Formation of large volume fractions of Mn-Ni-Si precipitates (MNSPs) causes excess irradiation embrittlement of reactor pressure vessel (RPV) steels at high, extended-life fluences. Thus, a new and unique, semi-empirical cluster dynamics model was developed to study the evolution of MNSPs in low- Cu RPV steels. The model is based on CALPHAD thermodynamics and radiation enhanced diffusion ki- netics. The thermodynamics dictates the compositional and temperature dependence of the free energy reductions that drive precipitation. The model treats both homogeneous and heterogeneous nucleation, where the latter occurs on cascade damage, like dislocation loops. The model has only four adjustable parameters that were fit to an atom probe tomography (APT) database. The model predictions are in semi-quantitative agreement with systematic Mn, Ni and Si composition variations in alloys character- ized by APT, including a sensitivity to local tip-to-tip variations even in the same steel. The model predicts that heterogeneous nucleation plays a critical role in MNSP formation in lower alloy Ni contents. Single variable assessments of compositional effects show that Ni plays a dominant role, while even small variations in irradiation temperature can have a large effect on theMNSP evolution. Within typical RPV steel ranges, Mn and Si have smaller effects. The delayed but then rapid growth of MNSPs to large volume fractions at high fluence is well predicted by the model. For purposes of illustration, the effect of MNSPs on transition temperature shifts are presented based on well-established microstructure-prop- erty and property-property models.
We characterise the influence of the segregation behaviours of two typical alloying elements, aluminium and nickel, on the interfacial cohesive properties of copper-rich precipitates in ferritic steels, with a view towards understanding steel embrittlement. The first-principles method is used to compute the energetic and bonding properties of aluminium and nickel at the interfaces of the precipitates and corresponding fracture surfaces. Our results show the segregation of aluminium and nickel at interfaces of precipitates are both energetically favourable. We find that the interfacial cohesion…
A relative contribution to irradiation hardening caused by dislocation loops and solute-rich precipitates is established for RPV steels of WWER-440 and WWER-1000 reactors, based on TEM measurements and mechanical testing at reactor operating temperature of 563 K. The pinning strength factors evaluated for loops and precipitates are shown to be much lower than those obtained for model alloys based on the room temperature testing as well as those evaluated by means of atomistic simulations in the temperature range of 300 to 600 K. This discrepancy is explained in the framework of a model of ther…
Ion irradiation provides a promising substitute to neutron tests for investigating the effects of radiation on materials for fission and fusion reactor plants. Here we show proton irradiation can quantitatively reproduce precipitation that leads to embrittlement in reactor pressure vessel steels, at dose rates 10,000 times greater than experienced in fission reactor operation. Small-angle neutron scattering (SANS) is used to characterize precipitate size distributions in copper-containing steels irradiated to average doses of approx. 7 mdpa with 5 MeV protons. Comparing our results with the li…
A comprehensive study of pulsed nitriding in AISI H13 tool steel at low temperature (400°C) is reported for several durations. X-ray diffraction results reveal that a nitrogen enriched compound (Epsilon-Fe2-3N, iron nitride) builds up on the surface within the first process hour despite the low process temperature. Beneath the surface, X-ray Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM) indicates relatively higher nitrogen concentrations (up to 12 at.%) within the diffusion layer while microscopic nitrides are not formed and existing carbides are not dissolve…
Since Reactor Pressure Vessel steels are ferromagnetic, they provide a convenient means to monitor changes in the mechanical properties of the material upon irradiation with high energy particles, by measuring their magnetic properties. Here, we discuss the correlation between these two properties (i.e. mechanical and magnetic properties) and microstructure, by studying the flux effect on the nuclear pressure vessel steel used in reactors currently under construction in Argentina. Charpy-V notched specimens of this steel were irradiated in the RA1 experimental reactor at 275°C with two lead fa…
