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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, α18-28, is found to change with Fe content, CFe, and can be approximated by the linear equation of α18-28 = −0.4789CFe + 29.927 around room temperature. This linear approximation suggests that the CTE of the alloy can be regulated to zero near CFe 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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