Invar

Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy. In one commercial grade called Invar 36 it consists of approximately 36% nickel and 64% iron,{{Cite web|url=https://www.vdm-metals.com/fileadmin/user_upload/Downloads/Data_Sheets/Data_Sheet_VDM_Alloy_36.pdf|title=VDM Alloy 36|publisher=VDM Metals|id=Material Data Sheet no. 7101|date=March 2022|access-date=2025-02-08}} has a melting point of {{cvt|1427|C}}, a density of {{val|8.05|u=g|up=cm3}} and a resistivity of {{val|8.2|e=-5|u=Ω·cm}}.{{cite news |url=https://www.americanelements.com/invar-36-alloy |title=Invar 36 Alloy }} The invar range was described by Westinghouse scientists in 1961 as "30–45 atom per cent nickel".{{cite journal |doi=10.1038/192962a0|title=A New Reversible Solid-State Transformation in Iron–Nickel Alloys in the Invar Range of Compositions|year=1961|last1=Ananthanarayanan|first1=N. I.|last2=Peavler|first2=R. J.|journal=Nature|volume=192|issue=4806|pages=962–963|bibcode=1961Natur.192..962A|s2cid=4277440}}

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 ({{val|1.2|ul=ppm|up=°C}}), while ordinary steels have values of around 11–15 ppm/°C.{{Cn|date=June 2023}} Extra-pure grades (<0.1% Co) can readily produce values as low as 0.62–0.65 ppm/°C.{{Cn|date=June 2023}} Some formulations display negative thermal expansion (NTE) characteristics.{{Cn|date=June 2023}} Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep.{{Cite journal |last1=Myslowicki |first1=Thomas |last2=Crumbach |first2=Mischa |last3=Mattissen |first3=Dorothea |last4=Bleck |first4=Wolfgang |date=August 2002 |title=Short time creep behaviour of Invar steel |url=https://onlinelibrary.wiley.com/doi/10.1002/srin.200200218 |url-access=subscription |journal=Steel Research |language=en |volume=73 |issue=8 |pages=332–339 |doi=10.1002/srin.200200218}}{{Cite journal |last1=Thackar |last2=Trivedi |first1=Romin A. |first2=Snehal V. |date=June 2017 |title=An Overview of Dimensional Stability of Invar 36 Material for Space Based Optical Mounting Applications |url=https://ijritcc.org/download/conferences/ICIIIME_2017/ICIIIME_2017_Track/1496822245_07-06-2017.pdf |journal=International Conference on Ideas, Impact and Innovation in Mechanical Engineering (ICIIIME 2017) |volume=5 |issue=6 |pages=147 |via=}}

Historically, the paramagnetic properties of certain iron-nickel alloys were first identified as a unique characteristic. These alloys exhibit a coexistence of two types of structures, whose proportions vary depending on temperature.{{Cite journal |last1=Weiss |first1=R.J. |date=May 1963 |title=The origin of the 'Invar' effect |url=https://iopscience.iop.org/article/10.1088/0370-1328/82/2/314/meta |url-access=subscription |journal=Proceedings of the Physical Society |language=en |volume=82 |issue=2 |pages=281–288 |doi=10.1088/0370-1328/82/2/314|bibcode=1963PPS....82..281W }}{{Cite journal |last1=Mohn |first1=P. |last2=Schwarz |first2=K. |last3=Wagner |first3=D. |date=February 1991 |title=Magnetoelastic anomalies in Fe-Ni Invar alloys |url=https://journals.aps.org/prb/abstract/10.1103/PhysRevB.43.3318 |url-access=subscription |journal=Physical Review B |language=en |volume=43 |issue=4 |pages=3318–3324 |doi=10.1103/PhysRevB.43.3318|pmid=9997641 |bibcode=1991PhRvB..43.3318M }}{{Cite journal |last1=Schilfgaarde |first1=M. van |last2=Abrikosov |first2=I.A. |last3=Johansson |first3=B. |date=July 1999 |title=Origin of the Invar effect in iron–nickel alloys |url=https://www.nature.com/articles/21848 |journal=Nature |url-access=subscription |language=en |volume=400 |issue=6739 |pages=46–49 |doi=10.1038/21848|bibcode=1999Natur.400...46V }} One of these structures is characterized by a high magnetic moment (ranging from {{val|2.2|to|2.5|u=μ_B}}) and a high lattice parameter, adhering to Hund's rules. The other structure, in contrast, has a low magnetic moment (ranging from {{val|0.8|to|1.5|u=μ_B}}) and a low lattice parameter. When exposed to a variable magnetic field, this dual-structure nature induces dimensional changes in the alloy. This phenomenon is particularly significant in the case of Invar alloys, which are renowned for their exceptional dimensional stability over a wide range of temperatures. However, to maintain this stability, it is crucial to avoid exposing the material to magnetic fields, as such exposure can disrupt the delicate balance between the two structures and lead to undesirable dimensional variations.

