Search Results (28)

Body-centered cubic (BCC) metals, extensively utilized in low-alloy high-strength steels and heat-resistant alloys, exhibit a pronounced ductile–brittle transition (DBT) at cryogenic temperatures, marked by a well-defined yet narrow ductile–brittle transition temperature (DBTT) window. This paper overviews the research progress regarding the DBT mechanism of BCC metals. This mechanism was recently found to be related to the mobility of screw dislocation relative to edge dislocation, a decrease in which can induce a critical drop in the proliferation efficiency of dislocation sources. Furthermore, this paper summarizes the current research on the dilute solution softening effect of BCC metals, which has been frequently observed and studied in refractory alloys. The mechanism of this effect involves the low-temperature mobility of screw dislocations that could be promoted by specific solute atoms through kink pair nucleation. This offers a potential strategy for reducing the DBTT of low-alloy steels using a dilute solution, namely microalloying in metallurgy. However, the current understanding of the relationship between the macroscopic ductility of BCC alloys and the dilute solution softening effect is limited. This review aimed to draw attention to this relationship and strengthen related research. Full article

The microstructural evolution and mechanical properties of WWER 440 steam generator steel GOST 22K after long-term operation were thoroughly examined in this study. The samples were taken directly from a steam generator using the small punch test method. The uniqueness of these samples lies in the fact that they were real operating materials used in a nuclear power plant with different years of operation. The microstructure was characterized using optical microscopy and transmission electron microscopy supplemented by selective electron diffraction and semi-quantitative chemical microanalysis. It was found that with the prolongation of the operation time of the steam generator, the density of carbides increased slightly, which was reflected in a decrease in the mean distance between particles, but these differences were very small, which indicates the microstructural stability of GOST 22K steel. The stability of this steel was also confirmed by measuring its mechanical properties, which changed only minimally depending on the years of operation. The tensile strength values were in the range of 508 to 579 MPa. In the case of the ductile-to-brittle transition temperature (DBTT), a slight increase was found after 6 years of operation. The DBTT did not change significantly with subsequent operation. Full article

Exposure of metals to neutron irradiation results in an increase in the yield strength and a significant loss of ductility. Irradiation hardening is also closely related to the fracture toughness temperature shift or the ductile-to-brittle transition temperature (DBTT) shift in alloys with a body-centered cubic (bcc) crystal structure. Ion irradiation is an indispensable tool in the study of the radiation effects of materials for nuclear energy systems. Due to the shallow damage depth in ion-irradiated materials, the nanoindentation test is the most commonly used method for characterizing the changes in mechanical properties after ion irradiation. Issues that affect the analysis of irradiation hardening may arise due to changes in the surface morphology and mechanical properties, as well as the inherent complexities in nanoscale indentation. These issues, including changes in surface roughness, carbon contamination, the pile-up effect, and the indentation size effect, with corresponding measures, were reviewed. Modeling using the crystal plasticity finite element method of the nanoindentation of ion-irradiated materials was also reviewed. The challenges in extending the nanoindentation test to high temperatures and to multiscale simulation were addressed. Full article

Chromium (Cr) metal has garnered significant attention in alloy systems owing to its exceptional properties, such as a high melting point, low density, and superior oxidation and corrosion resistance. However, its processing capabilities are hindered by its high ductile–brittle transition temperature (DBTT). Recently, powder bed fusion-laser beam for metals (PBF-LB/M) has emerged as a promising technique, offering the fabrication of net shapes and precise control over crystallographic texture. Nevertheless, research investigating the mechanism underlying crystallographic texture development in pure Cr via PBF-LB/M still needs to be conducted. This study explored the impact of scan speed on relative density and crystallographic texture. At the optimal scan speed, an increase in grain size attributed to epitaxial growth was observed, resulting in the formation of a <100> cubic texture. Consequently, a reduction in high-angle grain boundaries (HAGB) was achieved, suppressing defects such as cracks and enhancing relative density up to 98.1%. Furthermore, with increasing densification, Vickers hardness also exhibited a corresponding increase. These findings underscore the efficacy of PBF-LB/M for processing metals with high DBTT properties. Full article

