistance. It is generally believed that the reason why cobalt-based alloys are better than nickel-based alloys in this respect is that the melting point of cobalt sulfide (such as Co-Co4S3 eutectic, 877℃) is higher than that of nickel.
The melting point of the substance (such as Ni-Ni3S2 eutectic at 645°C) is high, and the diffusion rate of sulfur in cobalt is much lower than in nickel. And because most cobalt-based alloys have higher chromium content than nickel-based alloys, it can form a protective layer of alkali metal sulfate (such as a Cr2O3 protective layer that is corroded by Na2SO4) on the surface of the alloy. However, the oxidation resistance of cobalt-based alloys is generally much lower than that of nickel-based alloys. Early cobalt-based alloys were produced by non-vacuum smelting and casting processes. Later developed alloys, such as Mar-M509 alloy, are produced by vacuum smelting and vacuum casting because they contain more active elements such as zirconium and boron.
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