Nickel-based alloys composed of other elements are called nickel alloys. Nickel has good mechanical, physical and chemical properties, adding suitable elements can improve its oxidation resistance, corrosion resistance, high temperature strength and improve some physical properties. Nickel alloys can be used as materials for electronic tubes, precision alloys (magnetic alloys, precision resistance alloys, electric heat alloys, etc.), nickel-based high-temperature alloys, nickel-based corrosion-resistant alloys, and shape memory alloys. Nickel alloys are widely used in energy development, chemical industry, electronics, navigation, aviation and aerospace.

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

1. nickel base alloy Definition

Nickel-based alloys are generally called alloys with Ni content exceeding 30wt%, and common products have Ni content exceeding 50wt%. Due to their superior high-temperature mechanical strength and corrosion resistance, they are collectively called superalloys (Superalloy) with iron-based and cobalt-based alloys. They are generally used in high-temperature environments above 540 ℃, and different alloy designs are selected according to their application occasions, it is mostly used for equipment with special corrosion-resistant environment, high-temperature corrosion environment and high-temperature mechanical strength. Often used in aerospace, energy, petrochemical industry or special electronics/optoelectronics and other fields.

Application areas

Product requirements and characteristics

Product use

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, insulation, heat treatment industry, oil and gas industry

petrochemical industry

Corrosion resistance to aqueous solution (acid, alkali, chloride ion)

Desalination plant, petrochemical pipeline

Electronics/Optoelectronics General Industry

Generally resistant to corrosion or low temperature environments

Battery shell, lead frame, computer monitor screen cover

2. Origin and Development

Nickel-based alloy was developed in the late 1930 s. Britain first produced nickel-based alloy Nimonic75(Ni-20Cr-0.4Ti) in 1941. In order to improve the creep strength and add Al, Nimonic 80(Ni-20Cr- 2.5 Ti-1.3Al) was developed. The United States developed nickel-based alloys in the mid -1940 s, Russia in the late 1940 s and China in the mid -1950 s. 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 1950 s, the development of vacuum melting technology created conditions for refining nickel-based alloys containing high Al and Ti, and led to a substantial increase in alloy strength and service temperature. In the late 1950 s, due to the increase in the operating temperature of the turbine blade, the alloy was required to have a higher high temperature strength, but the strength of the alloy was high, it was difficult to deform, or even deform, so the precision casting technology was used to develop a series of good high temperature strength Cast alloy. In the mid-1960s, better directional crystalline and single crystal superalloys were developed, as well as powder metallurgy superalloys.

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

Characteristics of 3. nickel-based alloys

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

Includes:

(1) Excellent performance: high strength can be maintained at high temperature, and has excellent mechanical properties such as anti-creep and fatigue resistance, as well as anti-oxidation and corrosion resistance and good plasticity and weldability.

(2) Alloy addition is extremely complicated: nickel-based alloys often add more than ten kinds of alloy elements to enhance the corrosion resistance of different environments; and solid solution strengthening or precipitation strengthening.

(3) The working environment is extremely harsh: Nickel-based alloys are widely used in various harsh conditions of use, such as high temperature and high pressure parts of the gas chamber of space flight engines, structural parts of nuclear energy, petroleum and marine industries, corrosion-resistant pipelines, etc.

MICROSTRUCTURE OF 4. NICKEL-BASE ALLOY

The crystal structure of nickel-based alloys is mainly high-temperature stable face-centered cubic (FCC) Vostian iron structure, in order to improve its heat resistance, a large number of alloying elements are added, these elements will form a variety of secondary phases, improve the high temperature strength of nickel-based alloys. The types of secondary phases include various forms of MC, M23C6, M6C, M7C3 carbides, mainly distributed at grain boundaries, as well as ordered (Ordering) mesometal compounds that are structurally integrated (Coherent), such as γ' or γ. The chemical composition of the γ' and γ'' phases is roughly Ni3(Al, Ti) or Ni3Nb, such ordered phases are very stable at high temperatures, and excellent potential damage strength can be obtained through their strengthening.

