Introduction to nickel base alloy knowledge, and in which fields is nickel base alloy applied?

Nickel-based alloy composed of other elements is called nickel alloy. Nickel has good mechanical, physical and chemical properties, and the addition of suitable elements can improve its oxidation resistance, corrosion resistance, high temperature strength and improve certain physical properties. Nickel alloys can be used as materials for electronic tubes, precision alloys (magnetic alloys, precision resistance alloys, electrothermal alloys, etc.), nickel-based high-temperature alloys, and nickel-based corrosion-resistant alloys and shape memory alloys. Nickel alloys are widely used in energy development, chemical, electronic, marine, aviation and aerospace sectors.

Nickel can be alloyed with copper, iron, manganese, chromium, silicon and magnesium to form a variety of alloys. Among them, nickel-copper alloy is the famous Monel alloy, it is high strength, good plasticity, in the atmosphere below 750 degrees, chemically stable, widely used in the electrical industry, vacuum tubes, chemical industry, medical equipment and marine industry, etc.

I. Definition of nickel-based alloys

Nickel-based alloys are generally known as alloys with Ni content of more than 30wt%, and common products with Ni content of more than 50wt%, because of their superior high-temperature mechanical strength and corrosion resistance, together with iron-based and cobalt-based alloys are called superalloy. Environment, high temperature corrosion environment, need to have high temperature mechanical strength of the equipment. It is often used in aerospace, energy, petrochemical industry or special electronic/optical fields.

Applications

Product Characteristics

Product Applications

Aerospace industry

Maintain good mechanical strength at very high temperatures

Aircraft engines, gas turbines, engine valves

Energy industry

Good resistance to high temperature vulcanization and high temperature oxidation

Furnace parts, heat insulation, heat treatment industry, oil and gas industry

Petrochemical industry

Resistant to corrosion of aqueous solutions (acid, alkali, chloride ions)

Seawater desalination plants, petrochemical pipelines

Electronics/electronic general industry

General corrosion resistance or low temperature resistance environment

Battery casing, wire frame, computer monitor net cover

II. Origin and development

Nickel-based alloys were developed in the late 1930s, the United Kingdom in 1941 first produced the nickel-based alloy Nimonic 75 (Ni-20Cr-0.4Ti); in order to improve the latent strength and add Al, the development of Nimonic 80 (Ni-20Cr- 2.5Ti-1.3Al); and the United States in the mid-1940s, Russia in the late 1940s, China in the mid-1950s has also been developed. China also developed nickel-based alloys in the mid-1940s, Russia in the late 1940s and China in the mid-1950s. The development of nickel-based alloys 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 melting technology, for the refining of nickel-based alloys containing high Al and Ti to create the conditions, and led to a significant increase in the strength of the alloy and the use of temperature. late 1950s, due to the increase in the working temperature of the turbine blade, the alloy is required to have higher high-temperature strength, but the strength of the alloy is high, it is difficult to deform, or even can not be deformed, so the use of precision casting technology, the development of a series of casting with good high-temperature strength A series of casting alloys with good high-temperature strength. the mid-60s developed better performance directional crystalline and single crystal high-temperature alloys, as well as powder metallurgy high-temperature alloys.

In order to meet the needs of ships and industrial gas turbines, since the 1960s also developed a number of high Cr nickel-based alloys with good thermal corrosion resistance and stable organization. In the period of about 40 years from the early 1940s to the end of the 1970s, the working temperature of nickel-based alloys increased from 700 to 1,100°C, an average increase of about 10°C per year. Today, the operating temperature of nickel-based alloys can exceed 1,100°C. From the aforementioned Nimonic 75 alloy with simple composition to the recently developed MA6000 alloy, the tensile strength at 1,100°C can reach 2,220 MPa and the yield strength 192 MPa; the endurance strength at 1,100°C/137 MPa is about 1,000 hours. It can be used for aero-engine blades.

3、Features of nickel-based alloys

Nickel-based alloys are the most widely used and strongest materials among superalloys. The name of superalloy is derived from the material characteristics.

Including.

(1) Super performance: high strength can be maintained at high temperatures, and has excellent mechanical properties such as resistance to subduction and fatigue, as well as oxidation and corrosion resistance and good plasticity and weldability.

