Microstructure in Martensitic Steel DIN 1.4926 after 800 MeV proton irradiation

Abstract

Two double-wall windows of martensitic steel DIN 1.4926 were irradiated with 800 MeV protons at Los Alamos National Laboratory (LANL) to a total number of charged particles of about 6.3 × 10 22 protons (2.8 Ah) in a temperature range from 50°C to 230°C. The corresponding maximum fluence at beam centre was of about 2.6 × 10 25 p/m 2 . Initial examinations on irradiation induced changes of hardness and microstructure have been performed. The preliminary results of microstructural investigation show that precipitates become amorphous after irradiation. There are no evident changes in precipitate size and compositions. Voids and helium bubbles are not observed. Keywords A06 P11 R02 R03 S06 1 Introduction For spallation neutron sources of a medium or high power (⩾∼1 MW) liquid metal targets are being studied as an alternative to heterogeneously cooled solid metal targets [1] . As there is very little experience on the effects of high energy proton irradiation on mechanical properties of materials, the suitability of austenitic steels and martensitic/ferritic steels as materials for the liquid metal container is being examined based on results of neutron irradiation [2–4] . High energy proton irradiation differs from neutron irradiation in such matters as: (1) a recoil spectrum of much higher energy [5] ; (2) high helium and hydrogen production rates [6, 7] ; and (3) high transmutation rates [7] . Investigation of the behaviour of materials in a realistic spallation radiation environments is therefore essential. The Pb–Bi liquid metal target concept for the Swiss spallation neutron source (SINQ) at the Paul Scherrer Institut (PSI) dates back to the 1980s [8] . Martensitic stainless steel was chosen for both the Pb–Bi container and the safety window. In order to study the behaviour of the safety window under high energy proton irradiation, two full scale windows were manufactured by PSI and irradiated at LANL [9] . Now the windows are being investigated jointly by Forschungszentrum Jülich (FZJ), PSI and LANL. In the present paper, first results on microstructural changes will be presented. 2 Experimental The windows were manufactured from the martensitic steel, DIN 1.4926. The composition of this material is in wt%: Fe + 0.20C, 0.36Si, 0.46Mn, 0.019P, 0.006S, 10.5Cr, 0.9Mo, 0.64Ni, 0.26V, 0.009W, ∼300 nm) show that the precipitates are rich in Cr (45–65 wt%), and Fe (25–35 wt%), with low Mo, V, Mn, Ni and Si contents. The precipitates in the non-irradiated material are mainly M 23 C 6 type, which is in agreement with observations on 9–12% Cr martensitic steels [10, 11] . 3.2 Microstructures in irradiated material There are no evident changes in the appearance of the precipitate structure after irradiation, as can be seen in Fig. 2 which shows an example observed after a dose corresponding to 3.4 dpa. EDX analysis shows that there is no significant change in compositions of the precipitates as compared to those in unirradiated samples. However, the crystal structure of these precipitates has changed significantly: from crystalline to amorphous . The observations on samples after 3.4 and 6.2 dpa are essentially the same. Fig. 3 shows a micrograph from a sample of 3.4 dpa. Diffraction patterns taken at two large precipitates in the figure are the same, showing only rings, as illustrated at the upper right corner. This shows that these precipitates are completely amorphous. Although nanoprobe technique has not been used, rings could also be clearly seen in the diffraction pattern taken from small precipitates. A dark field image, presented in Fig. 4 , indicates that the small precipitates, like the larger ones, are amorphous. The amorphous transformation starts at low doses. In the samples of 0.39 dpa the transformation has started but not come to completion. This can be seen in Fig. 5 . Diffraction pattern 1 was taken from the precipitate indicated by an arrow, and consisted of both spots and rings, as shown in the upper part of Fig. 5 (a). The spot pattern demonstrates a fcc crystalline structure, which means the spots are diffracted from the M 23 C 6 precipitate [12] . The image shown at the lower right corner of Fig. 5 (a) is formed with the diffraction spot marked by an arrow in diffraction pattern 1. The bright image of the precipitate on the dark background indicates the crystalline structure of the precipitate. On the other hand, the rings, with the same diameter as that shown in Fig. 3 (camera length is different for the two cases), show the amorphous structure formed in the precipitate. In Fig. 5 (b), the dark field image is obtained with the diffraction spot from the matrix indicated in diffraction pattern 2 at the lower left corner of Fig. 5 (a), where the dark image of the precipitates also implies that the precipitate is amorphous. This preliminary investigation shows that dislocation structures in non-irradiated material have changed substantially following irradiation, which can be seen in Fig. 2 . Small dislocation loops produced by irradiation are observed, as shown in Fig. 6 taken from a sample of 6.2 dpa. There are no helium bubbles or voids observed although the calculated He concentration is as high as 600 appm. 4 Discussion The main result found in this study is a crystalline to amorphous transformation of M 23 C 6 precipitates in martensitic steel DIN 1.4926 irradiated with 800 MeV protons at temperatures of about 230°C and below. The amorphization can be already observed at a dose as low as 0.39 dpa and appears to be complete at 3.4 dpa. At the lower dose of 0.39 dpa, the amorphous structure is found coexisting with crystalline structure in the same precipitate. The changes in precipitate structures in martensitic steels with 8–12% Cr induced by neutron [10, 11, 13] and ion (of low energy) [14, 15] irradiation at temperatures above 300°C have been well studied. There, new...