Microstructural effect of solute addition for Fe-15Cr-20Ni steels irradiated in Joyo

Full text: The relationship between the addition of minor element and the swelling behavior was investigated using nine heats of Fe-15Cr-20Ni austenitic model alloys, Fe-15Cr-xNi(x = 20, 25 and 30)and a PNC1520 steel irradiated at 480, 570, 620 and 700oC in the Joyo to doses of 20-56 dpa. Phosphorus and boron suppressed the appearance of swelling for these experimental irradiation conditions. Additions of titanium and niobium were effective for suppression of swelling at low temperature, below 570 deg. C. Complicated microstructural dependence of nickel contents for Fe-15Cr-xNi alloys could be seen. It is required that the core materials for fast breeder reactors (FBR) have good mechanical properties at high temperature and high dimensional stability against huge exposure to fast neutrons. Austenitic stainless steels have superior high temperature strength, but less swelling resistance compared with ferritic steels or martensitic stainless steels.The second-generation advanced austenitic steel for near-term application for FBR has been selected as Fe-15Cr-20Ni-0.25Ti-0.1Nb, designated PNC1520 by the Japan Nuclear Cycle Development Institute (JNC). In PNC1520, void swelling resistance and high temperature creep properties were improved by adjusting the amount of cold working and the minor elements such as titanium, niobium, phosphorous, boron. In order to make clear what element is effective in improving the stability of stainless steels, such as irradiation hardening, creep and void swelling under neutron irradiation, it is important to investigate the microstructural evolution of Fe-Cr-Ni alloys after neutron irradiation using model alloys designed to extract the effect of minor element addition. In the previous work, it was shown that the suppression of void swelling behavior could be seen for the PNC1520 alloys irradiated in Joyo and found that the radiation-induced precipitation, especially phosphide was main contribution for void swelling suppression at high temperature during neutron irradiation. In this study, we used a set of Fe-15Cr-20Ni-M (M=B,C,Ti,Nb,P,Si and Mo) alloys to describe the effect of minor element addition on swelling behavior of Fe-15Cr-20Ni base alloys. Nine different 15Cr20Ni-austenitic alloys modified by various minor solute element additions, three different nickel contents of 15Cr-xNi-austenitic alloys and a PNC1520 steel were prepared. The amounts of minor element additions in the modified alloys were based on the content of minor element in PNC1520 steels. The specimen form was that of a 3 mm diameter, 0.2 mm thick TEM disk annealed at 1050 deg. C for 2 or 3 min in a vacuum. Irradiations were conducted in Joyo at 480, 570, 620 and 700 deg. C to doses of 20-56 dpa. Microstructural analysis was performed using a JEOL-2010. Precipitate identification was performed by combined information obtained from diffraction patterns, energy dispersive X-ray analysis (EDX) and two-dimensional ASID-EDX chemical analysis. From microstructural analyses of Fe-15Cr-20Ni-M model alloys, two interesting features were obtained: (1) Si and Ti aggregation around the Frank loops and (2) suppression of void swelling in 0.004B-added model alloy at high temperature. It is well known that the silicon, titanium and phosphorous additions can affect loop/network dislocation behaviors and that the silicon, titanium and niobium additions appear to enhance Frank loop formation in SA alloys. Aggregation of Si and Ti atoms inside the Frank loops and enrichment of Ni atoms around them were observed clearly. However, no Frank loop was observed in 0.1Nb-added alloys in any condition in this work, and large Frank loops were observed in 0.25Ti-added model alloys irradiated below 570 deg. C. That is, titanium plays an important role for nucleation and growth of Frank loops. The void swelling in the 0.004B-added alloy during neutron irradiation was suppressed at high temperatures. However it is interesting that no precipitate, Frank loop nor coarsened dislocation network could be seen in the void-free specimens. A few faint traces of boron carbides were remained in the grain, but all of boron carbides were dissolved. In the ternary alloy of Fe-15Cr-xNi alloy, the amount of swelling for the Fe-15Cr-25Ni alloys was the lowest among them irradiated in the range of 450 to 650 deg. C up to 40 dpa. The void formation was suppressed following by reduction of dislocation density in the ternary alloys.The same tendency was also reported in the previous work, but the reason why 25% of nickel content for Fe-Cr-xNi ternary alloys irradiated at high temperature affects the reduction of void and dislocation formation is not still clear. Detailed information and discussion will be presented in the conference. (author)