With the increase of the degree of alloying, the change of microstructure has the following trend: the number of γ' phase gradually increases, the size gradually increases, and from spherical to cubic, the same alloy appears in the size and shape of the γ' phase is not the same. In addition, γγ' eutectic formed in the solidification process also appears in the cast alloy, and the grain boundary precipitates discontinuous granular carbides and is surrounded by γ' phase film. These microstructural changes improve the properties of the alloy. In addition, the chemical composition of modern nickel-based alloys is very complex, the saturation of the alloy is very high, so it is required to strictly control the content of each alloying element (especially the main strengthening element), otherwise it will be easy to precipitate other harmful intermetallic phases during use, such as σ and Laves, which will damage the strength and toughness of the alloy.

Role and grade of 5. alloying elements

Nickel-based alloys are the most widely used and high-strength alloys in high-temperature alloys. The addition of a large amount of Ni is a stable element of the Vostian iron phase, which makes the nickel-based alloy maintain the FCC structure and can dissolve more other alloy elements, but also maintain good organizational stability and plasticity of the material; while Cr, Mo and Al have oxidation resistance and corrosion resistance, and have a certain strengthening effect. The strengthening of nickel-based alloys can be divided:

(1) Solid solution strengthening elements, such as W, Mo, Co, Cr and V, etc., strengthen the material by causing local lattice strain at the base of the Ni-Fe due to the difference between the atomic radius and the substrate;

(2) Precipitation strengthening elements such as Al,Ti, Nb and Ta can form integrated and orderly A3B type intermetallic compounds, such as Ni3(Al,Ti) and other strengthening phases (γ'), so that the alloy can be effectively strengthened and 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 enhance the high temperature properties of the alloy. Generally, the brand of nickel-based alloy is named by its developer, such as Ni-Cu alloy, also known as Monel alloy, common such as Monel 400, K-500, etc. Ni-Cr alloys are generally referred to as Inconel alloys, which are common nickel-based heat-resistant alloys. They are mainly used under oxidizing medium conditions, such as Inconel 600 and 625. If a higher amount of Fe is added to the Inconel alloy to replace Ni, it is a Incoloy alloy. Its high temperature resistance is not as good as that of nickel-based precipitation hardening alloy, but it is cheap and can be used for components with lower temperature in injection engines and petrochemical plant reactors, such as Incoloy 800H, 825, etc. If precipitation strengthening elements, such as Ti, Al, Nb, etc., are added to Inconel and Incoloy, they become precipitation hardening (iron) nickel-based alloys, which can still maintain 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 (Hastelloy), in which Ni-Cr-Mo is mainly used under the condition of reducing medium corrosion. Hastelloy representative brands such as C- 276, C- 2000 and so on.

Properties of 6. 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,

Including the use of the aforementioned ion and covalent bond, at room temperature with high melting point, high strength γ' or γ'' and other precipitation phase, with the sliding system and ductile Voss Tian iron phase base, the concept of composite materials to obtain both strength and plasticity of the excellent mechanical properties, so that the application temperature of nickel-based alloys to become the highest metal materials.

2. Latency strength

The phenomenon that the latent material slowly produces plastic deformation under the constant load of high temperature (T/Tm>0.5) is widely used in various high temperature environments due to the best resistance to high temperature latent change.

The three stages of latent deformation and the intensity-applied temperature diagram of the effect of temperature on the latent deformation.

It can be divided into three stages. In the initial creep (Primary Creep) stage, the deformation rate is relatively large, but it slows down with the increase of strain. When the deformation rate reaches a certain minimum value and is close to a constant, it is called the second stage latent change, or the steady-state stage latent change (Secondary or Steady-StateCreep), which is due to the result of the equilibrium between work hardening and dynamic recovery, and the latent strain rate required in the design of engineering materials refers to the strain rate at this stage. In the third stage (Tertiary Creep), due to the necking phenomenon, the strain rate increases exponentially with increasing strain, and finally reaches failure.

