Comparison of swelling and irradiation creep behavior of fcc-austenitic and bcc-ferritic/martensitic alloys at high neutron exposure

It is well-known that ferritic and ferritic/martensitic steels develop much less swelling than austenitic steels during neutron or charged particle irradiation. The prevailing assumption is usually that the steady-state swelling rate of bcc steels is inherently much lower than that of fcc steels, at least a factor of ten or more. It is shown in this paper that this perception is incorrect, with bcc steels having steady-state swelling rates perhaps only a factor two to four lower. It is thought to be significant that the creep compliance of the two types of alloys also differs only by a factor of about two. The lower swelling observed in bcc steels relative to fcc steels is shown in this paper to be primarily a consequence of much longer transient regimes prior to the onset of steady-state swelling. Several other commonly held perceptions concerning the swelling of both bcc and fcc steels are examined in this paper and also shown to require revision. These involve the effect of cold-work on swelling, the extent of the temperature regime of swelling, and the possibility that swelling might saturate eventually. It also appears that the use of well-controlled in-reactor materials tests that employ active temperature control tend to yield significantly lower values of void swelling compared to that obtained under more representative conditions typical of actual reactor operation. The role of previously ignored differences in displacement rate to determine the duration of the transient regime of void swelling is also shown to require reevaluation, especially for bcc steels.