Mn-Ni-Si precipitates (MNSPs) are known to be responsible for irradiation-induced hardening and embrittlement in structural alloys used in nuclear reactors. Studies have shown that precipitation of the MNSPs in 9-Cr ferritic-martensitic (F-M) alloys, such as T91, is strongly associated with heterogeneous nucleation on dislocations, coupled with radiation-induced solute segregation to these sinks. Therefore it is important to develop advanced predictive models for Mn-Ni-Si precipitation in F-M alloys under irradiation based on an understanding of the underlying mechanisms. Here we use a cluster…
Combined effect of Carbon and Nitrogen levels and isothermal holding temperature on the microstructure, precipitation and the tensile properties of Vanadium microalloyed steels with 0.05 weight percent Vanadium were studied. Two different Vanadium steels, one having higher Carbon and lower Nitrogen content, HCLN steel, and the other having lower Carbon and higher Nitrogen content, LCHN steel, were prepared and subjected to isothermal holding treatment over a temperature range of 500 to 750 degree Celsius, after hot-deformation. Maximum precipitation strengthening from fine Vanadium carbonitrid…
The modification of steel (AISI 316L and AISI 4140) surface morphology and underlying inter-crystalline grains strain due to Xe$^+$ ion bombardment are reported to affect nitrogen diffusion after a pulsed plasma nitriding process. The ion bombardment induces regular nanometric patterns and increases the roughness of the material surface. The strain induced by the noble gas bombardment is observed in depths which are orders of magnitude larger than the projectiles' stopping distance. The pre-bombarded samples show peculiar microstructures formed in the nitrided layers, modifying the in-depth ha…
Near-atomic investigation on the elemental redistribution during co-precipitation of nano-sized kappa phase and B2 phase in an Al-alloyed lightweight steel
University of Kassel（卡塞尔大学）；RWTH Aachen University（亚琛工业大学）
The formation of modified Z-phase in a 12Cr1MoV (German grade: X20) tempered martensite ferritic (TMF) steel subjected to interrupted long-term creep-testing at 550$^{\circ}$C and 120 MPa was investigated. Quantitative volumetric measurements collected from thin-foil and extraction replica samples showed that modified Z-phase precipitated in both the uniformly-elongated gauge ($f_v$: 0.23 $\pm$ 0.02 %) and thread regions ($f_v$: 0.06 $\pm$ 0.01 %) of the sample that ruptured after 139 kh. The formation of modified Z-phase was accompanied by a progressive dissolution of MX precipitates, which d…
Mechanism of solute-enriched clusters formation in neutron-irradiated pressure vessel steels is proposed and developed in case of Fe-Cu model alloys. We show that the obtained results are in a good agreement with available experimental data on the parameters of clusters enriched with the alloying elements. The suggested solute-drag mechanism is analogous to the well-known zone-refining process. Our model explains why the formation of solute-enriched clusters does not happen in austenitic stainless steels with fcc lattice structure. It also allows to quantify the method of evaluation of neutron…
The T91 grade and similar 9Cr tempered-martensitic steels (also known as ferritic-martensitic) are leading candidate structural alloys for fast fission nuclear and fusion power reactors. At low temperatures (300 to 400 $^\circ$C) neutron irradiation hardens and embrittles these steels, therefore it is important to investigate the origin of this mode of life limiting property degradation. T91 steel specimens were separately neutron irradiated to 2.14 dpa at 327 $^\circ$C and 8.82 dpa at 377 $^\circ$C in the Idaho National Laboratory Advanced Test Reactor. Atom probe tomography was used to inves…
Effective post-heat treatment is critical to achieve desired microstructure for high-performance in additively manufactured (AM) components. In this work, the influence of homogenization on microstructure-property relationship in 17-4PH steels has been investigated. Precipitation of NbC, oxides, and ε-Cu were observed in the as-built 17-4PH steels. To design an optimum post-heat treatment, homogenization was performed at 1050oC for different times followed by aging at 482oC for 1 hour. It was identified that homogenization for 1 hour followed by aging leads to the best combination of strength …
