Boundary Cohesion Research Articles

First-principles study of the effect of N on the ∑5 (210) [001] grain boundary of γ-Fe

High-nitrogen austenitic steels possess excellent properties owing to the solid solution strengthening of nitrogen, which is widely used in many fields. So it’s significant to investigate the effect of nitrogen on the properties of high austenitic nitrogen steels. In order to get atomic scale understanding of the influence of nitrogen, the first principles method was used to investigate the energetics, site preference, and segregation of nitrogen at γ-Fe ∑5 (210) [001] grain boundary (GB) and different atomic layers near GB plane. It is found that the nitrogen atom prefers to occupy the octahedral site. As nitrogen locates at different atomic layers away from the GB plane, the segregation energy presents increase first and then decrease. The mechanical properties were analyzed by Rice-Wang theory and theoretical tensile stress test, which shows that nitrogen at ∑5 (210) [001] GB has an embrittling effect on GB cohesion, while nitrogen located away from the GB plane presents a strengthening effect. The underlying mechanism was analyzed by bond length, charge density, and density of states (DOS). The segregation of nitrogen at ∑5(210) [001] grain boundary has negative effect on grain boundary cohesion, while nitrogen locating away from grain boundary plane presents strengthening effect on grain boundary.

In-situ investigation on the microstructure evolution of Mg-2Gd alloys during the V-bending tests

In this work, the in-situ observation of the microstructure in the Mg-2Gd (wt%) alloys during V-bending tests was operated. The microstructure and texture evolutions were characterized by the in-situ electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique. The results revealed a unique microcracks nucleation mechanism in the Mg-Gd alloys compared with the traditional Mg alloy (AZ31 alloy). The microcrack was nucleated at the grain boundary for the Mg-2Gd alloy and in the intragranular for the AZ31 alloy during the bending process. This difference mode between the Mg-Gd and the AZ31 alloys was mainly attributed to the more random topology of the grain boundary network, the lower grain boundary cohesion and the enhanced hinder ability for the dislocations due to the segregated Gd atoms in the Mg-Gd alloy. Furthermore, the microcracks that had a large angle θ with the extruded direction (ED) in the Mg-Gd alloys were preferred nucleated since grain boundaries had the larger normal stress during the bending process.

Effect of Boron Content on the Transverse Stress Rupture Properties of a Directionally Solidified Superalloy

The effect of boron content on the transverse stress rupture properties of a directionally solidified columnar superalloy is studied in present research. The experimental results indicate that the transverse stress property of the alloy containing 0.015% boron reaches the peak value at 870°C /380MPa while that of the alloy with 0.005% boron is the best at 1100°C/40MPa. At 870°C, dislocation slip is the main deformation mode. Stress concentration is induced when dislocations are hindered by precipitates at grain boundaries and interdendritic regions. And then cracks nucleate due to the stress concentration. In the alloy containing 0.015% boron, no borides precipitate and boron strengthens the grain boundary cohesion by solid solution. At 1100°C, grain boundary movement becomes the main deformation mode. In the alloy with 0.005% boron, the strengthening of grain boundary cohesion decreases comparing with that of alloy with 0.015% boron, and the resistance to grain boundary movement reduces with decreasing B content. Therefore, it results in compatible deformation between adjacent grains, resulting in the longest stress rupture lifetime.

Role of vibrational entropy in impurity segregation at grain boundaries in bcc iron

In this paper we study segregation of six sp−impurities to the Σ5(310)[001] grain boundary in bcc iron from first principles. First we evaluate segregation energy for interstitial and substitutional positions using the potential energy of a static lattice. Then we also evaluate Helmholtz free energy of harmonic phonons in order to check its effect at elevated temperatures. The results show that vibrational contribution to the crystal energy can be relatively high in some cases. However, the segregation preference of Al, Ga, In, Sn, Sb, and Te does not change with increasing temperature. Finally, based on calculations of the cleavage energy, we predict reduction of grain boundary cohesion due to the presence of segregated impurity atoms.

