High-temperature alloys are metal materials based on iron, nickel, and cobalt that can work for extended periods at temperatures above 600°C and under certain stresses. They possess high-temperature strength, excellent oxidation and corrosion resistance, good fatigue performance, fracture toughness, and other comprehensive properties. High-temperature alloys have a single austenitic structure, ensuring good structural stability and reliability across various temperatures.
Due to these properties and their high alloy content, high-temperature alloys are also known as “superalloys” and are widely used in aerospace, aviation, petroleum, chemical, and marine industries. Based on the base element, high-temperature alloys can be classified into iron-based, nickel-based, and cobalt-based high-temperature alloys. Iron-based high-temperature alloys typically withstand temperatures up to 750-780°C. For components that operate at even higher temperatures, nickel-based and refractory metal-based alloys are used. Nickel-based high-temperature alloys hold a special and significant position in the field, often used to manufacture the hottest parts of jet engines and industrial gas turbines.
Characteristics of High-Temperature Alloys
In high-temperature environments, various degradation rates of materials accelerate, leading to structural instability, deformation, crack growth under temperature and stress, and oxidation and corrosion on the material surface.
- High Temperature and Corrosion Resistance
The high-temperature and corrosion resistance of high-temperature alloys depend mainly on their chemical composition and microstructure. For instance, GH4169, a nickel-based wrought superalloy, contains high levels of niobium. The distribution of niobium in the alloy is directly related to the metallurgical process. GH4169 has a Ni-Cr solid solution matrix, with a nickel content of over 50%, allowing it to withstand temperatures around 1000°C, similar to Inconel 718. The alloy consists of a γ matrix phase, δ phase, carbides, and strengthening phases γ’ and γ”. The chemical elements and matrix structure of GH4169 demonstrate its superior mechanical properties, including yield strength and tensile strength surpassing 45 steel by several times, and better plasticity. Its stable lattice structure and numerous strengthening factors contribute to its excellent mechanical properties. Chemical composition of GH4196: Ni(51.96%), Cr(17.98%), Mo(3.07%), Ti(0.95%), Nb(4.82%), Al(0.45%), Fe(Balance). - High Machining Difficulty
Due to the complex and harsh working environments of high-temperature alloys, the integrity of the machined surface is crucial to their performance. High-temperature alloys are notoriously difficult to machine due to their high microhardness, severe work hardening, high shear strength, and low thermal conductivity. These characteristics result in high cutting forces and temperatures during machining, often leading to poor surface quality and significant tool wear.
Main Types of High-Temperature Alloys
Traditional classifications of high-temperature alloys are based on the following criteria: base element type, alloy strengthening type, and material forming method.
- By Base Element Type
- Iron-Based High-Temperature Alloys: Also known as heat-resistant alloy steels, these have an iron base with small amounts of Ni and Cr. They can be further categorized into martensitic, austenitic, pearlitic, and ferritic heat-resistant steels based on their normalizing requirements.
- Nickel-Based High-Temperature Alloys: Containing over 50% nickel, these alloys are suitable for conditions above 1000°C. They undergo solid solution and aging processes to significantly enhance creep resistance and yield strength. Nickel-based alloys dominate the high-temperature alloy field, widely used in jet engine turbine blades and industrial gas turbines’ hottest parts.
- Cobalt-Based High-Temperature Alloys: With around 60% cobalt content, these alloys include elements like Cr and Ni to improve heat resistance. Despite their excellent high-temperature properties, their usage is limited due to the scarcity and difficulty in processing cobalt. They are typically used in high-temperature (600-1000°C) and high-stress components like turbine blades and combustion chamber parts in aerospace engines.
- By Alloy Strengthening Type
- Solid Solution Strengthened Alloys: These alloys involve adding alloying elements to iron, nickel, or cobalt base materials, forming a single-phase austenitic structure. The solute atoms distort the matrix lattice, increasing slip resistance and strengthening the alloy.
- Age Precipitation Strengthened Alloys: These undergo solution treatment and plastic deformation at high temperatures, followed by aging to enhance their properties. For example, GH4169 alloy achieves maximum yield strength at 650°C and can be used for turbine blades up to 950°C.
- By Material Forming Method
- Cast High-Temperature Alloys: These are directly fabricated into parts through casting. They include iron-based, nickel-based, and cobalt-based cast alloys, categorized by their crystallization methods into polycrystalline, directionally solidified, directionally eutectic, and single-crystal cast alloys.
- Wrought High-Temperature Alloys: Widely used in aerospace engines, they represent the largest production volume in China and abroad. GH4169 is a prominent example, used extensively in turbine shaft engines. While the usage of wrought high-temperature alloys may decrease with the maturity of other alloy products, they will remain dominant for decades.
- New High-Temperature Alloys: Including powder metallurgy, titanium-aluminum intermetallic compounds, oxide dispersion strengthened alloys, corrosion-resistant high-temperature alloys, and nano materials, these alloys are highly specialized and in various development stages.
Applications of High-Temperature Alloys
- Aerospace Sector
Developing advanced engines for independent aerospace industries increases the demand for high-end and new high-temperature alloys. High-temperature alloys are crucial in aviation engines, which require materials with lightweight, high strength, toughness, high-temperature resistance, oxidation resistance, and corrosion resistance. In advanced aviation engines, high-temperature alloys constitute over 50% of the materials used. - Energy Sector
High-temperature alloys have extensive applications in energy production. Supercritical power plant boilers require high-temperature alloy tubes for their superheaters and reheaters due to their excellent creep resistance, steam-side oxidation resistance, and flue gas-side corrosion resistance. In gas turbines, high-temperature alloys with superior heat corrosion resistance and long-term stability are essential for turbine blades and guide vanes. Nuclear power requires high-temperature alloys with good solution corrosion resistance for steam generator heat transfer tubes. High-temperature alloys are also vital in coal gasification, energy conservation, and emissions reduction due to their excellent high-temperature corrosion and abrasion resistance. In deep oil and gas drilling, high-temperature alloys resistant to acidic environments and wear are essential.
By leveraging their unique properties, high-temperature alloys play a critical role in advancing technology and ensuring the reliability and performance of high-stress, high-temperature applications across various industries.
