Brittle-to-ductile Transition Research Articles

Molecular dynamics simulation of stretching-induced ductility for polystyrene

Some glassy polymers have a mechanical behavior that stretching can induce the brittle-to-ductile transition (BDT). For polystyrene (PS) film, after pre-stretching above glass transition temperature (Tg), the property at room temperature transforms to ductile, showing higher elongation at break and higher yield strength. In this work, we have investigated this phenomenon of stretching induced ductility by molecular dynamics (MD) analysis referring to our experimental findings. PS specimens in united-atom model with different chain lengths were loaded by deformation at two-step temperature setting, which was 393 K at first and then 300 K, to simulate tensile behaviors with various per-stretching conditions. The result has shown that as deformations beginning at 393 K and proceeding at 300 K, root mean square radius of gyration (Rg) and the degree of orientation (P2,x) reach the maximum value higher than solely stretching at 393 K or 300 K. Therefore, the stretching-induced BDT is attributed to changing of both chain conformation and coil configuration in polymer chains in stepwise stretching, which can change their coil configuration at the pre-stretching, adjusting their chain network in a more stretchable position, leading the whole polymer amorphous from brittle to ductile. The simulation result is accordant with our experiment and previous BDT theory to glassy polymers.

Response of interlayer-bonded bilayer graphene to shear deformation

We report results on the mechanical and structural response to shear deformation of nanodiamond superstructures in interlayer-bonded twisted bilayer graphene (IB-TBG) and interlayer-bonded graphene bilayers with randomly distributed individual interlayer C–C bonds (RD-IBGs) based on molecular-dynamics simulations. We find that IB-TBG nanodiamond superstructures subjected to shear deformation undergo a brittle-to-ductile transition (BDT) with increasing interlayer bond density (nanodiamond fraction). However, RD-IBG bilayer sheets upon shear deformation consistently undergo brittle failure without exhibiting a BDT. We identify, explain, and characterize in atomic-level detail the different failure mechanisms of the above bilayer structures. We also report the dependence of the mechanical properties, such as shear strength, crack initiation strain, toughness, and shear modulus, of these graphene bilayer sheets on their interlayer bond density and find that these properties differ significantly between IB-TBG nanodiamond superstructures and RD-IBG sheets. Our findings show that the mechanical properties of interlayer-bonded bilayer graphene sheets, including their ductility and the type of failure they undergo under shear deformation, can be systematically tailored by controlling interlayer bond density and distribution. These findings contribute significantly to our understanding of these 2D graphene-based materials as mechanical metamaterials.

Microscopic defect dynamics during a brittle-to-ductile transition

Deformation of all materials necessitates the collective propagation of various microscopic defects. On Earth, fracturing gives way to crystal-plastic deformation with increasing depth resulting in a "brittle-to-ductile" transition (BDT) region that is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. Here, we show that the crossing of a BDT in marble during deformation experiments in the laboratory is accompanied by systematic increase in the frequency of acoustic emissions suggesting a profound change in the mean size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle-ductile transition. As pressure increases, long-period signals are suppressed and short-period signals become dominant. At higher pressures, signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types. Complex mixed-mode events indicate that interactions between the defects are common over the whole pressure range, in agreement with postmortem microstructural observations. Our measurements provide unique, real-time data of microscale dynamics over a broad range of pressures (10 to 200 MPa) and can inform micromechanical models for semi-brittle deformation.

Brittle-to-ductile transition in nanoscratching of silicon and gallium arsenide using Berkovich and Conical tips

Determining the brittle-to-ductile transition (BDT) threshold is crucial for developing efficient machining processes for brittle materials. In this study, nanoscratch tests were performed on the (001) face of single-crystal silicon (Si) and gallium arsenide (GaAs) using a sharp Berkovich and a blunt Conical tip. The aim was to understand the effects of tip shape and microstructural variation on their BDT behaviour. The result showed that the critical cutting depth for BDT (dc) was affected by not only material microstructure but also tip shape. The stress distribution within the deformed sublayer varied with tip shape, ultimately impacting the microstructure evolution and thus deformation/removal mechanism. Using the sharp tip, Si had a slightly smaller dc than GaAs due to the ease of cracking along the {111} plane. In contrast, with the blunt tip GaAs had a smaller dc than Si, attributed to its relatively high brittleness index and higher density of crystallite defects. The defects in GaAs caused strain hardening and hence reduced ductility, while the amorphization of Si enhanced its ductility.

