Superalloys are alloys with higher mechanical properties, oxidation resistance and hot corrosion resistance at high temperatures. Superalloys can be divided into nickel-based superalloys, iron-nickel-based superalloys and cobalt-based superalloys according to the matrix composition. Among them, nickel-based superalloys are the fastest and most widely used, followed by iron-nickel-based superalloys. According to the strengthening method, it is divided into solid solution strengthening alloy and separation strengthening alloy (or aging deposition strengthening alloy) and so on. According to the forming method and production process, it is divided into deformed alloy, casting alloy, powder metallurgy alloy and mechanical alloying alloy.

The matrix of the solid solution strengthened superalloy is a solid solution of face-centered cubic lattice. In the range of its solid solubility, elements such as chromium, cobalt, molybdenum, tungsten, niobium and other elements are added to improve the interatomic bonding force, resulting in lattice distortion and reducing the heap. The stacking fault energy prevents the movement of dislocations and increases the recrystallization temperature to strengthen the solid solution. The effect of solid solution strengthening depends on the atomic scale and the amount of alloying elements. Molybdenum and tungsten with larger atomic radius and higher melting point have better solid solution strengthening effect, and the total content of both can reach 18% to 20%. Chromium can prevent high temperature oxidation and hot corrosion, but if the content is too high, it will reduce the solid solubility of γ phase and reduce the thermal strengthening of the alloy. Nickel-based solid solution strengthened superalloys generally have excellent anti-oxidation and hot corrosion resistance, high plasticity and good welding performance, but relatively low thermal strengthening. Iron-nickel-based solid solution strengthened superalloys, although slightly inferior in thermal strength, oxidation resistance and hot corrosion resistance compared with nickel-based solid solution strengthened superalloys, still have excellent mechanical properties and better cold and hot processing technology. function and welding function.

The separation-strengthening superalloy is carried out on the basis of the solid solution strengthening superalloy by adding more molybdenum, titanium, niobium and other elements. In addition to strengthening the solid solution, these elements combine with nickel to form a coherent and stable Ni3 (Al, Ti) phase (that is, a γ phase with a long-range ordered face-centered cubic structure) or Ni3 (Nb, Al, Ti) phase (that is, γ" phase, ordered body-centered tetragonal structure) intermetallic compound, together with tungsten, molybdenum, chromium and other elements and carbon to form various carbides (such as MC, M6C, M23C6, etc.). Because γ The existence of '(γ") phase and carbides greatly improves the thermal strength of the alloy. In addition, trace amounts of boron, zirconium and rare earth elements can also be added to these alloys to form void phases and strengthen grain boundaries. Some alloys developed in recent years are often strengthened by solid solution, separation and grain boundary methods, so that the alloys have excellent general properties. With the increase in the content of γ (γ") phase constituent elements such as Al, Ti, Nb, the strengthening effect is also increased, and the thermal strength is improved, but the cold and hot working performance and welding performance of the alloy are decreased. It is generally believed that Al Ti Welding of superalloys with a content of more than 6% (atomic percent) is difficult.

The nickel-based separation-strengthened superalloy has good thermal strength, oxidation resistance and corrosion resistance. As mentioned above, the cold and hot heating and welding functions are poorer than those of the solution-strengthened superalloy. However, in the solid solution state, some nickel-based sub-strengthened superalloys still have good plasticity and weldability. The iron-nickel-based separation-strengthened superalloy has high thermal strength, excellent anti-oxidation and hot corrosion resistance at medium temperature, and in the solid solution state, the cold and hot working performance and welding performance are the same as the nickel-based separation-strengthened superalloy similar. Regardless of whether the nickel-based separation-strengthened superalloy is an iron-nickel-based separation-strengthened superalloy, when more aluminum, titanium, boron and other strengthening elements are added, the cold and hot working plasticity will decrease, and it can only be formed by casting. Welding of alloys is more difficult.

Oxide dispersion strengthening is to add a certain amount of finely dispersed oxide particles in the matrix to strengthen the matrix, so that the alloy has high strength and certain characteristics. The alloys TDNi and TDNiCr are strengthened by adding about 2% thorium oxide (ThO2) particles in the nickel-chromium base, because the thorium oxide in this alloy is not easy to aggregate and grow at high temperature and is insoluble in the matrix. At the same time, the melting point of the alloy is high, and the grain size is extremely high. Thin, still has high strength at 1000 ~ 1200 ℃, high fatigue resistance, small notch sensitivity, good room temperature plasticity, and can be rolled into bars and sheets. Oxide dispersion strengthened (ODS) alloys are alloys strengthened with oxides (such as Y2O3 and Al2O3). These alloys are produced by a special powder metallurgy process and are forged and rolled into products. Oxide dispersion-strengthened alloys have high durable creep properties and are promising new high-temperature materials. Its defects are low success rate, poor plasticity, weldability and corrosion resistance, which need to be dealt with.

The function of superalloy mainly depends on the alloy composition and its structure. As mentioned above, the refractory metal elements Mo, W and Co play a solid solution strengthening effect, and the γ constituent elements such as Al, Ti, Nb play a separation strengthening effect. . It is generally believed that the strengthening effect should account for the total amount of W Mo and γ constituent elements, while Co and Cr occupy an unnecessary position, and the durability strength of the alloy increases with the increase of the total amount of alloy elements. At present, many studies have shown that the addition of trace elements such as B, Zr, Ce and Mg in superalloys can significantly improve the grain boundary conditions and improve the creep performance of the alloy, but it should be noted that the amount of these elements must be strictly controlled, otherwise it will Detrimental effects occur, such as embrittlement of alloys, formation of low melting point compounds, etc.

Most of the cast superalloys are alloys that can only be formed by casting and are not easily deformed by cold and hot working. With the continuous improvement of the working temperature and strength requirements of superalloys, the content of strengthening elements in the alloys continues to increase, and the composition becomes more complex. Cast superalloys for casting.

Powder superalloy is a superalloy produced by powder metallurgy process. It is difficult to process hot and cold superalloys, and the segregation of the as-cast alloys is serious, which leads to uneven microstructure and anisotropy of mechanical properties. Excellent superalloy material. Because powder alloys greatly improve the thermal processing properties of alloys, even the strongest casting alloys (such as Mar, M246) can be transformed into deformed superalloy materials through powder metallurgy process. In a sense, powder metallurgy removes the current boundaries between wrought and cast alloys. Powder superalloys cannot be welded yet. The process flow of powder superalloy is roughly as follows: pre-alloyed powder manufacturing → compaction (hot pressing, hot isostatic pressing, kneading, etc.) → hot deformation (forging, rolling, etc.) → heat treatment.