Dose Rate Effects in a Precipitation Hardened Nickel-Aluminum Alloy

A Ni-14 at. % Al alloy containing γ′ (Ni3Al) precipitates was irradiated at 725°C with 2.8-MeV 58Ni+ ions to determine precipitate stability under irradiation. The doses examined were 0.81, 2.5, and 8.1 displacements per atom (dpa) at both of two dose rates: 4.4 x 10-2 dpa/sec and 4.4 x 10-4 dpa/sec. Specimens were examined by transmission electron microscopy. The precipitates in the lower-dose-rate samples were seen to develop complex contrast features after irradiation but to retain their original size. The original precipitates in the higher-dose-rate samples developed a ragged appearance around the edges as if pieces were being dissolved, and a second smaller class of precipitates appeared in the matrix, and by 8.1 dpa the entire original precipitate size distribution had been converted to the new precipitates. These differences due to dose rate are not explained by current models of the behavior of precipitates under irradiation.