Ductile-brittle Transition Temperature Research Articles

Lowering the ductile-to-brittle transition temperature to -36 °C via fine-grained structures in chromium

The inherent ductile-to-brittle transition (DBT) behavior of body-centered cubic (BCC) metals severely limits their applications, and leads to low-temperature brittleness. To enhance the low temperature toughness of BCC metals, we take chromium as an example and design a fine-grained structure with average grain size of 5 μm, which shows a substantial decrease of DBT temperature to -36 °C. Compared with other samples with larger average grain sizes (45–135 μm), the fine-grained chromium still exhibits excellent toughness at -20 °C, and has the highest fracture energy and the yielding load. Our results reveal that numerous grain boundaries act as dislocation sources to emit easy glide edge dislocations at low temperature and also effectively slow down crack propagation, all together toughen ambient brittle chromium.

High-temperature compression deformation behavior of the high Nb-containing TiAl alloys doped with Sn

In this work, the high-temperature compression behavior of high Nb-containing TiAl–Sn alloys was investigated at deformed temperatures of 700 °C–1000 °C and strain rates of 0.002 s−1–0.2 s−1. The true stress-strain curves, microstructures, and constitutive equations of TiAl-xSn (x = 0, 0.5, 1, 1.5) alloys under different deformation conditions were systematically compared. Sn addition can increase the flow stress of TiAl alloys at the same deformation temperature, which is caused by the solid solution strengthening effect. At 800 °C/0.002 s−1, the peak flow stress of TiAl–1Sn alloy increases by 180 MPa compared with TiAl–0Sn alloy of 600 MPa. Below the peak stress, the flow stress increases with the increase of strain. Above the peak stress, the flow stress first decreases and then gradually trends to a steady state as the increase of deformation. Furthermore, the Sn addition decreases the brittle-ductile transition temperature of TiAl alloys. The TiAl-xSn (x = 0.5, 1, 1.5) alloys displays obvious flow softening behavior at 700 °C while TiAl–0Sn alloy fractures before reaching the target deformation. The relationship between of the flow stress, strain rate, and deformation temperature for TiAl–0Sn and TiAl–1Sn alloys is suitable for the hyperbolic sinusoidal equation. The dominant softening mechanism of hot deformation for TiAl-xSn (x = 0, 1) alloys is dynamic recrystallization (DRX). The activation energies Q of hot deformation for TiAl–0Sn and TiAl–1Sn alloys are 311 kJ mol−1 and 429 kJ mol−1 at condition of 700–1000 °C/0.002–0.2 s−1, indicating that the TiAl–1Sn alloy exhibits a better stability under the same application condition.

Effect of grain size and grain boundary on ductile to brittle transition behavior of Fe-6.5wt.%Si alloy under miniaturized three-point bending tests

This research investigates the influence of grain size and grain boundary on the ductile to brittle transition (DBT) behavior using three-point bending tests. With increasing grain size from 91 μm to 255 μm, the ductile to brittle transition temperature (DBTT) increases by approximately 80 °C, accompanied by a corresponding 0.196 eV increase in DBT activation energy. An evident linear reciprocal relationship exists between DBTT and grain boundary ratio. The Hall-Petch equation is employed to determine the quantitative relationship between the DBTT and grain size, offering novel guidance for determining the rolling temperature of Fe-6.5wt.%Si alloy.

A 3D rheological model for the Aegean Region: Mechanical layering and seismotectonic implications

We implemented a 3D thermo-rheological model for the broader Aegean Region, with special focus on determining the depth of the brittle-ductile transition (BDT) and the thickness of the brittle, potentially-seismogenic layers. In order to gain a more precise and comprehensive view on the thermo-physical characteristic of the main rheological transition, the model provides strength and temperature distribution along depth, as well as the relative values at the BDT depth. In particular, the reconstruction of a 3D model, allows to identify major trends and peculiarities, and to characterize the BDT depth, temperature and strength variations within this sector of the Mediterranean realm. The results indicate that a shallow transition (10–15 km) is common in the internal Aegean Region and most of continental Greece, while the external regions geodynamically associated to the Hellenic subduction show much deeper BDTs (around 30–35 and 40–45 km in continental and oceanic lithosphere sectors, respectively). The numerical modelling factors appearing to exert the greatest control on the BDT depth, and more in general on the spatial distribution of the rheological properties, at the scale of the whole study area, are the surface heat flow (as a proxy for the geothermal gradient) and the nature of the considered crust (continental vs oceanic). As a direct application of the results, the reconstructed 3D model of the shallower BDT is then exploited within the Aegean Region for defining the thickness of the seismogenic layer and its lateral variations. This in turn allows to independently constrain the maximum width of GreDaSS seismogenic sources and, by applying empirical relationships, to better quantify the maximum magnitudes expected on these sources.