Radiation-induced embrittlement of nuclear steels is one of the main limiting factors for safe long-term operation of nuclear power plants. In support of accurate and safe reactor pressure vessel (RPV) lifetime assessments, we developed a physics-based model that predicts RPV steel hardening and subsequent embrittlement as a consequence of the formation of nano-sized clusters of minor alloying elements. This model is shown to provide reliable assessments of embrittlement for a very wide range of materials, with higher accuracy than industrial correlations. The core of our model is a multiscale…
The development of new fire-resistant steels represents a challenge in materials science and engineering of utmost importance. Alloying elements such as Nb and Mo are generally used to improve the strength at both room- and high-temperatures due to, for example, the formation of precipitates and harder microconstituents. In this study we show alternatively that the addition of small amounts of boron in Nb-microalloyed steels may play a crucial role in maintaining the mechanical properties at high temperatures. The 66\,\% yield-strength criteria for fire resistance is achieved at $\approx 574$\…
The Fe-Cu system has attracted much attention over the last several decades due to its technological importance as a model alloy for Cu steels. In spite of these efforts several aspects of its phase diagram remain unexplained. Here we use atomistic simulations to characterize the polymorphic phase diagram of Cu precipitates in body-centered cubic (BCC) Fe and establish a consistent link between their thermodynamic and mechanical properties in terms of thermal stability, shape, and strength. The size at which Cu precipitates transform from BCC to a close-packed 9R structure is found to be stron…
Impact of the lead factor of neutron irradiation on the magnetic properties of RPV steels
Comisión Nacional de Energía Atómica（国家原子能委员会）；Universidad Tecnológica Nacional（国立技术大学）；Universidad Nacional de Cuyo, Instituto Balseiro（门多萨国立大学，巴尔塞罗研究所）
Data-efficient U-Net for Segmentation of Carbide Microstructures in SEM Images of Steel Alloys
Helmholtz AI Team Matter (FWCC-A)（海德堡人工智能团队物质（FWCC-A））；Helmholtz-Zentrum Dresden-Rossendorf（德罗斯登-罗斯托克研究中心）；Institute of Resource Ecology, Structural Materials Department（资源生态研究所，结构材料部门）
Based on the earlier proposed mechanism of formation of solute-enriched clusters in irradiated pressure vessel steels, it has been demonstrated that it is possible to suppress this process by the method of light alloying of PVS with a chemical element with specially selected properties such as solubility, activity, and mobility. It is shown that it is possible to change the thermal spike molten zone solidification mechanism and suppress the "solute drag" leading to the formation of solute-enriched clusters. It must be noted that such a solute-enriched cluster formation mechanism is appropriate…
Unlike the vast amount of irradiated material data that exists for stainless steel internals from LWRs, with high fast neutron flux and an irradiation temperature of~330oC, the CANDU reactor is unique with a high thermal spectrum stainless steel components peripheral to the core. In particular, the CANDU design contains an austenitic stainless steel calandria vessel, which contains the heavy water moderator at a temperature of 60-80oC. This article explores the effects of low (60-80oC) and moderate (300-360oC) irradiation temperature on irradiation induced defects and defect sinks, both in ter…
A 10 wt.% nickel-steel has been developed for high pressures and low-temperature applications, due to its high strength, excellent toughness, and low ductile-to-brittle transition temperature (DBTT). Under dynamic loading conditions this steel is, however, prone to shear localization that manifests as adiabatic shear bands (ASBs). The temperature increases and thermoplastic microstructural evolution in the ASB are studied in detail, from the macroscopic length scale to the atomic-scale employing correlative electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM), and at…
The current work combines the Cluster Dynamics (CD) technique and CALPHAD-based precipitation modeling to address the second phase precipitation in cold-worked (CW) 316 stainless steels (SS) under irradiation at 300-400 C. CD provides the radiation enhanced diffusion and dislocation evolution as inputs for the precipitation model. The CALPHAD-based precipitation model treats the nucleation, growth and coarsening of precipitation processes based on classical nucleation theory and evolution equations, and simulates the composition, size and size distribution of precipitate phases. We benchmark t…
