What is high temperature alloy

High-temperature alloys are divided into three types of materials: 760℃ high-temperature materials, 1200℃ high-temperature materials and 1500℃ high-temperature materials, with a tensile strength of 800MPa. In other words, it refers to high-temperature metal materials that work for a long time at temperatures above 760--1500°C and under certain conditions. They have excellent high-temperature strength, good resistance to oxidation and hot corrosion, good fatigue performance, fracture toughness and other comprehensive properties. It has become an irreplaceable key material for hot-end components of military and civilian gas turbine engines.

High-temperature main alloys are used to manufacture high-temperature components such as turbine blades, blades, turbine discs, high-pressure gas turbine discs and combustion chambers for aviation, naval and industrial gas turbines. They are also used to manufacture aerospace vehicles, rocket engines, nuclear reactors, petrochemical equipment and coal. conversion and other energy conversion devices.

Since the late 1930s, Britain, Germany, the United States and other countries have begun to study high-temperature alloys. During World War II, in order to meet the needs of new aerospace engines, the research and use of high-temperature alloys entered a period of vigorous development. In the early 1940s, the United Kingdom first added a small amount of aluminum and titanium to the 80Ni-20Cr alloy to form a γ phase for strengthening, and developed the first nickel-based alloy with high high-temperature strength. During the same period, in order to meet the needs of the development of turbochargers for piston aeroengines, the United States began to use Vitallium cobalt-based alloys to make blades.

In addition, the United States has also developed Inconel nickel-based alloy to make the combustion chamber of jet engines. Later, in order to further improve the high-temperature strength of the alloy, metallurgists added tungsten, molybdenum, cobalt and other elements to the nickel-based alloy, increased the aluminum and titanium content, and developed a series of alloy brands, such as the British "Nimonic", the American "Mar-M" and "IN", etc.; in cobalt-based alloys, elements such as nickel and tungsten are added to develop a variety of high-temperature alloys, such as X-45, HA-188, FSX-414, etc. Due to the lack of cobalt resources, the development of cobalt-based superalloys is restricted.

In the 1940s, iron-based superalloys were also developed. In the 1950s, brands such as A-286 and Incoloy901 appeared. However, due to poor high-temperature stability, development has been slow since the 1960s. The Soviet Union began to produce "ЭИ" brand nickel-based superalloys around 1950, and later produced "ЭП" series deformed superalloys and ЖС series cast superalloys. China began trial production of high-temperature alloys in 1956, and gradually formed the "GH" series of deformed superalloys and the "K" series of cast superalloys. In the 1970s, the United States also used new production processes to manufacture directional crystallized blades and powder metallurgy turbine disks, and developed high-temperature alloy components such as single-crystal blades to meet the increasing needs of aircraft engine turbine inlet temperatures.

So far, all countries are researching new high-temperature alloys, making continuous breakthroughs, innovations, and expanding application fields. In the development of modern industry, they have obviously become an indispensable alloy material.

(1) Iron-based high-temperature alloy

Iron-based high-temperature alloys can also be called heat-resistant alloy steels. Its matrix is ​​Fe element, with a small amount of Ni, Cr and other alloying elements added. Heat-resistant alloy steel can be divided into martensite, austenite, pearlite, ferrite heat-resistant steel, etc. according to its normalizing requirements.

(2) Nickel-based high-temperature alloy

Nickel-based high-temperature alloys contain more than half of the nickel and are suitable for working conditions above 1000°C. Using solid solution and aging processing, the creep resistance and compressive yield strength can be greatly improved. At present, based on the analysis of superalloys used in high-temperature environments, the scope of using nickel-based superalloys far exceeds the use of iron-based and cobalt-based superalloys. At the same time, nickel-based high-temperature alloy is also the high-temperature alloy with the largest production and largest usage in my country. Many turbine engine turbine blades and combustion chambers, and even turbochargers also use nickel-based alloys as preparation materials. For more than half a century, the high-temperature capabilities of high-temperature materials used in aerospace engines have increased from 750°C in the late 1940s to 1200°C in the late 1990s. It should be said that this huge improvement has also prompted improvements in casting processes, surface coatings, etc. Rapid development.

