ASTRONOMICAL+SPIE. TELESCOPES
INSTRUMENTATION
July 17-22 2022 Montréal, Québec

Low thermal expansion at cryogenic temperature inFe39−xCo49+xCr10Ni2 alloy used for astronomical telescopes

Hiromichi T. Fujiia, Naoki Sakaguchia, Kotaro Onaa
Fumihiro Uraguchib, Yutaka Hayanob, d, Toshihiko Yokoyamaca Shinhokoku Material Corporationb National Astronomical Observatory of Japanc Institute of Molecular Scienced Graduate University for Advanced Study (SOKENDAI)

新報国マテリアル株式会社
Shinhokoku Material Corporation

Outline Introduction✓ Observation accuracy in large astronomical telescopes✓ Precise control of thermal expansion

 Experimental procedure Results and discussion✓ Effect of Co content on microstructural evolution✓ Anomalously low thermal expansion at cryogenic temperature✓ Magnetic spin transition of Co atoms

 Conclusions

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Observation accuracy in large astronomical telescopes
 Thirty meter telescope (TMT)✓ New class of extremely large telescope✓ Observation of cosmic objects with
unprecedented sensitivity

 Infrared imaging spectrograph (IRIS)✓ Cooling down to cryogenic temperature
to reduce the thermal noise✓ Reduction of dimensional change down
to cryogenic temperature Low thermal expansion alloys✓ Invar alloy (Fe-36mass%Ni)✓ CTE at cryogenic temperature is still not
low enough for infrared instruments
Thirty meter telescope (TMT) planned for construction in Hawaii(Courtesy of NAOJ)

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Precise control of thermal expansion

 Previous research results*✓ Fe39Co49Cr10Ni2 was found to have controllable
low thermal expansion at cryogenic temperatureby subtle adjustment of chemical composition✓ Dimensional change between 100 K and room
temperature of a developed alloy was matchedwith that of a fused silica

Detailed investigations of magnetic spin transition, which
is origin of low thermal expansion (invar effect), areessential for the further development of low thermalexpansion alloys used at cryogenic temperatures.
A relative thermal expansion curve obtained from the Fe38.4Co49.7Cr10.1Ni1.8 alloy with
precisely controlled CTE between 100 K and room temperature. For comparison, thecurve obtained from the commercial fused silica is also shown.

Shinhokoku Material Corporation* Fujii H.T. et al., Proc. SPIE, 11451, 1145118 (2020).

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Objective

We clarify the magnetic spin transition of Fe and Co atoms in the Fe39Co49Cr10Ni2 alloy
to establish academic foundation for the development of the Fe-based low thermalexpansion alloys used at cryogenic temperature for infrared instruments mounted on
astronomical telescopes.

 Research issues✓ Microstructural characterization of Fe39+xCo49−xCr10Ni2 alloys✓ Effect of Co atom on thermal expansion characteristics at low temperature✓ Origin of low thermal expansion

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Experimental procedure Sample preparation✓ Materials: Fe39−xCo49+xCr10Ni2 (−1.0 ≤ x ≤ 2.3)
✓ Methods: Induction melting, castingand forging✓ Chemical analysis: Titration method

 Microstructural characterization✓ Optical microscopy✓ X-ray diffractometry

 Evaluation of material properties✓ Precise dilatometer with cooling bath
✓ SQUID magnetometer

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Results and discussion7

Effect of Co content on microstructural evolution

 Invar effectMartensite + austenite

Austenite✓ Remarkable in the ferromagnetic Fe fcc
phases with chemical composition near to aboundary between bcc and fcc phases✓ Fe lattice contraction with increasing
temperature caused by spin transition

 Microstructural characteristics✓ Lath martensite: bcc (Dark regions)✓ Residual austenite: fcc (Bright regions)✓ Single austenite: CCo ≥ 49%
Optical micrographs taken from the annealed Fe39−xCo49+xCr10Ni2 (x = −1.0, −0.1, 0.4, 0.7, 1.4
and 2.3) alloys. Dark and bright regions indicate lath martensite and austenite phases,
respectively.

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Quantitative evaluation of fcc phase fraction Evaluation of area fraction✓ Integrated peak intensity ratio between
martensite and austenite phases✓ CCo < 49%: Martensite + austenite✓ 49% ≤ CCo: Austenite

Materials

Single fcc austenite

Area fraction of phases [%]bcc martensite

fcc austenite

Fe39Co51Cr1071

29Fe39Co50Cr10Ni

57

43x = −1.0

3466

x = −0.1

1288

x = 0.70100

Fe39−xCo49+xCr10Ni2

XRD profiles obtained from the annealed Fe39−xCo49+xCr10Ni2 (x = −1.0,−0.1, and 0.7), the Fe39Co51Cr10 and the Fe39Co50Cr10Ni alloys.