Mechanical performance of doped W–Cu nanocomposites

Nanocomposite materials containing a soft and hard metal phase are a promising strategy to combine ultra-high strength, ductility and fracture toughness. However, given the rather brittle intercrystalline fracture mode, the true potential of these materials is only accessible after strengthening the vast number of interfaces within the composite. In this work, this is realized by doping a W–75Cu nanocomposite with either C, B, Hf or Re, elements that show promising effects on grain boundary cohesion in ab-initio calculations. The samples are fabricated from powders using severe plastic deformation and characterized using electron microscopy. Subsequently, various small-scale mechanical experiments are utilized to investigate the effect of the doping on strength, ductility and fracture toughness. While doping with C and B only leads to slight changes in mechanical properties, it was found that Hf increases the strength of the composite tremendously, most likely via the formation of nanosized oxides. Doping with Re showed an increase in strength and a major improvement in bending ductility, exhibiting “super-ductile” behavior in some cases. In microtensile tests this behavior was reduced, yet an increase in strength and ductility compared to the undoped composite was also apparent in these experiments. Interestingly enough, the fracture toughness of all doped variants did not change compared to the undoped W–Cu composite. This indicates that doping with Re improves resistance against crack initiation but not against crack propagation, making the materials properties highly sensitive to pre-existing defects and probed sample volume.

Microstructure and Mechanical Properties of Intergranular Boride Precipitation-Toughened HfMoNbTaTiZr Refractory High-Entropy Alloy.

To develop strong refractory high-entropy alloys for use at elevated temperatures as well as to overcome grain-boundary brittleness, an equimolar HfMoNbTaTiZr alloy was prepared, and a minor amount of boron (0.1 at.%) was added into the alloy. The microstructures of the alloys were characterized, and their macro-to-microscale mechanical properties were measured. The microstructural observations indicated that the matrices of both the alloys were composed of a body-centered cubic solid-solution structure, and the added boron induced the precipitation of hexagonal close-packed borides (most likely the (Hf, Zr)B2) at the grain boundaries. The modulus and hardness of differently oriented grains were about equivalent, suggesting a diminished anisotropy, and many small slips occurred on multiple {110} planes. While the hardness of the matrix was not increased, the intergranular precipitation of the borides markedly raised the hardness of the grain boundaries. Owing to the enhanced grain boundary cohesion, the work hardenability and ductility were effectively improved with the addition of boron.

Improving toughness of medium-Mn steels after friction stir welding through grain morphology tuning

• The FSW steel showed a higher toughness than the TIG-welded steel and base material. • Morphology tuning by FSW eliminates a weak prior γ boundary and improves the toughness. • The preserved γ R grains after FSW contribute to the improved toughness by active TRIP. This work demonstrated the viability of friction stir welding for the welding of medium-Mn steels when used as cryogenic vessel materials for liquefied gas storage. We used an intercritically annealed Fe-7Mn-0.2C-3Al (wt.%) steel with a dual-phase ( α ′ martensite and γ R retained austenite) nanolaminate structure as a base material and systematically compared its microstructure and impact toughness after friction stir and tungsten inert gas welding. The friction stir welded specimen exhibited a large amount of γ R phase owing to a relatively low temperature during welding, whereas the tungsten inert gas welded specimen comprised only the α ′ phase. Furthermore, the friction stir welded steel exhibited a tuned morphology of nanoscale globular microstructure at the weld zone and did not exhibit any prior austenite grain boundary due to active recrystallization caused by deformation during welding. The preserved fraction of γ R and morphological tuning in the weldment improved the impact toughness of the friction stir welded steel at low temperatures. In the steel processed by tungsten inert gas welding, the notch crack propagated rapidly along the prior austenite grain boundaries—weakened by Mn and P segregations—resulting in poor impact toughness. However, the friction stir welded steel exhibited a higher resistance against notch crack propagation due to the slow crack propagation along the ultrafine ferrite/ferrite ( α / α ) interfaces, damage tolerance by the active transformation-induced plasticity from the large amount of γ R , and enhanced boundary cohesion by suppressed Mn and P segregations. Graphical Abstract .

