Void swelling and radiation-induced phase transformation in high purity Fe-Ni-Cr alloys

Three high purity Fe-Ni-Cr alloys were irradiated with high energy Ni ions and with high energy electrons over a temperature range of 450 to 650°C to doses up to 30 displacements per atom (dpa) for heavy ions and 60 dpa for electrons. The alloy compositions were Fe-10Cr-15Ni, Fe-16Cr-15Ni, and Fe-20Cr-15Ni with very low carbon levels. These alloy compositions were chosen to bracket a range of compositions where large differences in void swelling are seen, and where alloy phase instabilities are predicted at lower temperatures. The results of transmission electron microscopy (TEM) of irradiated specimens showed that none of the phase transformations predicted by computed phase diagrams occurred during irradiation. In one case, M₂₃C₆ was formed under irradiation. A more significant finding was a martensitic reversion reaction which occurred under irradiation of ferritic regions of the lOCr alloy at 450 and 550°C. This reaction was found for both heavy ion and electron irradiations. The void swelling behavior of these alloys was examined in detail. Voids were found only after irradiation at 650°C for the heavy ion cases, but all irradiation temperatures with electrons. The amount of swelling was found to be dependent on composition, but the HVEM studies indicate that these differences may be due to differences in incubation doses between alloy compositions.