Evolution of microstructure in advanced ferritic-martensitic steels under irradiation: the origin of low temperature radiation embrittlement

Abstract

Advanced reduced activation ferritic/martensitic steels and oxide dispersion-strengthened steels exhibit significant radiation embrittlement under low temperature neutron irradiation. In this study we focused on atom probe tomography (APT) of Eurofer97 and ODS Eurofer steels irradiated with neutrons and heavy ions at low temperatures. Previous TEM studies revealed dislocation loops in the neutron-irradiated f\m steels. At the same time, our APT showed early stages of solid solution decomposition. High density (10 24 m -3 ) of ~3-5 nm clusters enriched in chromium, manganese, and silicon atoms were found in Eurofer 97 irradiated in BOR-60 reactor to 32 dpa at 332°C. In this steel irradiated with Fe ions up to the dose of 24 dpa, pair correlation functions calculated using APT data showed the presence of Cr-enriched pre-phases.APT study of ODS Eurofer found a significant change in the nanocluster composition after neutron irradiation to 32 dpa at 330 °C and an increase in cluster number density. APT of ODS steels irradiated with Fe ions at low temperatures revealed similar changes in nanoclusters.These results suggest that irradiation-induced nucleation and evolution of very small precipitates may be the origin of low temperature radiation embrittlement of f\m steels.

Atom probe characterisation of segregation driven Cu and Mn–Ni–Si co-precipitation in neutron irradiated T91 tempered-martensitic steel

“…Our analyses highlight the parallels that can be drawn between the much more limited database on MNSPs in neutron irradiated for Fe-Cr alloy systems, including model binary alloys and 9-12%Cr steels like T91 and HT9, with the much more extensive literature on precipitate evolution in RPV steels [16,41,52,[55][56][57]59,42,43,[46][47][48][49][50][51]. Notably, precipitates in some very low Ni RPV steels have compositions that fall near the Si(Mn,Ni) phase field.…”

“…where the subscript s denotes that the solutes are in solution. Thus, the reaction solute product is SP = {[XNi] 16 [XMn] 6 [XSi] 7 } 1/29 , where Xi are the mole fractions of the dissolved Ni, Mn and Si. The equilibrium solute product (SPe), or phase boundary, at 377°C is ≈ 5x10 -3 [41].…”

“…MNSPs also develop in low Cu steels, although even trace amounts of Cu are known to be potent catalysts for their formation [53]. The first experimental proof of MNSP formation in irradiated RPV steels was reported in 2004 [54]; and, since then, MNSPs have been widely observed [16,47,50,51,[55][56][57][58]. Notably, CRPs and MNSPs are well predicted by thermodynamic and kinetic modelling [45,46,59].…”

Nanocluster evolution and mechanical properties of ion irradiated T91 ferritic-martensitic steel

“…Microstructural and microchemical evolution is driven by neutron irradiation [2]. Characteristic microstructural features that result from neutron irradiation of T91 steel are the formation of Mn, Ni and Si -rich precipitates (MNSP) [3][4][5][6][7][8] (also known as G-phase Mn6Ni16Si7), Cr-rich alpha prime (α') formation [4,7,[9][10][11], voids [12] and solute segregation towards dislocations [13][14][15]. These features are a resultant of radiation-induced precipitation and radiation-enhanced diffusion mechanism [16].…”

“…Therefore, it then naturally follows to investigate the clustering dependence on irradiation dose on such a commercially used steel. Refs [3,30] did not include any Mn alloying addition and it is known for T91 steel that Mn clusters with Ni and Si [3][4][5][6][7][8] to form MNSPs [3][4][5][6][7][8]32,33]. In terms of specific issues in commercial Fe-Cr systems requiring greater examination, the role of Mn, Ni, Si and P in T91 steel has not been studied at single specific temperature across a range of ion irradiation doses to understand the role of each solute and impurity atoms have on the evolution of the microstructure.…”