Fracture energy of high-Poisson's ratio oxide glasses

The apparent relationship between Poisson's ratio and fracture energy has been used to guide the discovery of ductile glasses with a brittle-to-ductile (BTD) transition at Poisson's ratio around 0.32. Most organic and metallic glasses possess Poisson's ratio above 0.32, and thus, feature fracture energy that is around three orders of magnitude higher than that of oxide glasses, which feature Poisson's ratio typically below 0.30. However, whether the BTD transition can also be observed in oxide glasses remains unknown due to the lack of fracture energy measurements on oxide glasses with high Poisson's ratio. In this work, we measure the fracture energy of six oxide glasses with high Poisson's ratio between 0.30 and 0.34. We find no clear relationship between the two parameters even in those that possess the same Poisson's ratio as ductile metallic glasses. This suggests that Poisson's ratio is not the main property to enhance the fracture energy of oxide glasses. To this end, we instead find a positive relation between fracture energy and Young's modulus of oxide glasses, and even for some metallic glasses, which could explain their absence of ductility.

Effect of surface brittle-to-ductile transition on high-temperature thermal shock resistance of Al2O3 ceramics

The effect of surface brittle-to-ductile transition (BDT) on the high-temperature thermal shock resistance (TSR) of Al2O3 ceramics was investigated by quenching in a cold compressed-air flow. The brittle-to-ductile transition (BDT) and transition temperature were determined from the transformation in fracture mechanism via measuring flexural strength at elevated temperatures. Effects of cooling rate on the residual strength and microstructural evolution of Al2O3 ceramics were systematically studied. Results showed that the BDT temperature in Al2O3 ceramics was around 1000 °C. The BDT mechanism is associated with the softening of the grain boundary glass phase, which permits a certain degree of crack passivation to reduce thermal stress concentration. Compared with the original strength of 375 MPa, the residual strength of the specimen decreased rapidly to 190 MPa at a final temperature of 700 °C, while the residual strength of the specimen was maintained to 352 MPa at a final temperature of 1000 °C. The appearance of surface BDT in Al2O3 ceramics during the high-temperature thermal shock process produces a favorable impact on improving the residual strength of specimens by blunting the crack tip and reducing the surface thermal stress. Therefore, the surface BDT in Al2O3 ceramics is beneficial to the improvement of the high-temperature TSR of Al2O3 ceramics.

Effect of V on the microstructure and brittle-to-ductile transition of directionally solidified high-Nb TiAl alloy

The influence of V addition on the microstructure and high-temperature tensile performance of Ti46Al7Nb0.4W0.6Cr alloy, fabricated by using the cold crucible directional solidification method, is comprehensively investigated. The results show that V addition can refine the columnar grain width and promote the brittle-to-ductile transition (BDT) of the TiAl alloy. After the addition of 2 at.% V, the ultimate tensile strength and elongation increased at 973–1173 K. The fracture mode change from interlamellar to translamellar; this is attributed to the decrease in the angle between the lamellar alignment and growth direction. The BDT behaviour of V-containing TiAl alloy at 1073 K results from the transition from dislocation-dominated to twin-dominated deformation as indicated by transmission electron microscopic investigations.

Phase‐field‐crystal study on the crack propagation behavior in a nanoscale two‐dimensional lattice in the presence of nonlinear disturbance strains

AbstractThe nanoscale crack propagation behavior in the presence of nonlinear disturbance strains is studied by using the phase‐field‐crystal method. The influences of amplitude A and frequency ω on fracture mode and crack growth are discussed. The simulation results suggest that the disturbance strains can make fracture mode change between brittle fracture and ductile fracture. When amplitude A is large, increasing frequency ω will lead to the brittle‐to‐ductile transition (BDT). Further increasing ω can make ductile‐to‐brittle transition (DBT) happen. Meanwhile, the value of A can influence the critical frequencies for BDT and DBT. Crack growth is also affected by the disturbance strains. When ω is small, increasing ω or A can accelerate crack growth. When ω is large enough, increasing A will retard it. Through this work, we provide a new way to effectively explore the nanoscale mechanism and behavior of crack propagation.

