Invar and Other Low-Expansion Alloys

In 1896, Charles-Edouard Guillaume of France made an interesting and important discovery that earned him the 1920 Nobel Prize in Physics; his discovery: an iron–nickel alloy that has a very low (near-zero) coefficient of thermal expansion between room temperature and approximately 230⁰C. This material became the forerunner of a family of “low-expansion”(also sometimes called “controlled expansion”) metal alloys. Its composition is 64 wt% Fe–36 wt% Ni, and it has been given the trade-name of “Invar” since the length of a specimen of this material is virtually invariable with changes in temperature. Its coefficient of thermal expansion near room temperature is 1.6 X 10^-6(⁰C)^-1.

One might surmise that this near zero expansion is explained by a symmetrical potential energy versus interatomic distance curve. Such is not so; rather, this behavior relates to the magnetic characteristics of Invar. Both iron and nickel are ferromagnetic materials .A ferromagnetic material may be made to form a permanent and strong magnet; upon heating, this property disappears at a specific temperature, called the “Curie temperature,” which temperature varies from one ferromagnetic material to another. As a specimen of Invar is heated, its tendency to expand is counteracted by a contraction phenomenon that is associated with its ferromagnetic properties (which is termed “magnetostriction”). Above its Curie temperature (approximately Invar expands in a normal manner, and its coefficient of thermal expansion assumes a much greater value. Heat treating and processing of Invar will also affect its thermal expansion characteristics. The lowest values (α) result for specimens quenched from elevated temperatures (near 800⁰C) that are then cold worked. Annealing leads to an increase in α. Other low-expansion alloys have been developed.

Other low-expansion alloys have been developed. One of these is called “Super–Invar” because its thermal expansion coefficient [0.72 X 10^-6(⁰C)^-1] is lower than the value for Invar. However, the temperature range over which its low expansion characteristics persist is relatively narrow. Compositionally, for Super–Invar some of the nickel in Invar is replaced by another ferromagnetic metal, cobalt; Super–Invar contains 63 wt% Fe, 32 wt% Ni, and 5 wt% Co.

Another such alloy, with the trade-name of “Kovar,” has been designed to have expansion characteristics close to those of borosilicate (or Pyrex) glass; when joined to Pyrex and subjected to temperature variations, thermal stresses and possible fracture at the junction are avoided. The composition of Kovar is 54 wt% Fe, 29 wt% Ni, and 17 wt% Co. These low-expansion alloys are employed in applications that require dimensional stability with temperature fluctuations; these include the following:

• Compensating pendulums and balance wheels for mechanical clocks and watches.

• Structural components in optical and laser measuring systems that require dimensional stabilities on the order of a wavelength of light.

• Bimetallic strips that are used to actuate microswitches in water heating systems.

• Shadow masks on cathode ray tubes that are used for television and display screens; higher contrast, improved brightness, and sharper definition are possible using low-expansion materials.

• Vessels and piping for the storage and piping of liquefied natural gas.

Environmental Properties: Invar - Nickel Iron Alloy
Resistance Factors : 1=Poor 5=Excellent
Flammability	5
Fresh Water	5
Organic Solvents	5
Oxidation at 500C	5
Sea Water	5
Strong Acid	4
Strong Alkalis	5

Material	Invar - Nickel Iron Alloy
Property	Minimum Value (S.I.)	Maximum Value (S.I.)	Units (S.I.)	Minimum Value (Imp.)	Maximum Value (Imp.)	Units (Imp.)
Atomic Volume (average)	0.0068	0.0071	m3/kmol	414.961	433.268	in3/kmol
Density	8.1	8.2	Mg/m3	505.667	511.91	lb/ft3
Energy Content	50	200	MJ/kg	5416.93	21667.7	kcal/lb
Bulk Modulus	106	112	GPa	15.374	16.2442	106 psi
Compressive Strength	240	725	MPa	34.8091	105.152	ksi
Ductility	0.06	0.45		0.06	0.45
Elastic Limit	240	725	MPa	34.8091	105.152	ksi
Endurance Limit	185	405	MPa	26.832	58.7402	ksi
Fracture Toughness	120	150	MPa.m1/2	109.206	136.507	ksi.in1/2
Hardness	1200	2400	MPa	174.045	348.091	ksi
Loss Coefficient	0.0003	0.0011		0.0003	0.0011
Modulus of Rupture	240	725	MPa	34.8091	105.152	ksi
Poisson's Ratio	0.28	0.3		0.28	0.3
Shear Modulus	54	58	GPa	7.83204	8.41219	106 psi
Tensile Strength	445	810	MPa	64.5418	117.481	ksi
Young's Modulus	137	145	GPa	19.8702	21.0305	106 psi
Glass Temperature			K			°F
Latent Heat of Fusion	270	290	kJ/kg	116.079	124.677	BTU/lb
Maximum Service Temperature	600	700	K	620.33	800.33	°F
Melting Point	1690	1710	K	2582.33	2618.33	°F
Minimum Service Temperature	0	0	K	-459.67	-459.67	°F
Specific Heat	505	525	J/kg.K	0.390798	0.406276	BTU/lb.F
Thermal Conductivity	12	15	W/m.K	22.4644	28.0805	BTU.ft/h.ft2.F
Thermal Expansion	0.5	2	10-6/K	0.9	3.6	10-6/°F
Breakdown Potential			MV/m			V/mil
Dielectric Constant
Resistivity	75	85	10-8 ohm.m	75	85	10-8 ohm.m