Ductile-brittle Transition Temperature Research Articles

Superior strength-ductility synergy of high potassium-doped tungsten rods with large swaging deformation

The high ductile-to-brittle transition temperature (DBTT) is a key factor limiting the use of tungsten (W) - based materials in future nuclear fusion reactors. To overcome this challenge, and to obtain W alloys with improved mechanical properties without sacrificing substantial thermal conductivity, a large-sized W rod with a total weight of 42 kg and with 96 ppm doped potassium (K) was fabricated through a powder metallurgy mass production route. Then high-temperature swaging processes were conducted to produce a rod with large plastic deformation of 94.7% (log. Strain of 2.95). The swaged rod showed good low-temperature ductility and high strength simultaneously. At 50 °C, the AKS-W rod fractured in brittle cleavage mode. The fracture changed to a mixture of brittle and ductile at 100 °C, indicating a DBTT of 50 °C - 100 °C. This value was lower than many other reported in pure W and K-doped alloys. The swaged W rod exhibited a high strength of larger than 1.0 GPa up to 100 °C, with good conductivity at low temperatures. The maximum total elongation of 30.24% was obtained at 200 °C. The superior strength-ductility synergy of AKS-W rod was observed to be attributed to its unique microstructure and K bubbles morphology, which evolved during severe hot plastic deformation.

Synthesis and Rapid Sintering of Ultra Fine TiB2-ZrO2 Composite

TiB2 is considered candidate materials for ultra-high temperature ceramics and cutting tools because of its a high thermal conductivity, a low coefficient of thermal expansion, a high hardness and high melting temperature. Despite these attractive properties, TiB2 applications are limited because it has a low fracture toughness below the brittle-ductile transition temperature. To improve on its mechanical properties, the approach universally utilized has been to add secondary materials to form a composite and to fabricate an ultra - fine material. A dense ultra - fine TiB2- ZrO2 composite was rapidly sintered using pulsed high current activated heating (PHCAH) methods within 3 min in one step from the mechanically synthesized the powders of TiB2 and ZrO2 . Consolidation was reached using an effective combination of mechanical pressure and the pulsed high current. A highly dense TiB2-ZrO2 material with relative density of 97.2% was made by the simultaneous application of 75 MPa pressure and a pulsed 2500 A current. The grain sizes of TiB2 and ZrO2 in the composite were 135 nm and 84 nm, respectively. The fracture toughness and hardness of the TiB2 -ZrO2 composite were 11.2 MPa.m1/2 and 957 kg/mm2 , respectively. The fracture toughness of the TiB2 -ZrO2 composite was three times higher than that of monolithic TiB2 .

Machine-Learning-Based Composition Analysis of the Stability of V–Cr–Ti Alloys

Machine learning methods allow the prediction of material properties, potentially using only the elemental composition of a molecule or compound, without the knowledge of molecular or crystalline structures. Herein, a composition-based machine learning prediction of the material properties of V–Cr–Ti alloys is demonstrated. Our machine-learning-based prediction of the stability of the V–Cr–Ti alloys is qualitatively consistent with the composition-dependent experimental data of the ductile–brittle transition temperature and swelling. Furthermore, our computational results suggest the existence of a composition region, Cr+Ti ~ 60 wt.%, at a significantly low ductile–brittle transition temperature. This outcome contrasts with a reportedly low Cr+Ti content of less than 10 wt.% in conventional V–Cr–Ti alloys. Machine-learning-based numerical stability prediction is useful for the design and analysis of metal alloys, particularly for multicomponent alloys such as high-entropy alloys, to develop materials for nuclear fusion reactors.

A unified model for ductile-to-brittle transition in body-centered cubic metals

Ductile-to-brittle transition (DBT) is a well-known phenomenon in body-centered-cubic (BCC) metals, intermetallics and semiconductor materials. A quantitative prediction of the DBT temperature, however, has so far remained intractable. Here, we propose a unified model based on the efficacy of dislocation multiplication as the controlling factor for DBT, with the dislocation source efficiency governed by the relative mobility of screw versus edge dislocations. The model successfully predicts the DBT temperature of iron, molybdenum and tungsten, and also covers the influence of grain size, initial dislocation density, and the multiplicity of dislocation sources. A comparison with experiments indicates that the model captures the key DBT features, providing new insight into the toughness of BCC metals.

Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.

