Ductile-brittle Transition Temperature Research Articles

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.

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.

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.

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.

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.

Investigating the ductile to brittle transition phenomenon in binary Fe-Ni systems using molecular dynamics simulation

Ductile to brittle transition (DBT) phenomenon is commonly observed in BCC Fe-based systems, which significantly deteriorates their low temperature fracture toughness. Ni is known to be an effective alloying element to improve the low temperature toughness by reducing the ductile to brittle transition temperature (DBTT). The present article investigates the role of Ni in tailoring the DBT curve of BCC Fe by employing molecular dynamics (MD) simulation. Here, we performed mode-I loading of different pre-cracked systems (pure Fe, Fe-2 wt% Ni and Fe-4 wt% Ni alloys) over a temperature range of 1–800 K to construct their DBT curves. The DBT curves are subsequently correlated with the temperature sensitivity of fracture stress, which is estimated using the concept of critical strain energy release rate (GIC). The role of Ni in reducing the fracture stress sensitivity is responsible for decreasing the DBTT as well as the slope of the DBT region. The underlying mechanism is discussed in terms of the crack blunting effect of Ni which effectively increases the GIC and hence the fracture stress, particularly at the low temperature regimes. Besides, the significantly large strain rate imposed in MD simulation is mainly responsible for the higher DBTT values when compared with the experimental results. The present paper tackles this strain rate issue by using an appropriate strain rate sensitivity factor to rescale the simulated DBTT values down to the experimental ones, resulting in a good match between the simulated and experimental DBTT.

Enhancement of strength and ductile-brittle transition temperature of SA508 Gr.3 low-alloy steel by controlling heat accumulation in laser powder-directed energy deposition

The safety and longevity of small modular reactors are affected by reactor pressure vessels, which are complex integral components made of SA508 Gr.3 low-alloy steel. In this study, the impacts of heat accumulation on the microstructural and mechanical characteristics (tensile properties and Charpy V-notched impact energy) of SA508 Gr.3 steel fabricated using laser powder-directed energy deposition were investigated. Rectangular samples were prepared using long raster and short raster scanning strategies for changing heat buildup, and the mechanical tests were conducted depending on build direction. The time-temperature profile measured at a fixed point in the long raster sample showed a lower maximum temperature and a higher cooling rate, indicating lower heat accumulation compared to that of the short raster sample. In each build direction, the yield strength of the long raster sample was 45.8 %–60.5 % higher and its ductile-brittle transition temperature was 76.8–103.8 °C lower than that of the short raster sample. Additionally, compared to conventionally made samples and without requiring heat treatment, the long raster sample exhibited over a 45 % increase in yield strength and a 22.7 °C reduction in the ductile-brittle transition temperature. The superior combination in long raster samples is induced by smaller effective grain size, smaller cementite, and a higher pre-existing dislocation density. The results emphasize the importance of controlling heat accumulation throughout the additive manufacturing process and provide valuable insights into the use of additive manufacturing for manufacturing reactor pressure vessels in the nuclear industry.

Influence of thermomechanical treatment on mechanical properties of CLAM steel

To enhance the creep resistance of CLAM steel and thus elevate its maximum service temperature, in our prior research, a superior thermomechanical treatment process was developed based on thermal simulation experiments conducted. This study employed forging to achieve the desired thermal deformation in accordance with the superior thermomechanical treatment process, followed by mechanical property testing post tempering. Comparative analysis with normalized and tempered CLAM steel (N + T CLAM steel) revealed notable improvements in yield strengths ranging from 31–55 %, ultimate tensile strengths from 30–46 %, and a reduction in total elongations by 12–21 % at both room temperature and elevated temperatures. Additionally, the ductile-to-brittle transition temperature (DBTT) increased from -65 °C to -27 °C. Creep behavior of N + T CLAM steel exhibited similarities to other RAFM steels, with stress exponents measured at 500 °C, 550 °C, 600 °C, and 650 °C recorded as 16.5, 15.0, 10.2, and 7.8, respectively. The creep activation energy was determined to be 556 kJ/mol, closely aligning with results from Eurofer 97 steel and F82H steel, and approximately twice the self-diffusion activation energy of iron. The Larson-Miller parameter method was employed to forecast stress-rupture limits under various conditions, with a predicted result of 120 MPa under typical design life conditions (550 °C / 100,000 h) for the cladding module, comparable to Eurofer 97 steel and F82H steel. TMT CLAM steel exhibited a stress exponent of 16.7 at 550 °C, with a creep activation energy of 578 kJ/mol. Under typical design life conditions (550 °C / 100,000 h) for the cladding module, the predicted stress-rupture limit increased to 176 MPa, representing a 47 % improvement over N+T CLAM steel. Moreover, under the same conditions (550 °C / 230 MPa), the rupture time of TMT CLAM steel was 54 times longer than that of N+T CLAM steel.

Additive manufacturing of refractory metals and carbides for extreme environments: an overview

Refractory metals and their carbides possess extraordinary properties when subjected to high temperatures and extreme environments. Consequently, they can act as key material systems for advancing many sectors, including space, energy and defence. However, it has been difficult to process these materials using the conventional routes of manufacturing. Additive manufacturing (AM) has shown a lot of potential to overcome the challenges and develop new material systems with tailored properties. This review provides a fundamental understanding of the challenges in the processing of refractory metals and their carbides, including microcracking, formation of brittle oxide phases and high ductile to brittle transition temperature (DBTT). We also highlight some of the novel approaches that have been taken to improve the processability of these challenging material systems using AM. These include in-situ reactive printing, ultrasonic vibration, laser beam shaping, multi-laser deposition and substrate pre-heating with a focus on microstructural changes to improve the properties of printed parts.

