Enrichment Of Grain Boundaries Research Articles

Unconventional grain fragmentation creates high-density boundaries for efficient CO2-to-C2+ electro-conversion at ampere-level current density

Electrocatalytic CO2 reduction reaction (CO2RR) to produce multi-carbon products (C2+) is one of the most sustainable manners to achieve net-zero carbon emissions. Among many approaches, enriching grain boundaries (GBs) in copper (Cu) catalysts has been demonstrated to enable enhancement for C2+ production. However, it still lacks effective strategies to controllably synthesize abundant GBs, rendering efficient C2+ production a persistent challenge, especially at ampere-level current density. Herein, we propose a novel strategy, which can achieve unconventional grain fragmentation during thermal annealing and thus create controllable GB densities. The catalyst with the utmost GB density exhibits a peak C2+ faradaic efficiency of ca. 70.0 % in H-type cell and 68.2 % in flow cell; even more impressively, it delivers an ultra-high C2+ current density of 0.768 A cm−2, outperforming most recently reported results. A combination of in situ spectroscopies and theoretical calculations reveal that the enrichment of GBs yields more active sites for a higher *CO coverage, leading to promotion of the *CO-*CO coupling process and ultimately high C2+ production performance.

Effect of the Segregation of Embrittling Impurities on the Local Strength of Grain Boundaries in a Martensitic Steel

The finite element method and the results of delayed fracture tests of Charpy specimens are used to determine the local strength of grain boundaries in 18Kh2N4VA steel at various contents of phosphorus, antimony, and tin impurities. The embrittlement of grain boundaries is attributed to the adsorption enrichment of grain boundaries in impurities in initial austenite, which leads to weakening of the intergranular cohesion. Phosphorus is found to have the highest susceptibility to intergranular segregation: during the same heat treatment, its grain-boundary concentration is higher than that in steel with antimony or tin. However, the segregation of antimony is more dangerous than the segregation of phosphorus, since, at its lower intergranular concentration, the local strength of grains of quenched steel decreases as in the case of phosphorus segregation.

Enhanced ionic conductivity in electroceramics by nanoscale enrichment of grain boundaries with high solute concentration.

The enhancement of oxygen ionic conductivity by over two orders of magnitude in an electroceramic oxide is explicitly shown to result from nanoscale enrichment of a grain boundary layer or complexion with high solute concentration. A series of CaxCe1-xO2-δ polycrystalline oxides with fluorite structure and varying nominal Ca2+ solute concentration elucidates how local grain boundary composition, rather than structural grain boundary character, primarily regulates ionic conductivity. A correlation between high grain boundary solute concentration above ∼40 mol%, and four orders of magnitude increase in grain boundary conductivity is explicitly shown. A correlated experimental approach provides unique insights into fundamental grain boundary science, and highlights how novel aspects of nanoscale grain boundary design may be employed to control ion transport properties in electroceramics.

Mechanical and microstructural characteristics of Ti6Al4V/AA2519 and Ti6Al4V/AA1050/AA2519 laminates manufactured by explosive welding

The aim of the work was to produce laminated structures consisting of Ti–6Al–4V alloy and AA2519 plates and to investigate their microstructure and mechanical properties with an emphasis on the role of an additional AA1050 interlayer. Explosive welding was selected as a joining technology. The microstructure and chemical composition of the explosively joined samples were investigated. Mechanical properties were evaluated in the tensile testing and by microhardness analysis.The results demonstrated that explosive welding is an effective way to produce Ti/Al laminates. Both Ti6Al4V/AA2519 and Ti6Al4V/AA1050/AA2519 laminated plates exhibit good quality of bonding without voids and major delamination. The explosive welding produced metallurgical bonding with a nanostructured zone consisting of Al3Ti and Al2Ti phases. This zone is thicker in the joint with additional AA1050 interlayer when compared to direct AA2519/Ti6Al4V bonding. In the latter, SEM and STEM analysis reviled the presence of net-like structure in the collision zone. Advanced EDX analysis shows the enrichment of grain boundaries in copper. The formation of this structure is widely discussed. In addition, the explosive welding introduces large plastic deformation which induces the process of grain refinement in aluminium plates. Tensile testing confirms that joining section is not the weakest element of the cladded plates.

Growth mechanisms and thickness effect on the properties of Al‐doped ZnO thin films grown on polymeric substrates

AbstractThe properties of Al‐doped ZnO (AZO) thin films deposited by pulsed DC magnetron sputtering under various target power on polyethylene terephthalate (PET) substrates have been investigated. In situ and real‐time spectroscopic ellipsometry (1.5–6.5 eV) has been employed to study the optical properties of the AZO films as well as the growth mechanisms taking place. With X‐ray diffraction technique under grazing‐incidence geometry, the structural characteristics profiles of the AZO films have been depicted. Nanoindentation measurements revealed information about the mechanical properties of the films and have been correlated to the conductivity measurements towards growth insights understanding. As results have shown, the increase of target power led to the increase of the carrier density as well as the hardness of the AZO films possibly both ascribed to dislocations induced. The stress during the deposition of AZO film under high target power favoured the island growth which is possibly both connected to the formation of defects as electron traps and dislocations as electron sources. Finally, the increase of AZO film thickness led to the increase of the resistivity possibly due to the enrichment of grain boundaries with defects as electron traps.

