Mukesh Bachhav

High chromium ferritic-martensitic (F-M) steels are one of the promising structural material classes for future nuclear power plants. These steels are designed to combine corrosion resistance, conferred by chromium, with low swelling, high resistance to irradiation damage as well as to retain adequate toughness and elevated-temperature strength during service [1]. However, the long-term use of these steels in intense neutron irradiation environments requires reliable predictions of the evolution of their microstructures and mechanical properties. Binary Fe-Cr alloys constitute a model system for high Cr ferritic/martensitic steels and have therefore generated lot of interest by allowing the systematic study on irradiation induced microstructural changes. In the present study, microstructural changes in neutron irradiated Fe-Cr binary alloys are investigated using atom probe tomography (APT). A series of six Fe-Cr alloys of nominal compositions 3, 6, 9, 12, 15, and 18 at.%Cr were irradiated at a neutron fluence (E>1 MeV) of 1.1 x 1021 n/cm2 at 563 ± 15K and to a damage level of 1.82 displacements per atom (dpa). Solute distributions revealed a' precipitation for alloys containing more than 9at.%Cr (Figure 1). Both the Cr concentration dependence of a' precipitation and the measured matrix compositions are in agreement with the recently published Fe-Cr phase diagrams [2]. An irradiation-accelerated precipitation process is strongly suggested for a' precipitation. Along with homogenously distributed Cr-enriched clusters of the a' phase, few clusters involving Si, P, Ni, and Cr, are observed in the matrix [3]. For Fe-6, 9, 12 at.%Cr, Si and Cr are found segregated to dislocation loops and information pertaining to number density, size, and habit plane were analyzed for Fe-6at.%Cr alloy[4]. Grain boundary chemistry for Fe-Cr alloys are quantitatively compared between the as-received and the neutron irradiated alloys. Zones depleted of a' clusters and Si are found at the interfaces of carbide and nitride precipitates and along grain boundaries in the vicinity of these precipitates. To study stability of clusters and observed features in irradiated samples, annealing is carried out at high temperatures. The results are discussed in the context of equilibrium segregation, radiation-enhanced diffusion, and/or radiation induced segregation.

We have investigated proton irradiation induced Nb redistribution in Zr-xNb alloy (x = 0.4, 0.5, 1.0) by using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). We have found by STEM that 2MeV proton irradiation at 350°C induces precipitation of Nb-rich needle-like particles in the Zr matrix. Initially without irradiation effect, the Zr matrix only contains βNb and Laves phase native precipitates. After irradiation, in addition to the needle-like particles, we have also found by APT that Fe- and Nb-rich nanoclusters (less than 20 nm diamter) are present in the Zr matrix for Zr-0.5Nb, 1000°C annealed Zr-0.5Nb and Zr-1.0Nb. Despite different irradiation dose level, the total Nb content in the entire APT tip for all the samples ranges from 0.24 – 0.40 at. %, which is below the maximum solubility limit of 0.6 at. % Nb in Zr solid solution. After cluster removal from the Zr matrix of the irradiated samples, Nb concentration in the Zr solid solution is shown to significantly decrease with irradiation dose, which is suspected to be responsible for the improved corrosion resistance of ZrNb alloy in the reactor environment at high burnup.

Microstructural changes in a Fe–15 at.%Cr model alloy neutron irradiated to 1.82 dpa at 290 °C were characterized by atom probe tomography. Homogenously distributed a' precipitates as well as fewer clusters containing Si, P, Ni, and Cr, were observed in the matrix. Grain boundary analyses before and after irradiation revealed segregation of Cr, with W-shape concentration profiles developing in the vicinity of grain boundary carbide and nitride particles. After irradiation, impurities such as C, Si and P were segregated to the grain boundaries. Zones depleted of a' clusters, and Si were found at the interfaces of carbide and nitride precipitates and along grain boundaries in the vicinity of these precipitates.

The microstructural and chemical changes in a Fe–6 at.%Cr binary model alloy neutron irradiated to 1.82 dpa at 290 °C were investigated using atom probe tomography. After irradiation, Si and Cr are found segregated to dislocation loops, which were analyzed in terms of number density, size, and habit plane. Grain boundary chemistry was quantitatively compared between the as-received and the neutron irradiated alloys. The results are discussed in the context of equilibrium segregation, radiation-enhanced diffusion, and/or radiation induced segregation.

Austenitic stainless steel D9 is a candidate for Generation IV nuclear reactor structural materials due to its enhanced irradiation tolerance and high-temperature creep strength compared to conventional 300-series stainless steels. But, like other austenitic steels, D9 is susceptible to irradiation- induced clustering of Ni and Si, the mechanism for which is not well understood. This study utilizes atom probe tomography (APT) to characterize the chemistry and morphology of Ni–Si nanoclusters in D9 following neutron or proton irradiation to doses ranging from 5–9 displacements per atom (dpa) and temperatures ranging from 430–683 ºC. Nanoclusters form only after neutron irradiation and exhibit classical coarsening with increasing dose and temperature. The nanoclusters have Ni3Si stoichiometry in a Ni core–Si shell structure. This core–shell structure provides insight into a potentially unique nucleation and growth mechanism—nanocluster cores may nucleate through local, spinodal-like compositional fluctuations in Ni, with subsequent growth driven by rapid Si diffusion. This study underscores how APT can shed light on an unusual irradiation-induced nanocluster nucleation mechanism active in the ubiquitous class of austenitic stainless steels.

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.

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.

A series of model Fe–Cr alloys containing 3–18 at.% Cr was neutron irradiated at a nominal temperature of 563 K to 1.82 dpa. Solute distributions were analyzed by atom probe tomography, which revealed a' precipitation for alloys containing more than 9 at.% Cr. Both the Cr concentration dependence of a' precipitation and the measured matrix compositions are in agreement with the recently published Fe–Cr phase diagrams. An irradiation-accelerated precipitation process is strongly suggested.