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Expansion Alloy

Products Description

Expansion alloy, also known as thermal expansion alloy, is a precision alloy with anomalous thermal expansion characteristics. Its core feature is that within a specific temperature range, the coefficient of thermal expansion is significantly lower than that of conventional metals or matches specific materials such as glass and ceramics.

The expansion of general metals and alloys increases linearly with temperature when heated, but the thermal expansion curve of expansion alloys shows a bending point at a certain temperature, and the coefficient of thermal expansion below the bending point is much lower than the normal value. This phenomenon is called abnormal thermal expansion characteristics

Relevant Standards

Standards Organization	Standard Number	Scope
ASTM	F15, F30, B753	Kovar, Alloy 42/52, Invar
UNS	K94610, K94100, K93600	Unified Numbering System (US)
GB	4J29, 4J36, 4J32, 4J42, 4J52	Chinese National Standards
DIN/W.Nr.	1.3981, 1.3912	German Standards
JIS	—	Japanese Industrial Standards

Products Classification

Types of Expansion Alloys

Expansion alloys are categorized into three main types based on their thermal expansion coefficients:

High Expansion Alloys: These alloys expand significantly with temperature increases, making them ideal for applications requiring precise thermal responsiveness, such as thermostats and sensors.
Low Expansion Alloys: Known for their minimal expansion under temperature fluctuations, these alloys are used in environments where dimensional stability is critical, such as precision instruments and optical devices.
Constant Expansion Alloys: These alloys maintain a consistent expansion rate over a wide temperature range, making them suitable for applications requiring predictable thermal behavior, such as aerospace components and scientific equipment.

Based on the magnitude of the coefficient of thermal expansion, expansion alloys are divided into three major categories:

Type	CTE Range (×10⁻⁶/°C)	Representative Alloy	Key Characteristic
Low Expansion Alloys	≤ 3 (typically < 2)	4J36 (Invar)	Dimensions hardly change with temperature near room temperature
Controlled Expansion Alloys	4 – 10	4J29 (Kovar), 4J42, 4J52	CTE matches hard glass or ceramics
High Expansion Alloys	> 15	—	Used for active layers in thermostatic bimetals

Fe-Ni System (Iron-Nickel Alloys)

This is the most fundamental and widely used expansion alloy system.

Grade	Common Name	Ni Content	Characteristic	Typical Application
4J36	Invar	~36%	Lowest CTE (α ≤ 1.5×10⁻⁶/°C, 20-100°C)	Precision instruments, standard rulers, resonators, LNG storage tanks
4J32	Super-Invar	~32% + Co	CTE near zero (even lower than Invar)	Higher precision measuring instruments, cryogenic platforms for astronomical telescopes

Fe-Ni-Co System (Iron-Nickel-Cobalt Alloys)

Adding cobalt to Fe-Ni allows further tuning of expansion characteristics while improving strength.

Grade	Common Name	Characteristic	Typical Application
4J29	Kovar	CTE matches hard glass (e.g., borosilicate glass) (α = 4.0-6.6×10⁻⁶/°C)	Glass-to-metal hermetic sealing for electron tubes, transistors, integrated circuits
4J33/4J34	—	CTE matches 95% Al₂O₃ ceramic	Ceramic-to-metal sealing applications

Fe-Ni-Cr System (Iron-Nickel-Chromium Alloys)

Adding chromium to the Fe-Ni base primarily improves corrosion resistance.

Typical Alloy Grade Cross-Reference

Common Name	UNS No.	Chinese Grade (GB)	German Standard (W.Nr.)	Expansion Characteristic	Main Application
Invar 36	K93600/K93601	4J36	1.3912	Low expansion	Precision instruments, standard gauges
Super-Invar	—	4J32	—	Ultra-low expansion (near zero)	High-precision astronomical, aerospace components
Kovar	K94610	4J29	1.3981	Controlled expansion (matches glass)	Glass-to-metal sealing
Alloy 42	K94100	4J42	—	Controlled expansion (matches ceramic/glass)	Semiconductor packaging, ceramic sealing
Alloy 52	N14052	4J52	—	Controlled expansion

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Core Performance

Beyond having a specific coefficient of thermal expansion, expansion alloys are also required to meet the following criteria depending on the application:

Sealability: Strong, hermetic bond when sealing to glass or ceramics
Weldability/Solderability: Facilitates assembly and connection of electronic components
Corrosion Resistance: Maintains stable performance in service environments
Machinability: Easily processed into rods, plates, strips, wires, tubes, and other forms using conventional machining methods
Microstructural Stability: No phase transformations that would significantly alter expansion characteristics within the service temperature range

Main characteristics of expansion alloys

Thermal expansion performance: It has abnormal thermal expansion characteristics and can maintain a low or constant coefficient of expansion within a specific temperature range.
Physical properties: In addition to a specific coefficient of thermal expansion, good sealing, weldability, corrosion resistance, machinability, and ease of cutting are also required.
Chemical composition: The chemical composition of expansion alloys plays a decisive role in their properties, such as Fe Ni, Fe Ni Co, and Fe Ni Cr alloys.

Typical materials of expansion alloys

4J36 (Invar alloy)

Main components: 36% nickel and balance iron, with small amounts of added elements such as carbon, manganese, silicon, etc.
Thermal expansion performance: The average coefficient of thermal expansion in the range of room temperature to 300 ℃ is about 1.6 × 10 ⁻⁶/℃, with excellent low thermal expansion characteristics.
Mechanical properties: The tensile strength is generally between 490-680 MPa, the yield strength is between 240-420 MPa, the elongation is between 30-40%, and it has good plastic deformation ability.

4J29 expansion alloy

The 4J29 expansion alloy sheet is mainly composed of three elements: iron (Fe), nickel (Ni), and cobalt (Co). The nickel content is about 29%, the cobalt content is about 17%, and the rest is iron. In addition, the alloy also contains trace amounts of carbon (C), silicon (Si), manganese (Mn) and other elements, and the precise ratio of these elements endows 4J29 expansion alloy sheet with unique physical and chemical properties.
The biggest feature of 4J29 alloy is that its thermal expansion coefficient can maintain a high degree of matching with materials such as glass and ceramics within a certain temperature range, usually between 20-400 ℃. By controlling the ratio of iron, nickel, and cobalt in the alloy, 4J29 alloy can maintain dimensional stability during heating or cooling, making it the preferred packaging material.
4J29 expansion alloy not only has the advantage of thermal expansion, but also has good mechanical properties, especially its strength, hardness, and fatigue resistance. The yield strength and tensile strength of this material are high, and it can adapt to equipment that works for long periods of time in high stress environments. The ductility of 4J29 expansion alloy enables it to be formed through cold and hot working processes such as stamping, welding, etc., which further enhances its wide application.

Application of Expansion Alloys

Electronic packaging: In the field of electronic packaging, expansion alloys are widely used to make the casing or sealing structure of electronic devices. Due to its excellent thermal expansion performance, it can ensure that electronic components maintain a stable packaging state during temperature changes, improving the reliability and stability of electronic devices.
Vacuum devices: In the field of vacuum devices, expansion alloys are widely used in the manufacturing of sealing structures for vacuum tubes, ion pumps, and other vacuum devices due to their excellent thermal expansion properties. It can be tightly sealed with materials such as glass, effectively preventing gas or liquid leakage and ensuring the normal operation of vacuum devices.
Optical instruments: In the field of optical instruments, the high-precision sealing performance of expansion alloys makes them an ideal material for manufacturing optical components and assemblies. When manufacturing optical components such as lenses and filters, expansion alloys can ensure a tight seal with glass or ceramic materials, improving the performance and accuracy of optical components.
Other fields: In addition to the above fields, expansion alloys are also widely used in aerospace, petrochemical and other fields. In the aerospace field, its excellent thermal expansion performance and stability make it an ideal material for manufacturing key components such as navigation systems; In the field of petrochemicals, its excellent corrosion resistance and thermal stability make it the preferred material for manufacturing sealing materials and structural materials in high-temperature and high-pressure environments.

Why Choose Expansion Alloys?

Tailored Thermal Properties: Customizable expansion rates to meet specific application requirements.
High Precision and Reliability: Ensures the production of components with exacting tolerances and consistent performance.
Versatility: Suitable for a wide range of industries, from electronics to aerospace and beyond.
Durability: Resists wear and deformation, even under extreme temperature conditions, ensuring long-term performance.

As one of the leading expansion alloy manufacturers in China, we warmly welcome you to buy high quality expansion alloy at competitive price from our factory. For more cheap products, contact us now.

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