Effect of Normalizing on Impact and Corrosion Resistance of Low-temperature Service Seamless Steel Pipe

The seamless line pipe, which has fine low temperature impact properties for Low-temperature Service and good H2S corrosion resistance, was investigated through chemical composition designing, optimizing smelting-rolling processes, and heat treatment processes following ASTM A333 standard. The influence of normalizing on the impact property and corrosion resistance was studied by optical microscope, scanning electron microscope, transmission electron microscope, mechanical test and corrosion test. It is demonstrated that the low temperature mechanical properties and anti-H2S corrosion properties of hot-rolled trial-pipe and normalized trial-pipe meet ASTM A333 standard. The microstructure of hot-rolled trial-pipe is pearlite and ferrite, and the grain distribution is uniform and grain size is fine. The dominated precipitates phase are TiN particles in hot-rolled trial-pipe. Ductile to brittle transition temperature is determined at -60 °C. The grain refinement is achieved after normalized and the main precipitates are TiN particles together with a few VN particles. Ductile tobrittle transition temperature was further lowered, at - 80 °C. The corrosion behavior of the material was examined by hydrogen induced cracking (HIC) corrosion test, sulfide stress corrosion (SSC) test and polarization test at various polarization scanning rates. It is observed that the anti-HIC corrosion performance and SSC corrosion resistance are similar for both hot-rolled and normalized pipes, but the performance of electrochemical corrosion of normalized pipe is better than hot-rolled pipe, which indicates that the normalized pipe is more suitable for demanding corrosive environments.

Influence analysis of alloy elements on irradiation embrittlement of RPV steel based on deep neural network

Reactor pressure vessel (RPV) is the most important core equipment in PWR. Its service life determines the service life of nuclear power plant and directly affects the economy and safety of nuclear power plant. Because RPV is serviced at high temperature, high pressure and high energy neutrons for a long time, the properties of RPV steel will significantly degrade, in which irradiation embrittlement is the most important factor for the structural integrity of RPV. In this work, about 700 groups of data such as composition, irradiation conditions and ductile brittle transition temperature of RPV steel are collected, and the data are cleaned and screened for modelling by machine learning. The deep neural network is used for establishing the correlation between key component and irradiation embrittlement of RPV steel. The results show that the lower flux of neutron irradiation will make the radiation embrittlement effect of RPV steel more obvious at the same neutron fluence. Cu, P and Ni are the key factors to influence the △DBTT of RPV steel. The synergistic effect of Cu and Ni on irradiation embrittlement is greater than that of Cu and Mn. These results will help to promote the optimization design of new RPV steel.

Ductile–Brittle Transition Temperature of Epoxy Asphalt Concrete of Steel Bridge Deck Pavement Based on Impact Toughness

To solve the common low-temperature cracking problem of epoxy asphalt concrete pavement on the steel bridge deck, based on the principle of fracture mechanics and energy method, this paper puts forward the impact toughness as the evaluation index of crack resistance of epoxy asphalt concrete and verifies the feasibility of this index through theoretical analysis and experiment. At the same time, based on the study of impact toughness, the ductile–brittle transition temperature of epoxy asphalt concrete was explored, and the working state of epoxy asphalt concrete at different temperatures was mastered. The results show that the impact toughness is closely related to the J integral, which can better evaluate the crack resistance of epoxy asphalt mixture. The Boltzmann function can accurately reflect the relationship between impact toughness and temperature and can determine the ductile–brittle transition temperature and transition interval of epoxy asphalt concrete. Impact toughness and ductile–brittle transition temperature can be used as the evaluation index of crack resistance of epoxy asphalt concrete, which provides a new design idea for the study of crack resistance of epoxy asphalt concrete and the scope of engineering application.

Pre-stored edge dislocations-enabled pseudo-toughness in chromium

Pre-deformation is an effective mean to tune the ductile-to-brittle transition (DBT) temperature of body-centered cubic metals. However, due to complex microstructural evolution during pre-straining, altering not only dislocation density but also grain size, the mechanism of reduction in DBT temperature remains intriguing. Here, we designed two types of chromium samples, with similar grain size but different initial dislocation density, to uncover the underlying mechanisms that govern the variation in DBT. Small-punch, quasi-in situ crack propagation, and in-situ nanomechanical tests conducted over a wide temperature range revealed that pre-stored edge dislocations only can make pseudo-toughness, i.e., increasing deformability but the sample is still intrinsically brittle-prone to fragile cracking. Activation of the easy glide pre-stored edge dislocations could enhance the crack resistance at the initial stage of mechanical loading, which makes a left-shift of the DBT temperature, while the toughness/brittleness of chromium is still controlled by the relative mobility of screw versus edge dislocations.

