Ductile-brittle Transition Temperature Research Articles

Comprehensive Screening of Plasma-Facing Materials for Nuclear Fusion

Plasma-facing materials (PFMs) represent one of the most significant challenges for the design of future nuclear fusion reactors. Inside the reactor, the divertor will experience the harshest material environment: intense bombardment of neutrons and plasma particles coupled with extremely large and intermittent heat fluxes. The material designated to cover this role in ITER is tungsten. While no other materials have shown the potential to match the properties of tungsten, many drawbacks associated with its application remain, including cracking and erosion induced by a low recrystallization temperature combined with a high ductile-brittle transition temperature and neutron-initiated embrittlement; surface morphology changes (helium bubbles and fuzz layer) due to plasma-tungsten interaction with subsequent risk of spontaneous material melting and delamination; and low oxidation resistance in the case of air contamination. Exploring alternatives to tungsten requires the design of a multivariable optimization problem. This work aims to produce a structured and comprehensive material screening of PFM candidates based on known inorganic materials. The method applied in this study to identify the most promising PFM candidates combines peer-reviewed data present in the PAULING FILE database and first-principles density-function theory calculations focusing on two key PFM defects—namely, the sputtering of surface atoms and the incorporation of interstitial hydrogen, respectively characterized by the surface binding energy and the interstitial formation energy. The crystal structures and their related properties, extracted from the PAULING FILE, are ranked according to the heat-balance equation of a PFM subjected to the heat loads in the divertor region of an ITER-like tokamak. The materials satisfying the requirements are critically compared with the state-of-the-art in plasma-facing materials research and in studies of refractory materials exposed to high temperatures, plasma, and neutron bombardment. This comparison assesses their thermo-mechanical properties under such conditions, identifying a subset of promising materials for first-principles electronic structure calculations. Most previously known and extensively studied PFMs, such as tungsten, molybdenum, and carbon-based materials, are captured by this screening process, confirming its reliability. Additionally, less familiar refractory materials suggest performance that calls for further investigations. Published by the American Physical Society 2024

New insights from crystallography into the effect of Ni content on ductile-brittle transition temperature of 1000 MPa grade high-strength low-alloy steel

The significant effect of Ni content (0.92, 1.94 and 2.94 wt%) on ductile-brittle transition temperature (DBTT) and microstructure in a 1000 MPa grade high-strength low-alloy (HSLA) steel was studied. Using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), Charpy impact test and low-temperature tensile test to study the fundamental reasons for the effect of Ni content on toughness. The results indicated that increasing the Ni content can reduce the DBTT of HSLA steel and improve the impact toughness at low temperatures. EBSD data post-processing analysis revealed that the key reason for the increase in low-temperature toughness is the refinement of the microstructural crystallographic structure, specifically the significant increase in the boundary density of the block and packet. With the increase of Ni content, the density of grain boundary with an orientation difference greater than 5° between two adjacent {110} crystal planes was higher, which can form a higher density of dislocation pile-up group, thus better reducing local stress concentration. Meanwhile, the stacking fault energy (SFE) increases with the increase of Ni content, which made the screw dislocation more prone to cross slip at low temperature, resulting in an increase in plasticity at low temperatures. These observed phenomena and reasons provided a theoretical explanation for the role of Ni content in reducing DBTT and enhancing the toughness of the core in heavy gauge plates.

Development of a novel approach to correlate small punch fatigue with uniaxial ratcheting fatigue

The miniaturization of fatigue specimens has seen significant advancements, realized either by scaling down conventional specimens while preserving their geometry or by experimenting with novel specimen shapes and loading configurations. In this study, the small punch test (SPT) technique, which is well-established for estimating tensile, creep, and ductile-to-brittle transition temperature (DBTT), is employed to predict the cyclic deformation behaviour of SS 316LN and modified 9Cr-1Mo (P91) steels. The small punch fatigue (SPF) experiments demonstrated continuous displacement accumulation, similar to strain accumulation in ratcheting. The study evaluates the influence of various load parameters, such as amplitude and mean load, on SPF outcomes and compares these to the effects of stress amplitude and mean stress on ratcheting results. The differences in deformation behaviour between ratcheting and small punch fatigue and the effect of biaxiality in SP configuration life is analysed. Finally, to establish a correlation between SPF and ratcheting results, the three life prediction models (Gerber, Goodman, and Smith-Watson-Topper) are examined and a new correlation is proposed by introducing a novel damage parameter.