(3) Cobalt-based high-temperature alloy

Cobalt-based high-temperature alloys use cobalt as the matrix, and the cobalt content accounts for about 60%. At the same time, elements such as Cr and Ni need to be added to improve the heat resistance of the high-temperature alloy. Although this high-temperature alloy has better heat resistance, due to various countries The output of cobalt resources is relatively small and processing is difficult, so the amount is not large. It is usually used in high-temperature conditions (600~1000℃) and high-temperature parts subject to extreme complex stress for a long time, such as the working blades of aeroengines, turbine disks, hot-end components of combustion chambers, and aerospace engines. In order to obtain better heat resistance, under normal conditions, elements such as W, MO, Ti, Al, and Co should be added during preparation to ensure its superior heat resistance and fatigue resistance.

According to the type of alloy strengthening, high temperature alloys can be divided into solid solution strengthened high temperature alloys and age precipitation strengthened alloys.

(1) Solid solution strengthened type

The so-called solid solution strengthening type is to add some alloying elements to iron, nickel or cobalt-based high-temperature alloys to form a single-phase austenite structure. The solute atoms distort the solid solution matrix lattice, which increases the slip resistance in the solid solution and strengthens it. Some solute atoms can reduce the stacking fault energy of the alloy system, increase the tendency of dislocation decomposition, make cross-slip difficult, and strengthen the alloy to achieve the purpose of strengthening high-temperature alloys.

(2) Aging precipitation strengthening

The so-called age precipitation strengthening is a heat treatment process in which the alloy workpiece undergoes solid solution treatment, cold plastic deformation, and is placed at a higher temperature or room temperature to maintain its properties. For example: GH4169 alloy has a maximum yield strength of 1000MPa at 650°C, and the alloy temperature for making blades can reach 950°C.

Divided by material forming methods: cast high-temperature alloys (including ordinary casting alloys, single crystal alloys, directional alloys, etc.), deformed high-temperature alloys, and powder metallurgy high-temperature alloys (including ordinary powder metallurgy and oxide dispersion-strengthened high-temperature alloys).

(1) Casting high temperature alloy

Alloy materials that use casting methods to directly prepare parts are called cast high-temperature alloys. According to the composition of the alloy matrix, it can be divided into three types: iron-based casting superalloy, nickel-based casting superalloy and diamond-based casting superalloy. According to the crystallization method, it can be divided into four types: polycrystalline cast superalloy, directional solidification cast superalloy, directional eutectic cast superalloy and single crystal cast superalloy.

(2) Deformed high temperature alloy

It is still the most commonly used material in aerospace engines and is widely used at home and abroad. my country's annual output of deformed superalloys is about 1/8 of that of the United States [2]. Taking GH4169 alloy as an example, it is the main variety with the most applications at home and abroad. In my country, bolts, compressors and wheels, and oil sling pans of turboshaft engines are mainly used as main parts. As other alloy products become increasingly mature, deformation occurs at high temperatures. The use of alloys may gradually decrease but will remain dominant in the coming decades.

(3) New high temperature alloy

Including powder high-temperature alloys, titanium-aluminum intermetallic compounds, oxide dispersion-strengthened high-temperature alloys, corrosion-resistant high-temperature alloys, powder metallurgy and nanomaterials and other product segments.

①The alloying degree of the third generation of powder superalloys has been improved, allowing it to take into account the advantages of the first two generations and achieve higher strength and lower damage. The production process of powder superalloys is becoming increasingly mature. In the future, it may be developed from the following aspects :Powder preparation, heat treatment process, computer simulation technology, dual-performance powder disk;

②Titanium-aluminum intermetallic compounds have been developed to the fourth generation, and are gradually expanding in the two directions of multi-element micro-element and large-element micro-element. The University of Hamburg in Germany, Kyoto University in Japan, and the GKSS Center in Germany have all conducted extensive research on titanium. Aluminum intermetallic compounds are now used in shipbuilding, biomedical, and sporting goods fields;

③Oxide dispersion-strengthened superalloys are part of powder superalloys. There are nearly 20 types under production and development. They have high high-temperature strength and low stress coefficients and are widely used in heat-resistant and anti-oxidation components of gas turbines, advanced aerospace engines, etc. Petrochemical reactor, etc.;

④Corrosion-resistant high-temperature alloys are mainly used to replace refractory materials and heat-resistant steel in the fields of construction and aerospace.

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