Exploiting an as-extruded fine-grained Mg-Bi-Mn alloy with strength-ductility synergy via dilute Zn addition

Here, a new Mg-1Bi-1Mn-0.3Zn (BMZ110, wt%) wrought alloy designed by the dilute addition of Zn subjected to an appropriate extrusion processing that showed an excellent strength-ductility synergy at room temperature. The main results included four aspects as follows: 1) With dilute Zn addition, the as-cast BMZ110 alloy was refined into an approximately equiaxed-grained structure, since the solid solution of Zn provided a relatively high growth restriction factor. 2) The as-extruded BMZ110 alloy exhibited the finer average grain size and weaker basal texture intensity than the as-extruded BM11 alloy without Zn. The co-segregation of Bi and Zn solutes into grain boundaries was observed to impede grain growth during extrusion and further weaken texture intensity. Moreover, Zn atoms incorporated into Mg3Bi2 phases by replacing Bi atoms increasing the nucleation rate of Mg3Bi2 phases and refining their sizes. 3) Compared with the as-extruded BM11 alloy, the as-extruded BMZ110 alloy demonstrated the higher yield strength (~ 283.4 MPa), ultimate strength (~ 366.2 MPa) and elongation-to-failure (~ 26%). The size refinement of grains and second phases contributed to an extra strength increments. Grain boundary segregation enhanced grain boundary cohesion not only reducing the number density of grain boundary cracks, but also activating more slip modes and grain rotation for increasing the ductility. 4) The as-extruded BMZ110 alloy had the higher grain growth resistance than the as-extruded BM11 alloy during annealing. Solute segregation offered an effective obstacle to restrict grain boundary migration enlarging the activation energy (~ 112.2 kJ/mol) and further delaying the grain growth. Therefore, this work will be helpful for the development of new high performance low-alloyed Mg-Bi based wrought alloys to achieve extensive applications in industries.

Effect of grain substructure refinement on tensile mechanical behavior of L21-strengthened Al-Cr-Fe-Ni-Ti-Mo high-entropy alloy

Generally, BCC-based high-entropy alloys containing L21-type precipitates show outstanding creep resistance at high temperatures. However, their weak grain boundary cohesion results in poor room-temperature ductility and limits their industrial applications. This study achieves room-temperature ductility in L21-strengthened Al10Cr13.3Fe59.5Ni11.2Ti4Mo2 alloy through grain substructure refinement with a suitable thermomechanical treatment. We found that the fine and homogeneously distributed L21 precipitates act as effective pinning sites for dislocation movement during hot-rolling, thereby forming a subgrain structure that is three orders of magnitude smaller than that of homogenized alloys. Further, short-term annealing after rolling relieves the internal strain energy by transforming the subgrain boundary structure with energetically favorable dislocation arrangement. Consequently, the L21-strengthened alloy with a refined substructure exhibits a greatly increased plastic strain (∼6%) owing to the reduced microcrack length, and detoured crack propagation around precipitates.

Effect of phosphorus segregation on carbides and creep damage of Ni–Fe-based GH984G alloy

The effect of phosphorus segregation on carbides and creep damage of Ni–Fe based GH984G alloy, for the application of advanced ultra-super critical (A-USC) power plant, were investigated under the creep condition of 750 °C and 200 MPa. The results indicate that the primary precipitates of GH984G alloy are MC, M23C6 and γ′ after standard heat treatment. P-doping has no effect on the grain size, γ′ size and morphology. Nevertheless, P atoms segregate at the grain boundaries and around the MC carbides, resulting in a change in the morphology of M23C6 carbides at grain boundary from film-like to granular and a decrease in the density and area percentage of MC carbides. The creep behavior of GH984G alloy at 750 °C/200 MPa changes significantly after P doping and the creep properties (creep life and elongation) are optimized with the increasing P content. The damage feature is microcrack at grain boundaries for P-free alloy, while the creep cavities in P-doped alloy. The fracture mode changes from intergranular fracture to transgranular fracture with the increasing P doping for GH984G alloy after creep at 750 °C/200 MPa. The optimizing of creep properties and transformation of fracture mode of GH984G alloys after P doping are mainly attributed to increase of grain boundary cohesion and the change of carbides caused by the P segregation behaviors during creep process.

