ASTRONOMICAL+SPIE. TELESCOPES
INSTRUMENTATION
December 14-18 2020 Digital Forum

Precise control of negative thermal expansion instainless invar type alloy for astronomical telescopes

Hiromichi T. Fujii1, Naoki Sakaguchi1, Kotaro Ona1
Yutaka Hayano2, Fumihiro Uraguchi2
1 Shinhokoku Steel Corporation

2 National Astronomical Observatory of Japan

新報国製鉄株式会社
SHINHOKOKU STEEL CORPORATION

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

 Infrared imaging spectrograph (IRIS)✓ Cooling down to cryogenic temperature
to reduce the thermal noise✓ Extremely low thermal expansion of thestructural components is essential

 Low thermal expansion alloys✓ Invar alloy (Fe-36mass%Ni)✓ CTE at cryogenic temperature is not low
enough for infrared instruments
Thirty meter telescope (TMT) planned for construction in Hawaii(Courtesy of NAOJ)

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Low thermal expansion alloys for cryogenic temperature use
 Conventional invar alloy (Fe-36%Ni)✓ Low thermal expansion around roomtemperature✓ Not sufficient low CTE at cryogenic
temperature as a structural material for IRIS

 Stainless invar alloy (Fe-52%Co-10%Cr)✓ Dimensional stability between cryogenic androom temperatures✓ Adjustable CTE close to fused silica Negative thermal expansion (NTE)✓ Occurred between 200 and 340 K in stainlessinvar alloy✓ Control of NTE could significantly reduce thedimensional difference to fused silica

Relative thermal expansion curves to a dimension at 100 K of the Fe66Ni34
and the Fe37.8Co51.8Cr10.4 alloys. For comparison, a curve of the measured
commercial fused silica is also shown.
* Chikazumi, S.: J. Mag. Mag. Mater., 10, 113-119 (1979).
** Masumoto, H.: J. Jap. Inst. Met., 2, 141-146 (1938).

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Optimum chemical composition of stainless invar alloys

 Microstructure✓ Single face centered cubic (fcc: γ) phase✓ Ferromagnetic phase Optimum chemical composition✓ Co 52%, Fe 38%, Cr 10%✓ Minimum CTE region lies along theboundary between α and γ phases

 Precise control of thermal expansion✓ Accurate understanding of the effect ofadditional elements and microstructure on
the thermal expansion characteristics
Ternary phase diagram of Fe-Co-Cr alloy and the coefficients of thermal
expansion at room temperature; Solid lines after Masumoto and dottedlines after Kӧster.

SHINHOKOKU STEEL CORPORATION* Masumoto, H.: J. Jap. Inst. Met., 2, 141-146 (1938).
** Kӧster, W.: Arch. Eisenhütt., 5, 17-23 (1932).
*** Saito, H., Fujimori, H. and Saito, T.: Trans. JIM, 11, 68-71 (1970).

4Objective

To obtain comprehensive knowledge on the negative thermal expansion characteristics
in Fe-Co-Cr based alloys to develop a novel structural material for IRIS.

 Research issues✓ Effect of Ni addition on thermal expansion behavior✓ Microstructural characterization of Fe-Co-Cr based alloys✓ Effect of Cr element on negative thermal expansion at low temperature✓ Origin of thermal expansion characteristics in Fe-Co-Cr based alloys

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Experimental procedure Sample preparation✓ Material: Fe39Co51-xCr9Nix (x = 0, 1, 2)✓ Methods: Induction melting, casting
and/or forging✓ Chemical analysis: Titration method

 Microstructural characterization✓ Optical microscopy✓ X-ray diffractometry

 Evaluation of material properties✓ Precise measurement of thermal expansion

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Results and discussion7

Effect of Ni element on thermal expansion behavior

 Drastic decrease of CTE✓ Relative thermal expansion decreased
with increasing Ni content✓ CTE (100 ~ 200 K): 0.93 x 10-6 /K✓ CTE (200 ~ 300 K): -0.71 x 10-6 /K

 Effect of Ni addition✓ One of the austenite (γ phase)stabilizing elements✓ Increase the ferromagnetic γ phase withmuch lower CTE

Relative thermal expansion curves to a dimension at 100 K obtained from theannealed Fe39Co51-xCr10Nix (x = 0, 1, 2) specimens. For comparison, a relative
thermal expansion curve of pure Fe is also shown by dotted line.

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Effect of Ni element on microstructure

Optical micrographs taken from the annealed Fe39Co51-xCr10Nix (x = 0, 1, 2) specimens. The dark and bright regions in the micrographs of the Fe39Co51Cr10 and theFe39Co50Cr10Ni specimens correspond to bcc α and fcc γ phases, respectively.