The formation mechanism of fracture surface separations and their effects on the toughness for ferrite-pearlite steels

The formation mechanism of fracture surface separations for ferrite-pearlite steels is investigated by combining experimental testing and numerical simulation, and the effects of fracture surface separations on the toughness are discussed. The fracture surface of Charpy impact samples for both the as-rolled and the normalized ferrite-pearlite steel were compared. The fracture surface separations occur in the as-rolled plate above the ductile-brittle transition temperature instead of the normalized plate. The fracture surface separations are triggered by transverse tensile stress, which is supported by band-shaped pearlite with poor deformability. It is demonstrated that the formation of fracture surface separations is not only dependent on band-shaped pearlite and temperature but also on the stress state. The normal stress S11 in the X-direction is the largest, and the normal stress S33 distribution concentrates on the plate center, which can explain the location of the fracture surface separations. In contrast to the dispersed distribution of the normal stress S33 for the as-rolled plate, the large normal stress in the normalized plate is in a linear arrangement, resulting in a layered crack. The effects of fracture surface separations on the materials toughness are highly dependent on the number, length, depth and location of the separations.

Study on Strain Hardening and Cryogenic Toughness of Marine 10Ni5CrMoV Steel during Tempering

The tempering process is applied in marine 10Ni5CrMoV steel to study microstructure, and mechanical properties by multi‐scale characterizations, strain hardening behavior, and cryogenic toughening mechanism are further investigated. As the tempering temperature increases from 590 to 630 °C, the dislocation density decreases by up to 25% and the austenite volume fraction decreases by up to 35%. Additionally, the morphology of austenite changes from strip to bulk, which weakened the pinning effect on the laths, leading to a coarsened martensite and an increase in the equivalent grain size. Based on the modified Crussard–Jaoul analysis, the specimens at different tempering temperatures exhibit a single‐stage strain hardening behavior during plastic deformation. The increase in strength is attributed to the continuous transformation‐induced plasticity effect and the increasing dislocation density. Furthermore, high austenite volume fraction and the proportion of grain boundary misorientation above 45° promote the release of stress concentration and hinder the propagation of cracks. This results in an increase in the ductile–brittle transition temperature (DBTT) of the specimens from −105 to −135 °C. At a tempering temperature of 610 °C, the specimen demonstrates an outstanding balance between yield strength (868 MPa) and cryogenic toughness (DBTT of −135 °C).

PPR/EPDM/POE-g-MA/OMMT nanocomposites with superior low-temperature toughness based on embedded OMMT within compound rubber dispersion phase

A new approach to produce polypropylene random copolymer/ethylene propylene diene monomer/polyolefin elastomers modified with maleic anhydride/organic modified nanomontmorillonite (PPR/EPDM/POE-g-MA/OMMT) nanocomposites with superior low-temperature toughness was conducted. The brittle-ductile transition temperature (Tbd) of PPR/EPDM/POE-g-MA/OMMT nanocomposites reduced from −20 °C to −40 °C. The incorporation of POE-g-MA results in the distribution of OMMT changing from the interfacial distribution into dispersed inside rubber phase and a compound rubber dispersed phase in PPR/EPDM/POE-g-MA/OMMT nanocomposites was comprised of EPDM, POE-g-MA and embedded OMMT. The synergistic effect of POE-g-MA and OMMT was considered to result from the special compound rubber dispersed phase consisting of EPDM and POE-g-MA as well embedded OMMT, which led to the reduction in space between rubber domains and the increase in internal friction and ultimately brought about the remarkable enhancement in mechanical properties.

Understanding the tensile ductility of a novel low-activation BCC high-entropy alloy deformed at intermediate temperature

Low-activation body-centered cubic (BCC) high-entropy alloys (HEAs) often possess poor tensile ductility. Here we report a novel low-activation VCrFeMn0.33 BCC HEA that exhibits good tensile ductility at 650 °C and above. However, it shows poor tensile ductility at 500 °C and below, indicating that the ductile-to-brittle transition temperature (DBTT) of the current HEA falls between 500 °C and 650 °C. At temperatures above DBTT, the current BCC HEA deforms via dislocation glide, similar to the tensile deformation observed in conventional ductile BCC alloys, which is rarely reported for non-refractory BCC HEAs.

