Editing Invar (section)

== Properties ==
Like other nickel/iron compositions, Invar is a [[solid solution]]; that is, it is a [[Phase (matter)|single-phase]] [[alloy]]. In one commercial grade called Invar 36 it consists of approximately 36% nickel and 64% iron,<ref name=vdm>{{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}}</ref> 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}}.<ref name="am1">{{cite news |url=https://www.americanelements.com/invar-36-alloy |title=Invar 36 Alloy }}</ref> The '''invar range''' was described by Westinghouse scientists in 1961 as "30–45 [[Mole fraction|atom per cent]] nickel".<ref name="peavler61">{{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}}</ref>

Common grades of Invar have a coefficient of thermal expansion (denoted <var>α</var>, and measured between 20&nbsp;°C and 100&nbsp;°C) of about 1.2&nbsp;×&nbsp;10<sup>−6</sup>&nbsp;[[Kelvin|K]]<sup>−1</sup> ({{val|1.2|ul=ppm|up=°C}}), while ordinary steels have values of around 11–15&nbsp;ppm/°C.{{Cn|date=June 2023}}  Extra-pure grades (<0.1% [[Cobalt|Co]]) can readily produce values as low as 0.62–0.65&nbsp;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 (deformation)|creep]].<ref>{{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}}</ref><ref>{{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=}}</ref>

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.<ref name=Weiss1963>{{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 }}</ref><ref>{{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 }}</ref><ref>{{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 }}</ref> One of these structures is characterized by a high magnetic moment (ranging from {{val|2.2|to|2.5|u=[[Bohr magneton|''μ''<sub>B</sub>]]}}) 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''<sub>B</sub>}}) 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.<ref>{{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 }}</ref><ref>{{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 }}</ref> 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.