Preparation of spherical tungsten powder with uniform distribution of lanthania by plasma spheroidization

The high ductile-brittle transition temperature of tungsten (W) makes it prone to crack during additive manufacturing. Incorporating lanthania (La2O3) to tungsten had been proven to be an efficient way to solve the cracking problem. However, the current incorporation methods are often accompanied by problems such as uneven dispersion of lanthania. In this study, W-La2O3 spherical powder with uniform distribution of lanthania was successfully prepared by spray granulation, heat treatment, and plasma spheroidization. The morphology, particle size distribution, flowability, apparent density, and element distribution of W-La2O3 powder at different stages were studied. The results show that the final obtained W-La2O3 powder has smooth surface and narrow particle size distribution, which also has excellent flowability (8.2 s/50 g) and apparent density (10.7 g/cm3). Moreover, it was observed that the plasma spheroidization resulted in the decrease of lanthania content in the powder, and the law of variation of lanthania content was obtained by the linear fitting method. The spheroidization mechanism of W-La2O3 composite powders was also proposed by studying the microstructure of incompletely spheroidized powders and the change of lanthania content.

Effects of thermal aging at 873 K on the impact properties of an ODS ferritic steel

This investigation presents the effects of long-term thermal aging on the impact properties of an ODS reduced activation ferritic steel. The ODS ferritic steel, with composition Fe–14Cr–2W–0.4Ti–0.3Y2O3 (wt. %), was manufactured by mechanical alloying, compacted by hot isostatic pressing and hot cross rolled. A 1273 K annealing treatment was applied, followed by thermal aging at 873 K for 2000 h in Ar, to evaluate the thermal stability of the alloy. Charpy impact testing was performed from 77 to 473 K to investigate the mechanical response of the material under dynamic loads. The ductile-to-brittle transition temperature (DBTT) showed a slight increase with values of 255 ± 12 K before aging, and 270 ± 20 K after aging. The aging treatment softened the steep character of the Charpy impact curves inducing a wider transition regime with mixed brittle and ductile fracture mechanisms. This behavior was confirmed by the analysis of Force vs. Displacement Charpy curves and fractographic observations. Hot cross rolling induced a crack divider geometry and secondary cracks formation by delamination. Ductile fracture was less evident after aging due to the redistribution and transformation of Cr-W-rich precipitates along grain boundaries. Electron Backscattered Diffraction highlighted the features of transgranular brittle cracks at temperatures below the DBTT and showed misoriented and deformed regions near the fracture surface at temperatures above the DBTT.

Identifying intrinsic factors for ductile-to-brittle transition temperatures in Fe–Al intermetallics via machine learning

The determination of ductile-to-brittle transition temperatures (DBTT) in intermetallic compounds is crucial for assessing their practical applications. In this study, we investigate the intrinsic factors influencing the DBTT of Fe–Al intermetallic compounds through feature engineering. We developed and evaluated two machine learning strategies for this task. Comparing the strategy that incorporates all features, including alloy compositions and atomic features, with the strategy utilizing selected features, it is found that the latter demonstrates superior computational efficiency and reduces overfitting. Specifically, surrogate models based on two selected features, namely cohesive energy and ionization energy, enable accurate prediction of the DBTT of Fe–Al intermetallics, achieving an accuracy of 95%. Additionally, through symbolic regression, we derived a functional expression that captures the relationship between variations in the DBTT and the selected features of intermetallic compounds. These findings have the potential to serve as a valuable guide for optimizing intermetallic compounds.

Solute-induced grain refinement and defect suppression in boron-modified molybdenum manufactured via laser powder-bed fusion