Nanoindentation Test of Ion-Irradiated Materials: Issues, Modeling and Challenges.

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.

DBTT and recrystallization behavior analyses for rolled and forged potassium-doped tungsten alloys

Tungsten-potassium (WK) alloys are regarded as one of the candidates for plasma-facing materials (PFMs) intended for the divertor and first wall of future fusion reactors. Determining the range of the ductile-to-brittle transition temperature (DBTT) and the recrystallization temperature (RCT) for the WK alloy is critical as it is closely related to the safe operational temperature window of the fusion reactor. Variable temperature tensile tests were conducted on rolled and high energy rate forging (HERF) WK alloy specimens to elucidate their DBTT. Additionally, gradient high-temperature isochronal annealing was performed on the HERF samples, coupled with Vickers hardness characterization, to ascertain their distinct RCT for various orientation planes within the same alloy. Subsequently, the DBTT of the as-rolled WK alloy is determined to be approximately between 100 °C and 150 °C, while that of the HERF WK alloy is found to be around lower than 200 °C. Furthermore, the RCT of the ND plane of the HERF WK alloy is approximately 1600 °C, whereas that of the RD counterpart exceeded 1700 °C. The internal mechanisms underlying these phenomena are comprehensively analyzed in this study. This work has identified the safe service temperature window for future fusion reactors with this kind of advanced WK alloy and provided a new perspective for determining the RCT of many other W-based materials.

Atomistic simulation of temperature dependence of tensile properties and estimation of DBTT of Cu–Ag core–shell nanowires

An atomistic model of Cu-Ag core–shell nanowire using molecular dynamics is proposed. The simulation model was utilized for the evolution of temperature dependent (in the range of 10–500 K) tensile behavior and the identification of ductile-to-brittle transition temperature (DBTT) of single-crystal Cu-Ag core–shell nanowire. The evolution of defects such as point defects, dislocations, and stacking faults, arising from tensile deformation in the mentioned temperature rage were analyzed. The Wigner-Seitz Defect analysis, Dislocation analysis, and Common Neighbor analysis methods were implemented in this study to estimate the various defects concentration in the nanowire. The study identified that Cu-Ag core–shell (core diameter = 4 nm and shell thickness = 1 nm nanowire) exhibits DBTT somewhere in the temperature between 25 K to 30 K. The modulus of elasticity found to be increasing from 19.7 TPa at 10 K to 23.7 TPa at 30 K (within brittle regime), followed by a steep fall to 14.9 TPa at 500 K where fracture seems to be in ductile mode. The observed mechanical properties of the nanowire are discussed in the article in view of the dislocation and stacking faults generation in the core–shell nanostructures.

Impact of neutron irradiation on the tensile properties of advanced EUROFER97-type steels

Tensile properties (strength and ductility) of three different experimental EUROFER97 variants and two standard EUROFER97 products were investigated before and after neutron irradiation performed in the temperature range 275–300 °C up to ∼3 dpa, i.e a damage dose comparable to the end-of-life of ITER operation. The grades with the chemical compositions within and outside the specifications of EUROFER97 steel followed alternative thermo-mechanical treatments (TMT) as compared to the reference EUROFER97. Mechanical properties were assessed with hardness and tensile tests, while fracture surface was investigated by scanning electron microscopy. Post irradiation examination was conducted to evaluate the hardening behavior, change in ductility and fracture surface after irradiation. Furthermore, Ductile-to-Brittle Transition Temperature (DBTT) shift due to neutron irradiation was estimated based on the correlation formerly established for EUROFER97 at the studied irradiation temperature.

