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Why Are Nickel-based Alloys Superalloys?

Update On Sep 28.2022

Nickel-base alloys contain nickel, chromium, and molybdenum and are known for their high strength and resistance to heat, corrosion, and wear. They are also easily welded and fabricated and provide superior resistance to cracking and stress corrosion. 

This article will tell you why nickel alloys are super alloys?


The Principle of Stability of Nickel-based Alloys

Because metals in general usually lose a lot of their strength at high temperatures. Ordinary alloys like steel melt at around 1400 degrees Celsius, but before that, it loses almost all of its strength. You can refer to the diagram below, with mechanical properties on the vertical axis and temperature on the horizontal axis.

We use "creep" - Creep to describe the behavior of these metals at high temperatures.

There are many kinds of creep, but the creep that dominates at higher temperatures usually depends on diffusion, basically atoms move more easily at higher temperatures, this applies both to the diffusion of individual atoms in the lattice to adjacent locations, Also applies to the movement of crystal defects called dislocations. Therefore, to make a metal suitable for ultra-high temperature applications, we need to suppress diffusion and dislocation motion.

Superalloys are divided into iron-based superalloys, nickel-based superalloys, and cobalt-based superalloys according to different substrates. Among them, nickel-based superalloys are referred to as nickel-based alloys for short.

Nickel is definitely the predominant metal element in superalloys, in part because nickel maintains the same structure, face-centered cubic, from room temperature to melting point. Both iron and cobalt undergo phase transitions. As shown in the figure below, from the left are iron, nickel, and cobalt.

Nickel-based alloys are currently the most widely used superalloys and the alloy with the highest high temperature resistance. In the current field of superalloys, nickel-based alloys have a very important and special status. As a result, nickel-based alloys have stronger high temperature resistance, and are more powerful than iron-based alloys and cobalt-based alloys in terms of oxidation resistance and corrosion resistance.

Nickel-based alloys refer to a class of alloys with comprehensive properties such as high strength and certain oxidation and corrosion resistance at high temperatures of 650-1000 degrees Celsius. According to the main properties, it is subdivided into nickel-based heat-resistant alloys, nickel-based corrosion-resistant alloys, nickel-based wear-resistant alloys, nickel-based precision alloys, and nickel-based shape memory alloys.


Corrosion Resistance of Nickel-based Alloys

Nickel-based alloy not only has excellent performance at high temperatures, but also has high chemical stability, hardly oxidizes below 500 degrees, and is not affected by warm air, water, and certain salt aqueous solutions.

The control of material corrosion has been recognized as the best way to practice material economy in the industry. The material selection at the design end of industrial equipment does not only consider the material price. The length of the cycle required for subsequent replacement and maintenance, the overall use efficiency, as well as more important issues such as safety, all need to be more accurately included in the design and selection considerations.

Nickel-based alloys have good corrosion resistance in strong reducing corrosion environments, complex mixed acid environments, and solutions containing halide ions. Nickel-based corrosion-resistant alloys can be represented by Hastelloy alloys. As mentioned above, the Ni element is Crystallographically, it can accommodate more alloys to improve the ability to resist corrosive environments; and Ni itself has certain corrosion resistance, such as excellent resistance to stress corrosion of Cl ions and caustic alkali corrosion. The passivation elements added in nickel-based alloys can form solid solutions with the substrate phase, which improves the corrosion potential and thermodynamic stability of the materials. For example, adding Cu, Cr, Mo, etc. to Ni can improve the corrosion resistance of the overall alloy.

Contains extremely low levels of silicon and carbon, and is generally regarded as a universal corrosion-resistant alloy with excellent corrosion resistance to most corrosive media in both oxidizing and reducing atmospheres, as well as excellent resistance to pitting corrosion, crevice corrosion, and stress cracking corrosion performance, such alloys can control the precipitation of carbides due to the reduction of C and Si, and further improve their corrosion resistance. Because of these characteristics, it is widely used as a material for applications in harsh environments such as chemical equipment. In addition, Ni-Cr-Mo-Cu alloy has the ability to resist both nitric acid and sulfuric acid corrosion and also has good corrosion resistance in some oxidation-reduction mixed acids.


Advantages of Nickel-Based Alloys

1. Excellent performance at high temperatures:

It can maintain high strength at high temperatures and has excellent mechanical properties such as creep resistance and fatigue resistance, as well as anti-oxidation and corrosion resistance characteristics, and good plasticity and weldability.

2. Many alloy modifications can be added:

Nickel-based alloys are often added with more than ten kinds of alloying elements to improve corrosion resistance in different environments; as well as solid solution strengthening or precipitation strengthening.

3. Resistant to the extremely harsh working environment:

Nickel-based alloys are widely used in various harsh conditions, such as high temperature and high-pressure parts of aerospace engine gas chambers, structural parts of nuclear energy, petroleum, and marine industries, and corrosion-resistant pipelines.