Quantifying physical parameters to predict brittle/ ductile behavior

The brittle to ductile transition (BDT) is difficult to predict without extensive fitting parameters or tuning to a particular material. Currently, predicting fracture through extensive fitting or computationally expensive algorithms is high in both cost and time required to capture the relevant deformation physics. Presented here is analysis using a comparatively high throughput analytical model to predict fracture behavior using relatively few key experimentally determined parameters: activation volume, shear stress, and activation energy. This approach could reduce the time scale to predict fracture and thus accelerate new materials discovery. The current work utilizes seminal studies to provide the inputs for validating our approach via two single crystal materials, Si and W, which both have marginal toughness at low temperatures. It is shown that knowledge of underlying deformation mechanisms (still in progress) coupled to rapid determination of physical quantities (shear stress, activation volumes, and dislocation shielding) promotes unique discovery and opportunities, including future application to polycrystalline materials and phenomena. The technique, using literature values for physical parameters, correlates well to experimental fracture behavior for these two different classes of materials, semiconductors and metals, offering new opportunities for broader study.

Effects of ethylene‐octene copolymer (POE) on the brittle to ductile transition of high‐density polyethylene/POE blends

AbstractThree kinds of ethylene‐octene copolymers (POE) were melt‐blended with high‐density polyethylene (PE‐HD) in different proportions. Detailed characterizations were conducted to analyze their structural differences of POE and its effects in toughening PE‐HD. The higher molecular weight POE can improve the toughness of PE‐HD. 60:40 PE‐HD/POE is elongated to break up to 700% while impact strength is 84.7 kJ/m2 at −30°C, which is 21‐fold of PE‐HD. In the brittle to ductile transition (BDT) during impact, the fracture mechanism changes from the crazing mode to the shear yield‐plastic deformation mode. The BDT temperature decreases as the POE molecular weight and its content increase. The interface strength in tension is estimated to access their effects. The Boltzmann‐type models were successfully extended to describe the typical S‐shaped curves in BDT of notched impact strength vs POE content or temperature. The supplementary decay model is suggested for the attenuation in toughening. Transition map in impact is proposed to select the use range of composition (c) and temperature (T) for high toughness. The curves are converted into 3D graph of T‐c‐impact strength for illustrating their coupling‐separate effects, and further into the contour map of impact strength in T‐c space for finding their partial equivalence.

The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

Abstract Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.

3D HR-EBSD Characterization of the plastic zone around crack tips in tungsten single crystals at the micron scale

High angular resolution electron backscatter diffraction (HR-EBSD) was coupled with focused ion beam (FIB) slicing to characterize the shape of the plastic zone in terms of geometrically necessary dislocations (GNDs) in W single crystal in 3 dimensions. Cantilevers of similar size with a notch were fabricated by FIB and were deformed inside a scanning electron microscope at different temperatures (21 ∘C, 100 ∘C and 200 ∘C) just above the micro-scale brittle-to-ductile transition (BDT). J-integral testing was performed to analyse crack growth and determine the fracture toughness. At all three temperatures the plastic zone was found to be larger close to the free surface than inside the specimen, similar to macro-scale tension tests. However, at higher temperature, the 3D shape of the plastic zone changes from being localized in front of the crack tip to a butterfly-like distribution, shielding more efficiently the crack tip and inhibiting crack propagation. A comparison was made between two identically deformed samples, which were FIB-sliced from two different directions, to evaluate the reliability of the GND density estimation by HR-EBSD. The analysis of the distribution of the Nye tensor components was used to differentiate between the types of GNDs nucleated in the sample. The role of different types of dislocations in the plastic zone is discussed and we confirm earlier findings that the micro-scale BDT of W is mainly controlled by the nucleation of screw dislocations in front of the crack tip.

The brittle-to-ductile transition in cold-rolled tungsten sheets: On the loss of room-temperature ductility after annealing and the phenomenon of 45° embrittlement

The high brittle-to-ductile transition (BDT) temperature of conventionally produced tungsten (W), challenges the design of W-based structural components. Recent studies have demonstrated the potential of cold rolling to produce W sheets, which are ductile at room temperature and exhibit a BDT temperature of 208 K. In order to assess the thermal stability of these materials, we conducted isothermal heat treatments (at 1300 K, for annealing durations between 0.1 h and 210 h) combined with studies on the evolution of mechanical properties and microstructure of a severely deformed undoped W sheet. With this work, we demonstrate the need for a stabilized microstructure before utilization of cold-rolled W in high-temperature applications can take place successfully. After annealing at 1300 K for 6 h, the material properties changed remarkably: The BDT temperature increases from 208 K to 473 K and the sharp BDT of the as-rolled condition transforms into a wide transition regime spanning over more than 200 K. This means in fact, an endangered structural integrity at room temperature. We also address the so-called phenomenon of 45° embrittlement of W sheets. Here we show that cleavage fracture in strongly textured W sheets always takes place with an inclination angle of 45° to the rolling direction, independent of the studied material condition, whether as-rolled or annealed. An in-depth study of the microstructure indicates a correlation between an increased BDT temperature caused by annealing and microstructural coarsening presumably by extended recovery. We conclude that 45° embrittlement needs to be comprehended as a combined effect of an increased spacing between grain boundaries along the crack front, leading to an increased BDT, and a high orientation density of the rotated cube component or texture components close to that, which determine the preferred crack propagation of 45° to the rolling direction.