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Low thermal expansion caused by invar effect Thermal expansion characteristics✓ The alloys with a lot of bcc phases show normal
thermal expansion.
✓ Thermal expansion of single fcc alloys issuppressed by lattice contraction caused bymagnetic spin transition.

bcc + fcc
Single fcc✓ Average CTE between 100 and 300 K of theFe38.6Co49.4Cr10Ni2 alloy is 0.0 × 10-6/K.

fcc + slight bcc

Thermal expansion curves obtained from the annealed Fe39−xCo49+xCr10Ni2 (x = −1.0,−0.1, 0.4, 0.7, 1.4 and 2.3) specimens. For comparison, the curves of the Fe39Co51Cr10
and the Fe39Co50Cr10Ni alloys are also shown by dotted line.

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Decrease in CTE caused by lattice contraction

 Temperature dependence of CTE✓ The CTEs modestly increase with temperature up
to 150 K and decrease up to around 300 K.
✓ Lattice contraction owing to the magnetic spintransition occurs more significantly above 150 K.
✓ Lattice contraction of Co atoms contributes tothe low thermal expasnion.

Temperature dependence of the CTEs in the Fe39−xCo49+xCr10Ni2 (x = 0.4, 0.7, 1.4
and 2.3) specimens. The CTEs were derived from the temperature derivative of therelative dilation in the measured thermal expansion curves.

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Effect of Co spin transition on thermal expansion

 SQUID measurements✓ The saturation magnetization monotonicallydecreases with increasing temperature.
✓ Decreasing rate of saturation magnetization islarger above 150 K.

 Magnetic spin transition✓ 150 K is the onset temperature of the magnetic
spin transition of Co atoms.

✓ The lattice contraction seems to become large
above 150 K.

Temperature dependence of saturation magnetization of the Fe39Co49Cr10Ni2 alloy measured
by SQUID magnetometer. For comparison, the thermal expansion curve is also shown.

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Magnetic spin transition of Fe and Co atoms

 Temperature dependence of spin ratio✓ HS state ratio was calculated by the PIECP MCsimulations.
✓ The magnetic spin transition of Fe atoms fromthe HS to the LS states occurs at a constant rate.
✓ All Co atoms are HS states below 150 K and thespin transition occurs above 150 K.

Both Fe and Co atoms in the Fe39−xCo49+xCr10Ni2 alloys
show the invar effect, which can be characterized by themagnetic spin transition between 100 and 300 K.
Temperature dependence of high spin ratio of Fe and Co atoms calculate by thePIECP MC simulations.

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13Conclusions

1.

The average CTEs between 100 and 300 K are less than 0.5 × 10-6/K in the chemical composition
around the Fe39Co49Cr10Ni2. The temperature dependence of CTE has been characterized as themodest increase up to 150 K and decrease up to 300 K.

2. The saturation magnetization of the developed alloy decreased with increasing temperature ranging
from 100 to 350 K and its decreasing rate in the temperature range above 150 K is larger than that
below 150 K.
3. It is found for the first time that the magnetic spin transition of Co atoms from the HS state withlarge volume to the LS state with small volume strongly contributes to the low thermal expansion
characteristics.

These are important understandings for the future development of the low thermal expansion alloys
used at cryogenic temperature as the structural components for astronomical telescopes.

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Thank you for your kind attention.

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Hiromichi T. Fujii+81-49-242-1955
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SPIE AS 2022 発表論文題 目： “Low thermal expansion at cryogenic temperature in Fe39−xCo49+xCr10Ni2 alloy used for astronomical telescopes”
著 者： Hiromichi T. Fujii, Naoki Sakaguchi, Kotaro Ona, Fumihiro Uraguchi, Yutaka Hayano, Toshihiko Yokoyama
巻 号： 12188
年： 2022頁： 1218806 (12 pages)
概 要；
Fe39Co49Cr10Ni2 has exhibited ultra-low thermal expansion with its average coefficient of approximately 0.13 × 10−6/K between 100 K and 300
K from cryogenic to room temperatures. This value is extremely close to that of the fused silica used for cameras and spectrographs
mounted on astronomical telescopes. The nature of the low thermal expansion of Fe39−xCo49+xCr10Ni2 has been clarified by the measurementsof the magnetic properties and the numerical simulations based on the path-integral effective-classical-potential theory. The saturation
magnetization is found to decrease with increasing temperature in the range from 100 K to 350 K. The rate of decrease becomes larger attemperatures over 150 K. The numerical calculation has shown that this magnetic behavior is attributed to the electronic state changes fromhigh spin state to low spin state in the Co atoms, which occurs at temperatures over 150 K. The electronic state changes in Co atoms arefound to strongly contribute to the low thermal expansion for the first time in Fe39−xCo49+xCr10Ni2.
DOI: https://doi.org/10.1117/12.2626196

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