Influence of minor alloying elements and stress on microstructural evolution and void swelling of austenitic steels under neutron irradiation

A pressurized tube experiment was carried out in the British Prototype Fast Reactor (PFR) at Dounreay, U.K. in order to determine swelling and stress-induced swelling of four heats of the commercial German austenitic steel DIN 1.4970, with different contents of the minor alloying elements Si, Ti and Ti/C ratio. The maximum doses achieved are 106 dpa(NRT) at 420 degrees C, 81 dpa(NRT) at 500 degrees C and 61 dpa(NRT) at 600 degrees C. The hoop stresses of the pressurized tubes were 0, 60 and 120 MPa at all irradiation temperatures. TEM-examinations have been carried out to determine the influence of the minor alloying elements and the stress on the void swelling and the microstructural evolution. All alloys exhibited the highest swelling values at 420 degrees C and no swelling at 600 degrees C. The measurements show the large effect of the minor alloying elements upon swelling and in-pile creep. The maximum swelling suppression is achieved by a high Si-content and an under-stoichiometric Ti/C ratio (understabilization). The analysis of the microstructural evolution shows that the swelling resistance is correlated to the formation and stability of very fine gamma'-precipitates. For the alloy with the highest swelling resistance these precipitates can be found up to 106 dpa(NRT) at 420 degrees C, whereas they dissolve in the course of irradiation for the alloys with lower swelling resistance. The estimates of the stress-induced swelling using the Soderberg theorem and the length measurements are compared with immersion density measurements and results of void structure analysis by TEM. All methods yield the same order of magnitude for the stress-induced linear swelling.