Ultimate Strengthening, Theoretical and Limit Tool Hardness

The influence of passing from a microcrystalline to a nanocrystalline structure on the mechanical properties of chromium deposited by magnetron sputtering is studied. The possibility of additional strengthening nanomaterials due to enrichment of grain boundaries by “useful” additives elements is established. A wide spectrum of materials in different structural states was investigated by the method of micromechanical tests. The notions of the “theoretical” hardness (largest hardness for the material) and “limit tool” hardness, connected with tool limitations in indentation, are introduced.

An x-ray spectromicroscopic study of electromigration in patterned Al(Cu) lines

We studied the surface properties of patterned Al(Cu) lines related to the electromigration phenomena using photoemission spectromicroscopy techniques. We stressed the lines for electromigration in situ in the ultrahigh vacuum microscope chamber and observed the changes on the line surface. Our results show surface precipitation of Cu beneath the Al2O3 layer on the line surface as well as on side walls. Enrichment of grain boundaries in Cu due to electromigration flux was observed in areas downstream of voids with respect to the electron flow.

Structure and strength of powder metallurgy aluminum alloy AK-6

The interface chemical compositions and fine structure of sintered aluminum alloys were studied by the methods of Auger-electron spectroscopy and transmission electron microscopy. The transfer of impurity elements from the surface of powders to grain boundaries in the sintered alloys was experimentally confirmed. Enrichment of grain boundaries in oxygen, sulfur, and calcium causes brittle intergranular fracture, and seems to be the main reason for the lower strength of sintered alloys. Increased strength can be obtained by the elimination of intergranular brittleness, and the formation of dispersed precipitates of second phases in the matrix.

Properties of Iron Atoms at Grain Boundaries in Fe and Fe72Al28

ABSTRACTLocation of diffused 57Co atoms in single crystals, bicrystals and polycrystals of pure iron and Fe72Al28alloy were investigated by means of emission Mössbauer spectroscopy. To interpret the results, first principles calculations of iron atom magnetic moments and hyper-fine field were carried out. From comparison of M6ssbauer spectra of single crystals with those of bicrystals and polycrystals, an information about grain boundary positions occupied by diffusing atoms is obtained. It is shown that about 5% of the diffusing atoms at the {112} grain boundary of iron are located at the positions either having impurity atoms in the nearest neighbourhood or characterized by larger atomic spacing in comparison with the bulk. In the Fe72Al28 a dominating portion of diffusing atoms have different surrounding than in grain volume. An enrichment of grain boundaries by aluminum could explain their hyperfine parameters.

Application of SSNTDs for study of boron distribution in alloys

A study of boron behavior and its distribution in boron doped iron-, nickel- and aluminum-base alloys subjected to various treatment conditions has been performed. For this purpose a technique based on the detection of 10B(n,α) 7Li reaction products with track etch detectors has been applied. High boron enrichment of grain boundaries of both iron base and ingot nickel base alloys has been observed. After hot and then cold rolling of the latter specimens boron rearrangement along with the proper deformation of the structure took place. Striation of boron in a hot rolled aluminum alloy along the rolling direction has been observed.

The contribution of grain boundary effects to low-alloy steel irradiation embrittlement

The contribution of irradiation-induced enrichment of grain boundaries by impurities to irradiation embrittlement of reactor pressure vessel materials is discussed. Possible mechanisms of impurities and the effect of alloying elements on irradiation embrittlement of reactor pressure vessel steels are considered. Nickel has been found to influence greatly the tendency to irradiation embrittlement of nickel-containing steels with Ni wt% > 0.9. Irradiation resistance of nickel-containing steels has been shown to decrease significantly with the increase of silicon concentration from 0.24–0.28 to 0.3–0.4 wt%. The model for irradiation-induced enrichment of grain boundaries by impurities is used in order to explain the effect of silicon and nickel on irradiation embrittlement. In terms of the model, Si and Ni themselves do not prove the embrittlement, but they only influence thermodynamic and kinetic parameters of the phosphorus gain boundary adsorption. The embrittlement process itself is a result of decreasing of grain boundary cohesion with formation of phosphorus irradiation-induced grain boundary segregation.