Molybdenum manufactured with laser powder bed fusion (LPBF) has an undesirable coarse-grained, columnar microstructure interspersed with intergranular cracks, high porosity, and poor mechanical strength. These defects result from a combination of the harsh LPBF process conditions and the disadvantageous properties of molybdenum, such as its high brittle-ductile transition temperature and low tolerance for oxygen impurities. In order to suppress these defect-forming mechanisms and improve the suitability for LPBF, alloy-side material adjustments with simultaneous process optimization are necessary. In this work, the effect of adjusting Mo by adding 3.5 at.% B is investigated experimentally.Mo-3.5 at.% B specimens can be produced entirely free of cracks, with a density of 99.8%. The specimens have a microstructure of fine, equiaxed grains with an average grain size of 31 μm and an aspect ratio of 1.3, thus achieving substantial refinement of the otherwise typically coarse-grained columnar, anisotropic microstructure of pure Mo in LPBF. Furthermore, the grains possess a honeycomb-like cellular subgrain structure. This structure is formed through the solute rejection effect of B during the solidification and consists of initially solidified pure α-Mo cells with a cell size <1 μm and a honeycomb-like network of an ∼100 nm thick intercellular Mo2B phase completely covering the α-Mo cells. In addition, the formation of boron oxide inclusions, presumably B2O3, with a size of <50 nm within the Mo2B phase, provides an effective mechanism for scavenging oxygen impurities, thus ensuring segregation-free grain boundaries in Mo-3.5 at.% B.The microstructural modifications substantially improve the mechanical properties. Under appropriate process conditions, with the substrate plate preheating temperature playing a crucial role, a bending strength of 1120 ± 172 MPa and a hardness of 379 ± 24 HV10 at room temperature can be achieved. At a test temperature of 600 °C, an increase in the bending strength to 2265 MPa is observed, and the bending angle simultaneously increases from 2° at room temperature to 35° at 600 °C. These findings indicate that the strength of Mo-3.5 at.% B is limited by the brittle behavior of the material at lower temperatures, at which residual defects are likely to initiate fracture.

Microstructure evolution and mechanical properties of directionally solidified Al0.5CoCr1.3FeNi1.3(Ti, Si, B, C)0.3 multiphase complex concentrated alloy

Microstructure evolution and mechanical properties of Al0.5CoCr1.3FeNi1.3(Ti, Si, B, C)0.3 multiphase complex concentrated alloy (CCA) prepared by directional solidification combined with quenching during solidification (QDS) technique were studied. The directionally solidified (DS) samples were prepared at constant growth rates V ranging from 2.78 × 10−6 to 5.56 × 10−5 m/s, the constant temperature gradient in liquid at the solid-liquid interface of GL = 16.5 × 103 °C/m and then quenched at a cooling rate of about 50 °C/s. The solidification of the studied CCA begins with the growth of columnar FCC(A1) dendrites and is finalised by the formation of a multiphase microstructure in the interdendritic region. The DS samples subjected to isothermal annealing at 900 °C for 25 h show a strong brittle-ductile transition (BDT) behaviour characterised by brittle-ductile transition temperature (BDTT) between 650 and 700 °C at a strain rate of 1 × 10−4 s−1. The tensile deformation curves show only a strain hardening stage at test temperatures up to 650 °C. At temperatures ranging from 750 to 900 °C, the alloy shows anomalous strain hardening behaviour characterised by one strain hardening stage and three well-distinguished strain softening stages.

Microstructure evolution of a forged TiAl-Nb alloy during high-temperature tensile testing

A forged TiAl-Nb alloy was investigated by means of measurements of properties related to performance at high temperatures, and the microstructure was observed by scanning electron microscopy (SEM) with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The brittle-ductile transition temperature of the alloy was between 800 °C and 820 °C. The alloy had high strength in the brittle stage, and the main deformation mechanisms of the alloy were dislocation movement and dynamic recovery (DRV) in the γ phase. In the ductile stage, the alloy had good deformation ability and the main deformation mechanism of the alloy was continuous dynamic recrystallization (CDRX) in γ phase and α2 phase, the integrity of the lamellar colonies was destroyed. At the same time, the phase transformation occurred.

Embrittlement of WCLL blanket and its fracture mechanical assessment

In the European fusion programme, the Water Cooled Lithium Lead breeding blanket (WCLL BB) uses EUROFER as a structural material cooled with water at temperatures between 295 °C–328 °C and a pressure of 155 bar. The WCLL BB will be significantly irradiated (>2 dpa), while some parts will not receive significant heat loads, e.g. the sidewalls or the back-supporting structures. The irradiation, together with the irradiation temperature of EUROFER below 350 °C, produces a shift of the ductile-to-brittle-transition temperature (DBTT) to levels above room temperature at neutron doses, causing material damage as low as 2–3 dpa. Even though the DBTT does not reach the operating temperature level, brittle/non-ductile fracture is a concern during in-vessel maintenance when the BB temperature is below the DBTT. Two loading scenarios were identified as severe in this respect: (i) re-pressurization of the WCLL BB cooling loop after in-vessel maintenance, and (ii) dead weight loads during lifting of the BB segment. The embrittlement of the WCLL BB was investigated by quantifying the local DBTT shift in its parts based on current knowledge of the embrittlement behaviour of EUROFER under neutron irradiation. Therefore, a suitable, not overly conservative procedure was derived considering dpa damage and transmuted helium effects. The results demonstrate the ability to identify the 3D spread of the severely embrittled zones in the structure whose impact on the structural integrity was assessed considering the risk of brittle/non-ductile fracture. Thereby, the fracture mechanics approach established in nuclear codes was applied assuming its applicability to EUROFER. The embrittled zones in the first wall (FW) and its sidewalls pass the criteria when assessing the relatively low stresses resulting from the coolant pressure. The assessment was then continued considering stresses appearing in the FW during maintenance, in particular, when lifting the BB segment and transporting it out of the vacuum vessel. In this context, the maximum tolerable flaw sizes were determined in a parameter study considering designs of the FW with different cooling channel wall thicknesses.