(2) Super alloying: Nickel-based alloys often add more than ten kinds of alloying elements to enhance the corrosion resistance of different environments; and solid solution strengthening or precipitation strengthening, etc.

(3) Harsh working environment: Nickel-based alloys are widely used in a variety of harsh conditions of use, such as the high-temperature and high-pressure parts of the gas chamber of aerospace engines, nuclear energy, oil, marine industry, structural parts, corrosion-resistant pipelines, etc.

4. Microstructure of Nickel-Based Alloys

The crystal structure of nickel-based alloys is mainly a high-temperature stable face-centered cubic (FCC) Worsted iron structure. In order to improve its heat resistance, a large number of alloying elements are added, which form various secondary phases and enhance the high-temperature strength of nickel-based alloys. The secondary phases include various forms of MC, M23C6, M6C, M7C3 carbides, which are mainly distributed at grain boundaries, and Ordering meso-metallic compounds such as γ' or γ'' which are structurally Coherent. These ordered phases are very stable at high temperatures and can be strengthened by them to obtain excellent latent damage strength.

As the degree of alloying increases, the microstructure changes have the following trends: the number of γ' phases gradually increases, the size gradually increases, and from spherical to cubic, and the γ' phases of different sizes and forms appear in the same alloy. In addition, γ+γ' eutectic formed during solidification occurs in cast alloys, and discontinuous granular carbides precipitate at grain boundaries and are surrounded by γ' phase films, and these changes in microstructure improve the properties of the alloy. In addition, the chemical composition of modern nickel-based alloys is very complex, the alloy is highly saturated, and therefore requires strict control of the content of each alloying element (especially the main strengthening elements), otherwise it will be easy to precipitate other harmful dielectric phases in the process, such as σ, Laves, etc., will damage the strength and toughness of the alloy.

V. The role of alloying elements and grades

Nickel-based alloys are the most widely used high-temperature alloys, the highest strength of a class of alloys. The addition of a large amount of Ni for the Worcester iron-phase stabilizing elements, making nickel-based alloys to maintain the FCC structure and can dissolve more other alloying elements, but also to maintain good organizational stability and material plasticity; while Cr, Mo and Al have antioxidant and anti-corrosion effect, and has a certain strengthening effect. Nickel-based alloy strengthening according to the role of the elements can be divided into.

(1) Solid solution strengthening elements, such as W, Mo, Co, Cr and V. By the difference between the radius of these atoms and the base material, local lattice strain is caused at the base of Ni-Fe to strengthen the material;

(2) Precipitation strengthening elements, such as Al, Ti, Nb and Ta, can form integrated and ordered A3B type intermetallic compounds, such as Ni3(Al,Ti) and other strengthening phases (γ'), so that the alloy can be effectively strengthened to obtain higher high temperature strength than iron-based high temperature alloys and cobalt-based alloys;

(3) grain boundary strengthening elements, such as B, Zr, Mg and rare earth elements, can strengthen the high temperature properties of the alloy. Ni-Cu alloys are also known as Monel alloys, such as Monel 400, K-500, etc. Ni-Cr alloys are generally known as Inconel alloys, which are common nickel-based heat-resistant alloys used mainly in oxidizing media, such as Inconel 600, 625, etc. If a higher amount of Fe is added to the Inconel alloy to replace Ni, it is Incoloy alloy, which is not as high temperature resistant as the nickel-based precipitation hardening alloy, but it is cheaper and can be used in the lower temperature components of jet engines and reactors of petrochemical plants, such as Incoloy 800H, 825, etc. If Inconel and Incoloy are added with precipitation strengthening elements, such as Ti, Al, Nb, etc., they become precipitation hardening (iron) nickel-based alloys, which can retain good mechanical strength and corrosion resistance at high temperatures, and are mostly used in jet engine components, such as Inconel 718, Incoloy A-286, etc. The Ni-Cr-Mo(-W)(-Cu) alloy is called Hastelloy, where Ni-Cr-Mo is mainly used under the condition of reducing medium corrosion.

Sixth, the performance of nickel-based alloys

1. High temperature (instantaneous) strength

Nickel-based alloys have high tensile strength at room temperature (TS=1,200-1,600; YS=900-1,300 MPa), and good ductility.