In recent years, advancements in material science have led to the development of non-ferromagnetic Invar alloys. These innovative materials have opened up new possibilities for applications in cutting-edge fields such as the semiconductor industry and aerospace engineering.{{Cite book |last1=Fujii |first1=Hiromichi T. |last2=Matsumura |first2=Shingo |last3=Sakaguchi |first3=Naoki |last4=Ohno |first4=Haruyasu |last5=Ona |first5=Kotaro |chapter=Unlocking the potential of non-ferromagnetic Invar-type alloys in space exploration |editor-first1=Ramón |editor-first2=Ralf |editor-last1=Navarro |editor-last2=Jedamzik |date=September 2024 |title=Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation VI |chapter-url=https://doi.org/10.1117/12.3017918 |language=en |volume=13100 |pages=131000L |doi=10.1117/12.3017918|bibcode=2024SPIE13100E..0LF |isbn=978-1-5106-7523-0 }}{{Cite book |last1=Fujii |first1=Hiromichi T. |last2=Matsumura |first2=Shingo |last3=Sakaguchi |first3=Naoki |last4=Ohno |first4=Haruyasu |last5=Ona |first5=Kotaro |editor-first1=Joern-Holger |editor-first2=Kurt G. |editor-first3=Paolo A. |editor-first4=Patrick P. |editor-first5=Toshiro |editor-last1=Franke |editor-last2=Ronse |editor-last3=Gargini |editor-last4=Naulleau |editor-last5=Itani |chapter=Exceptional dimensional stability of non-ferromagnetic Invar alloy for advanced semiconductor manufacturing equipment |date=September 2024 |title=International Conference on Extreme Ultraviolet Lithography 2024 |chapter-url=https://doi.org/10.1117/12.3034312 |language=en |volume=13215 |pages=132150Y |doi=10.1117/12.3034312|isbn=978-1-5106-8155-2 }} By eliminating the influence of magnetic fields on dimensional stability, non-ferromagnetic Invar alloys have the potential to significantly enhance the performance of optical instruments and other precision devices.

Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, color-television tubes' shadow-mask frames,{{cite web |title=Nickel & Its Uses |publisher=Nickel Institute |work=Nickel Magazine |date=3 May 2005 |url=http://www.nickelinstitute.org/index.cfm/ci_id/12313.htm |access-date=20 March 2011 |url-status=dead |archive-url=https://web.archive.org/web/20101219005643/http://www.nickelinstitute.org/index.cfm/ci_id/12313.htm |archive-date=19 December 2010 }} valves in engines and large aerostructure molds.{{Citation |title=Boeing 787 Fuselage (MIE-375) | date=30 October 2018 |url=https://www.youtube.com/watch?v=VyWFiUnoT5E |access-date=2023-06-29 |language=en}}

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 1898 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 1930s.

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.{{Cite book |last1=Baričević |first1=Sergej |last2=Barković |first2=Đuro |last3=Zrinjski |first3=Mladen |last4=Staroveški |first4=Tomislav |chapter=Development of Levelling Staff Scale Calibration Method by Integrating a CCD Camera |date=2022 |editor-last=Ademović |editor-first=Naida |editor2-last=Mujčić |editor2-first=Edin |editor3-last=Akšamija |editor3-first=Zlatan |editor4-last=Kevrić |editor4-first=Jasmin |editor5-last=Avdaković |editor5-first=Samir |editor6-last=Volić |editor6-first=Ismar |title=Advanced Technologies, Systems, and Applications VI |chapter-url=https://link.springer.com/chapter/10.1007/978-3-030-90055-7_40 |series=Lecture Notes in Networks and Systems |volume=316 |language=en |location=Cham |publisher=Springer International Publishing |pages=514–521 |doi=10.1007/978-3-030-90055-7_40 |isbn=978-3-030-90055-7}}{{Cite web |title=ISO 12858-1:2014 Optics and optical instruments — Ancillary devices for geodetic instruments — Part 1: Invar levelling staffs |url=https://www.iso.org/standard/57606.html |access-date=2023-09-02 |website=ISO |language=en}} Invar struts were used in some pistons to limit their thermal expansion inside their cylinders.{{cite book|title=Internal combustion engines illustrated|year=1947|publisher=Odhams Press Limited|location=Long Acre, London|page=85}} 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.[https://www.aero-mag.com/tooling-mould-die/ Tooling to mould and die for!] {{Webarchive|url=https://web.archive.org/web/20180410202335/https://www.aero-mag.com/tooling-mould-die/ |date=10 April 2018 }}, Mike Richardson, Aerospace Manufacturing, 6 April 2018, accessed 10 April 2018.{{Cite book|first1=Hiromichi T.|last1=Fujii|first2=Haruyasu|last2=Ohno|first3=Naoki|last3=Sakaguchi|first4=Shingo|last4=Matsumura|first5=Kotaro|last5=Ona|first6=Junichi|last6=Go|first7=Umito|last7=Yoshioka|title=Camx Proceedings |chapter=Revolutionizing molding precision for aviation and urban air mobility: The power of low thermal expansion tooling in CFRTP press forming |year=2024|chapter-url=https://www.nasampe.org/store/viewproduct.aspx?id=24547917|volume=TP24|pages=237|doi=10.33599/nasampe/c.24.0237 }}

