Grain boundary pseudopartial wetting

Usually one distinguishes partial and complete wetting of surfaces or interfaces. In case of partial wetting contact angle θ > 0 and the liquid droplet is surrounded by “dry” surface or interface. In the majority of cases the direct transition occurs from partial wetting into complete wetting, for example by increasing temperature or decreasing pressure. However, in some cases the state of pseudopartial wetting occurs between partial and complete wetting. In this case the contact angle θ > 0, the liquid droplet does not spread over the substrate, but the thin (few nm) precursor film exists around the droplet and separates substrate and gas. Such precursor film is very similar for the liquid “pancake” in case of complete wetting and deficit of the liquid phase. The pseudopartial wetting has been observed before only for liquid/liquid mixtures (alcanes/water solution of salt or glucose) or Pb and Bi on the Cu surface. We observed the pseudopartial wetting of Al/Al grain boundaries (GBs) by solid Zn in the Al – 10 wt.% Zn ultra-fine grained polycrystals. The solid Zn partially wets Al/Al GBs (with non-zero contact angle). Nevertheless, the Al/Al GBs contain the 2 nm thin uniform Zn-rich layer connected with Zn grains. Such thin layers are the reason of high ductility of ultra-fine grained Al–Zn alloys at room temperature. This phenomenon opens the way for development of novel light-weight alloys. The pseudopartieal GB wetting by a liquid phase exists also in the WC–Co hard alloys. The pseudopartieal GB wetting by various liquid and solid phases also controls the properties of Nd–Fe–B-based hard magnetic alloys

Coexistence of Intermetallic Complexions and Bulk Particles in Grain Boundaries in the ZEK100 Alloy

Magnesium-based alloys are highly sought after in the industry due to their lightweight and reliable strength. However, the hexagonal crystal structure of magnesium results in the mechanical properties’ anisotropy. This anisotropy is effectively addressed by alloying magnesium with elements like zirconium, zinc, and rare earth metals (REM). The addition of these elements promotes rapid seed formation, yielding small grains with a uniform orientation distribution, thereby reducing anisotropy. Despite these benefits, the formation of intermetallic phases (IP) containing Zn, Zr, and REM within the microstructure can be a concern. Some of these IP phases can be exceedingly hard and brittle, thus weakening the material by providing easy pathways for crack propagation along grain boundaries (GBs). This issue becomes particularly significant if intermetallic phases form continuous layers along the entire GB between two neighboring GB triple junctions, a phenomenon known as complete GB wetting. To mitigate the risks associated with complete GB wetting and prevent the weakening of the alloy’s structure, understanding the potential occurrence of a GB wetting phase transition and how to control continuous GB layers of IP phases becomes crucial. In the investigation of a commercial magnesium alloy, ZEK100, the GB wetting phase transition (i.e., between complete and partial GB wetting) was successfully studied and confirmed. Notably, complete GB wetting was observed at temperatures near the liquidus point of the alloy. However, at lower temperatures, a coexistence of a nano-scaled precipitate film and bulk particles with nonzero contact angles within the same GB was observed. This insight into the wetting transition characteristics holds potential to expand the range of applications for the present alloy in the industry. By understanding and controlling GB wetting phenomena, the alloy’s mechanical properties and structural integrity can be enhanced, paving the way for its wider utilization in various industrial applications

Wetting of grain boundaries in ultrafine-grained copper by liquid bismuth

In the present work, we studied the effect of liquid Bi on the microstructure evolution of ultrafinegrained Cu at elevated temperatures

Structure Refinement and Fragmentation of Precipitates under Severe Plastic Deformation: A Review

During severe plastic deformation (SPD), the processes of lattice defect formation as well as their relaxation (annihilation) compete with each other. As a result, a dynamic equilibrium is established, and a steady state is reached after a certain strain value. Simultaneously, other kinetic processes act in opposite directions and also compete with each other during SPD, such as grain refinement/growth, mechanical strengthening/softening, formation/decomposition of solid solution, etc. These competing processes also lead to dynamic equilibrium and result in a steady state (saturation), albeit after different strains. Among these steady-state phenomena, particle fragmentation during the second phase of SPD has received little attention. Available data indicate that precipitate fragmentation slows down with increasing strain, though saturation is achieved at higher strains than in the case of hardness or grain size. Moreover, one can consider the SPD-driven nanocrystallization in the amorphous phase as a process that is opposite to the fragmentation of precipitates. The size of these crystalline nanoprecipitates also saturates after a certain strain. The fragmentation of precipitates during SPD is the topic of this review

Grain Boundary Wetting Phenomena in High Entropy Alloys Containing Nitrides, Carbides, Borides, Silicides, and Hydrogen: A Review

In this review, we analyze the structure of multicomponent alloys without principal components (they are also called high entropy alloys-HEAs), containing not only metals but also hydrogen, nitrogen, carbon, boron, or silicon. In particular, we discuss the phenomenon of grain boundary (GB) wetting by the melt or solid phase. The GB wetting can be complete or incomplete (partial). In the former case, the grains of the matrix are completely separated by the continuous layer of the second phase (solid or liquid). In the latter case of partial GB wetting, the second solid phase forms, between the matrix grains, a chain of (usually lenticular) precipitates or droplets with a non-zero value of the contact angle. To deal with the morphology of GBs, the new GB tie-lines are used, which can be constructed in the two- or multiphase areas of the multidimensional HEAs phase diagrams. The GBs in HEAs in the case of complete or partial wetting can also contain hydrides, nitrides, carbides, borides, or silicides. Thus, GB wetting by the hydrides, nitrides, carbides, borides, or silicides can be used in the so-called grain boundary chemical engineering in order to improve the properties of respective HEAs.This research was funded by the Russian Ministry of Science and Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013) Support from the University of the Basque Country under the GIU19/019 project is also acknowledged