Using Acoustic Emission Pulses to Assess the Ductile–Brittle Transition Temperature of Aluminum Matrix Composites at Negative Temperatures

Using Acoustic Emission Pulses to Assess the Ductile–Brittle Transition Temperature of Aluminum Matrix Composites at Negative Temperatures

Predicting Ductile–Brittle transition temperatures for polyolefins using convolutional neural networks and instrumented notched Charpy experiments

For polyolefins, a transition from ductile- to brittle fracture behavior can be observed at decreasing temperatures, which is typically characterized by the Ductile–Brittle Transition Temperature (DBTT). For each material, the DBTT needs to be determined by performing multiple impact tests across wide temperature ranges. In this study, we utilize Convolutional Neural Networks (CNN) to predict the DBTT from single Instrumented Notched Charpy (INC) experiments instead. Our dataset consists of over 6732 INCs encompassing 132 Polyethylene (PE) and Polypropylene (PP) compounds. For each of these compounds, we examine 51 impact tests at temperatures ranging from -40 °C to 60 °C. We train models on combinations of instrumentation signals and fracture surface images. We test the trained networks on 1683 INCs, where we achieve a R2-score of 0.96 and an average error below 5 °C of predictions compared to experimentally determined DBTTs. The presented methodology provides a flexible way of predicting DBTTs in case of non-instrumented testing or if thermal specimen conditioning is not possible. This allows for fast screening of new materials.

A quantitative relation for the ductile-brittle transition temperature in pipeline steel

An accurate description of pipeline steel at low temperatures requires a comprehensive understanding of its ductile-brittle transition temperature (DBTT). In this work, we collect a data set of low-temperature toughness for pipeline steel and reduce the dimensionality of the data set using several feature screening approaches. Multiple machine learning models, validated via ten-fold cross-validation, are then employed to fit and predict the DBTT. Symbolic regression allows us to derive a relation for DBTT appropriate for pipeline steel. Such a formula is shown to provide a versatile model to estimate the DBTT of pipeline steel. It not only serves as a guide to predict the low-temperature properties of pipeline steel but also lays the groundwork for further research on other steel materials.

Crystal plasticity finite element model of the brittle–ductile transition temperature and fracture toughness of irradiated A508-3 steel

Crystal plasticity finite element model of the brittle–ductile transition temperature and fracture toughness of irradiated A508-3 steel

Polyolefin ductile-brittle transition temperature predictions by machine learning

Polymers show a transition from ductile-to brittle fracture behavior at decreasing temperatures. Consequently, the material toughness has to be determined across wide temperature ranges in order to determine the Ductile-Brittle Transition Temperature This usually necessitates multiple impact experiments. We present a machine-learning methodology for the prediction of DBTTs from single Instrumented Puncture Tests Our dataset consists of 7,587 IPTs that comprise 181 Polyethylene and Polypropylene compounds. Based on a combination of feature engineering and Principal Component Analysis, relevant information of instrumentation signals is extracted. The transformed data is explored by unsupervised machine learning algorithms and is used as input for Random Forest Regressors to predict DBTTs. The proposed methodology allows for fast screening of new materials. Additionally, it offers estimations of DBTTs without thermal specimen conditioning. Considering only IPTs tested at room temperature, predictions on the test set hold an average error of 5.3°C when compared to the experimentally determined DBTTs.

Nonlinear Impact Damage Evolution of Charpy Type and Analysis of Its Key Influencing Factors

The current research of Charpy impact mainly focuses on obtaining the ductile brittle transition temperature of materials by experiments. Compared with experiments, numerical simulation can study many problems with harsh conditions. However, there are still few studies on the influence of geometric factors such as side grooves. In this paper, the geometry of standard Charpy impact test is designed. Specimens with different widths and side grooves are tested. The finite element model of Charpy impact was established by ABAQUS software. Use test results and simulation results to verify each other. The effects of sample width, side groove depth and side groove bottom fillet on the impact fracture resistance of the sample were studied. The results show that the specimen width is positively correlated with the impact toughness of the specimen. The side groove greatly reduces the impact toughness of the material; the toughness of side groove decreases with the increase of depth; the fracture toughness of side groove decreases with the increase of fillet at the bottom of side groove. The proportion of toughness energy to impact energy of samples was analyzed. The results show that the toughness energy accounts for about 70% of the impact energy of the sample, which has little to do with the geometric characteristics of the sample. This study presents a reliable method for studying Charpy impact tests. The influence of geometric parameters is obtained, which provides a reference method for the study of impact toughness of high toughness materials.

Study of fractography of ferritic ductile iron at different temperatures and loading conditions

This study characterizes the fractography of ferritic ductile iron under various loads, including tensile, fatigue and bending, and impact conditions. The results indicate that ductile fracture is the primary mechanism observed during tensile testing at room temperature. The fractography resulting from fatigue testing exhibits characteristics similar to cleavage fracture, and explains the formation of fatigue striations caused by the joint effects of dislocation slip and oxidation under crack tip stress. Under impact testing, the main fracture mechanism transitions from ductile to brittle with decreasing temperature. At high temperatures, fractography is mainly characterized by elliptical dimples with graphite nodules at the center that deform along the stress direction. In the ductile-brittle transition temperature range, a mixed fracture mechanism involving both dimples and cleavage patterns is observed. At low temperatures, the fracture mechanism is cleavage fracture, cleavage fracture is mainly caused by the deformation twin, inducing crack nucleation. These findings further validate D.O.Frenandino’s quantitative analysis method for determining the main crack propagation direction of ductile fracture and brittle fracture. By employing larger statistical datasets, it is shown that this method yields high accuracy in determining the main crack propagation direction of ferritic ductile iron, thereby promoting its application as a general method for impact fractography analysis.