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Introduction:
Nickel-base cast superalloys have emerged as the go-to option for high-temperature applications due to their excellent oxidation and creep resistant properties. They are also relatively easy to cast and have a unique ability to withstand cyclic loading in both high temperature and high-strain-rate environments. In short, nickel-base cast superalloys offer an ideal balance between strength, ductility, and fatigue resistance.
History and Development:
The development of nickel-base cast superalloys began in the 1930s when nickel-chromium alloys, mostly based on an iron-nickel matrix, were developed to overcome the cracking and oxidation issues of pure nickel. These new nickel-chromium alloys contained chromium, molybdenum, tungsten, vanadium, and other elements to increase their corrosion and oxidation resistance. Over time, new alloys have been developed to meet higher temperature and strength requirements and in the 1980s, “superalloys” began to be developed with very high levels of microalloying, often containing more than 10 alloying elements.
Chemical Requirements:
Nickel-base cast superalloys have chemical requirements that must be met in order to achieve the desired properties at high temperatures. The nickel content of these alloys must be in a range of about 50-61 wt.%, and the chromium content must be between 12-17 wt.%, molybdenum between 3.6-11 wt.%, and tungsten up to 6% wt.. Often, cobalt is also added to provide additional strength and hardness.
Physical Characteristics:
Nickel-base cast superalloys have excellent mechanical properties, especially at elevated temperatures. They exhibit high strength, low creep rates, and good oxidation resistance. However, even at high temperatures, they typically have low ductility. They also experience significant gamma prime-phase precipitation hardening, which can lead to high amounts of residual stress.
Manufacturing Process:
As nickel-base cast superalloys can be difficult to form by traditional wrought fabrication methods due to their high strengths, casting has become the preferred method of manufacture. Sand casting, vacuum induction melting, vacuum arc remelting and electro slag remelting are the most commonly used processes for producing Ni-base cast superalloys.
Advantages and Disadvantages:
The main advantages of Ni-base cast superalloys are their excellent oxidation and creep resistant properties, good strength at high temperatures, and their ability to be easily machined and cast. However, these alloys can also experience significant gamma prime-phase precipitation hardening, leading to high amounts of residual stress, and have low ductility due to the relatively coarse microstructure specified for high temperature applications.
Conclusion:
In conclusion, nickel-base cast superalloys have emerged as the go-to option for high-temperature applications due to their excellent oxidation and creep resistant properties. Their chemical makeup requires a balance of nickel, chromium, molybdenum, tungsten, and sometimes cobalt to provide the desired properties. Casting is the preferred method for their manufacture due to the difficulty of forming these materials by traditional wrought fabrication methods. Nickel-base cast superalloys have many advantages, including their excellent oxidation and creep resistant properties, good strength at high temperatures, and machinability, however, they can suffer from significant gamma prime-phase precipitation hardening and low ductility.
