Nickel-Based alloy with other elements is called nickel alloy. Nickel has good mechanical, physical and chemical properties. Adding appropriate elements can improve its oxidation resistance, corrosion resistance, high temperature strength and some physical properties. Nickel alloy can be used as electronic tube material, precision alloy[TJC STEEL] (magnetic alloy, precision resistance alloy, electric heating alloy, etc.), nickel base superalloy and nickel base corrosion resistance alloy and shape memory alloy. In energy development, chemical, electronics, navigation, aviation and aerospace sectors, nickel alloys are widely used.
Nickel can form many alloys with copper, iron, manganese, chromium, silicon and magnesium. Nickel copper alloy is the famous Monel alloy, it has high strength, good plasticity, in the atmosphere below 750 degrees, stable chemical properties, widely used in electrical industry, vacuum tube, chemical industry, medical equipment and Marine industry.
A.Definition of Nickel-Based Alloys:
Nickel-based alloy is generally referred to as the alloy with Ni content of more than 30wt%, and the Ni content of common products is more than 50wt%. Due to its super high temperature mechanical strength and corrosion resistance, the[TJC STEEL] alloy combined with iron and cobalt alloy is called Superalloy. It is generally applied in the high temperature environment above 540℃, and according to its application occasions, Selection of different alloy design, mainly used in special corrosion resistance environment, high temperature corrosion environment, need to have high temperature mechanical strength equipment. It is often used in aerospace (aircraft engine, gas turbine, engine valve), energy (furnace parts, heat insulation, heat treatment industry, oil and gas industry), petrochemical industry (seawater desalination plant, petrochemical transmission pipeline), or special [TJC STEEL]electronic/photoelectric (battery shell parts, wire frame, computer monitor mesh cover) and other fields.
B.Origin and Development:
The nickel-based alloy was developed in the late 1930s. The first nickel-based alloy Nimonic75(Ni-20Cr-0.4Ti) was produced in Britain in 1941. Nimonic 80(Ni-20Cr-2.5Ti-1.3Al) was prepared by adding Al to improve the submersible strength. The United States in the mid-1940s, Russia in the late 1940s, China in the mid-1950s have also developed nickel-based alloys. The development of nickel base alloy includes two aspects, namely the improvement of alloy composition and the innovation of production technology.
For example, in the early 1950s, the development of vacuum smelting technology created conditions for the refining of nickel-based [TJC STEEL]alloys containing high Al and Ti, which led to a substantial increase in the strength and operating temperature of alloys. In the late 1950s, due to the increase in the working temperature of turbine blades, higher high-temperature strength of alloys was required. However, high strength of alloys made it difficult or even impossible to deform. Therefore, a series of casting alloys with good high-temperature strength were developed by adopting precision casting technology. In the mid-1960s, directional crystals and single crystal superalloys with better properties were developed, as well as powder metallurgy superalloys.
In order to meet the needs of ships and industrial gas turbines, a batch of high Cr nickel base alloys with good thermal corrosion resistance and stable microstructure have been developed since the 1960s. In a period of about 40 years from the early 1940s to the late 1970s, the operating temperature of the nickel-based alloy [TJC STEEL]increased by 1,100℃ from 700, an average increase of about 10℃ per year. Today, nickel-based alloys can be used at temperatures in excess of 1,100℃, from the initial Nimonic75 alloy with simple composition, to the recently developed MA6000 alloy with tensile strength of 2,220MPa and yield strength of 192MPa at 1,100℃. Its endurance strength at 1,100℃/137MPa is about 1,000 hours, which can be used in aero-engine blades.
C.Characteristics of Nickel-Based Alloys:
Nickel-based alloy is the most widely used and strongest material in superalloys. The name superalloy derives from the material characteristics.
(1) Excellent performance: it can maintain high strength at high temperature, and has excellent mechanical properties such as anti-creep and [TJC STEEL]anti-fatigue, as well as oxidation and corrosion resistance and good plasticity and weldability.
(2) Super complicated alloy addition: more than ten alloying elements are often added to nickel-based alloys to improve corrosion resistance in different environments; And solid solution strengthening or precipitation strengthening.
(3) Extremely harsh working environment: Nickel-based alloy is widely used in various harsh working conditions, such as the high-temperature and high-pressure part of the gas chamber of aerospace engine, the structural parts of nuclear energy, petroleum, Marine industry, corrosion resistant pipelines, etc.
