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In the realm of controlled thermal expansion, there is a new challenger to Invar&#;s long held title. ALLVAR Alloys offer a new approach to solve thermal expansion issues. The similarity in name is no coincidence. Invar is an iron-nickel alloy that exhibits a very low, isotropic CTE (Coefficient of Thermal Expansion) while ALLVAR Alloy 30 is a titanium alloy that exhibits a highly negative, anisotropic CTE. As a result, ALLVAR Alloy 30 is non-magnetic, lower density, and corrosion resistant compared to Invar. As we like to say, &#;ALLVAR Alloy 30 is all the VAR that Invar isn&#;t.&#;
When discovered in by Swiss physicist Charles Guillaume, Invar&#;s low coefficient of thermal expansion enabled a technological leap in precision instruments that won Guillaume a Nobel Prize . Since its first discovery, various forms and compositions of Kovar, Invar, and Super Invar have been developed and extensively studied. Near Room temperature CTE&#;s for these alloys range from ~0.720 ppm/°C for Super Invar (25°C to 96°C), ~1.6 ppm/°C for Invar (25°C to 96°C), and ~5.2 ppm/°C for Kovar (25°C to 200°C). Invar&#;s thermal expansion coefficient is isotropic, which means it is the same value in every direction. This means a complex component can be machined from a single block of Invar and it will exhibit the same CTE in all directions.
Since their original discovery in , ALLVAR Alloys have been developed to offer a wide range of thermal expansion values for various products and applications. ALLVAR Alloy 30 was designed to have as negative a CTE as possible with a value of -30 ppm/°C. Using this novel property, designers and engineers can compensate for the natural thermal expansion of other materials in an assembly. For the first time, a designer can choose a CTE anywhere between -30 ppm/°C and the CTE of the most positive material in their system by simply changing the length of the ALLVAR Alloy component. The tradeoff is that ALLVAR Alloys exhibit anisotropic CTE, which means the thermal expansion is different in various directions. A typical bar of ALLVAR Alloy 30 will exhibit -30 ppm/°C along its length and +31 ppm/°C in the diameter.
To ensure CTE and dimensional stability in service, Invar requires special machining processes that include stress-relieving heat treatments for a consistent CTE in a precision machined part. These heat treatments add significant cost and time to making Invar components. Additionally, Invar is difficult to machine due to rapid wear of cutting tools. While Invar has a relatively low raw material cost, these requirements significantly increase the cost of a precision component. There are several excellent resources available if you need help machining Invar alloys.
ALLVAR Alloys are easier to machine than Invar components, but there are some things you need to know before machining it. ALLVAR Alloys machine like a beta-titanium. With the right feeds and speeds and a knowledgeable machinist experienced with titanium alloys, it machines well. Additionally, ALLVAR Alloy 30 has a maximum operating temperature of 100°C. Special care should be given to flood the cutting surface with coolant and &#;peck&#; when drilling holes. ALLVAR Alloys are also similar to commonly used wrought alloys like aluminum or normal titanium in that a stabilizing process is required for high precision applications. The ALLVAR team has developed proprietary stabilizing processes for normal (0°C to 50°C), medium (-40°C to 80°C), and extreme (cryogenic to 100°C) operating temperature ranges. If machining titanium is not in your company&#;s wheelhouse and stability is key, ALLVAR can provide your parts already machined and stabilized.
The operating temperature range should be considered when choosing either ALLVAR Alloy 30 or any variant of Invar. ALLVAR Alloy 30 is suitable from very cold cryogenic temperatures to 100°C. The material&#;s CTE performance cannot be guaranteed above 100°C.
The low thermal expansion of Invar, Super Invar, and Kovar is dictated by their magnetic properties. Therefore, they lose their low thermal expansion when cooling to colder temperatures or going above their Curie temperature. For example, Super INVAR can lose its low CTE properties at very low temperatures, while Invar-36 will drift away from its low CTE outside a -70 to 100°C temperature window.
Invar is very susceptible to corrosion if not handled properly and components typically require nickel plating or other coatings to prevent corrosion in service. ALLVAR Alloys on the other hand have excellent corrosion resistant properties like other titanium alloys. ALLVAR Alloys can also be anodized and coated like other titanium alloys.
It depends on your application! Each alloy has its advantages for specific applications. If Invar&#;s low thermal expansion works and you need the same isotropic thermal expansion properties in every component direction, INVAR is likely better suited for your project. If your goal is to target thermal stability in one direction or compensate for thermal mismatch of dissimilar materials, consider using ALLVAR Alloy 30.