Solute interface segregation measurement: Cross comparison between four different analytical methods

Phosphorus intergranular segregation is known to influence the fracture properties of steels by decreasing grain boundary cohesion and induce intergranular fracture. Different techniques such as Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS), Wavelength Dispersive Spectroscopy (WDS), Energy Dispersive X-ray Spectroscopy coupled with Scanning Transmission Electron Microscope (STEM-EDX), and Atom Probe Tomography (APT) can be used to quantify intergranular segregation. Although many studies of this phenomenon were conducted over the last decades, there are rarely direct comparisons between different techniques and there is still a need of reliable and comparable quantification methods for grain boundary segregation. This study cross-compares four available techniques (AR-XPS, WDS, STEM-EDX, and APT) to quantify phosphorus interfacial segregation within the same grain of a sample. This was done by fabricating a Fe-P-Fe sandwich specimen with phosphorus segregated at the interface. Attention was paid to the way of expressing the results of the different techniques so that they can be compared with one another. The quantification results from the different techniques show reasonable agreement.

Effects of Mn Segregations on Intergranular Fracture in a Medium‐Mn Low‐Density Steel

Al‐bearing medium‐Mn low‐density steels possess great potential in the automotive industry because of their excellent mechanical properties based on transformation‐induced plasticity and low specific weight. Reducing the austenite stability against deformation‐induced martensitic transformation enables a high strain‐hardening capacity to be obtained; however, undesirably low stability often results in considerably reduced tensile ductility and brittle fracture. Herein, the brittle fracture that occurs with increasing annealing temperature for a Fe−0.3C–9Mn−5Al (wt%) steel is investigated in relation to Mn segregation at the phase boundaries between ferrite and austenite. The results demonstrate that annealing at 850 and 900 °C leads to ductile fractures with 72% and 95% tensile elongation, respectively, whereas only 25% elongation is achieved for the specimen annealed at 950 °C, exhibiting predominant intergranular facets. 3D atom probe tomography reveals that annealing at 950 °C promotes considerable Mn segregation at the ferrite/austenite phase boundaries with a peak composition of ≈19 at%, which is sufficient to reduce the boundary cohesion for intergranular fracture. Thermodynamic moving boundary simulation reveals that intercritical annealing is not a prerequisite for segregations; however, low‐temperature and prolonged holding should be accompanied, such as the coiling procedures.

First principles calculations of cohesive energy of fission-product-segregated grain boundary of UO2

Segregation of fission products and their effect on Σ3[110]/(111) UO2 grain boundary cohesive energy were analyzed via first principles calculations. The most stable segregation sites were investigated for zirconium, molybdenum, cesium, and xenon in UO2. Molybdenum, cesium, and xenon showed segregation tendencies on the Σ3[110]/(111) UO2 grain boundary but zirconium did not. The grain boundary cohesive energy was analyzed for the structure in which each segregation element was located at the most preferred segregation site. The results showed that zirconium and molybdenum strengthened the grain boundary cohesion, whereas cesium and xenon weakened it. This study shows which fission products cause the weakening of the irradiated UO2 grain boundary, which could help model irradiated fuel performance.

Tuning mechanical properties of ultrafine-grained tungsten by manipulating grain boundary chemistry

Tungsten is, due to a combination of high strength and good physical properties, frequently considered for high-performance applications in the harshest environments. Oftentimes its inherent brittleness and low ductility stand in the way of a successful deployment in these fields. Since tungsten has been proposed as divertor material for nuclear fusion reactors, an improvement of ductility and fracture toughness is essential. An obvious first step to increase these properties is to reduce the grain size to the ultrafine-grained regime. As this still leaves the material with a relatively low-energy intercrystalline fracture mode, this work takes a step further. With the help of doping elements, which are identified from ab-initio simulations, an attempt to increase grain boundary cohesion of ultra-fine grained tungsten to improve ductility is made. After fabrication of the doped samples from powders using severe plastic deformation, thorough microstructural investigations and extensive mechanical characterization, utilizing various small-scale testing techniques, are combined to assess the properties of the materials. We report that the addition of boron and hafnium can significantly increase the bending strength and bending ductility of ultra-fine grained tungsten. An additional heat treatment of the boron doped sample amplifies this effect even further, drastically increasing the strength and overall mechanical properties due to a combination of hardening-by-annealing and increased grain boundary segregation. Thus, an effective way to adaptively improve the mechanical properties of tungsten by manipulating grain boundary chemistry is reported, validating grain boundary segregation engineering as a powerful tool for enhancing damage tolerance in brittle materials.