By using the ionic and covalent bonding mentioned above, the precipitated phases such as γ' or γ'' with high melting point and high strength at room temperature, together with the ductile iron phase base of Worcester with many slip systems, we can obtain excellent mechanical properties of both strength and plasticity with the concept of composite materials, making the application temperature of nickel-based alloys the highest among metallic materials.

2. Subduction strength

Latent change is the phenomenon of plastic deformation of a material slowly under constant load at high temperature (T/Tm>0.5), which is a material alloy with the best resistance to high temperature latent change, and is widely used in various high temperature environments as a load-bearing part.

The three stages of latent deformation and the influence of temperature on the strength of latent deformation - application temperature diagram

In the Primary Creep stage, the deformation rate is relatively large, but it slows down as the strain increases and process hardening occurs. When the deformation rate reaches a minimum value and approaches a constant, it is called the Second or Steady-StateCreep, which is the result of the balance between hardening and dynamic recovery. In the Tertiary Creep, the strain rate increases exponentially as the strain increases due to the necking phenomenon, and finally reaches failure.

The relationship between stress and strain rate varies depending on the mechanism of latent transformation. In general, an increase in temperature or stress increases the rate of deformation and shortens the life of the steady-state latent transformation. The mechanism of latent change can be divided into (1) Dislocation latent change: With the help of high temperature, the dislocation may slip along the slip surface and then deformation occurs. (2) Diffusion latent change: caused by the movement of atoms, along the grain scattering is called Nabarro-Herring Creep, at high temperature is the main mechanism. The diffusion along the grain boundaries is called Coble Creep, which is the main mechanism at low temperatures. Therefore, the smaller the grain size, the more likely it is that diffusion is latent. (3) Grain Boundary Slip: Because the grain boundary is weak at high temperature, the material is prone to slip along the grain boundary, causing cracks along the crystal. Therefore, the smaller the grain size at high temperature, the more likely to produce grain boundary slip latent transformation and cracking along the crystal. The latent deformation of metals is often the interaction of differential latent transformation and grain boundary slip. Nickel-based alloys can significantly suppress differential latent transformation due to the precipitation of meso-metallic phases, while the precipitation of carbides on grain boundaries can help resist the latent transformation caused by grain boundary slip.

Comparison of latent change properties of different alloy materials

In addition, if the traditional casting method is changed to unidirectional solidification of long columnar crystals, the resistance to high-temperature subduction will increase, and if it is further grown into a single crystal, the resistance to subduction will be greatly improved.

3. Corrosion resistance

The control of material corrosion has been regarded as the best way to practice economic savings in materials in industry. The selection of materials in the design of industrial equipment is not only the price of materials, but also the length of time required for subsequent replacement and maintenance, the overall efficiency of use, and more importantly, the safety issues, all of which need to be more accurately included in the design and selection considerations. Nickel-based alloys have good corrosion resistance in strong reducing corrosive environments, complex mixed acid environments, and solutions containing halogen ions, and nickel-based corrosion resistant alloys can be represented by Hastelloy alloys, as mentioned earlier, the Ni element in crystallography can accommodate more alloys to enhance the ability to resist corrosive environments; and Ni itself has certain corrosion resistance, such as stress corrosion against Cl ions and Caustic alkali corrosion has excellent resistance. The addition of passivating elements in Ni-based alloys can form solid solution with the base material, which enhances the corrosion potential and thermodynamic stability of the material. For example, the addition of Cu, Cr, Mo, etc. to Ni improves the overall corrosion resistance of the alloy.

Schematic diagram of the corrosion potential of different alloy materials

In addition, the alloying elements can promote the alloy surface to generate a dense protective film of corrosion products, such as the formation of Cr2O3, Al2O3 and other oxide layer, providing materials to resist various types of corrosive environments of the protective layer, so nickel-based corrosion-resistant alloys usually contain Cr, Al one of the two elements or both, especially when the strength of the alloy is not the main requirement, pay special attention to the alloy's resistance to high-temperature oxidation and thermal corrosion performance Although the high temperature oxidation behavior of high temperature alloys is complex, the oxidation kinetics and changes in the composition of the oxide film are usually used to indicate the oxidation resistance of high temperature alloys, and here the corrosion resistance of pure nickel and major nickel-based alloys are divided as follows.