D.Microstructure of Nickel-Based Alloys:
The crystal structure of nickel-based alloy is mainly high temperature stable face-centered cube (FCC) structure. In order to improve its heat resistance, a large number of alloy-elements are added. These elements will form various secondary phases and improve the high temperature strength of Nickel-based alloy. The secondary phase consists of Coherent metallitic compounds such as MC, M23C6, M6C, and M7C3 in various [TJC STEEL]forms, usually distributed in grain boundaries, and coherent Ordering such as γ' or γ'. The chemical composition of γ' and γ' phase is roughly Ni3(Al, Ti) or Ni3Nb. This ordered phase is very stable at high temperature, and excellent latent failure strength can be obtained through their strengthening.
With the increase of alloying degree, the microstructure of γ' phases has the following trend: the number of γ' phases increases gradually, the size increases gradually, and the γ' phases change from spherical to cubic, and the size and shape of different γ' phases appear in the same alloy. In addition, γ+γ' eutectic is formed in the solidification process, and discontinuous granular carbide precipitates from the grain boundary and is surrounded by γ' thin films. These micromicrostructure changes improve the [TJC STEEL]properties of the alloy. In addition, the chemical composition of modern nickel-based alloys is very complex, and the saturation of the alloys is very high, so it is necessary to control the content of each alloy element (especially the major strengthening elements). Otherwise, other harmful intermediate metal phases, such as σ and Laves, may be precipitated during use, which will damage the strength and toughness of the alloy.
E.Role and Brand of Alloying Elements:
Nickel-based alloy is one of the most widely used superalloys with the highest strength. The addition of a large amount of Ni is the stable element of Wastian iron phase, which makes the nickel-based alloy maintain FCC structure and can dissolve more other alloy elements, and also maintain good microstructure stability and plasticity of the material. While Cr, Mo and Al have oxidation and corrosion resistance, and have [TJC STEEL]a certain strengthening effect. The strengthening of nickel-based alloys can be divided into:
(1) Solid solution strengthening elements, such as W, Mo, Co, Cr and V, cause local lattice strain at the base of Ni-Fe by the difference between the atomic radius and the base material;
(2) The precipitation of strengthening elements, such as Al,Ti, Nb and Ta, can form integrated and ordered A3B intermetallic compounds, such as Ni3(Al,Ti) and other strengthening phases (γ '), so that the alloy can be strengthened effectively and obtain higher high-temperature strength than iron base superalloy and cobalt base alloy.
(3) Grain boundary strengthening elements, such as B, Zr, Mg and rare earth elements, can enhance the high temperature properties of the alloy. Generally, the brand of nickel base alloy is named by its development manufacturer, such as Ni-Cu alloy, also known as Monel alloy, common such as Monel 400, K-500 and so on. Ni-Cr alloy is commonly known as Inconel alloy, which is a common nickel-based heat-resistant alloy. It is mainly used in oxidizing medium conditions, such as Inconel 600, 625, etc. The Inconel alloy is not as hot as the nickel-based alloy, but it is also an inexpensive alternative to the Inconel alloy, which is used in the lower temperature components of jet engines and in petrochemical reactors, such as Incoloy 800H and 825. The Inconel and Incoloy Coloy are separated out [TJC STEEL]with enhanced elements, such as Ti, Al, and Nb, so that the alloy can maintain good mechanical strength and corrosion resistance at high temperatures. It is also widely used in jet engine components, such as Inconel 718 and Incoloy A-286. Ni-Cr-Mo(-W)(-Cu) alloy is known as Hastelloy corrosion resistant alloy, wherein Ni-Cr-Mo is mainly used in reducing medium corrosion conditions. Hastelloy is represented by brands such as C-276, C-2000, etc.
F.Properties of Nickel-Based Alloys:
1. High Temperature (Instantaneous) Intensity:
Nickel-based alloys have higher tensile strength at room temperature (TS= 1200-1,600; YS= 900-1,300 MPa), and has good ductility. It includes [TJC STEEL]the use of the above ionic and covalent bonded γ-γ 'or γ-γ' precipitates with high melting point and high strength at room temperature, coupled with a large number of slip systems and ductile Wastein iron phase base, the concept of composite materials to obtain excellent mechanical properties with both strength and plasticity.
2. Latent Strength:
Under the constant load at high temperature (T/Tm>0.5), the material alloy slowly produces plastic deformation phenomenon, because the material alloy has the best resistance to high temperature diving ability, and is widely used in a variety of high temperature environment, as a load bearing parts.
