Generally, cobalt-based superalloys lack coherent strengthening phases. Although the strength at medium temperature is low (only 50-75% of nickel-based alloys), they have higher strength, good thermal fatigue resistance, and thermal corrosion resistance above 980°C. And abrasion resistance, and has better weldability. It is suitable for making guide vanes and nozzle guide vanes for aviation jet engines, industrial gas turbines, naval gas turbines, and diesel engine nozzles.
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 phase and Laves, are harmful and make the alloy brittle. Cobalt-based alloys seldom 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°C). 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 superior to 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 that in nickel. And because most cobalt-based alloys have higher chromium content than nickel-based alloys, they 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. The alloys developed later, such as Mar-M509 alloy, are produced by vacuum smelting and vacuum casting because they contain more active elements such as zirconium and boron.
The wear of alloy workpieces is largely affected by the contact stress or impact stress on the surface. Surface wear under stress depends on the interaction characteristics of dislocation flow and contact surface. For cobalt-based alloys, this feature is related to the lower stacking fault energy of the matrix and the transformation of the matrix structure from face-centered cubic to hexagonal close-packed crystal structure under the effect of stress or temperature. Metals with hexagonal close-packed crystal structure Material, abrasion resistance is better. In addition, the content, morphology and distribution of the second phase of the alloy, such as carbides, also have an impact on the wear resistance. Because the alloy carbides of chromium, tungsten and molybdenum are distributed in the cobalt-rich matrix and part of the chromium, tungsten and molybdenum atoms are solid-dissolved in the matrix, the alloy is strengthened, thereby improving wear resistance. In cast cobalt-based alloys, the size of carbide particles is related to the cooling rate. Faster cooling means finer carbide particles. In sand casting, the hardness of the alloy is low, and the carbide particles are also coarser. In this state, the abrasive wear resistance of the alloy is significantly better than that of graphite casting (fine carbide particles), and the adhesive wear resistance of both There is no significant difference, indicating that coarse carbides are beneficial to improve the ability of abrasive wear resistance.
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