The brittle-to-ductile transition in cold-rolled tungsten sheets: Contributions of grain and subgrain boundaries to the enhanced ductility after pre-deformation

• In cold-rolled W, a striking fraction of straight dislocations spans across the grains. • These segments are presumably ½〈111〉 screw type and the result of rolling below the knee temperature. • The contributions of CSL boundaries to the reduction in BDT temperature seems negligible. • Hall–Petch-like attempts describe the rolling-induced reduction in BDT temperature. • Considering LABs and HABs instead of HABs only results in more promising correlations. One of the key demands on tungsten (W) as designated plasma-facing material (PFM) is the capability to fulfill a structural function. Since W has refused ductilization strategies by alloying alone, the production of W materials with enhanced ductility has come into focus considering tailored microstructures. This work addresses the rolling-induced microstructural modifications of warm- and cold-deformed W sheets and is supplemented by a comprehensive fracture mechanical study as a fundament for correlations between the spatial distribution of boundaries and brittle-to-ductile transition (BDT) temperature. Here we show that an extended Hall–Petch-like relationship is well suited to describe the rolling-induced reduction in BDT temperature and moreover has the potential to reflect the anisotropic nature of the transition temperature in severely rolled W sheets. Using the data of warm- and cold-rolled W sheets and also of strongly recovered W, best description of the BDT temperature was achieved by using as microstructural variables (i) the mean spacing between boundaries which intersect with the crack front and (ii) the mean boundary spacing along the normal of the crack plane. Taking into account the similarity to recent simulative-derived relationships, our findings support the theory suggesting the stimulated dislocation nucleation at boundaries as the decisive factor for more effective shielding of the crack tip in UFG materials and, in consequence, significantly reduced BDT temperatures. Besides, this work gives strong indications that the reduction of the BDT temperature in UFG W is not related to coincidence site lattice (CSL) boundaries.

Relating activation of brittle-to-ductile transition to β relaxation in Cu46Zr44Al7Y3 metallic glass

A series of compression tests were performed on the Cu46Zr44Al7Y3 metallic glass (MG) at different strain rates and a wide temperature range in which the β relaxation can be observed. A distinct boundary that can be used to describe the brittle-to-ductile transition (BDT) of the MG in compression was summarized. Furthermore, it was found that both the BDT and the β relaxation are in accordance with the Arrhenius relations, by which two energies with similar values reflecting the apparent activation energies of the BDT and the β relaxation are determined, respectively. Therefore, the BDT and the β relaxation are co-related. In addition, the origin of BDT was discussed in terms of the intrinsic correlation among the potential shear transition zones, the β relaxation, and the inhomogeneous atomic structure in MGs.

The brittle-to-ductile transition in cold-rolled tungsten sheets: the rate-limiting mechanism of plasticity controlling the BDT in ultrafine-grained tungsten

Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures.

Effects of neutron irradiation on the brittle to ductile transition in single crystal tungsten

Only limited data exist on the effect of neutron irradiation on the brittle to ductile transition (BDT) in tungsten. This work investigates the increase in brittle to ductile transition temperature (BDTT) following neutron irradiation to 1.67 displacements per atom, using four-point bend tests over a range of temperatures (623–1173 K) and strain rates (3.5 × 10−7 - 2.5 × 10−5 s−1). The BDTT was found to increase by 500 K after irradiation. The activation energy for the BDT was determined using Arrhenius analysis of the four-point bend tests. Nanoindentation strain-rate jump tests were used to characterise the activation volume for dislocation motion. These were quantified as 1.05 eV and 4.6 b3 respectively, very close to values found for unirradiated tungsten. This suggests that kink-pair formation is the controlling mechanism for the BDT before and after irradiation. This work also carries out a unique verification of inventory-code-modelling (via FISPACT-II) of transmutation of tungsten to rhenium and osmium under neutron irradiation using two independent techniques (X-ray and gamma-ray spectroscopy). These results show that modelling can correctly predict this transmutation, provided that an accurate neutron spectrum is used. This is a critical result given the widespread use of inventory codes such as FISPACT-II, and the associated nuclear data libraries, for modelling transmutation of tungsten.