Influence of calcium consumption and content on the quality of 10KhSND steel made from continuously cast slabs

The effect of calcium consumption during out-of-furnace processing and its content in the finished metal on the quality of sheet steel grade 10KhSND, obtained from continuously cast slabs, has been studied. The research is based on the results of evaluating the macrostructure of slabs obtained from a continuously cast billet, as well as after testing the finished metal to determine impact strength, contamination with non-metallic inclusions, and ductile-brittle transition temperature. The paper analyzes the effect of consumption and calcium content in the metal.

Unforeseen influence of the prior austenite grain size on the mechanical properties of a carbide-free bainitic steel

The reasons behind the limited impact toughness of medium manganese, medium carbon carbide-free bainitic steels has been discussed in this manuscript. Various hot-rolling schedules have been employed to obtain microstructures with three different prior austenite grain sizes (PAGSs): coarse (CG = 45 ± 5 μm), medium (MG = 18 ± 4 μm) and fine (FG = 8 ± 2 μm) grain sizes. Although the coarse and middle grain size microstructures present similar levels of strength (tensile strength ∼1000 MPa) and ductility (36% and 41%, respectively), the impact toughness measured at room temperature improves significantly from 17 J to 83 J in the middle grain size one because of the lower ductile-to-brittle transition temperature (DBTT) temperature. Compared to the other microstructures, the fine grain size one contains higher contents of blocky unstable retained austenite, which are responsible for the enhanced strain hardening behavior under tensile testing but can be harmful for the toughness. Besides, during the first stages of straining these blocky austenite islands transform effortlessly, deteriorating the ductility (30%) and toughness (58 J) in spite of keeping a similar DBTT as the microstructure with the medium grain size.

Synergistic effect of different testing environments on the hydrogen-induced mechanical degradation to establish the role of microstructural features on the failure of X70 pipeline steel

Hydrogen-induced mechanical degradation was evaluated in different environmental conditions on two X70 pipeline steels having the same composition but different thermomechanical processing parameters, and a further correlation between microstructural features and hydrogen embrittlement (HE) susceptibility was established. HE susceptibility was determined by performing the Slow Strain Rate Tensile Test (SSRT) in air, ex-situ hydrogen-charged, in corrosive media, in-situ hydrogen-charged and Charpy impact tests before and after hydrogen charging in a wide range of temperatures.The SSRT results indicated that the synergistic action of stress and hydrogen in in-situ charged sample leads to maximum HE susceptibility in both X70 steels. Charpy results demonstrated that ductile-to-brittle transition temperature (DBTT) increases after hydrogen charging. The texture analysis revealed that X70-2 with a higher volume fraction ratio of Ƴ-fiber/{100}<011> demonstrated improved HE resistance, whereas X70-1 with higher fractions of {113}<110> and {100} || ND demonstrated a higher DBTT than before and after hydrogen charged X70-2.

Effect of B and N Content and Austenitization Temperature on the Tensile and Impact Properties of Modified 9Cr-1Mo Steels

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.

Effect of V, Nb, and Ti microalloying on low–temperature impact fracture behavior of non–quenched and tempered forged steel

In this study, three kinds of non-quenched and tempered forged steels with V, V–Nb and V–Nb–Ti microalloying elements were designed. The microstructure and low–temperature impact fracture behavior of the three steels were comprehensively compared and analyzed. The results showed that the addition of Nb and Ti elements can significantly refine the grains and form precipitated particles to improve the strength of the steel. The fracture behavior of the three steels types at a low temperature of −28 °C was dominated by brittle cleavage fractures. V–Nb–Ti steel exhibited the worst low–temperature impact toughness. The SEM analysis result showed that coarse (V, Nb, Ti) (C, N) particles present in the steel acted as local cleavage initiation sites. No cleavage fracture caused by (V, Nb) (C, N) particles occurred in the V–Nb steel. The results showed that the high impact energy (70.4 J) at room temperature and low ductile–brittle transition temperature (DBTT, −20 °C) of the V–Nb steel were mainly attributable to the grain refinement of Nb. The micron–sized coarse (V, Nb, Ti) (C, N) particles were the main factor responsible for the low impact energy (35.5 J) at room temperature and high DBTT (−7 °C) of V–Nb–Ti steel and can act as the local cleavage initiation sites to dominate the cleavage fracture at low temperature and decrease the impact toughness.