Effect of tantalum addition on the mechanical properties of tungsten/tantalum composite thin films

Tungsten (W) when used as a plasma-facing material (PFM) in a tokamak will face harsh neutron radiation and high-temperature fluctuations. Due to its ductile-to-brittle transition temperature (DBTT), W behaves as a brittle material in lower temperatures and thus helps in propagating cracks easily resulting in dust formation and failure of material. In this study, an attempt has been made to improve the ductility, hardness, and toughness of W thin films by adding tantalum (Ta) to it, resulting in composite thin films. W/Ta composite thin films deposited on reduced-activation ferritic martensitic steel (RAFM) steel by RF magnetron sputtering method are compared with W thin films deposited on the same substrate. Taguchi orthogonal array is used to get the best properties for the composite thin films and nanoindentation technique is used to evaluate the mechanical properties of developed thin films. With the least hardness of 7.19 Gpa, composite film hardness is 452% lesser than the maximum hardness of (39.54 Gpa) W films. The reduction in hardness for composite thin films has not depleted the toughness as the Ef/Es ratio is higher than cracking threshold of 1.3. The maximum plastic energy absorbed for yield by composite films during nanoindentation is (1710 mN nm) 14% higher than W thin film (1500 mN nm), indicating the composite film is a ductile and tougher than the W film.

DBTT and tensile properties of as-sintered tungsten alloys reinforced by yttrium-zirconium oxide

Tungsten alloys with 0.5wt%Y2O3 and 0-2.0wt%ZrO2 was prepared by spark plasma sintering (SPS). The grain size of W-0.5wt%Y2O3-0.5wt%ZrO2 (WYZ05) is about 1.95 µm, which is much smaller than that of W-0.5wt%Y2O3 (WY). The tensile tests results at high temperature show that compared with pure W and WY alloys, WYZ05 alloy has higher high temperature hardness and lower ductile-to-brittle transition temperature (DBTT). At 500°C, the tensile strength of WYZ05 alloy is 525.2 MPa, which is 67.53% higher than that of WY. The DBTT of W-0.5wt%Y2O3-(0.5, 1.0, 2.0)wt%ZrO2 material is between 300°C and 400°C. According to the microstructure analysis, it is considered that the reason for improving the mechanical properties of WYZ05 alloy is the fine grain strengthening and Orowan strengthening caused by Y2O3/ZrO2 particles.

Microstructure Evolution by Thermomechanical Processing in the Fe-10Al-12V Superalloy

Nowadays, great efforts are being made to develop bcc-superalloys for medium- and high-temperature applications. However, the high brittle-to-ductile transition temperatures (BDTT) have restricted their application. Therefore, designing hot-processing routes to obtain a refined grain in these new superalloys is required. Particularly in the Fe-10Al-12V (at%) alloy, we have recently tested the BDTT shifting and, using physical models, it was indicated that a combination of L21-precipitate sizes with small grain sizes could shift the BDTT below room temperature. Here, we will present the study that allowed us to design the processing route for grain refinement in the tested superalloy. Molds of different geometry and with metallic and sand walls were used to test two different types of casting. Carbide conditioning treatments for improving the sizes and distribution were studied. The recrystallization process was explored first by hot rolling and post-annealing in stepped geometry samples with two different columnar grain orientations. Finally, we analyzed the grain microstructure obtained along a hot processing route consisting of carbide conditioning treatment, forging into a squared bar, and hot rolling up to a 2.8 mm thickness strip.