Pure nickel materials such as Ni 200/201 (UNS N02200/ UNS N02201) are commercially pure nickel (>99.0%). The corrosion resistance of Ni 200 makes it particularly useful in applications such as food, man-made fibers and caustic soda where purity of the product is required. It is also used extensively in structural applications where corrosion resistance is a major consideration. Other uses include sky and missile components. Nickel-based corrosion resistant alloys include Hastelloy and Ni-Cu alloys. The main alloying elements are Cr, Mo, Cu, etc., which have good overall performance and can resist various acid and stress corrosion. The earliest application of Ni-Cu component of Monel; in addition, there are Ni-Cr alloy (that is, nickel-based heat-resistant alloys, corrosion-resistant alloy in the corrosion-resistant alloy), Ni-Mo alloy, Ni-Cr-Mo alloy (that is, Hastelloy C series) and so on. In terms of corrosion resistance, Ni-Cu alloy has better corrosion resistance than Ni in reducing media, and better corrosion resistance than Cu in oxidizing media, and is the best material to resist high temperature fluorine gas, hydrogen fluoride and hydrofluoric acid in the absence of oxygen and oxidizing agents; Ni-Cr alloy is mainly used in oxidizing media. Can resist high-temperature oxidation and corrosion of gases containing sulfur, vanadium, etc. The amount of Cr in the alloy is greater than 13% to cause effective corrosion resistance, and the higher the Cr content, the better its corrosion resistance, but in non-oxidizing media such as hydrochloric acid, corrosion resistance is poor, this is because non-oxidizing acids are not easy to make the alloy generate oxide film, while there is a dissolving effect on the oxide film.

Ni-Cr-Mo(-W) alloy has both the above mentioned properties of Ni-Cr and Ni-Mo alloys. These alloys have good corrosion resistance in high-temperature hydrogen fluoride gas, in hydrochloric and hydrofluoric acid solutions containing oxygen and oxidizing agents, and in wet chlorine gas at room temperature. The importance of Mo-containing nickel-based corrosion-resistant alloys is that they can resist both oxidizing and reducing acids, such as titanium and stainless steel, which are resistant only to oxidizing acids, for example, Hastelloy C-276 or C-2000 alloy is a Ni-Cr-Mo alloy containing W

Data on the corrosion resistance of different alloys in reducing acids (HCl)

Containing very low silicon and carbon, these alloys are generally considered to be universal corrosion resistant alloys, having excellent corrosion resistance to most corrosive media in both oxidizing and reducing atmospheres, as well as excellent resistance to pore corrosion, crevice corrosion and stress cracking corrosion. Because of these characteristics, they are widely used as materials 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.

7、Production technology of nickel-based alloys

The traditional production process of nickel-based alloys is nickel raw material → nickel alloy ingot (melting) → secondary refining → processing → finished products → downstream applications

Flow chart of general nickel-based alloy production

Other special technologies such as directional solidification, single crystal casting, powder metallurgy, etc. have been developed for special needs such as aerospace applications. This article is a brief introduction to the key technologies traditionally used to produce nickel-based alloys, such as melting, thermal processing, heat treatment, etc.

The composition of Ni-based alloys is mainly Ni-Cr-Fe, and other elements such as Cu, Si, Mn, Al, Ti, Nb, W, C, etc. are added. The influence of these elements on superalloys can be understood from the literature, but to reorganize or add new alloy components and understand their interaction in the microstructure, recently there is a material property simulation software that can perform thermodynamic and kinetic calculations of alloy systems to help provide cost-effective directions and improve the efficiency of alloy design. Nickel-based alloy melting is mainly distinguished by the general grade of Electric Arc Furnace (EAF) + Electro-Alag Remelting (EAR) and the high grade of Vacuum Induction Melting (VIM) + Electro-Lag Remelting (EAR). VIM) + electroslag remelting refining products. In order to obtain a more pure alloy melt, reduce the gas content and harmful element content; at the same time, due to the presence of some alloys with easy to oxidize elements such as Al, Ti, etc., smelting in a non-vacuum way is difficult to control; moreover, in order to obtain better thermoplasticity, nickel-based alloys are usually produced by vacuum induction furnace melting, or even by vacuum induction melting plus vacuum self-consumption furnace or electroslag furnace remelting. Among them, VIM

