Classification of Nickel-based Alloys

I. Classification by main application purpose (most commonly used)

Nickel-based high-temperature alloys:

Core characteristics: Maintain high strength at high temperatures (usually above 600°C), excellent creep resistance, fatigue resistance, oxidation resistance and hot corrosion resistance.

Application areas: Aeroengines/gas turbines (turbine blades, guide vanes, combustion chambers, turbine disks), rocket engines, nuclear reactor heat exchange tubes, industrial gas turbines, etc.

Representative grades:

Solid solution strengthening type: Haynes 230, Inconel 600, Inconel 625 (both with corrosion resistance).

Precipitation strengthening type: Inconel 718 (most widely used), Inconel 713, René series (such as René N5), Waspaloy, Udimet 720, Mar-M247, CMSX single crystal series (such as CMSX-4).

Key elements: high Cr (anti-oxidation/corrosion), Al, Ti (forming γ' phase Ni3(Al,Ti) strengthening), Mo, W, Co, Ta, Re (solid solution strengthening and stabilizing γ'), C (carbide strengthening).

Nickel-based corrosion-resistant alloys:

Core characteristics: High resistance to uniform corrosion, pitting, crevice corrosion, and stress corrosion cracking in harsh chemical environments (strong acid, strong alkali, halogen ion solution, seawater, reducing or oxidizing media).

Application areas: Chemical processing (reactors, pipelines, valves, pumps), oil and gas (deep sea, high sulfur environment), marine engineering, flue gas desulfurization (FGD), pulp and paper, pollution control, pharmaceuticals, etc.

Representative grades:

Ni-Cu series: Monel 400 (Ni66Cu33), Monel K-500 (precipitation strengthening type).

Ni-Mo series: Hastelloy B-2/B-3 (resistant to reducing acid).

Ni-Cr-Mo series: Most widely used:

Hastelloy C series (C-276, C-22, C-2000) - All-round corrosion resistant, especially resistant to pitting/crevice corrosion.

Inconel 625 (both high temperature performance), Incoloy 825 (containing Fe).

Ni-Cr series: Inconel 600, Inconel 690 (high Cr, resistant to high temperature oxidation and stress corrosion cracking).

Nickel-based precision alloys/functional alloys:

Core characteristics: Utilize the specific physical properties of nickel-based alloys.

Main types:

Expansion alloys: Such as Ni36Fe (Invar/Invar alloy), extremely low thermal expansion coefficient, used for precision instruments, laser cavities, liquefied gas storage tanks.

Soft magnetic alloys: Such as Ni80Fe20 (Permalloy), high magnetic permeability, low coercivity, used for transformer cores and magnetic shielding.

Electric heating alloys: such as Ni80Cr20 (Cr20Ni80), high resistivity, high temperature oxidation resistance, used for electric heating elements.

Shape memory alloys: such as NiTi (Nitinol), with shape memory effect and superelasticity, used for medical devices and actuators.

Hydrogen storage alloys: Some nickel-based alloys can reversibly absorb and release hydrogen.

2. Classification by main alloying elements

Ni-Cr system:

Base system, providing good oxidation resistance and corrosion resistance, high temperature strength mainly depends on solid solution strengthening (Cr, Mo, etc.). Such as Inconel 600, Inconel 690.

Ni-Cr-Fe system:

High iron content (usually >10%), relatively low cost, with certain high temperature strength and good corrosion resistance. Often called "iron-nickel-based alloy", but nickel is still the main matrix. Such as Incoloy 800/800H/800HT, Incoloy 825.

Ni-Cr-Mo series:

The addition of molybdenum significantly improves the corrosion resistance to reducing media, pitting/crevice corrosion resistance and high temperature strength. It is the main force of corrosion-resistant alloys. Such as Hastelloy C series, Inconel 625.

Ni-Cr-Co series:

Cobalt can increase the dissolution temperature of the γ' phase, stabilize the microstructure, and significantly improve the high temperature creep strength. It is mainly used for high-end high-temperature alloys. Such as Waspaloy, Udimet 720, René series.