Analysis of Ductile-to-Brittle Transition Characteristics of Reactor Pressure Vessel Steels

Ductile-to-brittle transition (DBT) characteristics of three steels for reactor pressure vessel (RPV) belt line application are analyzed from new parameters based on model functions describing the strength and toughness characteristics of the materials. In order to estimate nil-ductility temperature (NDT) from strength property, a strain rate–compensated temperature parameter based on the thermally activated deformation of materials is adopted. A measure of NDT is determined from tensile strength properties for the first time assuming an estimated notch tip strain rate at the lower shelf. It is estimated to be 110, 42, and 106 K for the Cr-Mo-V-Ni, 20MnMoNi55, and A533B steels, respectively. The measure of ductile-to-brittle transition temperature (DBTT) in steels using 41-J Charpy impact-absorbed energy on the basis of a logistic class of functions is compared and shown to be equivalent with those obtained from fitting the tanh model equation. A bi-logistic function based on the concept of separable parameters representing the fracture of ductile and brittle zones in steels within the DBTT regime was applied to model the Charpy impact energy behavior of the three steels. The bi-logistic function-fitting parameters yielded a new measure of brittleness as a DBT characteristic of steels that correlated well with other measures of transition temperature of the selected RPV steels. The parameters from the hyperbolic and logistic fitting were used to develop a model relationship suitable for the generation of a master curve based on Charpy energy in exponential form that unifies the transition temperature behavior of the selected western and eastern RPV materials. The model relationship is also found to closely predict ~5 K of the reference temperature To determined as per American Society for Testing and Materials standard E1921 of the selected RPV steels.

Study of cleavage fracture in ferritic stainless steels Part II: Cleavage micro-mechanisms and critical stresses

Cleavage fracture and the mechanisms involved in different ferritic steel model microstructures are investigated according to grain sizes, precipitates and solute atoms in the matrix. For each case, one of these parameters is varied. Estimation of the critical stress for cleavage was investigated and discussed: it is shown to be determined from a simple Griffith inspired analysis or by employing the Smith's description. The description to adopt depends on careful analysis of the involved mechanisms. This aims at better understanding the governing parameters of the Ductile to Brittle Transition Temperature (DBTT) and to suggest routes for alloys processing to reduce the DBTT in ferritic steels.

The effect of thermal aging on fracture properties of a narrow-gap Alloy 52 dissimilar metal weld

In nuclear power plants, dissimilar metal welds belong to safety class 1. The effect of thermal aging at 400 °C up to 15000 h on ductile-to-brittle transition (DBT) region was investigated for an Alloy 52 dissimilar metal weld (DMW). The cracks and the notches were placed close to the fusion boundary between the low-alloy steel (LAS) and the weld metal. In previous work for DMWs, the shifts in different DBT temperatures have not been investigated, including reference temperature T0, impact toughness-based T28J and reference temperature for arrest toughness, TKIa. The results show that the T0 reference temperature does not change significantly with aging time, only 10 °C, but the shift in T28J is 49 °C after 15000 h. The shift in TKIa is 35 °C. The results indicate that the aging mechanism affects more the crack propagation and arrest properties than brittle fracture initiation.

Deformation behaviour and notch sensitivity of a super duplex stainless steel at different strain rates and temperatures

For the successful development of a structure, understanding of mechanical behaviour of the selected material is an important issue to prevent damage or accidents. Mechanics of the material failure depends on its chemical composition, heat treatment, grain size, strain rate, type of loadings etc. Apart from material properties, the performance of the structure is also affected by its geometry, crack/notch/hole/groove and manufacturing processes. In construction of offshore, marine, power plant and oil-gas industry structures, super duplex stainless steel is one of the suitable materials. Therefore, the objective of the paper is to study the deformation behaviour and notch sensitivity of a super duplex stainless steel, SDSS 2507 at different strain rates (0.0001–0.1s−1) and temperatures (25–200 °C) under tensile, compressive and flexural (three-point bending) loads. Heating rate and soaking time are 8 °C/min and 15 min respectively. Experiments are conducted on electromechanical universal testing machine using suitable fixtures. The steel is found positive sensitive to the strain rate and temperature, where the phenomenon of dynamic strain ageing (DSA) is observed during thermal effects. Fractography analysis is correlated with the typical stress-strain curves. Heat treatment (heating rate 12 °C/min and soaking time 1–3 h) enhances the tensile strength of the steel upto 18% at 400–600 °C for strain rate 0.001s−1, whereas at 700 °C, the strength decreases by 31% and the energy dissipation decreases by 88% due to its brittle behaviour. Notch profiles (C, U and V) on the flat tensile specimens of the steel decreases the energy dissipation by nearly 67–74% at 0.001s−1. Ductile-brittle transition temperature exists between 600℃-700℃ in tensile tests. Compression tests are conducted for varying geometries of cylindrical specimens. Flexural behaviour of the steel is investigated for 120–160 mm span lengths and flat-transverse orientations. Existing material models (Cowper-Symonds and Johnson-Cook) are evaluated.

