Characterisation of the influence of vanadium and tantalum on yttrium-based nano-oxides in ODS Eurofer steel

J Fu,T.P Davis,A Kumar,I.M Richardson,M.J.M Hermans

doi:10.1016/j.matchar.2021.111072

Abstract

Oxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y2O3 stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y2O3) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters.

Highlights

Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for structural applications in high temperature and nuclear applications including advanced fission and fusion reactors [1]
energy dispersive spectrometer (EDS) analysis indicated that the pre cipitates were rich in Fe, Cr, W and C, which can be identified as M23C6 carbides (M = Fe, Cr and W) based on their size and chemical compo sition
The pinning effect by these nanoparticles on the dislocations is clearly manifested in the micrograph, which could lead to an enhanced creep resistance compared to non-Oxide dispersion strengthened (ODS) steels

Summary

Introduction

Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for structural applications in high temperature and nuclear applications including advanced fission and fusion reactors [1]. By the small addition (0–0.5 wt%) of nanosized yttrium oxide (Y2O3) in the steel matrix, the operating temperature of these RAMF steels can be increased by 100–200 K due to a significant increase in the creep and fatigue properties [2,3,4]. These fine and thermally stable dispersoids hinder the motion of dislocations and pin grain boundaries, and act as effective trapping sites for both point defects and helium atoms generated during irradiation [5]. The nanoclusters in ODS Eurofer were characterised and investigated by means of scanning electron microscopy (SEM), TEM and APT. Providing insights into possible mechanisms of nanocluster formation, this paper will contribute to the optimisation of ODS steels with a desired microstructure and superior mechanical performance for nu clear applications

Materials

SEM and TEM observations

APT analysis

Conclusions