Excellent high-temperature strength and ductility of the ZrC nanoparticles dispersed molybdenum

The interface control is critical in the development of high-performance molybdenum (Mo) alloys for high-temperature applications like space reactors. In this work, nanostructured Mo-ZrC alloy with excellent mechanical properties at both room temperature and high temperatures was fabricated by nanoscale ZrC dispersion and interface control. The average grain size of Mo-ZrC alloy is only 0.67 μm owing to the homogeneous dispersion of nanoscale ZrC particles. At room temperature, the ultimate tensile strength (UTS) and total elongation (TE) of the Mo-ZrC alloy are 928 MPa and 34.4%, respectively. At 1000 °C, the UTS is still as high as 562 MPa, which is significantly higher than those reported in oxide dispersion-strengthened Mo alloys, while the TE remains a high value of 23.5%. Additionally, the recrystallization start temperature of Mo-ZrC alloy is about 1400 °C, indicating remarkable thermal stability. First-principles calculations revealed that the interstitial oxygen reduces grain boundary cohesion, while C and ZrC could increase the fracture strength of the boundaries owing to the strong Mo-C bonds. The excellent mechanical properties and thermal stability of the Mo-ZrC alloy can be attributed to a synergistic effect of nanoscale ZrC particles dispersion strengthening, fine-grain strengthening, and grain boundary purification. The strategy could be applied to design other refractory alloys with both high strength and ductility for high-temperature applications.

Grain boundary segregation in Si-doped B-based ceramics and its effect on grain boundary cohesion

Boron carbide and boron suboxide are attractive for their low densities and ultrahigh hardness values, yet they exhibit poor fracture resistance. Strategies to improve mechanical performance aim to control bulk microstructures and properties by incorporating Si-based additives. A related approach to toughen boron carbide is tailoring the phase-like states of grain boundaries by applying the concept of grain boundary complexion engineering. This work directly investigates the potential of grain boundary complexion engineering to improve fracture resistance by systematically evaluating grain boundary segregation behavior of a polycrystalline silicon hexaboride / boron carbide diffusion couple using aberration-corrected scanning transmission electron microscopy and ζ-factor microanalysis. A main result was that all grain boundaries in the diffusion couple exhibited Si segregation that, depending on the bulk Si concentration, approached up to 3 monolayers of coverage, thereby resulting in nanolayer complexions (oftentimes known as intergranular films). Furthermore, upon analyzing 110 distinct grain boundaries throughout the diffusion zone and an impurity boron suboxide region, free energies of Si segregation in boron carbide and boron suboxide were experimentally determined using the Brunauer, Emmett and Teller (BET) multilayer grain boundary segregation theory. Subsequently, changes in grain boundary energy due to Si segregation were quantified and a maximum energy reduction of 51% and 81% were observed in boron carbide and boron suboxide, respectively, which led to decreases in works of adhesion of 18% and 17%, respectively. In summary, this work uncovers grain boundary processing-structure-property relationships that can be used to improve the fracture resistance in icosahedral boron-based ceramics.

The effect of solute atoms on the bulk and grain boundary cohesion in Ni: Implications for hydrogen embrittlement

Solute atoms dissolved in the Ni matrix can segregate to grain boundaries and have a pronounced effect on the performance of Ni-base alloys. Some elements, like, hydrogen lead to deterioration of interatomic bonding in the alloys resulting in a phenomenon known as hydrogen embrittlement. In this paper, we investigate the effect of 83 impurity atoms on the bulk and grain boundary decohesion resistance of Ni by calculating changes in the partial cohesive energy and in the grain boundary work of separation from density functional theory. The calculated results are used as training sets for two machine learning models. Both models are based on the use of the partial cohesive energies as one of the key features and posses very good predictive capability. The calculated data are analyzed in detail to identify the effect of all computed solutes on the grain boundary cohesive properties in Ni general and their effect on the overall resistance of Ni to hydrogen enhanced decohesion in particular.

