Ductile-to-brittle Transition 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.

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.

Unveiling the intrinsic rhenium effect in Tungsten

Body-centered cubic refractory metals, as a class of critical structural materials, are often restricted by poor low-temperature toughness, which is a common, undesirable phenomenon that continues to be the focus of intense research. Historically, rhenium (Re) has been considered the most promising alloying element in improving ductility and reducing the ductile-to-brittle transition (DBT) temperature of group-VI metals, known as the “Re effect”. However, whether this is indeed possible remains uncertain so far. Here, in the modelled W-xRe (x = 0, 3, 5, 10 wt.%) alloys, we reveal that only in a limited temperature range of 50 °C∼200 °C does the Re softening occur, imparting a positive but small effect on fracture toughness. At higher temperatures (>300 °C), the dynamic interactions between Re solutes and dislocations produce profuse jogs and sessile loops that strongly hinder dislocation motion and cause hardening. The resultant difficulty in plastic flow significantly degrades the high-temperature (>300 °C) toughness of W-Re alloys, leading to an undesirable increase of the DBT temperature, which shifts higher with increasing Re content. Our discovery of the solution softening-to-hardening transition, including its dislocation mechanism, clarifies the intrinsic effect of Re on the toughness of recrystallized W alloy, providing essential knowledge for the compositional design of refractory alloys in general.

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.

Enhanced size effects on shear performance and fracture behavior of BGA structure micro-scale Cu/Sn–3.0Ag–0.5Cu/Cu joints at low temperatures

To assess the serving reliability of shrinking-sized solder joints in space electronics (especially cryogenic electronics), the effects of joint size on the shear performance of Cu/Sn–3.0Ag–0.5Cu/Cu joints with ball grid array (BGA) structure at different temperatures (25 to −125 °C) were investigated. As the joint size decreased, shear strength of the solder joint increased, showing prominent size effect, which was enhanced at low temperatures. Moreover, a ductile-to-brittle transition (DBT) fracture occurred as the test temperature declined, and the critical temperature decreased as the joint size decreased. All of the results suggest that the small-sized joints are more robust at low temperatures under shear loading.

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.

Temperature Dependence of Cleavage Fracture Toughness for an Ultrahigh Strength Martensitic Steel

Abstract This work conducts an exploratory evaluation of the brittle fracture behavior for an ultrahigh strength martensitic steel using conventional three-point bend SE(B) and precracked Charpy V-notch (PCVN) specimens. A primary purpose of this study is to verify the effectiveness of the Master Curve methodology in providing a reliable estimate of the reference temperature (T0) derived from fracture toughness data sets measured in the ductile-to-brittle transition (DBT) region of an ultrahigh strength, low alloy martensitic steel. Fracture toughness testing conducted on three-point bend SE(B) specimens and PCVN configurations at different test temperatures in the DBT region provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc, and its corresponding KJc-values for the tested material. Although this class of ultrahigh strength steel having a martensitic microstructure is currently beyond the reach of ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, the analyses described here show that the predicted normalized curves of median fracture toughness versus temperature are in good agreement with the experimental measurements.

Effect of cold working on mechanical properties of pure-304L stainless steel with variable temperature - an experimental study

ABSTRACT In the current investigation, cold work influence on mechanical strength, microstructure, hardness and ductile to brittle transition (DBT) in pure (P) 304 L-stainless steel (SS) (with ultra-low content of sulphur (0.001%) and phosphorus (nil) at several levels of cold work (5%, 10%, 20% and 30%) are reported, which provide valuable information regarding beneficial effects of reduced level of sulphur for the material selection in structural design. The cold work results in dual-phase (γ +) with distinct values of dislocation densities and grain size that are used in the modified Hall-Petch equation to obtain the corresponding change in yield strength. Cold working up to 30% increased yield strength () from 280 to 900 MPa, whereas ultimate tensile strength (), increased from 620 to 950 MPa, i.e. an increase of 34.73%. The hardness values increased from 172 to 360 Hv, i.e. an enhancement of 110.5% for a cold work of 30%. The material showed a remarkable enhancement in the impact energy absorption from 200 to 390 J for a temperature change from 77 to 473 K. It is observed that cold working at 5% level reduces the impact energy absorption from 390 to 250 J, i.e. reduction of 35.89% at 298 K. The value of impact energy absorbed by P-304 L-SS was 37% higher than the commercial grade 304 L-SS (sulphur (0.03%), phosphorous (0.04%). The study succinctly brings out the importance of reduced content of sulphur and phosphorus in obtaining superior mechanical properties through cold work.

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 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.

