Effect of MeV ion irradiation on the coefficient of thermal expansion of Fe-Ni Invar alloys: A combinatorial study

The effect of low-dose MeV ion irradiation on the thermal conductivity and coefficient of thermal expansion (CTE) of Fe-Ni alloys has been studied over the entire composition range using a combinatorial method. With our experimental precision of ±5%, the thermal conductivity of Fe-Ni alloys is unchanged by 2.3 MeV Ar⁺ irradiation at a fluence of 1.2 × 10¹⁴ ions cm^{- 2}. The CTE of Fe-Ni Invar alloys with Ni concentrations between 30 and 37 at.% increases significantly upon 2.3 MeV Ar⁺ irradiation. At the Invar composition of Fe₆₅ Ni₃₅, the CTE increases from 0.5 × 10^{- 6} K^{- 1} before ion irradiation to 4.3 × 10^{- 6} K^{- 1} after irradiation for an ion fluence of 1.2 × 10¹⁴ ions cm^{- 2} or, equivalently, 0.1 displacements per atom. We attribute this increase in CTE to changes in atomic short-range order created by ion irradiation.