Synergistic effect of residual stress and phosphorus grain-boundary segregation on embrittlement of coarse-grained heat-affected zone in a novel Ni-Cr-Mo reactor pressure vessel steel

SA508Gr.4N steel is a novel Ni-Cr-Mo low-alloy steel for the new-generation reactor pressure vessel. Herein, a coarse-grained heat-affected zone (CGHAZ) in a P-doped SA508Gr.4N steel is simulated using Gleeble thermal cycling simulation with a peak temperature of 1320 ℃ under a heat input of 100 kJ cm−1. For the as-simulated specimen, due to the synergistic effect of high residual stress and P grain-boundary segregation, the fracture appearance changes in the order of ductile fracture to intergranular fracture, and then to cleavage fracture with decreasing impact temperature. With the residual stress relieved by annealing at 620 ℃, the specimen toughens to a certain extent even after ageing at 560 ℃, causing a decrease in ductile-to-brittle transition temperature (DBTT), although the boundary cohesion declines to a certain degree owing to the increase of P segregation. Simultaneously, the intergranular-to-cleavage fracture transition temperature (ICFTT) shifts to a lower temperature below –190 ℃, enabling the specimen to exhibit intergranular fracture even at −190 ℃. With the ageing temperature lowered to 500 ℃, the boundary segregation of P is raised to a high level, leading to a large increase in DBTT and a further decrease in ICFTT. Accordingly, the synergistic effect of residual stress and P boundary segregation affects the fracture nature of CGHAZ, thereby making fracture appearance have different characteristics. A mechanism diagram for the above effect is proposed based on the experimental results.

An understanding of hydrogen embrittlement in nickel grain boundaries from first principles

Here, the segregation and accumulation of hydrogen in Ni grain boundaries, and its effects on cohesion and tensile mechanical strength were studied by means of density functional theory simulations. Three model grain boundaries were considered: the Σ3(11¯1)[110], Σ5(120)[001] and Σ11(11¯0)[113], as representatives for the highly coherent twin, high energy random high angle, and “special” low energy highly coherent grain boundaries, respectively. Hydrogen segregation was found to be favourable in the Σ5 and Σ11 grain boundaries, but not in the Σ3. Hydrogen accumulation studied via a comprehensive site-permutation analysis revealed the mechanisms for how H accumulation capacity varies as a function of grain boundary character. We show that the interfacial cohesion of boundaries can diminish by between 6.7–37.5% at varying levels of H-accumulation. The cohesion of the grain boundaries was analysed using a novel chemical bond-order based approach, enabling a quantitative atomistic determination of the fracture paths arising from hydrogen embrittlement. These simulations explain the details of why grain boundary character is the principal determinant of the likelihood of hydrogen segregation and accumulation, and hence their vulnerability to hydrogen-enhanced decohesion. This knowledge can be used in the design of thermomechanical processes to achieve grain boundary engineering for resistance to hydrogen embrittlement.

Towards pressureless sintering of nanocrystalline tungsten

The challenge of sintering ultrafine-grained tungsten to >99% density and towards nanocrystallinity is hereby addressed by optimized pressureless two-step sintering. It successfully produced tungsten samples with 99.3% theoretical density and 290 nm average grain size, with a uniform grain structure, good grain boundary cohesion, and 7.8 GPa hardness that is the highest in all pressurelessly sintered tungsten. The critical role of initial powders and the resultant green bodies was noted, which greatly affects later-on sintering kinetics and microstructural uniformity. Several key questions concerning two-step sintering of metallic tungsten were addressed, including the selection of the first- and second-step sintering temperatures, the thermodynamically required critical density to start the second-step sintering, and grain growth kinetics during sintering. The lessons learned here should be directly transferable to other refractory metals and their alloys and would help address the ultimate task of pressureless sintering of bulk nanocrystalline refractory metals/alloys.