It can be divided into three stages. In the stage of Primary Creep, the deformation rate is relatively high, but it slows down with the increase of strain due to work hardening. When the deformation rate reaches a certain minimum value and is close to a constant, it is called Secondary or Steady-StateCreep, which is the result of balance between work hardening and dynamic recovery. The latent strain rate required in engineering material design is the strain rate at this stage. At the third stage (Tertiary Creep), the strain rate is increased exponentially with the increase in strain due to necking, and finally failure is achieved.
The relationship between stress and strain rate varies with the different mechanisms of submersion. Generally speaking, the increase of temperature or stress will increase the deformation rate of steady-state submersion and shorten the submersion lifetime. The mechanism of submersion can be divided into (1) differential displacement submersion: With the help of high temperature, the differential displacement may slip along the slip plane and then deformation occurs. (2) Diffusion Creep: Nabarro-Herring Creep, which is caused by atom movement and dispersed along grain, is the main mechanism at high temperature. Diffusion along grain boundaries, called Coble Creep, is the dominant mechanism at low temperatures. Therefore, the smaller the grain size, the more easily the diffusion latent occurs. (3) grain boundary slip: Because the grain boundary is weak at high temperature, the material is easy to slip along the grain boundary, resulting in intergranular cracks. Therefore, the smaller the grains at high temperature, the easier it is to produce grain boundary slip slip and intergranular cracks. The deformation of metals is often an interaction between differential displacement and grain boundary slip. Nickel base alloy can greatly inhibit differential displacement due to the precipitation of medimetallic phase, and carbide precipitated on grain boundary can help to resist the displacement caused by grain boundary slip, which makes nickel base alloy has better resistance to displacement than other metal materials.
In addition, when the traditional casting method is changed to unidirectional solidified long columnar crystal, the resistance to high temperature diving will be improved, and when it is further grown into single crystal, the resistance to high temperature diving will be greatly improved. Therefore, special technologies such as directional eutectic solidification, single crystal casting and powder metallurgy have been developed for nickel-based alloys, which further enhance the resistance to high temperature diving of nickel-based alloys.
3. Corrosion Resistance:
Corrosion control of materials has been regarded as the best way to practice material economy in industry. The selection of materials for industrial equipment at the design end is not just about the price of materials; issues such as the time required for replacement and maintenance, as well as the overall efficiency of use and, more importantly, safety, need to be more accurately considered in design and selection. Nickel-based alloys have good corrosion resistance in strong reducing corrosion environment, complex mixed acid environment, and solutions containing halogen ions. Nickel-based corrosion resistant alloys can be represented by Hastelloy alloy. As mentioned above, Ni elements can accommodate more alloys in crystallography to improve their ability to resist corrosion environment. Moreover, Ni itself has certain anti-rot properties, such as excellent resistance to stress corrosion and caustic corrosion of Cl ions. The addition of passivated elements in nickel-based alloy can form solid solution with the substrate phase, which improves the corrosion potential and thermodynamic stability of the material. For example, Cu, Cr and Mo are added to Ni to improve the corrosion resistance of the whole alloy.
In addition, alloying elements can promote the formation of dense corrosion product protective film on the alloy surface, such as the formation of Cr2O3, Al2O3 and other oxide layers, providing a protective layer for materials to resist various types of corrosion environment. Therefore, nickel-based corrosion resistant alloys usually contain Cr and Al one or both of these two elements, especially when the strength of the alloy is not the main requirement. Special attention should be paid to the high-temperature oxidation resistance and thermal corrosion resistance of the alloy. The oxidation resistance of the superalloy varies with the alloying element content. Although the high-temperature oxidation behavior of the superalloy is very complicated, the oxidation resistance of the superalloy is usually expressed by the oxidation kinetics and the composition of the oxide film.
