Resonant creep enhancement in austenitic stainless steels due to pulsed irradiation at low doses

Creep responses of austenitic stainless steels to cyclic 10 MeV deuteron irradiation have been examined, for solution-annealed (SA) or 20% cold-worked (CW) 316 stainless steel (SS) and Fe25Ni15Cr alloy. After the strain rate reached a steady state under a long-term continuous irradiation, the irradiation mode was switched to square waveforms (pulse width τP = 10 ms–1000 s). Anomalous large creep enhancement due to the cyclic irradiation was observed at a particular pulse width range around τP = 100 s, even at a low damage rate of 2.0 × 10−7 dpa s−1, whereas the specimen gave zero or negative strain changes with respect to the steady state level at τP < 1 s or τP > 200 s. Consequently, the pulse width dependence of strain change had a resonant feature with a peak around τP = 100 s. The SA-316 SS and SA- and CW-Fe25Ni15Cr alloys exhibited strain increments of about 10−4 for 10−3 dpa, which were larger by one to two orders of magnitude than the steady state creep. The mechanism of the large creep is ascribed to pulse-induced point defect enrichment, by the aid of a rate theory. It was thus demonstrated that first-wall materials in a plasma operation of about 102 s may suffer from unexpected transient creep and that the dynamic aspects for damaging effects on the first-wall materials should be taken into account, especially for fusion devices to be operated at low temperatures such as ITER.