Experimental and modeling study on irradiation effect of A508-Ⅲ steel

Irradiation can induce microscopic defects in reactor pressure vessel (RPV) steel, resulting in macroscopic hardening and embrittlement. This paper proposes a micro‑meso-macroscale calculation framework for the mechanical properties of irradiated RPV steel based on experimental and numerical investigations. The mean size and number density of Fe-ion-irradiation-induced dislocation loops were measured using transmission electron microscopy (TEM). These parameters were described by a power-function relationship with the irradiation dose at various temperatures and were used as input parameters for the crystal plasticity finite element model (CPFEM). Nano-indentation tests of A508-Ⅲ steel under different irradiation conditions were successfully simulated. In addition, microstructural characterization and molecular dynamics (MD) simulations revealed the phenomenon of dislocation pinning and the sweeping of irradiation defects during dislocation migration, implying that the dislocation-loop interactions could be responsible for the formation of dislocation channels and curved dislocation lines. Furthermore, increase in yield stress and the ductile-brittle transition temperature (DBTT) at various temperatures were investigated. Results indicated that yield stress, DBTT, and the degree of irradiation hardening and embrittlement increased with the irradiation dose and decreased with irradiation temperature. Finally, the adjusted reference temperature (ART), a significant design parameter for nuclear reactors, was calculated and evaluated using the multiscale framework. The framework can serve as a reference text for extending the life of nuclear reactors.

Effect of La2O3 on the properties of molybdenum rhenium alloy

Adding rhenium to molybdenum can improve room temperature processing performance and reduce the ductile brittle transition temperature. Adding lanthanum oxide to molybdenum can refine grain size and improve strength. This work analyzes the effect of adding different amounts of lanthanum oxide to molybdenum rhenium alloys on their properties. The results indicated that with the increase of lanthanum oxide, the tensile strength increased, but the room temperature elongation only increased at 0.6%; as the temperature increases, the trend of tensile strength and elongation at 900 °C and 1100 °C is similar; at 1300 °C, the strength decreases significantly but the elongation increases sharply; the corresponding changes can be explained by the fracture morphology.

Oxygen solutes-regulated low temperature embrittlement and high temperature toughness in vanadium

The plasticity of body-centered cubic (BCC) metals is sensitive to interstitial trace impurities. Owing to high oxygen affinity, vanadium (V) shows a tendency of embrittlement with increasing oxygen concentration. However, how oxygen solutes affect the ductile-to-brittle transition (DBT) behavior of V remains unclear. In this study, we investigate the DBT behavior of V with different oxygen solute concentrations using small-punch test. As oxygen content increases, the DBT temperature (DBTT) rises incrementally accompanied by a widening of the semi-brittle transition zone. The reduction of the lower fracture energy plateaus indicates a classical low-temperature embrittlement, while the rising of the upper fracture energy plateaus manifests a remarkable high-temperature toughening. Below DBTT, owing to the strong pinning effect of oxygen-vacancy complexes on dislocations, only a limited number of slip systems were activated with very low dislocation density, and all screw dislocations are immobile. Above DBTT, owing to the intensive interactions between dislocation and oxygen-vacancy complexes, frequent dislocation cross-slips trigger multiple slip systems and accelerate dislocation multiplication and storage, all of which contribute to the high-temperature toughness. These findings clarify the effect of oxygen solute on the DBT of V and guide the design of high-performance refractory metals.

Synergistic toughening by a layered microstructure and texture of Si-enriched ferritic/martensitic(F/M) heat resistant steel

In order to achieve favorable heat corrosion resistance, it is effective to add high Si contents to F/M heat resistant steel. Unfortunately, the high Si content results in a significant deterioration of impact toughness. Determining how to balance heat corrosion resistance and impact toughness has been a challenge. In this paper, we optimize austenite isothermal hot-rolling (AIHR) technique to the 10Cr1SiY F/M dual-phase heat resistant steel with 1% Si(wt.), to obtain layered microstructure with thread-like δ-ferrite. This decreases ductile-brittle transition temperature (DBTT) from 149 °C to 15 °C, and increases impact energy from 8 J to 107 J, compared to the quenched and tempered steel with the same chemical composition. According to the instrumented Charpy impact tests, higher impact energy derives from the absorbing energy during crack propagation, accounting for 94.9% of the total absorbing energy. The weak interfaces in the layered microstructure effectively deflect cracks, while the {110} slip plane induced by strong textures from multi-pass rolling promoted dislocation slip, thus rendering plastic deformation and weakening the crack tip stress; synergistic interaction is responsible for the improved toughness, resulting in the delamination fracture of the steel. The reduced grain size and M23C6 phases also contributes to the improved toughness.