 Mixed microstructure Single microstructure✓ Both bcc α and fcc γ phases wereobserved by the contrast of dark
and bright regions

SHINHOKOKU STEEL CORPORATION✓ Many linear twin boundaries
formed on fcc γ phase wereobserved

9Crystallographic microstructure

Materials

Area fraction [%]bcc α phase

fcc γ phase

Fe39Co51Cr10

71.4

28.6

Fe39Co50Cr10Ni

57.1

42.9

Fe39Co49Cr10Ni20100

 Stabilization of fcc phase✓ Ni addition of 2% eliminated bcc phase✓ Full fcc phase microstructure is essential
to obtain low thermal expansion alloy inFe39Co51-xCr10Nix alloy

XRD profiles obtained from the annealed Fe39Co51-xCr10Nix (x = 0, 1, 2) specimens.
Circles and squares indicate the diffraction peaks from fcc γ and bcc α phases,respectively.

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10Negative thermal expansion caused by Cr element

 Invar effect✓ Thermal excitation from high spin tolow spin states with temperature rise
✓ High spin state: Ferromagnetic andhigh volume✓ Low spin state: Antiferromagnetic andlow volume

 Effect of Cr on thermal expansion✓ Antiferromagnetic element✓ Acceleration of thermal excitation tolow spin state between 200 and 340 K→ Negative thermal expansion
Relative thermal expansion curves to dimensions at 100 K and 150 K obtained
from the annealed Fe39Co49Cr10Ni2 and the Cr94Fe5Sn specimens, respectively.
The curve of the Fe66Ni34 is also shown for comparison.

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Origin of negative thermal expansion

 Natural thermal expansion✓ Anharmonic effects of lattice oscillation(curve c)✓ All atoms are kept as only high spin state Invar effect✓ Thermal excitation from high spin to low spinstates with contraction✓ Cancellation of natural thermal expansion bymagnetic contraction (curve b)

 Thermal expansion in stainless invar alloy✓ Negative thermal expansion caused byacceleration of thermal excitation from highspin to low spin states (curve a)

Potential energy curves predicted from Morse’s equation in Fe atoms of high spin and low
spin states. Inter-atomic distances at some temperatures in the stainless invar type alloy,
the invar type alloy and the natural thermal expansion are shown by circles.

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Precise control of thermal expansion

 Control of thermal expansion✓ Precise adjustment of chemical composition
based on the experimental results✓ Dimensional change between 100 K androom temperature of a stainless invar alloy
was matched with that of a fused silica✓ Average CTE between 100 K and room
temperature was approximately 0.13 x 10-6 /K

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.

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13Conclusions

1.

The average CTE between cryogenic and room temperatures drastically decreased by the addition of
2% Ni in the Fe39Co51-xCr10Nix alloy.

2. Antiferromagnetic Cr elements included in the Ni-added stainless invar alloys would accelerate
thermal excitation of the electrons from ferromagnetic high spin state to antiferromagnetic low spinstate.
3. The desired dimensional change including zero between cryogenic and room temperatures can beobtained by precise control of negative thermal expansion in the Ni-added stainless invar alloys.

Observation accuracy of infrared instrument mounted on astronomical telescope could be drastically
improved by the structural components made by the Ni-added stainless invar alloys.

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14Thank you for your kind attention.

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Hiromichi T. Fujii+81-49-242-1955

fujii@shst.co.jp
www.shst.co.jp

SPIE AS 2020 発表論文

題 目： “Precise control of negative thermal expansion in stainless invar type alloy for astronomical telescopes”
著 者： Hiromichi T. Fujii, Naoki Sakaguchi, Kotaro Ona, Yutaka Hayano, Fumihiro Uraguchi
巻 号： 11451
年： 2020頁： 1145118 (10 pages)
概 要；
Thermal expansion characteristics of Fe39Co51Cr10 based stainless invar alloys were investigated with the aim of developing a novel structural
material used at cryogenic temperature for infrared instruments mounted on astronomical telescopes. Ni-added stainless invar type alloys
Fe39Co49Cr10Ni2 were found to exhibit anomalous thermal expansion behavior in the low temperature range between 100 K and room
temperature. According to the measured thermal expansion curves, these alloys expanded with increasing temperature from approximately
100 K to 200 K and contracted from 200 K to 300 K. The coefficients of thermal expansion in each temperature range were estimated to beapproximately 0.93 x 10-6 /K and −0.71 x 10-6 /K, respectively. As the results of thermal expansion measurements of Cr based alloys, similar
negative thermal expansion behavior was observed in the same temperature range between 200 K and 300 K. These findings suggest that
the Cr element plays a key role in the negative thermal expansion in the Fe39Co49Cr10Ni2 alloys. We concluded that the dimensional change
between 100 K and room temperature could be precisely controlled in the extremely low thermal expansion range by controlling the negative
thermal expansion. The coefficient of thermal expansion between 100 K and 300 K was achieved to be approximately 0.13 x 10 -6 /K in theFe38.4Co49.7Cr10.1Ni1.8 alloys developed in this study. As this value was close to that of the fused silica used for infrared instruments, thedeveloped alloys can be a powerful tool for high precision observation in astronomical telescopes.
DOI: https://doi.org/10.1117/12.2561193

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