Revisiting the Dependence of Poisson's Ratio on Liquid Fragility and Atomic Packing Density in Oxide Glasses.

Poisson’s ratio (ν) defines a material’s propensity to laterally expand upon compression, or laterally shrink upon tension for non-auxetic materials. This fundamental metric has traditionally, in some fields, been assumed to be a material-independent constant, but it is clear that it varies with composition across glasses, ceramics, metals, and polymers. The intrinsically elastic metric has also been suggested to control a range of properties, even beyond the linear-elastic regime. Notably, metallic glasses show a striking brittle-to-ductile (BTD) transition for ν-values above ~0.32. The BTD transition has also been suggested to be valid for oxide glasses, but, unfortunately, direct prediction of Poisson’s ratio from chemical composition remains challenging. With the long-term goal to discover such high-ν oxide glasses, we here revisit whether previously proposed relationships between Poisson’s ratio and liquid fragility (m) and atomic packing density (Cg) hold for oxide glasses, since this would enable m and Cg to be used as surrogates for ν. To do so, we have performed an extensive literature review and synthesized new oxide glasses within the zinc borate and aluminoborate families that are found to exhibit high Poisson’s ratio values up to ~0.34. We are not able to unequivocally confirm the universality of the Novikov-Sokolov correlation between ν and m and that between ν and Cg for oxide glass-formers, nor for the organic, ionic, chalcogenide, halogenide, or metallic glasses. Despite significant scatter, we do, however, observe an overall increase in ν with increasing m and Cg, but it is clear that additional structural details besides m or Cg are needed to predict and understand the composition dependence of Poisson’s ratio. Finally, we also infer from literature data that, in addition to high ν, high Young’s modulus is also needed to obtain glasses with high fracture toughness.

Deformation induced complete amorphization at nanoscale in a bulk silicon

Solid state amorphization is induced by shock, irradiation and deformation, while deformation induced complete amorphization remains a challenge in a bulk solid. Brittle-to-ductile transition (BDT) mechanism is elusive at loading speeds of m/s at nanoscale depth of cut. Existing formula has no effects of shape and radius of cutting edges on the critical depth of cut at BDT. In this study, a new route of deformation induced complete amorphization at nanoscale is proposed in a bulk solid confirmed by transmission electron microscopy (TEM). This is performed by a novel approach of ultraprecision grinding, conducted on a specially designed setup. The grinding is carried out by a developed single diamond grain with a cutting edge radius of 2.5 μm, at depth of cut of 24 nm under a loading speed of 40 m/s. BDT takes place at depth of cut of 419 and 172 nm for Si (100) respectively, ground by single diamond grains with tip radii of 5 and 2.5 μm correspondingly. A new model is suggested for BDT, considering the effects of radius and shape of cutting edges. The findings provide new insights for design and fabrication of high performance devices used in flexible electronics, nanodevices, microelectronics and optoelectronics.

“Refractory Metal, RM – Wrap”: A tailorable, pressure-less joining technology

The RM-wrap (RM = Refractory Metal) is a pressure-less, versatile and tailorable joining process: it consists of wrapping Si foils inside a refractory metal wrap (i.e., Mo, Nb, Ta) in order to prevent molten silicon from leaking outside the joined region and infiltrating the facing materials during the joining process.RM-wrap (RM = Mo, Nb, Ta) has been successfully applied to join C/SiC composites in this work: optimized joining treatment consisted of heating to 1450 °C with a heating rate of 1000 °C/h followed by a dwell time of 5 min in a non-reactive environment of Argon flow.The joints were characterized by morphological analysis and lap shear tests at room temperature and 1000 °C.Microscopical analysis revealed an in-situ formed composite joint consisting of a silicon matrix reinforced with silicides of the refractory metals. Joining material exhibited continuous and cracked free bonding with C/SiC irrespective of composite fibre orientation.Joints lap shear strength values at 1000 °C were higher than at room temperature, probably due to the brittle to ductile transition (BTDT) of silicon and silicides.Vickers microhardness on refractory metal disilicides measured inside the joints showed a trend similar to their mechanical strength, with higher lap shear strength and hardness for Mo-Wrap and lower for Ta-wrap joints.