Temperature-dependent deformation and fracture properties of low-carbon martensitic steel in different stress states

The temperature-dependent ductile-brittle transition (DBT) in low-carbon martensitic steel is investigated in different stress states. To capture the effects of the stress state, tensile tests have been performed with flat specimens and three-point bending tests have been conducted with Charpy specimens at dynamic (impact) and quasi-static loading rates, covering a broad temperature range of 77 K–423 K. The results reveal that ductile-brittle transition temperature (DBTT) is significantly affected by stress triaxiality, wherein the transition temperature in uniaxial tension is ∼142 K lower than that in plane strain condition of Charpy specimens. The energy absorption capacity in the ductile resistance domain during three-point bending tests is sensitive to the loading rate. Moreover, dynamic strain aging poses a decrease in ductile resistance upon quasi-static loading when the temperature exceeds 323 K and thus without a specific upper shelf plateau. Tensile strength and ductility tend to increase with decreasing temperature, and the associated failure mode changes from shear-dominated to nearly pure tension due to the specific discrepancy of temperature influence on the critical normal and shear strengths, wherein the formation of shear bands that dominate the tensile deformation could be strictly inhibited by the formation of nanoscale dislocation cells based on dislocation analysis. The failure mechanisms in tensile tests correlate with the tensile specimens' macroscopic fracture angle. In the end, a new local parameter is proposed for temperature-dependent toughness estimation in tensile tests.

Experimental studies of the influence of biaxial bending on the crack resistance of reactor pressure vessel steel

Background: to assess the residual life of nuclear reactor hulls in Ukraine, normative temperature dependences of the crack resistance of hull steels obtained on standard samples with a deep through crack under plane strain conditions are used. However, it is known that the thickness of the wall of the reactor body is subject to biaxial loading, which affects the value of the crack resistance characteristics. Therefore, in order to substantiate the possibility of lengthening beyond the design resource of VVER-1000 reactor hulls, an experimental study of the effect of biaxial load on the crack resistance characteristics of hull reactor steel 15X2NMFAA is relevant. Objective: development of an experimental technique and investigation of crack resistance under biaxial loading on small-sized samples in the the brittle-ductile transition temperature range of reactor steels. Methods: on the basis of numerical and engineering calculations, a specimen was developed, the corresponding additional equipment for its tests under biaxial loading on a uniaxial servo-hydraulic machine. The influence of biaxial bending on the fracture toughness of reactor steel 15X2NMFAA was experimentally investigated. Results: the results obtained on cruciform specimen are calculated according to the ASTM 1921 standard and plotted on the Master curve (temperature dependence of fracture toughness). The analysis showed that the data on crack resistance obtained on samples with "short" (a/W=0.1...0.2) linear and surface semi-elliptical cracks exceed the upper limit of the confidence interval of the Master curve, and biaxial loading of cruciform specimen reduces crack resistance compared to uniaxial loading.

The ductile-brittle transition behaviors of W/Ta multilayer composites

Tungsten (W) usually shows a high ductile-brittle transition temperature (DBTT). To overcome this shortcoming, W-based multilayer composites have been designed. In this study, the ductile-brittle transition behaviors of W/Ta multilayer composites are investigated by uniaxial tensile and three-point bending tests under temperatures ranging from room temperature to 600 ℃. For uniaxial tensile tests, the DBTT of composites bonded at 1000 ℃ is about 200 ℃. However, the DBTT of composites bonded above 1200 ℃ is about 300 ℃. When testing temperature is lower than the DBTT, both the strength and ductility are improved with increasing temperature. The composites exhibit the highest tensile strength at 300 ℃. Meanwhile, the fracture modes of the composites, especially the W layer, changes dramatically at different testing temperature. For the composites bonded at 1000 ℃, the fracture mechanism of W layer changes from brittle cleavage fracture to delamination fracture, and then to ductile shear fracture. For the composites bonded at a temperature above 1200 ℃, the fracture mode of W layer evolves from intergranular fracture to cleavage fracture, then to shear fracture. Compared with uniaxial tensile tests, the DBTT is lower during three-point bending.

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.