Application Areas of Nickel-based Alloys

Nickel-based alloys are mostly used in special corrosion-resistant environments, high-temperature corrosion environments, and equipment that requires high-temperature mechanical strength.

● Often used in aerospace, energy, petrochemical industry or special electronics / optoelectronics and other fields

● Aircraft engines, gas turbines, engine valves - good mechanical strength at very high temperatures

● Energy industry - good resistance to high temperature vulcanization and high temperature oxidation

● Furnace parts, thermal insulation, heat treatment industry - damage resistance at very high temperatures

● Petrochemical industry, desalination plants, petrochemical pipelines - corrosion resistance to aqueous solutions (acids, alkalis, chloride ions)

● Electronics/photonics, battery housings, lead frames, computer monitor grilles—generally environments with low corrosion resistance or high temperature resistance


Why spend so much time and money developing and upgrading super alloys?

Application of Nickel-Based Alloys in Aerospace

In order to meet the needs of ships and industrial gas turbines, since the 1960s, a number of high Cr nickel-based alloys with good thermal corrosion resistance and stable structure have been developed. In about 40 years from the early 1940s to the late 1970s, the working temperature of nickel-based alloys increased by 1,100 °C from 700, with an average increase of about 10 °C per year. Today, the service temperature of nickel-based alloys can exceed 1,100°C. From the aforementioned Nimonic75 alloy with a simple composition to the recently developed MA6000 alloy, the tensile strength at 1,100°C can reach 2,220MPa and the yield strength is 192MPa; Its lasting strength under the condition of 1,100℃/137MPa is about 1,000 hours, which can be used for aero-engine blades.

The most important application of superalloys is turbine blades. It's clear that jet engines have very high internal temperatures, and we'd like to see them get even higher. Internal combustion engines are all controlled by the Brayton cycle, and the greater the temperature difference between the combustion chamber and the external air reservoir, the more efficient the engine will be. To maximize the safe temperature at which turbines can operate, modern jet engine and gas turbine blades use a single crystal superalloy with cooling runners and a ceramic coating on the surface. The development of superalloy-related technologies has resulted in a significant increase in the maximum temperature of jet engines over the past three decades. The latest superalloys can withstand long-term exposure temperatures in excess of 1100 degrees Celsius. The actual maximum temperature inside the turbine is higher due to thermal insulation coatings and cooling runners, and given the amount of aviation fuel used each year, the development of these materials may have saved millions of liters of fuel, resulting in economic and environmental cost savings , the economic and environmental impact of saving fuel is truly astounding. This is part of the reason why foreign countries call superalloys super alloys.

With the development of China's industrialization construction, nickel-based superalloys are also gradually used in civil industry energy power, transportation, petrochemical, metallurgical mining and glass building materials and other sectors. At present, nickel-based superalloys are mainly used in turbochargers for diesel engines and internal combustion engines, industrial gas turbines, internal combustion engine valve seats, steering rollers, etc.


Combination of Investment Casting Technology and Nickel-Based Alloy Materials

For example, in the early 1950s, the development of vacuum melting technology created conditions for refining nickel-based alloys containing high Al and Ti, which led to a substantial increase in alloy strength and operating temperature. In the late 1950s, due to the increase in the working temperature of turbine blades, the alloy was required to have higher high-temperature strength, but the alloy with high strength was difficult to deform, or even could not be deformed, so precision casting technology was used to develop a series of high-temperature strength. of casting alloys. In the mid-1960s, directional crystalline and single crystal superalloys with better performance were developed, as well as powder metallurgy superalloys.

It is usually melted in a vacuum induction furnace to ensure the composition and control of the gas and impurity content, and the parts are made by vacuum remelting-precision casting technology. For superalloy workpieces, the choice of smelting method will affect the impurity zone (that is, the abnormal segregation of components). Generally speaking, the impurity and defects (such as pores) are related to the alloy composition and casting technology.


Besser Casting: Nickel-Based Alloy Casting Services

Besser vacuum casting line

Besser Casting has two smelting production lines, vacuum casting and non-vacuum casting. The non-vacuum casting smelting can provide castings with a thickness of 2.5mm and ensure that the castings can pass the X-ray flaw detection test. Vacuum casting melting can cast high-temperature nickel-based alloy castings with a limit thickness of 0.5mm and ensure no defects.

Strict chemical composition control

The modification of many superalloys results from the mobilization of chemical elements in very small proportions. In terms of chemical composition control of high-temperature nickel-based alloys, we have been recognized by customers in Japan and the United States. 

Besser Casting pursues the quality of castings and uses the nickel-based channel of the German Spectrometer to detect the chemical composition to ensure that each element meets the requirements of customized nickel-based alloy materials, so as to ensure that the customized castings can be maintained during the long-term use of the client. Consistently excellent performance and state, to provide customers with the most cost-effective castings.

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