Still unsure if Invar or ALLVAR Alloys are better for your project? We are a team of materials scientists at heart. Whether it is Invar or ALLVAR Alloys, we would love to help you find the right material for your next project. Please contact us by clicking the orange button below and we will steer you in the right direction. Your success is what&#;s most important to us.
We are also happy to provide a spec-sheet with example CTEs across a wide range of temperatures or if you are interested in our experimental ALLVAR Alloys with higher upper operating temperatures.
Don&#;t forget to follow ALLVAR on our LinkedIn page, check out our YouTube page, and subscribe to our newsletter to stay up to date with our latest news and events! As a team of passionate material scientists and engineers, we would love to connect and answer any questions you may have about our revolutionary material.
Samples of Invar The coefficient of thermal expansion of nickel/iron alloys is plotted here against the nickel percentage (on a mass basis) in the alloy. The sharp minimum occurs at the Invar ratio of 36% Ni.
Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel&#;iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes,[1] and is a registered trademark of ArcelorMittal.[2]
The discovery of the alloy was made in by Swiss physicist Charles Édouard Guillaume for which he received the Nobel Prize in Physics in . It enabled improvements in scientific instruments.[3]
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Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy. In one commercial grade called Invar 35 it consists of approximately 36% nickel and 64% iron,[4] has a melting point of C, a density of 8.05 g/cm3 and a resistivity of 8.2 x 10-5 Ω·cm.[5] The invar range was described by Westinghouse scientists in as "30&#;45 atom per cent nickel".[6]
Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20 °C and 100 °C) of about 1.2 × 10&#;6 K&#;1 (1.2 ppm/°C), while ordinary steels have values of around 11&#;15 ppm/°C.[citation needed] Extra-pure grades (<0.1% Co) can readily produce values as low as 0.62&#;0.65 ppm/°C.[citation needed] Some formulations display negative thermal expansion (NTE) characteristics.[citation needed] Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep.[7][8]
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Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, color-television tubes' shadow-mask frames,[9] valves in engines and large aerostructure molds.[10]
One of its first applications was in watch balance wheels and pendulum rods for precision regulator clocks. At the time it was invented, the pendulum clock was the world's most precise timekeeper, and the limit to timekeeping accuracy was due to thermal variations in length of clock pendulums. The Riefler regulator clock developed in by Clemens Riefler, the first clock to use an Invar pendulum, had an accuracy of 10 milliseconds per day, and served as the primary time standard in naval observatories and for national time services until the s.
In land surveying, when first-order (high-precision) elevation leveling is to be performed, the level staff (leveling rod) used is made of Invar, instead of wood, fiberglass, or other metals.[11][12] Invar struts were used in some pistons to limit their thermal expansion inside their cylinders.[13] In the manufacture of large composite material structures for aerospace carbon fibre layup molds, Invar is used to facilitate the manufacture of parts to extremely tight tolerances.[14]
In the astronomical field, Invar is used as the structural components that support dimension-sensitive optics of astronomical telescopes.[15] Superior dimensional stability of Invar allows the astronomical telescopes to significantly improve the observation precision and accuracy.
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There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:
citation needed
][example needed
]α &#; 5.3 ppm/°C
, matching that of silicon, is widely used as lead frame material for integrated circuits, etc.[citation needed
]5 ppm/°C
) and form strong bonds with molten borosilicate glass, and because of that are used for glass-to-metal seals, and to support optical parts in a wide range of temperatures and applications, such as satellites.[citation needed
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A detailed explanation of Invar's anomalously low CTE has proven elusive for physicists.
All the iron-rich face-centered cubic Fe&#;Ni alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy composition. Scientists had once proposed that Invar's behavior was a direct consequence of a high-magnetic-moment to low-magnetic-moment transition occurring in the face centered cubic Fe&#;Ni series (and that gives rise to the mineral antitaenite); however, this theory was proven incorrect.[16] Instead, it appears that the low-moment/high-moment transition is preceded by a high-magnetic-moment frustrated ferromagnetic state in which the Fe&#;Fe magnetic exchange bonds have a large magneto-volume effect of the right sign and magnitude to create the observed thermal expansion anomaly.[17]
Wang et al. considered the statistical mixture between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs) in Fe
3Pt with the free energies of FM and SFCs predicted from first-principles calculations and were able to predict the temperature ranges of negative thermal expansion under various pressures.[18] It was shown that all individual FM and SFCs have positive thermal expansion, and the negative thermal expansion originates from the increasing populations of SFCs with smaller volumes than that of FM.[19]
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