Anisotropic cutting mechanisms on the surface quality in ultra-precision machining of R-plane sapphire

The machined surface-layer quality of sapphire is influenced by its anisotropic properties. This work investigates the anisotropic cutting mechanisms on the surface quality of R-plane sapphire in ultra-precision machining. Plunge-cutting experiments were conducted to reveal ductile-to-brittle transition (DBT) depth. The groove topography and profiles show that the DBT depth with machining direction perpendicular to A-plane (type PER) can reach 200 nm, while the DBT depth with machining direction parallel to A-plane (type PAR) is <143.51 nm. However, the TEM images show a crack of 560 nm depth existing in the subsurface of type PER groove although its surface exhibits ductile characteristics. Besides, HRTEM results reveal that the dislocation defects determine the affected-layer depth for type PER groove, while the twinning defects dominate the affected-layer depth for type PAR groove. Moreover, overlap-cutting experiments highlight that the cutting force with type PER is larger than that with type PAR. MD simulation results indicate that the plastic deformation depth for type PER groove is higher than that for type PAR groove. In regards to the precision machining of sapphire, comprehensive evaluation of DBT depths, subsurface damage and cutting forces is essential for optimizing the process parameters.

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.

Cleavage fracture micromechanisms in simulated heat affected zones of S690 high strength steels

High strength steels are widely used for structural applications, where a combination of excellent strength and ductile-to-brittle transition (DBT) properties are required. However, such a combination of high strength and toughness can be deteriorated in the heat affected zone (HAZ) after welding. This work aims to develop a relationship between microstructure and cleavage fracture in the most brittle areas of welded S690 high strength structures: coarse-grained and intercritically reheated coarse-grained HAZ (CGHAZ and ICCGHAZ). Gleeble thermal simulations were performed to generate three microstructures: CGHAZ and ICCGHAZ at 750 and 800 °C intercritical peak temperatures. Their microstructures were characterised, and the tensile and fracture properties were investigated at − 40 °C, where cleavage is dominant. Results show that despite the larger area fraction of martensite-austenite (M-A) constituents in ICCGHAZ 750 °C, the CGHAZ is the zone with the lowest fracture toughness. Although M-A constituents are responsible for triggering fracture, their small size (less than 1 μm) results in local stress that is insufficient for fracture. Crack propagation is found to be the crucial fracture step. Consequently, the harder auto-tempered matrix of CGHAZ leads to the lowest fracture toughness. The main crack propagates transgranularly, along {100} and {110} planes, and neither the necklace structure at prior austenite grain boundaries of ICCGHAZs nor M-A constituents are observed as preferential sites for crack growth. The fracture profile shows that prior austenite grain boundaries and other high-angle grain boundaries (e.g., packet and block) with different neighbouring Bain axes can effectively divert the cleavage crack. Moreover, M − A constituents with internal sub-structures, which have high kernel average misorientation and high-angle boundaries, are observed to deflect and arrest the secondary cracks. As a result, multiple pop-ins in load-displacement curves during bending tests are observed for the investigated HAZs.

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.

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.

Low-Temperature Deformation and Fracture of Cr-Mn-N Stainless Steel: Tensile and Impact Bending Tests

The paper presents the results of tensile and impact bending tests of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel in temperatures ranging from −196 to 20 °C. The steel microstructure and fracture surfaces were investigated using transmission and scanning electron microscopes, as well as X-ray diffraction analysis. The steel experiences a ductile-to-brittle transition (DBT); however, it possessed high tensile and impact strength characteristics, as well as the ductile fracture behavior at temperatures down to −114 °C. The correspondence between γ–ε microstructure and fracture surface morphologies was revealed after the tensile test at the temperature of −196 °C. In this case, the transgranular brittle and layered fracture surface was induced by ε-martensite formation. Under the impact bending test at −196 °C, the brittle intergranular fracture occurred at the elastic deflection stage without significant plastic strains, which preceded a failure due to the high internal stresses localized at the boundaries of the austenite grains. The stresses were induced by: (i) segregation of nitrogen atoms at the grain boundaries and in the near-boundary regions, (ii) quenching stresses, and (iii) reducing fcc lattice volume with the test temperature decrease and incorporation of nitrogen atoms into fcc austenite lattice. Anisotropy of residual stresses was revealed. This was manifested in the localization of elastic deformations of the fcc lattice and, consequently, the stress localization in &lt;100&gt;-oriented grains; this is suggested to be the reason of brittle cleavage fracture.

