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

Abstract

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 be approximately 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 the Fe_38.4Co_49.7Cr_10.1Ni_1.8 alloys developed in this study. As this value was close to that of the fused silica used for infrared instruments, the developed alloys can be a powerful tool for high precision observation in astronomical telescopes.