Phase Stability in Fe-15 Ni-x Cr Alloys Under Ion or Electron Irradiation

Three high-purity Fe-Ni-Cr alloys have been irradiated with heavy ions and with electrons at 450, 550, and 650°C to doses up to 60 dpa. The heavy ion irradiations were carried out with Ni6+ ions, and specimens were irradiated both with and without helium preinjection. The electron irradiations were carried out in a high-voltage electron microscope (HVEM) on specimens without helium preinjection. The alloy compositions were Fe-15 Ni-10 Cr, Fe-15 Ni-16 Cr, and Fe-15 Ni-20 Cr, all with low carbon levels. The latter two of these compositions are austenitic at room temperature, whereas the first can include martensite on rapid cooling. According to the computer calculated phase diagrams of Watkins et al, all of these alloy compositions would be metastable at the lowest irradiation temperature in the present study, but the austenitic phase structure should be stable in all of the alloys at the highest irradiation temperature.Voids were formed in all of these alloys during irradiation at 650°C to 30 dpa, and swelling levels at that dose were independent of the presence or absence of preinjected helium. The more interesting irradiation behavior consisted of irradiation induced phase transformations, predominantly during irradiation at 450°C. The Fe-15 Ni-16 Cr alloy precipitated a fine distribution of M23C6 after 30 dpa at 450°C under heavy ion irradiation. Similar precipitates were not found in either of the two other alloys under similar irradiation conditions. The formation of these precipitates in a low carbon alloy was surprising. The precipitates were nucleated uniformly and were not associated with dislocation loops or other microstructural features.The more striking irradiation induced phase transformation was the evolution of austenitic regions in the martensitic laths in the Fe-15 Ni-10 Cr alloy during irradiation at 450°C. This martensitic reversion reaction seems to involve a mechanism associated with dislocation loop formation. The martensitic reversion reaction was observed for materials irradiated both by HVEM and by heavy ions. The austenitic precipitates were stable and grew with irradiation at 450°C. They were found to nucleate in all orientations consistent with the Nishiyama-Wassermann relation in the body centered cubic matrix. This paper develops an explanation of this behavior based on dislocation loop morphology and relative phase stability.