Fundamental insights on ductile to brittle transition phenomenon in ferritic steel

Ductile to brittle transition (DBT) is a well-known phenomenon responsible for deteriorating the low temperature fracture toughness of ferritic steels. It is unambiguous to state that the transition phenomenon is a direct consequence of the strong temperature and strain rate sensitivity of flow stress, which in turn is regulated by the thermally activated motion of screw dislocation at the atomic scale. Due to very high Peierls stress, screw dislocation motion is accompanied by the nucleation and propagation of kink-pairs along the dislocation line. Thus, the activation energy required for kink-pair nucleation is an essential ingredient for correctly predicting the flow stress sensitivity and hence the ductile to brittle transition temperature (DBTT) of ferritic steel. However, experimental determination of kink-pair activation energy is rather tedious and there exists a strong discrepancy between atomistic simulation results (using Density Functional Theory and empirical potentials). Therefore, the present review is aimed at understanding the mechanism of screw dislocation motion in BCC Fe lattice and comparing the results obtained for kink-pair activation energy through experiments and simulations. The effect of different alloying elements on the overall flow stress sensitivity of Fe is also discussed with reference to the interaction of screw dislocation with solute atoms or clusters. Finally, the importance of screw dislocation on predicting the DBTT is explored to identify the present knowledge gaps and the future research directions.

Nano-to-microscale ductile-to-brittle transitions for edge cracking suppression in single-diamond grinding of lithium metasilicate/disilicate glass-ceramics

To suppress grinding-induced edge cracks in dental lithium metasilicate/disilicate glass-ceramics (LMGC/LDGC), this paper established a contact stress model for single-diamond grinding (SDG) to relate their crack generation and ductile-to-brittle transition (DBT) thresholds with the mechanical properties, diamond tool profiles and process variables. Nanoindentation, friction test and SDG were conducted to unravel material responses and dynamic diamond grit-workpiece interactions to determine the DBT thresholds. The nanoindentation revealed significant indentation size effects (ISEs) on the hardness and elastic moduli of the ceramics. SDG clearly elucidated their DBT behaviors, wherein edge cracks initiated at diamond peripheries when the concurrent contact stresses reached the DBT thresholds. Accordingly, the indicated critical cutting depths for DBT may be changed from the nanoscale to the microscale by increasing the tool radius and reducing the machining speed. This research contributes to edge crack suppression for ductile machining of brittle materials at large removal rates.

Effects of Si on the Ductile‐to‐Brittle Transition Behavior of As‐Cast Advanced High‐Strength Steels

High Si concentrations in third‐generation advanced high‐strength steels (AHSS) are known to cause cracking in continuous cast slabs during cooling at temperatures below 300 °C. To investigate the mechanism connecting Si to this embrittlement, impact toughness tests are conducted on as‐cast Fe–0.2C–3.0Mn steels with 0.5, 1.5, and 3.0 wt% Si at temperatures between 100 and 400 °C. The propagation path of the cracks through the microstructure of the specimens is examined. The ductile‐to‐brittle transition (DBT) behaviors of the three steels are compared. Higher Si concentrations raise the DBT temperature of the steels. The influence of Si on both solid‐solution strengthening and autotempering during cooling likely contributes to this by increasing the hardness of these as‐cast microstructures. In addition, the precipitation of pro‐eutectoid ferrite (αPE), which is promoted by higher Si concentrations, significantly lowers the upper shelf energy of the DBT curve. The αPE phase also alters the propagation path of brittle fracture in the specimens, potentially contributing to the increase in DBT temperature. The increase in DBT temperature and decrease in upper shelf energy may contribute to the as‐cast AHSS slab embrittlement at low temperatures observed in the industry.

Revealing the ductile-to-brittle transition mechanism in polycrystalline body-centered tetragonal tin (Sn) for cryogenic electronics

The inherent ductile-to-brittle transition (DBT) of body-centered tetragonal Sn at cryogenic temperatures restricts the use of Sn-based solders in the interconnection of cryogenic electronics, but little is known about the deformation behaviors accompanying with the transition and the underlying transition mechanism. In this work, the deformation features before cryogenic brittle fracture and the DBT mechanism in polycrystalline Sn were studied through uniaxial tensile experiments at different temperatures. Compared to the softening process stimulated by dynamic recovery and dynamic recrystallization before ductile fracture at room temperature (~293 K), a high strain hardening rate (~5% of the shear modulus) is maintained during the linear hardening period preceding brittle fracture at the liquid nitrogen temperature (~77 K) due to the pronounced intersecting of {301} deformation twins. But the irreconcilable velocity difference between dislocation glide (~3 µm/s) and twin thickening (~10 µm/s) at 77 K leads to a premature brittle fracture in the midst of the linear hardening, and indeed the DBT. The suggested specific DBT mechanism is substantiated by the fact that a significant increase in the velocity (~1500 µm/s) with the increasing temperature (123 K) allows the dislocation slip to readily accommodate the shear strains due to {301} twin thickening at the grain boundaries, thereby resulting in ductile fracture rather than brittle fracture. This deep understanding about the DBT in polycrystalline Sn may help forge a new path to design ductile and strong Sn-based solders and solder joints for cryogenic electronics by deformation twinning.