Pure nickel materials such as Ni 200/201(UNS N02200/ UNS N02201) are commercial pure nickel (>99.0%). It has good mechanical properties and excellent corrosion resistance, and other useful physical properties, including its magnetic properties, magnetostrictivity, high thermal and electrical conductivity. Ni 200's resistance to corrosion makes it particularly useful in applications where purity is required, such as food products, man-made fibers, and caustic soda. It is also widely used in structural applications where corrosion resistance is a major consideration. Other uses include sky and missile parts. Nickel base corrosion resistant alloys include Hastelloy alloy and Ni-Cu alloy, the main alloy elements are Cr, Mo, Cu, etc., has good comprehensive performance, can withstand various acid corrosion and stress corrosion. Monel, the earliest application of Ni-Cu component; In addition, there are Ni-Cr alloy (nickel base heat resistant alloy, corrosion resistant alloy in corrosion resistant alloy), Ni-Mo alloy, Ni-Cr-Mo alloy (C series of Hastelloy alloy) and so on. In terms of corrosion resistance, Ni-Cu alloy has better corrosion resistance than Ni in reducing medium, and better corrosion resistance than Cu in oxidizing medium. Under the condition of no oxygen and oxidant, it is the best material to resist high temperature fluorine gas, hydrogen fluoride and hydrofluoric acid. Ni-Cr alloy is mainly used in oxidizing medium conditions. Can resist high temperature oxidation and corrosion containing sulfur, vanadium and other gases, Cr content in the alloy is greater than 13% to cause effective corrosion resistance, and the higher the Cr content, the better the corrosion resistance, but in the non-oxidizing medium such as hydrochloric acid, corrosion resistance is poor, this is because non-oxidizing acid is not easy to make the alloy oxide film, at the same time there is a dissolution effect on the oxide film.
The addition of elements containing Mo and Cu in the nickel base alloy can improve the corrosion resistance of the reducing acid of the protective layer. For example, Ni-Mo alloy is mainly used under the corrosion condition of reducing medium, and it is the best kind of alloy resistant to hydrochloric acid corrosion, but in the presence of oxygen and oxidant, the corrosion resistance will decrease significantly. Ni-cr-mo (-W) alloy has the properties of Ni-Cr and Ni-Mo alloy mentioned above, and is mainly used in mixed medium of oxidation and reduction. This kind of alloy has good corrosion resistance in high temperature hydrogen fluoride gas, hydrofluoric acid solution containing oxygen and oxidant, and wet chlorine gas at room temperature. The importance of Mo nickel-based corrosion resistant alloys is that they can resist both oxidizing and reducing acids. For example, titanium and stainless steel are only resistant to oxidizing acids. For example, Hastelloy C-276 or C-2000 alloy is a Ni-Cr-Mo alloy containing W.
Containing very low silicon and carbon, is generally considered to be universal corrosion resistant alloy, has in oxidation and reduction two atmosphere state, has excellent corrosion resistance to most corrosive media, as well as excellent corrosion resistance to pore corrosion, crack corrosion and stress cracking corrosion, such alloy because of the reduction of C, Si, so can control carbide precipitation, but also improve its corrosion resistance. Because of this kind of characteristics, so widely used as chemical equipment and other harsh environment application materials. In addition, Ni-Cr-Mo-Cu alloys have the ability to resist both nitric acid and sulfuric acid corrosion, and have good corrosion resistance in some oxidation-reducing mixed acids.
G.Production Technology of Nickel-Based Alloys:
The traditional production process of nickel-based alloy is nickel raw material → nickel alloy ingot (smelting)→ secondary refining → processing → finished product → downstream application.
Other special technologies such as directional solidification, single crystal casting and powder metallurgy have been developed to meet the special needs of aerospace applications. In this paper, the traditional key technologies for the production of nickel-based alloys, such as melting, hot working and heat treatment, are briefly introduced.
The composition of nickel base alloy is mainly Ni-Cr-Fe, and the addition of other elements such as Cu, Si, Mn, Al, Ti, Nb, W, C, etc. The effects of these elements on superalloying materials are generally known from the literature. However, in order to recombine or add new alloy components and understand their interactions in microstructures, recently available material property simulation software can be used to calculate the thermodynamics and dynamics of alloy systems, helping to provide cost-effective direction, which can improve the efficiency of alloy design. The realization of alloy design must be completed by melting technology. The smelting of nickel-based alloy is mainly divided into Electric Arc Furnace (EAF)+ Electro-Alag Remelting, EAR) and high grade Vacuum Induction Melting (VIM)+ electroslag remelting refined products. In order to obtain more pure and purified alloy steel liquid during smelting, reduce the content of gas and harmful elements; At the same time, due to the existence of easy oxidizing elements such as Al and Ti in some alloys, non-vacuum smelting is difficult to control; In order to obtain better thermoplasticity, nickel-based alloys are usually smelted by vacuum induction furnace, or even produced by vacuum induction melting and vacuum consumable furnace or electroslag furnace remelting.