For the application of forged medium-Mn steels on flywheel rotors, the effect of annealing temperatures from 300 °C to 650 °C on the impact toughness of 0.16C-6.5Mn forged steel was investigated to demonstrate the microstructural characteristics and austenite reverse transformation determining the impact toughness. The results obtained through standard Charpy V-notch impact tests at ambient temperature show that the impact absorbed energy holds at lower than 10 J almost constantly at annealing temperatures of 300 °C to 500 °C, and a representative intergranular fracture is presented. At an annealing temperature of 600 °C, the impact absorbed energy increases to 147 J, with the ductile fracture characteristics showing plenty of fine dimples, and the high impact toughness is attributed to the high volume fraction above 30% and the moderate stability of reverted austenite. Subsequently, the annealing temperature rises higher than 600 °C, the impact absorbed energy decreases, and the fracture morphology shows brittleness characterized by more flat facets of intergranular fractures and small quasi-cleavage facets, presumably corresponding to the insufficient transformation and twinning-induced plasticity effect due to weakening the Mn partitioning from quenched martensite to reverted austenite, which results in lower austenitic stability. Furthermore, the ductile-to-brittle transition temperature (DBTT) of the 0.16C-6.5Mn forged steel annealed at 600 °C, which holds the highest impact absorbed energy, and is explored for the possibility of flywheel rotor application in a service environment. The DBTT reaches −21 °C, obtained through the Boltzmann function, and the impact absorbed energy is approximately 72 J. Full article

The nondestructive determination of the neutron-irradiation-induced embrittlement of nuclear reactor pressure-vessel steel is a very important and recent problem. Within the scope of the so-called NOMAD project funded by the Euratom research and training program, novel nondestructive electromagnetic testing and evaluation (NDE) methods were applied to the inspection of irradiated reactor pressure-vessel steel. In this review, the most important results of this project are summarized. Different methods were used and compared with each other. The measurement results were compared with the destructively determined ductile-to-brittle transition temperature (DBTT) values. Three magnetic methods, 3MA (micromagnetic, multiparameter, microstructure and stress analysis), MAT (magnetic adaptive testing), and Barkhausen noise technique (MBN), were found to be the most promising techniques. The results of these methods were in good agreement with each other. A good correlation was found between the magnetic parameters and the DBTT values. The basic idea of the NOMAD project is to use a multi-method/multi-parameter approach and to focus on the synergies that allow us to recognize the side effects, therefore suppressing them at the same time. Different types of machine-learning (ML) algorithms were tested in a competitive manner, and their performances were evaluated. The important outcome of the ML technique is that not only one but several different ML techniques could reach the required precision and reliability, i.e., keeping the DBTT prediction error lower than a ±25 °C threshold, which was previously not possible for any of the NDE methods as single entities. A calibration/training procedure was carried out on the merged outcome of the testing methods with excellent results to predict the transition temperature, yield strength, and mechanical hardness for all investigated materials. Our results, achieved within the NOMAD project, can be useful for the future potential introduction of this (and, in general, any) nondestructive evolution method. Full article

The behavior of the EK-181 low-activation ferritic-martensitic reactor steel (Fe–12Cr–2W–V–Ta–B) in the states with different levels of strength and plastic properties after traditional heat treatment (THT) and after high-temperature thermomechanical treatment (HTMT) in the temperature range from −196 to 25 °C, including the range of its cold brittleness (ductile–brittle transition temperature, DBTT) is studied. The investigations are carried out using non-destructive acoustic methods (internal friction, elasticity) and transmission and scanning electron microscopy methods. It is found that the curves of temperature dependence of internal friction (the vibration decrement) of EK-181 steel after THT and HTMT are similar to those of its impact strength. Below the ductile–brittle transition temperature, it is characterized by a low level of dislocation internal friction. The temperature dependence curves of the steel elastic modulus increase monotonically with the decreasing temperature. In this case, the value of Young’s modulus is structure-sensitive. A modification of the microstructure of EK-181 steel as a result of HTMT causes its elastic modulus to increase, compared to that after THT, over the entire temperature range under study. The electron microscopic studies of the steel microstructure evolution near the fracture surface of the impact samples (in the region of dynamic crack propagation) in the temperature range from −196 to 100 °C reveal the traces of plastic deformation (increased dislocation density, fragmentation of the martensitic structure) at all of the temperatures under study, including those below the cold brittleness threshold of EK-181 steel. Full article

