Mr. Haruyasu Ohno Profile

Haruyasu Ohno

at Shinhokoku Material Corp

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Proceedings Article | 21 November 2023 Poster + Paper

Achievement of ultimate zero thermal expansion in super invar based alloy for stable optical systems

Hiromichi Fujii, Naoki Sakaguchi, Haruyasu Ohno, Kotaro Ona

Proceedings Volume 12750, 127500T (2023) https://doi.org/10.1117/12.2686329

KEYWORDS: Alloys, Iron, Carbon, Thermal stability, Temperature metrology, Semiconductors, Manufacturing, Lithography, Heat treatments, Cryogenics

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A novel alloy, exhibiting extremely low thermal expansion, high temporal stability and superior Young’s modulus, has been developed based on the super invar (Fe₆₄Ni₃₁Co₅). The average coefficient of thermal expansion (CTE) of the alloy, α_18-28, is found to change with Fe content, C_Fe, and can be approximated by the linear equation of α_18-28 = −0.4789C_Fe + 29.927 around room temperature. This linear approximation suggests that the CTE of the alloy can be regulated to zero near C_Fe of 62.5%, in contrast to the conventional super invar alloy, which is frequently used as the structural support components for stable optical systems and has a CTE of approximately 0.5 × 10⁻⁶ K⁻¹ around room temperature. Based on these findings, we have successfully achieved an extraordinarily low thermal expansion of −0.046 × 10⁻⁶ K⁻¹ average CTE in the temperature range between 18°C and 28°C through precise control of the chemical composition. the carbon content was reduced to below 0.02% to suppress the carbon-dependent expansion over time, which is known to occur in high carbon invar-based alloys. As a result, the dimensional change over the years of our developed cast alloy is less than 0.05 ppm/year. The low Young’s modulus due to the coarse and columnar grains microstructure in the cast alloys can be improved by 30% with a grain refinement technique using cryogenic and recrystallization thermal treatments.

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