Structural and chemical evolution in neutron irradiated and helium-injected ferritic ODS PM2000 alloy

Abstract

An investigation of the influence of helium on damage evolution under neutron irradiation of an 11 at% Al, 19 at% Cr ODS ferritic PM2000 alloy was carried out in the High Flux Isotope Reactor (HFIR) using a novel in situ helium injection (ISHI) technique. Helium was injected into adjacent TEM discs from thermal neutron ⁵⁸Ni(n_th,γ) ⁵⁹Ni(n_th,α) reactions in a thin NiAl layer. The PM2000 undergoes concurrent displacement damage from the high-energy neutrons. The ISHI technique allows direct comparisons of regions with and without high concentrations of helium since only the side coated with the NiAl experiences helium injection. The corresponding microstructural and microchemical evolutions were characterized using both conventional and scanning transmission electron microscopy techniques. The evolutions observed include formation of dislocation loops and associated helium bubbles, precipitation of a variety of phases, amorphization of the Al₂YO₃ oxides (which also variously contained internal voids), and several manifestations of solute segregation. Notably, high concentrations of helium had a significant effect on many of these diverse phenomena. These results on PM2000 are compared and contrasted to the evolution of so-called nanostructured ferritic alloys (NFA).

Introduction

Irradiation of structural materials with energetic neutrons at elevated temperatures results in structural and chemical changes beginning at the nanoscale that degrade their performance sustaining properties [1], [2], [3], [4]. Prediction of material performance in intense irradiation environments, such as those experienced by a fusion first wall or in fission reactor core components, requires an understanding of events that occur over an enormous range of spatial and temporal scales. This poses a significant materials challenge, both from the perspective of conducting meaningful experiments in suitable timeframes, as well as developing predictive models. Neutron-induced displacement damage produces isolated self-interstitials, vacancies and clusters of these defects that interact to yield an evolving microstructure, and in many cases leads to substantial material degradation over the operating service lifetime. Key challenges that can arise due to the structural and chemical changes that occur under neutron irradiation include loss of ductility and fracture toughness, large increases in yield strength, dimensional instabilities due to swelling and irradiation creep, changes in deformation behavior as well as thermal and electrical conductivity. However, unlike the fission environment, a fusion first wall suffers from high-energy neutron n,α reactions that generate large concentrations of helium at rates of 10 appm He/dpa, where dpa is displacements per atom.

Target end-of-life dpa levels correspond to thousands of appm of helium. Helium is an undesirable gaseous transmutant atom that first precipitates as pressurized, nanometer sized gas bubbles that initially grow only with the addition of helium. Yet, upon reaching a critical size of approximately 3 nm, the bubbles can transition into faceted voids that grow unstably via a bias driven flux of excess vacancies without the need for additional helium [1], [5], [6], [7], [8]. While the helium bubbles can themselves lead to substantial degradation of the mechanical properties, the transition to voids and resulting void swelling can lead to dimensional changes that further compromise structural integrity. To address these issues, microstructural design of new iron-based alloys is being pursued to control the large helium inventory. The ultimate goal is twofold, 1) limit the ability of helium to form bubbles that can reach the critical size for unstable void growth, and 2) minimize the diffusion of helium to grain boundaries where it can eventually lead to various manifestations of embrittlement.

Oxide-dispersion-strengthened (ODS) ferritic alloys have emerged as an attractive class of structural materials for both advanced nuclear fission and fusion applications. In particular, the nanostructured ferritic alloy (NFA) variants manifest a combination of high tensile, creep, and fatigue strengths, coupled with a high resistance to the effects of irradiation [4], [6], [7]. This is achieved because the NFAs contain an ultrahigh number density (>10²³/m³) of 1–5 nm Y-Ti-O oxides, have fine grain sizes on the order of 1–2 μm, and a high dislocation density.

PM2000 is a different ODS ferritic alloy that contains a higher Cr concentration compared to NFA, and has Al added to improve its corrosion resistance. This mechanically alloyed FeCrAl alloy also has ∼0.2 wt% Y₂O₃, but during thermomechanical processing this oxide phase is converted to yield a much different distribution of ODS particles than found in NFAs. Mechanical alloying of the starting powders and hot consolidation leads to a dispersion of Y-Al-O oxide precipitates with an average diameter of 20–30 nm at a density of ∼10²¹ m⁻³, considerably more coarse than NFA [[8], [9], [10], [11]]. This coarser distribution of oxide particles is less effective in stabilizing both small grain sizes and high dislocation densities and gives overall lower strength as compared to NFAs. The advantage of PM2000 is that the Al addition imparts better corrosion and oxidation resistance due to a durable passivating Al₂O₃ surface scale, which supports its consideration for applications such as lead-bismuth eutectic cooled advanced fission reactors and as accident tolerant fuel cladding for commercial light water reactors [12], [13], [14]. The relative irradiation tolerance of this latter class of ODS ferritic stainless steels has yet to be systematically explored, especially in the presence of high levels of helium. While the Al content precludes PM2000 from being considered as a reduced activation alloy and thus as a candidate for fusion first wall applications, the different grain structure and ODS distribution does provide a useful basis for comparison of dpa and He effects in the NFAs versus ODS steels.

Limited studies of the microstructural response of PM2000 to self-ion and α particle irradiation have been reported [15], [16], [17], [18], [19], [20], [21], [22], but there is little to no data on neutron-irradiation effects. Fields et al. [23] and Edmondson et al. [24] reported on the microstructural response of four FeCrAl alloys irradiated with neutrons at 593 K, but these were not ODS alloys. The self-ion and α-particle irradiations suggest that the Y-Al-O precipitates are very stable under charged particle irradiation, but can trap helium that leads to bubble and void formation on matrix/particle interfaces. One of these studies also found evidence of a strong association between helium bubbles and dislocation loops that formed when PM2000 was implanted with α particles under uniaxial stress [20].

To properly study the response of PM2000 under fusion first wall conditions requires specially designed neutron irradiation experiments. An in situ helium injection (ISHI) technique [25], [26], [27], [28] was used to produce simultaneous helium and displacement (dpa) damage in PM2000 in mixed fast-thermal spectrum irradiations in the High Flux Isotope Reactor. We present the first results obtained for PM2000 that explore the microchemical and microstructural changes that occur both with and without concurrent helium injection.