Takuya Yamamoto

Tensile test results at 25 and 300 °C on five 9-12Cr tempered martensitic steels and one 14Cr oxide dispersion strengthened alloy, that were side-by side irradiated to 6.5 dpa at 295 °C in the Advanced Test Reactor (ATR), are reported. The engineering stress–strain curves are analyzed to provide true stress–strain constitutive σ(ε) laws for all of these alloys. In the irradiated condition, the σ(ε) fall into categories of: strain softening, nearly perfectly plastic and strain hardening. Increases in yield stress (Δσy) and reductions in uniform strain ductility (eu) are observed, where the latter can be understood in terms of the alloy's σ(ε) behavior. Increases in the average σ(ε) in the range of 0–10% strain are smaller than the corresponding Δσy, and vary more from alloy to alloy. The data are also analyzed to establish relations between Δσy and coupled changes in the ultimate stresses as well as the effects of both test temperature and the unirradiated yield stress (σyu). The latter shows that higher σyu correlates with lower Δσy. In five out of six cases the effects of irradiation are generally consistent with previous observations on these alloys. However, the particular heat of the 12Cr HT-9 tempered martensitic steel in this study has a much higher eu than observed for earlier heats. The reasons for this improved behavior are not understood and may be microstructural in origin. However, it is noted that the new heat of HT-9, which was procured under modern quality assurance standards, has lower interstitial nitrogen than previous heats. Notably lower interstitial solute contents correlate with improved ductility and homogenous deformation in broadly similar steels.

A series of Fe-Cr alloys, with 3 at.% - 18 at.% Cr, was irradiated in the Advanced Test Reactor (ATR) at the Idaho National Labs (INL), USA, up to a dose of ~6.7 dpa at a temperature of ~456 ◦C. Transmission electron microscopy (TEM) samples were extracted using a focused ion beam (FIB) instrument, and the resulting microstructural defects, such as voids, dislocation loops, network dislocations, Cr rich precipitates, etc., were characterized using a TEM. It was found that the size and number density of these defects varied widely over the different alloys with varying Cr content. As expected, there were no Cr rich precipitates in samples with Cr up to 9 %, and they started appearing only in samples with 12 at.% Cr and above. The particle size decreased from about 15 nm at 12 % Cr to 8 nm at 18 % Cr, while the number density increased from ~7e20 /m3 to 6e22 /m3 for the same Cr contents. Grain boundary segregation of Cr, along with a precipitate free zone, was observed in the cases where a boundary was present in the sample. Large voids (>1–2 nm) were almost invisible in the Fe-3at.% Cr sample, while the average void size remained almost constant between 15 and 18 nm for samples with 6–15 at.% Cr and increased slightly at 18 at.% Cr to ~22 nm. Fe-3 %Cr showed a high density of small voids(<2 nm), estimated to be about 1e23/m3. The number density of large voids increased from ~0 at 3 % Cr to a peak of ~6.4e20 /m3 at 12 % Cr, then decreased to about 1.1e20 /m3 at 18 % Cr. The dislocation loops, which appeared in linear arrays in the Fe-9 %Cr sample, were analysed in detail using both invisibility criteria and image simulations using the Oxford University TEMACI software, and it was found that they are most likely to be ½ 〈111〉 type loops on {100} planes. These loops seem shaped like sections of helices in some places and are most likely formed by loops near screw dislocations climbing into a helix and then collapsing into a loop array. Similar dislocation analysis was performed in the other samples as well wherever feasible, and it was found that there is a mixture of ½ 〈111〉 and [100] dislocation loops.