Schematic diagram of vacuum induction melting and electroslag remelting refining equipment

The main purpose is to precisely hit 7-12 alloy components and remove impurity elements and harmful gases, and then use ingot solidification control technology to maintain a dense structure without surface defects, as the alloy is melted in a vacuum environment, it can limit the formation of non-metallic oxidation inclusions, and remove unwanted trace elements and dissolved gases such as oxygen, hydrogen and nitrogen with high vapor pressure to obtain a precise and uniform alloy composition. VIM finished melting ingots can be used as ESR electrodes for refining, and the purpose of ESR (Figure 10) process is to obtain purer and lower impurity ingots, i.e. to remove coarse inclusions by slag/refining control technology, and then to achieve the goal of pure composition, dense structure and uniform microstructure by ingot solidification control technology. Usually, vacuum induction furnace melting is used to ensure the composition and control the gas and impurity content, and the parts are made by vacuum remelting and precision casting technology. In the case of processed superalloys, the choice of melting method affects the impurity zone (i.e., abnormal segregation of the composition) and, in general, the impurity and defects (e.g., porosity) are related to the alloy composition and casting technology.

Nickel-based alloys are often used in the processing of forging, rolling and other ways type, for poor thermoplastic alloy even use extrusion open embryo after rolling or with soft steel (or stainless steel) package set of direct extrusion technology. The purpose of general deformation is to break the casting organization and optimize the microstructure. The high deformation resistance and the instability of thermal ductility at high temperatures increase the difficulty of the process of nickel-based alloys. Generally, nickel-based alloys have high strength and are not easy to process cold or hot. In addition to high-temperature deformation resistance, it is also necessary to consider the occurrence of different deformation resistance or inclusions in the area of thermal ductility at different temperatures, and the impure area will harm the high-temperature mechanical properties of the alloy.

The data curve of thermal ductility and deformation resistance of nickel-based alloy Inconel 601 at different temperatures shows that processing at temperatures below 60% of thermal ductility is likely to cause cracks to occur.

The temperature range that allows the processing of superalloy castings with both resistance and thermal ductility can be considered as the working range of the thermal processing process. The purpose of solution heat treatment for nickel-based alloys is to control the grain size depending on the product properties (e.g. toughness or latent change) and to promote recrystallization and stress relief at high temperatures, as well as to retreat the undesirable phases precipitated in the process, such as M23C6, δ, η, etc. In the case of solution-strengthened nickel-based alloys, the heat treatment procedure is to (1) raise the temperature to the temperature at which the precipitates can be melted back, (2) hold the temperature to achieve the required grain size, and (3) control the cooling rate to avoid the precipitation of the sensitized phase M23C6, etc.

Generally speaking, the mechanical properties after solid solution treatment are affected by the grain size and precipitates along the crystal, and the temperature and time of solid solution treatment should be adjusted according to the alloy composition and the previous process to achieve the desired properties. In addition, Cr-containing nickel-based alloy will precipitate chromium carbide (M23C6) at the grain boundaries during the heat treatment at 400~800oC, resulting in the formation of Cr-depletion zone around the grain boundaries, which will lead to the reduction of corrosion resistance in this zone, called sensitization, and will easily lead to along-grain erosion (IGA) and along-grain stress corrosion cracking (IGSCC). On the other hand, the heat treatment of Worcester Fe-based precipitation-reinforced nickel-based alloys consists of (1) a solid solution stage at which the temperature is raised to the temperature at which the precipitates are returned to solution and (2) an aging stage at which the temperature is held in the γ/γ' two-phase region. The solid solution stage makes the precipitates re-solvate, and the elements required for γ' analysis in the base increase, and achieve the homogenization of each added element, and control the grain size of γ' phase of the base material; while the aging stage can hold temperature, time, cooling rate and multi-stage aging to control the volume fraction, shape, size and distribution of γ', the main distribution and shape of precipitates can affect the latent change and corrosion resistance. Generally speaking, the strengthening phase is often nano-scale, and it is not easy to observe by general metallographic methods. It is often necessary to use high magnification penetrating electron microscopy (TEM) to grasp the shape of precipitates.