High Entropy Alloys for Energy Conversion and Storage: A Review of Grain Boundary Wetting Phenomena

This research was funded by the Russian Ministry of Science and Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013). Support from the University of the Basque Country (project GIU19/019) and from the Basque Government (project IT1714-22) is also acknowledged.The multicomponent alloys with nearly equal concentration of components, also known as high entropy alloys (HEAs), were first proposed 22 years ago. The HEAs quickly became very important in materials science due to their unique properties. Nowadays, the HEAs are frequently used in energy conversion and storage applications. HEAs can consist of five, six or more components. Plasma cladding permits coating of the large surfaces of cheap substrates with (often expensive) HEAs and to enlarge, in such a way, their application area. The large-area coatings deposited by plasma cladding possess multiple advantages such as low thermal distortion, very high energy density, as well as low dilution of the substrate material. Plasma cladding ensures good metallurgical bonding between coating and substrate. The costs of operation and equipment are also very attractive. During plasma cladding, the mixed powders are blown by carrier gas into a plasma torch or are positioned on a substrate. This powder mixture is then melted in or under the plasma torch. The plasma torch, in turn, sequentially scans the substrate. After finalizing the crystallization process, the solid polycrystal appears which contains few residual melts. This remaining melt can completely or incompletely wet the grain boundaries (GBs) in solid phase of the polycrystal. These completely or incompletely wetted GBs can strongly influence the microstructure of HEA coatings and their morphology. In this review we analyze the GB wetting HEAs containing one phase in HEAs with two, three and more phases, as well as in HEAs reinforced with particles of carbides, nitrides, borides, or oxides. We also analyze the microstructure of the rather thick coatings after plasma cladding after additional laser remelting and observe how GB wetting changes over their thickness.--//-- Published under the CC BY 4.0 licence.Russian Ministry of Science and Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013); University of the Basque Country (project GIU19/019); Basque Government (project IT1714-22); Institute of Solid State Physics, University of Latvia as the Center of Excellence acknowledges funding from the European Union’s Horizon 2020 Framework Programme H2020- WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Ferromagnetic behaviour of ZnO: The role of grain boundaries

The possibility to attain ferromagnetic properties in transparent semiconductor oxides such as ZnO is very promising for future spintronic applications. We demonstrate in this review that ferromagnetism is not an intrinsic property of the ZnO crystalline lattice but is that of ZnO/ZnO grain boundaries. If a ZnO polycrystal contains enough grain boundaries, it can transform into the ferromagnetic state even without doping with “magnetic atoms” such as Mn, Co, Fe or Ni. However, such doping facilitates the appearance of ferromagnetism in ZnO. It increases the saturation magnetisation and decreases the critical amount of grain boundaries needed for FM. A drastic increase of the total solubility of dopants in ZnO with decreasing grain size has been also observed. It is explained by the multilayer grain boundary segregation

Grain boundary oxide layers in NdFeB-based permanent magnets

The microstructure of grain boundaries (GBs) in the commercial NdFeB-based alloy for permanent magnets has been studied. It is generally accepted that the unique hard magnetic properties of such alloys are controlled by the thin layers of a Nd-rich phase in Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs. These GB layers ensure the magnetic isolation of Nd$_{2}$Fe$_{14}$B grains from each other. It is usually supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial NdFeB-based permanent magnets frequently contain a tangible amount of neodymium oxide Nd$_{2}$O$_{3}$ at the triple junctions between Nd$_{2}$Fe$_{14}$B grains. The goal of this work was to check whether the Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs could also contain the thin layers of Nd$_{2}$O$_{3}$ oxide phase. Indeed, the screening with EELS-based elemental analysis permitted to observe that some of these Nd-rich layers in Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs contain not only neodymium, but also oxygen. More detailed analysis of such GBs with high-resolution transmission electron microscopy (HR TEM) showed these GB layers are crystalline and have the lattice of neodymium oxide Nd$_{2}$O$_{3}$. In turn, the Lorentz micro-magnetic contrast in TEM permitted to observe that the Nd-oxide GB layers prevent the migration of domain walls from one Nd$_{2}$Fe$_{14}$B grain to another during remagnetization. This finding proves that the GB oxide layers, similar to those of metallic Nd or Nd-rich intermetallic compounds, can ensure the magnetic isolation between Nd$_{2}$Fe$_{14}$B grains needed for high coercivity. Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets

Grain Boundary Wetting by a Second Solid Phase in the High Entropy Alloys: A Review

In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomponent phase diagrams for HEAs. The GB wetting by the second solid phase can be used to improve the properties of HEAs by applying the so-called grain boundary engineering methods.This research was funded by the Russian Ministry Of Science And Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013). Support from the University of the Basque Country under the GIU19/019 project is also acknowledged