Ni-Fe series:

Mainly used for precision alloys (such as Invar, Permalloy) or specific corrosion-resistant alloys (partial Monel).

Ni-Cu series:

Monel alloy, the outstanding feature is resistance to seawater, hydrofluoric acid, alkaline solution, etc. Such as Monel 400, Monel K-500.

Ni-Mo series:

Hastelloy B series, the leader in strong reducing acid (hydrochloric acid, sulfuric acid) corrosion resistance. Such as Hastelloy B-2, B-3.

3. Classification by strengthening mechanism (mainly for high-temperature alloys)

Solution-strengthened alloys:

Mainly rely on adding a large amount of Mo, W, Cr, Co and other elements dissolved into the nickel matrix to cause lattice distortion and hinder dislocation movement for strengthening. Usually cannot be heat-treated and strengthened, with good formability and weldability, and medium and high temperature strength is lower than precipitation-strengthened type. Such as Haynes 230, Hastelloy X.

Precipitation-strengthened alloys:

Key mechanism: By adding Al, Ti, Nb, Ta and other elements, fine, dispersed, coherent intermetallic compound γ' phase (Ni3(Al,Ti)) or γ'' phase (Ni3Nb) is precipitated during heat treatment (aging) to obtain extremely high temperature strength.

Processing technology: Requires complex heat treatment (solid solution treatment + aging treatment).

Representatives: Inconel 718 (γ'' strengthening dominated), Waspaloy, René series (γ' strengthening dominated).

Oxide dispersion strengthened alloy:

Through special powder metallurgy processes (such as mechanical alloying), small and stable oxide particles (such as Y2O3, ThO2) are uniformly dispersed into the nickel matrix. These particles can remain stable when close to the melting point of the alloy, providing extremely high high-temperature creep strength. Such as MA754, MA6000. Mainly used for parts in extremely high temperature environments (such as turbine blade liners).

IV. Classification by forming process

Deformed alloy:

Formed by hot/cold processing processes such as forging, rolling, and extrusion. It has good comprehensive mechanical properties and can produce various specifications of bars, plates, tubes, wires, strips and forgings. Most nickel-based alloys belong to this category.

Casting alloy:

Directly cast into parts with complex shapes required by investment casting, sand casting, etc. More strengthening elements can be added to obtain higher high-temperature strength, but the plastic toughness is usually lower than that of deformed alloys.

Classification:

Equiaxed crystal casting alloy: Such as Inconel 713C, K418.

Directionally solidified columnar alloy: The grain boundaries are arranged in parallel along the direction of the principal stress, eliminating the transverse grain boundaries and improving the longitudinal creep and thermal fatigue performance. Such as the DZ series.

Single crystal alloy: The entire part is composed of one grain, completely eliminating the grain boundary (the weakest link), and has the highest high-temperature creep strength and thermal fatigue resistance. Such as the CMSX series, DD series. Mainly used for the turbine blades at the front end of aircraft engines.

V. Classification by crystal structure

Face-centered cubic structure: This is the crystal structure of all nickel-based alloy matrices. It has many slip systems, which gives the alloy good plastic toughness and processing performance foundation.

Summary and material selection points

The diversity of nickel-based alloys is the key to its ability to meet extremely demanding service requirements. In practical applications, material selection often requires comprehensive consideration:

Service environment: Temperature, pressure, medium (composition, concentration, pH, redox), and stress state (static/cyclic) are the primary considerations. High-temperature alloys are preferred for high-temperature environments, and corrosion-resistant alloys are preferred for corrosive environments.

Performance requirements: Clarify the specific requirements and priorities for strength (room temperature/high temperature, transient/creep), plasticity, toughness, fatigue, corrosion resistance (uniform corrosion/localized corrosion), oxidation resistance, wear resistance, physical properties, etc.

Process performance: The machinability (casting, forging, welding, machining) and heat treatability of materials are crucial to manufacturing feasibility and cost.

Standard specifications: Follow the requirements of relevant international (such as ASTM, ASME, ISO), national (such as GB) or industry standards for material composition, performance, and inspection.

Economic efficiency: Consider material cost, manufacturing cost, service life, and maintenance cost while meeting performance requirements.