Effect of Laser Scan Speed on Defects and Texture Development of Pure Chromium Metal Fabricated via Powder Bed Fusion-Laser Beam.

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.

Impact Toughness Dependent on Annealing Temperatures in 0.16C-6.5Mn Forged Steel for Flywheel Rotors

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.

Selection of EU-DEMO divertor operating conditions: water cooling thermal-hydraulic parameters and power exhaust capabilities

In the context of EUROfusion activities for the development of the DEMO reactor project, the divertor design is a major challenge. It must sustain very high heat, ion particle and neutron fluxes allowing, at the same time, the shielding of the vacuum vessel and the vacuum pumping for reducing the plasma pollution. The conceptual divertor design is based on the use of EUROFER97 for the divertor cassette body, while tungsten monoblocks bonded to CuCrZr pipes are used for plasma-facing targets. EUROFER97 was selected considering its reduced long-term activation and superior creep and swelling resistance under neutron irradiation. However, depending on the operating temperature under neutron irradiation, a pronounced shift of the Ductile to Brittle Transition Temperature (DBTT) is expected. At the same time, for the plasma-facing targets, the coolant temperature has to be identified such to allow sufficient heat removal capacity at the strike point. This study explores alternative cooling conditions for the divertor system that are able to ensure the fulfillment of functional and system requirements and to allow for divertor cassette body re-use during plant lifetime. The main aim is to identify the best water cooling thermal-hydraulic conditions avoiding material embrittlement (for EUROFER 97) and softening/hardening (for copper alloy pipes). At the same time, the goal is to reduce the inventories (enthalpy of the cooling circuit) and the radwaste at the end of divertor lifetime.

Enhancing the Tensile Properties and Ductile-Brittle Transition Behavior of the EN S355 Grade Rolled Steel via Cost-Saving Processing Routes.

Structural rolled steels are the primary products of modern ferrous metallurgy. Consequently, enhancing the mechanical properties of rolled steel using energy-saving processing routes without furnace heating for additional heat treatment is advisable. This study compared the effect on the mechanical properties of structural steel for different processing routes, like conventional hot rolling, normalizing rolling, thermo-mechanically controlled processing (TMCP), and TMCP with accelerating cooling (AC) to 550 °C or 460 °C. The material studied was a 20 mm-thick sheet of S355N grade (EN 10025) made of low-carbon (V+Nb+Al)-micro-alloyed steel. The research methodology included standard mechanical testing and microstructure characterization using optical microscopy, scanning and transmission electronic microscopies, energy dispersive X-ray spectrometry, and X-ray diffraction. It was found that using different processing routes could increase the mechanical properties of the steel sheets from S355N to S550QL1 grade without additional heat treatment costs. TMCP followed by AC to 550 °C ensured the best combination of strength and cold-temperature resistance due to formation of a quasi-polygonal/acicular ferrite structure with minor fractions of dispersed pearlite and martensite/austenite islands. The contribution of different structural factors to the yield tensile strength and ductile-brittle transition temperature of steel was analyzed using theoretical calculations. The calculated results complied well with the experimental data. The effectiveness of the cost-saving processing routes which may bring definite economic benefits is concluded.

The effects of interstitial oxygen on mechanical properties of TiZrNb medium-entropy alloy over a wide temperature range

Interstitial oxygen plays an important role in the microstructure and mechanical properties of recently developed refractory high-entropy or complex concentrated alloys. However, the mechanical properties of the oxygen-doped complex concentrated refractory alloy over a wide temperature range remain unclear. In this work, the effect of interstitial oxygen on the mechanical behavior of TiZrNb medium-entropy alloys (MEAs) has been investigated. At room temperature (RT), the interstitial oxygen improves the yield strength and tensile ductility owing to interstitial solid-solution strengthening and strain-hardening enhancement. However, simultaneous improvement of the RT strength-ductility by adding interstitial oxygen comes at the expense of increasing the ductile-brittle transition temperature (DBTT). Furthermore, the high-temperature yield strength of TiZrNb alloy can be improved up to 800 °C with the addition of 2 at% oxygen. The results above provide valuable insight into the mechanical properties of TiZrNb MEA, which would contribute to optimizing the high-performance refractory complex concentrated alloys in various applications.