The present study investigates the relative effect of B and N concentrations and the austenitization temperature on the microstructure and mechanical properties (tensile and Charpy impact) of modified 9Cr-1Mo (P91) steels. Initially, a B-free P91 steel (with 500 ppm N) and four different B-containing steels (25–100 ppm) with varying N concentrations (20–108 ppm) were hot-rolled, normalized from different austenitization temperatures (1000–1100 °C/1 h) and finally tempered at 760 °C for 1 h. A Charpy impact test shows that the ductile–brittle transition temperature (DBTT) of all the B-added steels decreases with an increase in the austenitization temperature, where the 100 ppm B steel offers the lowest DBTT (−85 °C). Similarly, the strength increases with the increase in the austenitization temperature (1100 °C), with a slight drop in ductility. The influence of precipitates on the microstructure and mechanical properties is explained considering the B enrichment at the precipitates and the thermodynamic stability of the precipitates. The 100 ppm B steel (containing the maximum B and minimum N), normalized from 1100 °C austenitization, shows the best combination of tensile and Charpy impact properties, owing to the effective dissolution of coarse M₂₃C₆ and MX precipitates during the normalization treatment and the formation of fine B-rich (Fe,Cr)₂₃(B,C)₆ precipitates during the subsequent tempering. Full article

Multiple-basis-element (MBE) alloy was defined as the entropy of mixing over 1R (R is the gas constant, 8.31 J/k), and contains at least three principal elements, each one at over 5%. Thus, MBE alloys can include high-entropy alloys (HEAs), medium-entropy alloys (MEAs), amorphous alloys, and some martensite stainless steels, which have been reported to possess excellent cryogenic properties. This paper reviews the progress of the cryogenic-mechanical properties and applications of MBE alloys. It has been concluded that, with the increase of entropy, the ductile-brittle-transition temperatures (DBTT) can be decreased to the liquid helium temperature (4.2 K). In summary, the cryogenic toughness of MBE alloys can be greatly enhanced by entropy adjustments, which is beneficial to their application at low temperatures. Full article

A novel nondestructive method called magnetic adaptive testing has been previously applied to detect the neutron-irradiation-generated structural changes in reactor pressure vessel steel material. This method has been found to be a useful tool for this purpose, and good correlation—as a tendency—has been found between the estimated ductile-to-brittle transition temperature and magnetic parameters. However, a significant scattering of measured points was also observed for the investigated set of Charpy specimens. The main result of the work was that by magnetic selection of samples, the scatter can be notably reduced. As a conclusion, the magnetically measured parameters seemed to be precise and reliable for the detection of embrittlement of the reactor pressure vessel steel, with lower scattering of points than in the conventionally used destructive mechanical characteristics (ductile-to-brittle transition temperature). This result is surprising and needs further verification. The purpose of the present work is to repeat the measurement on irradiated reactor steel blocks. In this work, instead of the DBTT transition temperature, individually measured Vickers hardness (VH) data were used to help characterize the mechanical properties of the material. The so-called “property transformation” is a known and applied technique in the nuclear industry. The mechanical property characterized by the transition temperature cannot be determined individually for each specimen; instead, it can be obtained only on a set of samples by statistical fitting. Therefore, the individually measured Vickers hardness values can be utilized in order to predict the individual transition temperature values by the help of the property transformation technique. In this paper, however, not these derived transition temperature values, but their origins, the Vickers hardness values, are studied in a direct manner. The same behavior of blocks was observed as in the case of Charpy specimens, which is considered to validate the previously published results. As a possible reason for the scattering of points, large magnetic inhomogeneity of samples cut even from the same block was also proved. The magnetic parameters and Vickers hardness correlate well with each other. This result justifies the potential future application of magnetic techniques in practice aimed at the regular inspection of nuclear reactors. Full article

The formation and propagation of cracks occur through irreversible dislocation movements at notches, material defects, and grain boundaries. Since this process is partly thermally controlled, the resistance to dislocation movements at low temperatures increases. This slows both fatigue initiation and fatigue crack propagation. From recent experimental data, it can be seen that fatigue crack growth is accelerated below the fatigue transition temperature (FTT) that correlates with the ductile-brittle transition temperature (DBTT) found by well-known fracture mechanics tests, i.e., Charpy impact, fracture toughness, and CTOD. Hence, this study investigates the relation between FTT and DBTT in S500 high-strength steel base material and welded joints at low temperatures using fatigue crack growth, fracture toughness tests as well as scanning electron microscopy. From the tests, an almost constant decrease in fatigue crack propagation rate is determined with decreasing test temperature even below the DBTT. At −100 °C, the fatigue crack propagation rate is about half of the rate observed at room temperature for both base material and weld metal. Full article