The relationship between stress and strain rate varies with the mechanism of creep, and in general, an increase in temperature or stress increases the deformation rate of steady-state creep and shortens the creep life. The mechanism of submersion can be divided into (1) differential row submersion: with the help of high temperature, the differential row may slip along the slip surface and then deform. (2) Diffusion creep: caused by atomic movement, scattered along the grain is called Nabarro-Herring Creep, which is the main mechanism at high temperature. Diffusion along grain boundaries is called Coble Creep, which is the main mechanism at low temperatures. Therefore, the smaller the grain size, the more prone to diffusion creep. (3) grain boundary slip: due to the weak grain boundary at high temperature, the material is easy to slip along the grain boundary, resulting in cracks along the crystal. Therefore, the smaller the grain size at high temperature, the easier it is to produce grain boundary slip and intergranular cracks. The latent deformation of metal is often the interaction between differential discharge latent transformation and grain boundary slip, nickel-based alloy can greatly inhibit the differential discharge latent transformation due to the precipitation of the medium metal phase, while the carbide precipitated on the grain boundary can help resist the latent transformation phenomenon caused by grain boundary slip,

Comparison of Latency Properties of Different Alloy Materials

In addition, changing from the traditional casting method to unidirectional solidification of long columnar crystals will increase the resistance to high temperature creep. If single crystals are further grown, the resistance to creep will be greatly improved. Therefore, nickel-based alloys have also developed special technologies such as directional eutectic solidification, single crystal casting, powder metallurgy, etc., which further enhance the resistance to high temperature creep of nickel-based alloys.

3. Corrosion resistance

The control of corrosion 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 does not only consider the price of materials, but also the length of the cycle required for subsequent replacement and maintenance and the overall efficiency of use, as well as more important issues such as safety, which need to be more accurately included in the design and selection considerations. Nickel-based alloys have good corrosion resistance in strong reducing corrosive environment, complex mixed acid environment and solutions containing halogen ions. Nickel-based corrosion-resistant alloys can be represented by Hastelloy alloys. As mentioned above, Ni elements can accommodate more alloys crystallographically to enhance the ability to resist corrosive environment. And Ni itself has certain corrosion resistance, for example, it has excellent resistance to stress corrosion and caustic corrosion of Cl ions. The passivation elements added to the nickel-based alloy can form a solid solution with the substrate phase, which improves the corrosion potential and thermodynamic stability of the material. For example, Cu, Cr,Mo, etc. are added to Ni to improve the corrosion resistance of the overall alloy.

Schematic diagram of corrosion potential of different alloy materials

In addition, alloying elements can promote the formation of dense corrosion product protective films on the alloy surface, such as the formation of Cr2O3, Al2O3 and other oxide layers, providing protective layers for materials to resist various corrosive environments. Therefore, nickel-based corrosion-resistant alloys usually contain one or both of Cr and Al, especially when strength is not the main requirement of the alloy, special attention should be paid to the high-temperature oxidation resistance and hot corrosion resistance of the alloy, the oxidation performance of high-temperature alloys varies with the content of alloy elements, although the high-temperature oxidation behavior of high-temperature alloys is very complex, but usually still in the oxidation kinetics and oxide film composition changes to express the oxidation resistance of high-temperature alloys, here will be pure nickel and the main nickel-based alloy corrosion resistance properties are described below.

Pure nickel materials such as Ni 200/201(UNS N02200/ UNS N02201) are commercially pure nickel (>99.0 percent). It has good mechanical properties and excellent corrosion resistance, and other useful physical properties, including its magnetic properties, magnetostrictive properties, high thermal and electrical conductivity. The corrosion resistance of Ni 200 makes it particularly useful in applications where the purity of the product needs to be assured, such as food, rayon and caustic. It is also widely used in structural applications when corrosion resistance is a major consideration. Other uses include sky and missile parts. Nickel-based corrosion-resistant alloys include Hastelloy and Ni-Cu alloys. The main alloying elements are Cr, Mo, Cu, etc., which have good comprehensive properties and can resist various acid corrosion and stress corrosion. Monel, the earliest application of Ni-Cu components; In addition, there are Ni-Cr alloys (I. e., nickel-based heat-resistant alloys, corrosion-resistant alloys in heat-resistant alloys), Ni-Mo alloys, Ni-Cr-Mo alloys (I. e., Hastelloy C series), etc. In terms of corrosion resistance, the corrosion resistance of Ni-Cu alloy is better than that of Ni in reducing medium, and the corrosion resistance is better than that of 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. It can resist high temperature oxidation and corrosion of sulfur, vanadium and other gases. The effective corrosion resistance can only be caused when the Cr content in the alloy is greater than 13%. The higher the Cr content, the better the corrosion resistance. However, in non-oxidizing media such as hydrochloric acid, the corrosion resistance is poor, because non-oxidizing acid is not easy to generate oxide film on the alloy and has dissolution effect on the oxide film.