Response of hydrogen diffusion and hydrogen embrittlement to Cu addition in low carbon low alloy steel

Hydrogen embrittlement (HE) has arisen as a critical issue for the development of high strength steel. The austenite and Cu-rich precipitates can provide stable trapping sites for diffusible hydrogen, and thus improved the resistance to HE. But the research on the synergistic effect of Cu and austenite on HE in low carbon low alloy steel has not been actively reported. In this study, four steels with varying Cu addition (0-3 wt%) were fabricated, and their resistance to HE was evaluated. The role of Cu addition was investigated by the ductile-brittle transition temperature (DBTT) value obtained from Charpy impact test and the loss of elongation measured from slow-strain-rate tensile (SSRT) test after hydrogen charging. Also, the hydrogen diffusivity and concentration were evaluated by electrochemical hydrogen permeation test. Cu addition resulted in a first decrease and subsequent increase of effective diffusion coefficient (D eff ), elongation loss and DBTT, and the minimum value was achieved in steel with 1.5 wt% Cu addition, indicating a higher resistance to HE. The unraveled mechanism was that Cu addition increased the content of retained austenite, which impeded the diffusion of hydrogen during deformation. But the stability of retained austenite decreased when the content of retained austenite reached a high value, which led to martensitic transformation and accordingly increased HE susceptibility. Moreover, Cu addition promoted the formation of Cu-rich precipitates in which Cu cluster/bcc Cu-rich precipitate coherent/semi-coherent with matrix can be strong trapping sites for hydrogen, and fcc Cu-rich precipitate acted as weak trapping sites for hydrogen. In addition, a higher density of dislocations was found in the steel with 1.5 wt% Cu addition, which may contribute to lower D eff and higher density of hydrogen traps (N T ). The present work suggested that Cu addition can enhance the resistance to HE during tensile and impact processes for the design of high strength steel. • Cu addition resulted in a first decrease and subsequent increase in the effective diffusion coefficient (D eff ) of hydrogen. • Cu addition increased the elongation rate and decreased the ductile-brittle transition temperature (DBTT) of the steels after hydrogen embrittlement. • The resistance to hydrogen embrittlement during tensile and impact process was determined by retained austenite, Cu-rich precipitates and dislocations in the microstructure. • The resistance to hydrogen embrittlement varied with the content and stability of austenite and the structure of Cu-rich precipitates.

On the brittleness of elementary semiconductors

It is shown that the brittleness of a single-component covalent crystal (diamond, Si, Ge) is due to the "duplicity" of the paired potential of interatomic interaction for elastic (reversible) and for plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by irreversible breaking than by reversible elastic stretching. It is indicated that the brittle-ductile transition in a single-component covalent crystal is accompanied by metallization of covalent bonds on the surface. It is shown that the brittle-ductile transition temperature (TBDT) for single-component covalent crystals under static load has an upper limit: TBDT/Tm<0.45, where Tm --- is the melting temperature. Keywords: interatomic covalent bond, brittleness, ductility, elementary semiconductors, brittle-ductile transition.

EFFECT OF TANTALUM ON THE IMPACT RESISTANCE OF 12% CR STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT

The effect of tantalum on the impact resistance of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from -40 to +120°C with determining the brittle-ductile transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the entire fracture toughness versus temperature curve is 30-50 J/cm2 higher over the entire temperature range, including the upper and lower shelf energies, and the brittle-ductile transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller initial austenite grain size, a larger average size but lower density of M23C6 carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.

О хрупкости элементарных полупроводников

It is shown that the brittleness of a single-component covalent crystal (diamond, Si, Ge) is due to the “duplicity” of the paired potential of interatomic interaction for elastic (reversible) and for plastic (irreversible) deformation. This leads to the fact that the specific surface energy during plastic deformation of a covalent crystal is more than two times less than the specific surface energy during elastic deformation. Therefore, with a small deformation of a covalent crystal, it is energetically more advantageous to create a surface by irreversible breaking than by reversible elastic stretching. It is indicated that the brittle-ductile transition in a single-component covalent crystal is accompanied by metallization of covalent bonds on the surface. It is shown that the brittle-ductile transition temperature (TBDT) for single-component covalent crystals under static load has an upper limit: TBDT/Tm < 0.45, where Tm is the melting temperature.