In the astronomical field, Invar is used as the structural components that support dimension-sensitive optics of astronomical telescopes.{{Cite book|first1=Hiromichi T.|last1=Fujii|first2=Naoki|last2=Sakaguchi|first3=Kotaro|last3=Ona|first4=Yutaka|last4=Hayano|first5=Fumihiro|last5=Uraguchi|title=Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation IV |chapter=Precise control of negative thermal expansion in stainless invar type alloy for astronomical telescopes |editor-first1=Roland |editor-first2=Ramón |editor-last1=Geyl |editor-last2=Navarro |year=2020|chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11451/2561193/Precise-control-of-negative-thermal-expansion-in-stainless-invar-type/10.1117/12.2561193.short|volume=11451|pages=1145118|doi=10.1117/12.2561193 |bibcode=2020SPIE11451E..18F |isbn=9781510636897 |s2cid=230575165 |accessdate=2021-05-08}} Superior dimensional stability of Invar allows the astronomical telescopes to significantly improve the observation precision and accuracy.

There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:

• Inovco, which is Fe–33Ni–4.5Co and has an α of 0.55 ppm/°C (from 20 to 100 °C).{{Cn|date=June 2023}}{{Examples|date=June 2023}}
• FeNi42 (for example NILO alloy 42), which has a nickel content of 42% and {{nowrap|α ≈ 5.3 ppm/°C}}, matching that of silicon, is widely used as lead frame material for integrated circuits, etc.{{Cn|date=June 2023}}
• FeNiCo alloys—named Kovar or Dilver P—that have the same expansion behaviour (~{{nowrap|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.{{Cn|date=June 2023}}
• Elinvar has a near-constant modulus of elasticity, making it valuable for wristwatch balance wheels, spring scales, and other spring-based measuring instruments.

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.{{cite journal |author1=K. Lagarec |author2=D.G. Rancourt |author3=S.K. Bose |author4=B. Sanyal |author5=R.A. Dunlap |url=http://fizz.phys.dal.ca/~dunlap/index_files/index_files/journal/c210_lagarec.pdf |title=Observation of a composition-controlled high-moment/low-moment transition in the face centered cubic Fe–Ni system: Invar effect is an expansion, not a contraction |journal=Journal of Magnetism and Magnetic Materials |volume=236 |year=2001 |issue=1–2 |pages=107–130 |doi=10.1016/S0304-8853(01)00449-8 |bibcode=2001JMMM..236..107L |url-status=dead |archive-url=https://web.archive.org/web/20120425145000/http://fizz.phys.dal.ca/~dunlap/index_files/index_files/journal/c210_lagarec.pdf |archive-date=25 April 2012 }} 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.{{cite journal|author1=D.G. Rancourt |author2=M.-Z. Dang |title= Relation between anomalous magneto-volume behaviour and magnetic frustration in Invar alloys|journal=Physical Review B|volume= 54|issue= 17|year=1996|pages= 12225–12231|doi=10.1103/PhysRevB.54.12225|pmid=9985084 |bibcode=1996PhRvB..5412225R}}

Wang et al. considered the statistical mixture between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs) in {{chem|Fe|3|Pt}} 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.Wang, Y., Shang, S. L., Zhang, H., Chen, L.-Q., & Liu, Z.-K. (2010). Thermodynamic fluctuations in magnetic states: Fe 3 Pt as a prototype. Philosophical Magazine Letters, 90(12), 851–859. https://doi.org/10.1080/09500839.2010.508446 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.{{Cite journal|doi=10.1038/srep07043|pmid=25391631|pmc=4229665|title=Thermal Expansion Anomaly Regulated by Entropy|journal=Scientific Reports|volume=4|pages=7043|year=2014|last1=Liu|first1=Zi-Kui|last2=Wang|first2=Yi|last3=Shang|first3=Shunli|bibcode=2014NatSR...4.7043L}}

• Constantan and Manganin, alloys with relatively constant electrical resistivity
• Elinvar, alloy with relatively constant elasticity over a range of temperatures
• Sitall and Zerodur, ceramic materials with a relatively low thermal expansion
• Borosilicate glass and Ultra low expansion glass, low expansion glasses resistant to thermal shock

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Category:Ferrous alloys

Category:Nickel alloys

Category:Surveying instruments

Category:Low thermal expansion materials