Adding elements such as Mo and Cu to the nickel-based alloy can enhance the corrosion resistance of the protective layer against reducing acid. For example, Ni-Mo alloy is mainly used under the condition of reducing medium corrosion and is the best alloy resistant to hydrochloric acid corrosion. However, in the presence of oxygen and oxidant, the corrosion resistance will decrease significantly. Ni-Cr-Mo(-W) alloy has the properties of the above Ni-Cr and Ni-Mo alloy, mainly in the oxidation and reduction of mixed medium conditions, this kind of alloy in high temperature hydrogen fluoride gas, in oxygen and oxidant hydrochloric acid, hydrofluoric acid solution and in wet chlorine at room temperature corrosion resistance is good. 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 only resistant to oxidizing acids, such as Hastelloy C- 276 or C- 2000 alloy, which is a Ni-Cr-Mo alloy containing W.

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

Containing very low silicon and carbon, it is generally considered to be a universal corrosion-resistant alloy. It has excellent corrosion resistance to most corrosive media in the oxidation and reduction atmosphere, and excellent resistance to pitting corrosion, crevice corrosion and stress cracking corrosion. Such alloys can control the precipitation of carbides and improve their corrosion resistance due to the reduction of C and Si. Because of such properties, 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.

Production Technology of 7. Nickel-base Alloy

The traditional production process of nickel-based alloy is nickel raw material → nickel alloy ingot (melting) → secondary refining → processing → finished product → downstream application.

General nickel-based alloy production flow chart.

Other special needs such as aerospace applications, such as the development of directional solidification, single crystal casting, powder metallurgy and other special technologies. This paper is a brief introduction to the key technologies of the traditional production of nickel-based alloys, such as melting, hot working and heat treatment.

The composition of nickel-based alloy is mainly Ni-Cr-Fe, and other elements are added such as Cu, Si, Mn, Al, Ti, Nb, W, C, etc. Generally, the influence of these elements on superalloy materials can be understood from the literature. However, if new alloy components are to be reorganized or added and their interaction in microstructures is to be understood, there has recently been a material property simulation software that can calculate thermodynamics and dynamics of alloy systems, assist in providing a direction of high cost performance, and can improve the efficiency of alloy design. The realization of alloy design must be completed by smelting technology. Nickel-based alloy smelting is mainly divided into general grade electric furnace (Electric Arc Furnace,EAF) electroslag remelting refining (Electro-Alag Remelting,EAR) and high grade vacuum induction Induction Melting (VIM) electroslag remelting refining products. In order to obtain a purer alloy steel solution during smelting and reduce the gas content and harmful element content; At the same time, due to the existence of easily oxidized elements such as Al and Ti in some alloys, it is difficult to control smelting in non-vacuum mode. In order to obtain better thermoplasticity, nickel-based alloys are usually melted in vacuum induction furnace, and even produced by vacuum induction melting plus vacuum consumable furnace or electroslag furnace remelting. Where vim

Schematic diagram of vacuum induction melting and electroslag remelting refining equipment