On the bainite structure in Cr‐Mo‐V rotor steel

The morphology of continuously cooled and isothermally transformed bainite structures formed in a Cr‐Mo‐V rotor steel has been studied using transmission electron microscopy. The samples were austenitised at 955°C for an hour followed by air cooling to room temperature. The isothermal transformation reaction was carried out at 450°C for up to 100 000 s. The microconstituents observed are predominantly lower bainite with very small amount of upper bainite and martensite (formed from untransformed austenite due to water quenching). Analysis of the selected area diffraction patterns confirm that the carbide in bainite is orthorhombic cementite and the orientation relationship between ferrite and cementite is consistent with that of Bagaryatskii. The carbide particles in isothermally transformed bainite are coarser than those of continuously cooled bainite. Tempering one hour at 670°C of continuously cooled steel samples exhibited formation of fine spheroidal MC type carbides. In addition tempering leads to the enrichment of prior austenite grain boundaries by cementite particles. Tempering ten hours at 670°C exhibited microstructures almost identical to those observed in one hour tempering.

The segregation of As, Sb and Bi in Cr-Ni steels and alloys with different Ni contents investigated using AES

This paper reports on an AES study of grain boundary segregation in five kinds of Cr-Ni corrosion-resistant steels and alloys with different Ni contents that were enriched by As, Sb or Bi. The results show that all mentioned impurities segregate to the grain boundaries and that the segregation level depends on the type of the impurity. The highest amount of segregant atoms was found in the case of Bi, while it appeared to be substantially lower for As and Sb. The segregation level of each impurity grows also with the Ni content. This increase is more pronounced for Bi and Sb than for As. In particular the results demonstrate that the enrichment of grain boundaries by a given impurity is strongly connected to the solubility of this element in the matrix.

The nucleation of high temperature brittle intergranular fracture in 2.25cr-1mo steel

An investigation of brittle intergranular crack nucleation at high temperatures in 2.25Cr-lMo steel is presented. A series of tests was designed to allow the independent variation of segregating impurities (e.g., sulfur and phosphorus) and locally applied stress levels to determine the effects of applied stress on impurity enrichment and fracture of grain boundaries in as-quenched material. The study utilized blunt notched specimens with suitable finite element stress analyses, together with high resolution Auger electron surface analysis techniques. It is shown that the imposition of a tensile hydrostatic stress enhances the segregation of elemental sulfur to grain boundariesprior to the nucleation of high temperature brittle intergranular cracks. The location of enriched grain boundaries relative to the peak stress regions in notched specimens is discussed, and possible mechanisms for the observed sulfur enrichment presented.

High resolution scanning auger microscopic investigation of intergranular fracture in As-quenched Fe-12mn

Previous research in this laboratory led to the conclusion that the low temperature intergranular fracture mode in Fe-Mn alloys is microstructurally determined, and does not require metalloid segregation or other chemical contamination. That conclusion was tested in the present investigation, which used high resolution scanning Auger microscopy to study the intergranular fracture surfaces. The fracture mode at liquid nitrogen temperature was found to be intergranular fracture whenever the alloy was quenched from the austenite field, irrespective of the austenization time or temperature. High resolution chemical analyses of the intergranular fracture surfaces failed to reveal any consistent segregation of P, S, O, or N. The occasional appearance of sulfur or oxygen on the fracture surface was found to be due to a low density precipitation of MnS and MnO2 along the prior austenite grain boundaries. Excepting these dispersed precipitates, there was no evidence of manganese enrichment of the prior austenite grain boundaries. A slight segregation of carbon was found along the grain boundaries, but does not appear to be implicated in the tendency toward intergranular fracture. The present results hence reinforce the conclusion that the low temperature intergranular fracture of Fe-12Mn is microstructurally determined.

Mechanism of grain boundary strengthening of steels

1. The mechanism of grain boundary strengthening depends on several factors, above all the grain size and the condition of the grain boundaries. 2. In fine-grained steels the effect of the barrier mechanism of grain boundary strengthening is negligible, while the effect of grain boundaries on resistance to deformation described by the Hall—Petch equation is manifest indirectly through the effect of strain hardening with a more rapid increase of dislocation density. 3. With decreasing relative length of grain boundaries (large grains) or enrichment of grain boundaries in impurities and precipitates of second phase the number of potential dislocation sources near grain boundaries decreases. In this case the fulfillment of the Hall—Petch relationship indicates the barrier mechanism of grain boundary strengthening. 4. The barrier effect of cell boundaries is evident in austenitic steels with large deformations, which leads to deviation of σf=f(d−1/2) from linear.

Effect of cyclic heat treatment on the density of cadmium with a bismuth admixture

A study has been made of the effect of cyclic heat treatment on the density of cadmium having a bismuth admixture. The effects of the heating rate, cooling rate, and time the cadmium is held at the maximum temperature were studied. It is concluded that thermal-structural stresses and admixture enrichment of grain boundaries play an important role in the density changes.