The main purpose is to accurately hit 7-12 alloy components and remove impurity elements and harmful gases, and then maintain the compact structure without surface defects with the ingot solidification control technology. Because the alloy is smelted in the true space environment, the formation of non-metallic oxide inclusions can be limited, and the unnecessary trace elements and dissolved gases, such as oxygen, hydrogen and nitrogen, can be removed with high vapor pressure. To get an accurate and uniform alloy composition. The ingot from VIM smelting can be used as electrode of ESR for refining. The purpose of the ESR process (FIG. 10) is to obtain a purer ingot with low impurity. The slag/refining control technology is used to remove coarse intermediates, and the ingot solidification control technology is used to achieve the goal of pure composition, compact structure and uniform microstructure. Vacuum induction furnace is usually used for melting to ensure composition and control gas and impurity content, and vacuum remelting - precision casting technology is used to make parts. In the case of superalloy processing, the choice of smelting method will affect the impurity zone (i.e. the abnormal segregation of the composition). In general, the impurity and defects (such as pores) are related to the alloy composition and casting technique.
Nickel-based alloys are often processed by forging, rolling, etc., and for the alloy with poor thermoplasticity, even by extrusion after opening rolling or direct extrusion technology with mild steel (or stainless steel) sheath. The general purpose of deformation is to break the casting structure and optimize the microstructure. The high deformation impedance and the instability of thermal ductility of nickel-based alloys at high temperature increase the difficulty in the process of nickel-based alloys. Generally, nickel-based alloys have high strength and are not easy to work in cold and hot. Taking C-276 as an example, the deformation impedance at high temperature is about 2.4 times that of stainless steel. And the high hardening rate of cold working makes its strength up to 2 times of stainless steel. In addition to the high temperature deformation impedance, the occurrence of different deformation resistance of thermal ductility or inclusion zone at different temperatures should be considered in hot working, and the impure zone will harm the high temperature mechanical properties of the alloy.
The temperature range in which both resistance and thermal ductility of superalloy castings are allowed to be processed can be regarded as the working range of the hot working process. After processing or partial casting alloys need heat treatment. The purpose of solution heat treatment of nickel-based alloys is to control the grain size according to the requirements of product properties (such as toughness or creep), and to promote recrystallization and stress relief at high temperature, as well as to precipitate bad phases, such as M23C6, δ, η, etc., during the process before dissolution. For solid solution enhanced nickel-based alloys, the heat treatment procedures are as follows: (1) the temperature is raised to the point at which precipitates can dissolve, (2) the temperature is held to achieve the desired grain size, and (3) the cooling rate must be controlled to avoid precipitation such as sensitized phase M23C6.
Generally speaking, the mechanical properties of solid solution treatment are affected by the grain size and intergranular precipitates, and the temperature and time of solid solution treatment should be adjusted according to the alloy composition and the pre-process condition to achieve the desired properties. In addition, when the Ni-base alloy containing Cr reached 400~800oC, chromium carbide (M23C6) precipitated into the grain boundary, which made chromium deficiency Zone formed around the grain boundary, and thus reduced the corrosion resistance of this zone. It is called sensitization, which easily leads to intergranular erosion (IGA) and stress corrosion fracture (IGSCC). On the other hand, the heat treatment of the Wastian Fe-series precipitation enhanced nickel-base alloy includes (1) the solution stage at which the precipitation is raised to the temperature of resolution and (2) the aging stage at the temperature holding in the γ/ γ' two-phase region. The solid solution makes the precipitates dissolve back, the elements required for γ' precipitation in the base increase, and achieve the homogenization of the added elements, and control the grain size of the substrate γ phase; In the aging stage, the volume fraction, morphology, size and distribution of γ' can be controlled by holding temperature, time, cooling rate and multi-stage aging. The distribution and morphology of the main precipitates can affect the creep and corrosion resistance properties. Generally speaking, the intensification phase is usually on the nanoscale, which is not easy to be observed by ordinary metallographic methods. The morphology of precipitates is often determined by penetration electron microscopy (TEM) with high power.
In recent years, the global production of nickel-based alloys will continue to increase, especially for petrochemical EAF grade and energy/aerospace VIM grade nickel-based alloys. The Asian market is growing most rapidly, and their applications in aerospace and energy will increase significantly.
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