The main purpose is to accurately hit 7-12 alloy components, remove impurity elements and harmful gases, and then use ingot solidification control technology to maintain a compact structure without surface defects. Because alloy smelting is carried out in a vacuum environment, it can limit the formation of non-metallic oxide inclusions, and remove unnecessary trace elements and dissolved gases, such as oxygen, hydrogen and nitrogen, with high vapor pressure to obtain accurate and uniform alloy composition. The ingot smelted by VIM can be used as the electrode of ESR for refining. The purpose of ESR (Figure 10) process is to obtain a purer ingot with low impurities, I .e. to remove coarse medium by slag/refining control technology, and then to achieve the goal of pure composition, compact structure and uniform microstructure by ingot solidification control technology. Vacuum induction furnace melting is usually used to ensure the composition and control of gas and impurity content, and vacuum remelting-precision casting technology to make parts. In the case of superalloy workpieces, the choice of melting method affects the impure zone (I. e., abnormal segregation of components). In general, impure and defects (e. g., pores) are related to alloy composition and casting technology.

Nickel-based alloys are often forged and rolled in processing. For alloys with poor thermoplasticity, they are even rolled after extrusion opening or directly extruded with mild steel (or stainless steel). The purpose of general deformation is to break the casting structure and optimize the microstructure. The high deformation resistance and thermal ductility instability of nickel-based alloys at high temperatures increase the difficulty of the nickel-based alloy process. General nickel-based alloy high strength, cold and hot processing is not easy, to C- 276, for example, high temperature deformation resistance is about 2.4 times that of stainless steel; and the high hardening rate of cold working makes its strength can be 2 times that of stainless steel. In addition to the high temperature deformation resistance, thermal processing needs to consider the occurrence of different deformation resistance or inclusion areas) of thermal ductility at different temperatures, while the impure zone will damage the high temperature mechanical properties of the alloy,

The data curves of thermal ductility and deformation resistance of nickel-based alloy Inconel 601 at different temperatures show that processing at temperatures below 60% of thermal ductility is prone to cracking.

The temperature range in which superalloy castings are resistant to thermal ductility and are allowed to be processed at the same time can be regarded as the working range of the thermal processing process. After processing or part of the cast alloy needs to be heat treated. The purpose of nickel-based alloy solid solution heat treatment is to control the grain size according to the requirements of product properties (such as toughness or latent change), and to promote recrystallization and stress relief at high temperature, as well as the undesirable phases precipitated in the pre-dissolution process, such as M23C6, δ, η, etc. In the case of solid-soluble nickel-based alloys, the heat treatment procedure is (1) heating to the temperature at which the precipitates can be dissolved back,(2) holding the temperature to achieve the required grain size, and (3) the cooling rate must be controlled to avoid precipitation such as the sensitized phase M23C6.

Generally speaking, the mechanical properties after solid solution treatment are affected by grain size and precipitates along the crystal, and the temperature and time of solid solution treatment need to be adjusted according to the alloy composition and pre-process conditions to achieve the required properties. In addition, when the Cr-containing nickel-based alloy is subjected to a thermal history of 400 ~ 800oC, chromium carbide (M23C6) will precipitate in the grain boundary, resulting in the formation of a chromium deficiency zone (Cr-depletion Zone) around the grain boundary, resulting in a decrease in corrosion resistance in this zone, which is called sensitization and easily leads to intergranular erosion (IGA) and intergranular stress corrosion cracking (IGSCC). On the other hand, the heat treatment of the Vostian iron-based precipitation-strengthened nickel-based alloy includes (1) the solid-soluble stage of heating to the temperature at which the precipitates are dissolved back and (2) the aging stage of holding the temperature in the γ/ γ' two-phase region. Among them, the solid solution makes the precipitates dissolve back, the elements required for γ' precipitation in the base are increased, and the homogenization of each added element is achieved, and the grain size of γ phase of the substrate is controlled. In the aging stage, the volume fraction, morphology, size and distribution of γ' can be controlled by holding temperature, time, cooling speed and multi-stage aging. The distribution and morphology of the main precipitates can affect the creep and corrosion resistance. In general, the strengthening phase is nano-scale, which is not easy to observe by general metallographic method. It is often necessary to use a transmission electron microscope (TEM) with a high magnification to grasp the morphology of the precipitates.