Tungsten (W) as a structural component has grown roots in many special applications owing to its radiation-shielding capabilities and its properties at elevated temperatures. The high ductile-brittle transition temperature (DBTT) and the very high melting point of tungsten however have limited its processability to certain technologies such as powder metallurgy. Laser powder bed fusion (LPBF) has been introduced in recent years as an alternative for manufacturing tungsten parts to overcome the design limitations posed by powder metallurgy technology. A review of the literature shows significant improvements in the quality of tungsten components produced by LPBF, implying a strong potential for manufacturing tungsten with this technology and a need for further research on this subject. This review paper presents the current state-of-the-art in LPBF of unalloyed tungsten, with a focus on the effect of process parameters on the developed structure/properties and identifies current knowledge gaps. Full article

Tungsten is a refractory metal with the highest melting temperature and density of all metals in this group. These properties, together with the high thermal conductivity and strength, make tungsten the ideal material for high-temperature structural use in fusion energy and other applications. It is widely agreed that the manufacture of components with complex geometries is crucial for scaling and optimizing power plant designs. However, there are challenges associated with the large-scale processing and manufacturing of parts made from tungsten and its alloys which limit the production of these complex geometries. These challenges stem from the high ductile-to-brittle transition temperature (DBTT), as well as the strength and hardness of these parts. Processing methods, such as powder metallurgy and additive manufacturing, can generate near-net-shaped components. However, subtractive post-processing techniques are required to complement these methods. This paper provides an in-depth exploration and discussion of different processing and manufacturing methods for tungsten and identifies the challenges and gaps associated with each approach. It includes conventional and unconventional machining processes, as well as research on improving the ductility of tungsten using various methods, such as alloying, thermomechanical treatment, and grain structure refinement. Full article

Reduced activation ferritic (RAF) steels are expected to be widely used in challenging nuclear industrial applications under severe thermo-mechanical regimes and intense neutron loads. Therefore, actual research panorama is facing the strengthening strategies necessary to maximize both performance and endurance under these conditions. Oxide dispersion strengthened (ODS) RAF steels are leader candidates as structural materials in fusion energy reactors thanks to the reinforcement obtained with a fine dispersion of nanosized oxides in their matrix. In this study, the influence of the initial powder particle size and the selected processing route on the final material has been investigated. Two RAF ODS steels coming from atomized pre-alloyed powders with nominal particle powder sizes of 70 and 30 µm and composition Fe-14Cr-2W-0.4Ti-0.3Y₂O₃ (wt. %) were manufactured by mechanical alloying. Alloyed powders were compacted by hot isostatic pressing, hot crossed rolled, and annealed at 1273 K. Initial powder particle size differences minimize after milling. Both steels present an almost completely recrystallized material and similar grain sizes. The same type and distributions of secondary phases, Cr-W-rich, Ti-rich, and Y-Ti oxide nanoparticles, have been also characterized by transmission electron microscopy (TEM) in both alloy samples. The strengthening effect has been confirmed by tensile and Charpy impact tests. The two alloys present similar strength values with slightly better ductile brittle transition temperature (DBTT) and ductility for the steel produced with the smaller powder size. Full article

This paper presents the results of an experimental investigation of a 12% Cr steel where a significant increase in Charpy impact toughness and a slight decrease in ductile-brittle transition temperature (DBTT) from 70 °C to 65 °C were obtained through thermo-mechanical processing, including interim hot forging at 1050 °C with long-term annealing at 1000 °C, as compared with conventional heat treatment. At lower temperatures ranging from −20 °C to 25 °C, the value of impact toughness comprised ~40 J cm⁻² in the present 12% Cr steel subjected to thermo-mechanical processing. The amount of δ-ferrite decreased to 3.8%, whereas the size of prior austenite grains did not change and comprised about 40–50 μm. The boundaries between δ-ferrite and martensitic laths were decorated by continuous chains of Cr- and W-rich carbides. M₂₃C₆ carbides also precipitated along the boundaries of prior austenite grains, packets, blocks and martensitic laths. Thermo-mechanical processing increased the mean size of M₂₃C₆ carbides and decreased their number particle densities along the lath boundaries. Moreover, the precipitation of a high number of non-equilibrium V-rich MX particles was induced by hot forging and long-term normalizing at 1000 °C for 24 h. Full article