Formation of a high density of Mn–Ni–Si nanoscale precipitates in irradiated Cu-free and Cu-bearing reactor pressure vessel steels could lead to severe unexpected embrittlement. Models long ago predicted that these precipitates, which are not treated in current embrittlement prediction models, would emerge only at high fluence. However, the mechanisms and variables that control Mn–Ni–Si precipitate formation, and their detailed characteristics, have not been well understood. High flux irradiations of six steels with systematic variations in Cu and Ni contents were carried out at ~295 °C to high and very high neutron fluences of ~1.3 × 1020 and ~1.1 × 1021 n cm-2. Atom probe tomography shows that significant mole fractions of Mn–Ni–Si-dominated precipitates form in the Cu-bearing steels at ~1.3 × 1020 n cm-2, while they are only beginning to develop in Cu-free steels. However, large mole fractions of these precipitates, far in excess of those found in previous studies, are observed at 1.1 × 1021 n cm-2 at all Cu contents. At the highest fluence, the precipitate mole fractions primarily depend on the alloy Ni, rather than Cu, content. The Mn–Ni–Si precipitates lead to very large increases in measured hardness, corresponding to yield strength elevations of up to almost 700 MPa.

Ferritic-martensitic steels are leading candidates for many nuclear energy applications. However, formation of nanoscale a' precipitates during thermal aging at temperatures above 450?°C, or during neutron irradiation at lower temperatures, makes these Fe-Cr steels susceptible to embrittlement. To complement the existing literature, a series of Fe-9 to 18 Cr alloys were neutron-irradiated at temperatures between 320 and 455?°C up to doses of 20 dpa. In addition, post-irradiation annealing treatments at 500 and 600?°C were performed on a neutron-irradiated Fe-18 Cr alloy to validate the a-a' phase boundary. The microstructures were characterized using atom probe tomography and the results were analyzed in light of the existing literature. Under neutron irradiation and thermal annealing, the measured a' concentrations ranged from ~81 to 96?at.% Cr, as influenced by temperature, precipitate size, technique artifacts, and, possibly, cascade ballistic mixing.

Nuclear reactor lifetimes may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation (NI) of pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases (embrittlement). Physical models of embrittlement must be based on characterization of precipitation as a function of the combination of metallurgical and irradiation variables. Here we focus on rapid and convenient charged particle irradiations (CPI) to both: a) compare to precipitates formed in NI; and, b) use CPI to efficiently explore precipitation in steels with a very wide range of compositions. Atom probe tomography (APT) comparisons show NI and CPI for similar bulk steel solute contents yield nearly the same precipitate compositions, albeit with some differences in their number density, size and volume fraction (f) dose (dpa) dependence. However, the overall precipitate evolutions are very similar. Advanced high Ni (>3 wt%) RPV steels, with superior unirradiated properties, were also investigated at high CPI dpa. For typical Mn contents, MNSPs have Ni16Mn6Si7 or Ni3Mn2Si phase type compositions, with f values that are close to the equilibrium phase separated values. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. Low Mn significantly reduces, but does not eliminate, precipitation in high Ni steels. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs

Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPVtype steels, with an unusually wide range of dissolved Mn (0.06e1.34 at.%) and Ni (0.19e3.50 at.%) contents, were irradiated at z 290
C to z 1.4 1020 n/cm2 at an accelerated test reactor flux of z3.6 1012 n/cm2 -s (E > 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with z Ni1.6Mn0.8. Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Ni-silicide phase type compositions are observed

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(nth,γ) 59Ni(nth,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).

Embrittlement of light water reactor pressure vessel (RPV) steels by fast neutron irradiation may limit extended nuclear plant life. Embrittlement, which is manifested as increases in various indexes of a ductile to brittle transition temperatures (ΔT), is primarily due to hardening by nanoscale precipitates containing Cu, Ni, Mn, and Si, which form under irradiation. In addition to these elements, P has also been found to play a role in embrittlement. While only slightly enriched in the precipitates, hardening and embrittlement increase with trace P concentrations in low-Cu steels. Here, we characterize the individual and synergistic irradiation precipitation and hardening mechanisms in a series of RPV steels containing no to low-Cu and with systematic variations in Ni and P. The steels were irradiated to a fluence of ∼ 1.38×1020 n/cm2 at ∼ 292 °C in the UCSB ATR-2 experiment. In nominally Cu-free medium and high-Ni RPV steels, atom probe tomography shows that P and Ni promote precipitation of P-Mn-Si-Ni and Mn-Si-Ni precipitates, respectively. The precipitate microstructure correlates with the observed irradiation hardening.