Orientation relationship of the fcc-to-bcc transformation in DIN 1.4970 austenitic stainless steels due to dissolution corrosion in liquid Pb-Bi eutectic

摘要

The deployment of Generation-IV lead-cooled fast reactors (Gen-IV LFRs) requires thecompatibility between the candidate structural/fuel cladding steels and the inherently corrosivelead-based alloy coolants. Therefore, understanding the liquid metal corrosion behaviour ofcandidate structural/cladding materials in contact with heavy liquid metal coolants is crucial for thedesign and safe operation of Gen-IV lead-fast reactors (LFRs). The Multipurpose hYbrid ResearchReactor for high-tech Applications (MYRRHA) system, currently under development at SCK•CEN,uses the lead-bismuth eutectic (LBE) alloy as both primary coolant and spallation target. TheMYRRHA candidate steels are: the 316L structural steel and the DIN 1.4970 fuel cladding steel;both steels are austenitic (fcc) stainless steels.At sufficiently high temperatures (T >450 °C) and low LBE oxygen concentrations (CO < 10~(-8)mass%), these steels are susceptible to dissolution corrosion, a particularly undesirable corrosionmechanism that can jeopardize the structural integrity of the reactor components. Dissolutioncorrosion is manifested by the progressive transfer of steel alloying elements to the LBE coolant;dissolution starts by the selective leaching of the highly soluble steel alloying elements (Ni, Mn,Cr), which in turn leads to the ferritization (fcc-to-bcc phase transformation) of the dissolutionaffectedzone due to the removal of the austenite stabilizers (Ni, Mn).In literature, the austenite to ferrite (γ→α) phase transformations have mainly been studied whendepending on the steel alloying and thermal processing of the steels. These transformations canoccur in a displacive manner by a coordinated movement of atoms or through a diffusion-drivenreconstructive transformation mechanism. Specific orientation relationships (ORs) have beenrepeatedly found between the two phases. In steels, for the vast majority of cases, the Kurdjumov-Sachs (K-S) and/or Nishiyama-Wassermann (N-W) ORs have been reported.In this work, the focus is on the less studied austenite-to-ferrite phase transformation, whichoccurs when austenitic stainless steels are exposed to heavy liquid metals (HLMs), such as leadand lead-bismuth eutectic (LBE), under specific exposure conditions (i.e., high temperatures, lowoxygenHLMs). A large statistical population of γ/α phase boundaries was analyzed by electronbackscatter diffraction (EBSD) on LBE-exposed austenitic stainless steels, in order to determinethe dominant OR between several selected ORs (i.e., Bain, K-S, N-W and Pitsch). The Pitschmodel was found to be the dominant orientation relationship, which is unique for γ→α bulktransformations in steels. The unusual nature of the transformation, which involves loss of alloyingelements and the presence of an interfacial liquid metal layer, is discussed to explain this finding.