INVAR Alloys

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Introduction to INVAR Alloys

Fe-Ni alloys with approximately 35% nickel concentration demonstrate an extraordinarily low thermal expansion coefficient, making their dimensions practically "invariable" near room temperature—hence their name, INVAR alloys. These materials exhibit various magnetic property anomalies, including deviation from the Slater-Pauling curve as outer shell electrons decrease, and strong dependence of the Curie temperature (TC) on the mean atomic distance within the alloy structure.

Invar alloys, also known generically as FeNi36 (or 64FeNi in the US), are nickel steel alloys distinguished by their uniquely low coefficient of thermal expansion (CTE or α). Swiss scientist Charles Édouard Guillaume invented these alloys in 1896, earning him the Nobel Prize in Physics in 1924—a testament to the material's significance in scientific instrumentation. While "Invar" remains a registered trademark of Arcelor Mittal, FeNi36 is manufactured by multiple companies worldwide. Unlike many alloys, Invar is a solid solution, meaning it exists as a single-phase alloy.

Composition and Crystal Structure

All alloys in the Invar family consist of nickel-iron or nickel-iron-cobalt compositions exhibiting face-centered cubic crystal structures. As nickel content increases beyond 36%, both thermal expansion coefficient and Curie temperature increase proportionally. The Curie temperature rises from approximately 280°C (536°F) for 36% nickel content to over 565°C (1050°F) for alloys containing 50% nickel.

Table 1. Typical properties and chemical composition of the Invar family of alloys

Property	Super Invar 32.5 (ASTM F-1684)	Carpenter Invar 36 (ASTM F-1684)	Free-Cut Invar 36 (ASTM F-1684)	Low Expansion 39 (ASTM B-753)	Low Expansion 42 (ASTM B-753)	Low Expansion 45 (ASTM B-753)	Low Expansion 49 (ASTM B-753)	Glass Sealing 52 (ASTM F-30)
Composition:
Carbon (%)	0.05	0.05	0.10	0.05	0.05	0.05	0.05	<0.01
Manganese (%)	0.35	0.35	0.90	0.40	0.10	0.50	0.50	0.50
Silicon (%)	0.30	0.30	0.35	0.25	0.20	0.25	0.40	0.25
Nickel (%)	32.0	36.0	36.0	39.0	42.0	45.0	47.5	50.5
Other Elements	Co ≤ 5.25, Fe bal.	Fe bal.	Se 0.20, Fe bal.	Fe bal.	Fe bal.	Fe bal.	Fe bal.	Fe bal.
Specific Gravity	8.10	8.05	8.05	8.08	8.12	8.20	8.25	8.30
Thermal Conductivity (W/cm °C)	10	10	10	11	11	16	17	16
Electrical Resistivity (µΩ-cm)	80	82	82	71	67	55	48	43
Curie Temperature (°C)	260	280	280	340	380	440	500	530
Specific Heat (cal/g-°C)	0.12	0.12	0.12	0.12	0.12	0.12	0.12	0.12
Typical Coefficients of Thermal Expansion (as annealed) (x10-4/°C):
25 to 93°C	0.72	1.6	1.8	2.3	5.8	7.9	8.5	10.0
149°C	/	2.0	2.3	2.7	3.6	7.7	8.5	10.1
260°C	/	4.1	4.5	3.2	5.4	7.4	8.5	10.2
371°C	/	7.2	7.9	5.8	5.8	7.2	8.5	10.1
Tensile Strength (MPa, ksi)	483 (70)	517 (75)	517 (75)	517 (75)	517 (75)	565 (82)	517 (75)	517 (75)
Yield Strength (MPa, ksi)	276 (40)	276 (40)	276 (40)	276 (40)	276 (40)	276 (40)	276 (40)	276 (40)
Elongation in 2 in. (%)	30	30	30	30	30	30	30	30
Hardness, Rockwell B Scale	80	80	80	80	80	80	80	80
Elastic Modulus (GPa, Msi)	144 (21.0)	141 (20.5)	141 (20.5)	144 (21.0)	148 (21.5)	144 (21.0)	166 (24.0)	165 (24.0)

Magnetic Properties and Scientific Significance

The unusual physical properties of Invar-composition alloys have attracted significant scientific interest. In 1960, Sedov and colleagues proposed that the magnetic anomalies in Invar alloys result from "latent antiferromagnetism." This hypothesis suggested that in the gamma phase of FeNi Invar alloy, the exchange interaction energy parameters between neighboring atoms (Ni-Ni and Ni-Fe) have opposite signs, with negative values for Fe-Fe atoms. Experimental evidence later confirmed the potential for antiferromagnetism in the gamma-iron lattice through characteristic temperature-dependent susceptibility measurements and Mössbauer spectroscopy.

Using rotational hysteresis measurements of Fe-Ni alloys, Nakamura estimated that the antiferromagnetically ordered regions demonstrate Néel temperatures (TN) between 35-50°K. Further neutron-diffraction investigations by Dubinin and colleagues confirmed the coexistence of paramagnetic and antiferromagnetic phases within Invar materials.

Thermal Expansion Characteristics

Invar (UNS K93601) and related binary iron-nickel alloys exhibit low expansion coefficients only within a specific temperature range, as illustrated in Figure 1. The thermal behavior can be characterized by several distinct regions:

• Region A to B: High coefficient of expansion at low temperatures
• Region B to C: Decreasing coefficient
• Region C to D: Minimum expansion (the "Invar effect" region)
• Region D to E: Increasing coefficient with rising temperature
• Region E to F: Normal expansion similar to constituent metals

The practical minimum expansivity exists only within the C to D temperature range.

Figure 1: Change in length of a typical Invar alloy over different ranges of temperature

Industrial and Technological Applications

The exceptional dimensional stability of Invar alloys makes them valuable in numerous precision applications:

• Scientific instrumentation and metrology devices
• Liquefied natural gas (LNG) containers and transfer lines
• Temperature-sensitive devices including bimetal thermostats and regulators
• Precision timekeeping components for pendulum clocks and balance wheels
• Telecommunications equipment including echo boxes and filters for mobile phones
• Electronic components such as shadow masks and deflection clips for CRTs
• Aerospace industry tools and molds for advanced composites
• Magnetic shielding applications
• Radar and microwave cavity resonators