Local chemical instabilities in 20Cr 25Ni Nb-stabilised austenitic stainless steel induced by proton irradiation

Abstract

We have assessed the local solute redistribution at defect sinks in 20Cr-25Ni Nb-stabilised austenitic stainless steel after proton irradiation at three temperatures, i.e. 420, 460 and 500°C, up to a maximum damage level of 0.8dpa. This material is currently being used as cladding in advanced gas-cooled reactors (AGR), and potential local Cr depletions would compromise its resistance to intergranular corrosion attack during wet storage of spent fuel elements. Irradiation induces the depletion of Cr, Fe and, to a lesser extent, Mn from grain boundaries, whereas Ni and Si become enriched at those locations. The elemental profiles are symmetric and primarily W-shaped at 420°C, whereas at higher temperatures asymmetric and double-peaked profiles are also detected, most likely as a result of grain boundary migration. High-angle grain boundaries with a misorientation angle 40°become mobile at 460°C and especially at 500°C, and also experience a relatively large solute redistribution, with local Cr contents in a significant number of boundaries falling below 12wt.% and profile widths 100nm. However, coincidence site lattice boundaries (CSL) Σ3 boundaries prove to be resistant to Cr depletion and to boundary mobility. Local elemental patterns at radiationinduced dislocations seem to mimic those at grain boundaries, but do not trigger the formation of Ni3Si precipitates. Additionally, Ni and Si form a shell-like structure around the pre-existing Nb(C,N) precipitates, potentially leading to the transition into G-phase at higher damage levels.

The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding

“…Although the stabilising effect of Nb has been studied in the past [16], the effect of these NbC precipitates on corrosion behaviour is not clear. Recent research on this material in open literature has focussed on irradiation effects [17] [18] [19], thermal processing [20] [21] [22], the use of novel corrosion techniques [23], evaluating the alloy in conditions relevant to a pond storage environment [24] [25] [26] [27] [28], dry storage [29] [30], and final geological disposal [31].…”

Aspects of Applied Chemistry Related to Future Goals of Safety and Efficiency in Materials Development for Nuclear Energy

“…They have also been considered as structural components in innovative nuclear fusion test reactors due to the very good results they have obtained in applications in extreme environments [ 117 , 118 , 119 ]. The 300 series steels (Fe-Cr-Ni alloy) have good corrosion and mechanical properties for applications at high temperatures, making them viable to be applied in aeroengine parts, turbochargers, oil and gas pipelines [ 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 , 131 ], and as structural components in nuclear reactors [ 116 , 117 , 132 , 133 ]. The 304 SS is one of the austenitic steels that is most used in nuclear reactors.…”

The Effect of Replacing Ni with Mn on the Microstructure and Properties of Al2O3-Forming Austenitic Stainless Steels: A Review

“…In addition, Ni is a highly active element. It will undergo transmutation and produce a large amount of He in the neutron irradiation environment, resulting in material swelling, while Mn is relatively stable [ 7 , 8 ].…”