Fabrication of Hollow AlAu2 Nanoparticles by Solid State Dewetting and Oxidation of Al on Sapphire Substrate

The Al-Au binary diffusion couple is a classic example of the system exhibiting Kirkendall voiding during interdiffusion. We demonstrate that this effect, which is a major reason for failures of the wire bonds in microelectronics, can be utilized for producing hollow AlAu2 nanoparticles attached to sapphire substrate. To this end, we produced the core-shell Al-Au nanoparticles by performing a solid state dewetting treatment of Al thin film deposited on sapphire substrate, followed by the deposition of thin Au layer on the top of dewetted sample. Annealing of the core-shell nanoparticles in air resulted in outdiffusion of Al from the particles, formation of pores, and growth of the AlAu2 intermetallic phase in the particles. We demonstrated that the driving force for hollowing is the oxidation reaction of the Al atoms at the Au-sapphire interface, leading to the homoepitaxial growth of newly formed alumina at the interface. We developed a kinetic model of hollowing controlled by diffusion of oxygen through the Au thin film, and estimated the solubility of oxygen in solid Au. Our work demonstrates that the core-shell nanoparticles attached to the substrate can be hollowed by the Kirkendall effect in the thin film spatially separated from the particles.Comment: 27 pages, 8 figure

Annealing-induced recovery of indents in thin Au(Fe) bilayer films

We employed depth-sensing nanoindentation to produce ordered arrays of indents on the surface of 50 nm-thick Au(Fe) films deposited on sapphire substrates. The maximum depth of the indents was approximately one-half of the film thickness. The indented films were annealed at a temperature of 700 °C in a forming gas atmosphere. While the onset of solid-state dewetting was observed in the unperturbed regions of the film, no holes to the substrate were observed in the indented regions. Instead, the film annealing resulted in the formation of hillocks at the indent locations, followed by their dissipation and the formation of shallow depressions nearby after subsequent annealing treatments. This annealing-induced evolution of nanoindents was interpreted in terms of annihilation of dislocation loops generated during indentation, accompanied by the formation of nanopores at the grain boundaries and their subsequent dissolution. The application of the processes uncovered in this work show great potential for the patterning of thin films

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Self-Healing and Shape Memory Effects in Gold Microparticles through the Defects-Mediated Diffusion

Some metal alloys subjected to irreversible plastic deformation can repair the inflicted damage and/or recover their original shape upon heating. The conventional shape memory effect in metallic alloys relies on collective, or “military” phase transformations. This work demonstrates a new and fundamentally different type of self-healing and shape memory in single crystalline faceted nano and microparticles of pure gold, which are plastically deformed with an atomic force microscope tip. It is shown that annealing of the deformed particles at elevated temperatures leads to nearly full recovery of their initial asymmetric polyhedral shape, which does not correspond to global energy minimum shape. The atomistic molecular dynamic simulations demonstrate that the shape recovery of the particles is controlled by the self-diffusion of gold atoms along the terrace ledges formed during the particles indentation. This ledge-guided diffusion leads to shape recovery by the irreversible diffusion process. A semiquantitative model of healing developed in this work demonstrates a good agreement with the experimental data

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

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

High Entropy Alloys Coatings Deposited by Laser Cladding: A Review of Grain Boundary Wetting Phenomena

High-entropy alloys (HEAs) are called also alloys without a main component or multiprincipal alloys. They consist of five, six or more components in more or less equal proportions and possess unique properties. Several dozens of thousands of publications have already been devoted to bulk HEAs, while HEA coatings are just beginning to develop. More than half of the works on the deposition of HEA coatings are devoted to laser cladding. In the laser cladding process, a mixture of powders on a substrate is melted in a focused laser beam, which sequentially scans the substrate. In the heated zone, the powder mixture melts. At the end of the crystallization process, a solidified polycrystal and a small amount of residual melt are found in the heated zone. It is possible that the grain boundaries (GBs) in the solidified polycrystal are incompletely or fully wetted by this liquid phase. In this way, the GB wetting with a melt determines the morphology and microstructure of HEAs coatings. This review analyzes GB wetting in single-phase HEAs, as well as in HEAs containing two or more phases. We analyze how the HEAs’ composition, laser scanning speed, laser beam power, external magnetic field or ultrasonic impact affect the microstructure and GB wetting. It is also shown how the microstructure and GB wetting change over the thickness of the rather thick as well as multilayer coatings deposited using a laser cladding.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

Severe Plastic Deformation and Phase Transformations in High Entropy Alloys: A Review

This review discusses an area of expertise that is at the intersection of three large parts of materials science. These are phase transformations, severe plastic deformation (SPD), and high-entropy alloys (HEA). First, SPD makes it possible to determine the borders of single-phase regions of existence of a multicomponent solid solution in HEAs. An important feature of SPD is that using these technologies, it is possible to obtain second-phase nanoparticles included in a matrix with a grain size of several tens of nanometers. Such materials have a very high specific density of internal boundaries. These boundaries serve as pathways for accelerated diffusion. As a result of the annealing of HEAs subjected to SPD, it is possible to accurately determine the border temperature of a single-phase solid solution area on the multicomponent phase diagram of the HEA. Secondly, SPD itself induces phase transformations in HEAs. Among these transformations is the decomposition of a single-phase solid solution with the formation of nanoparticles of the second phase, the formation of high-pressure phases, amorphization, as well as spinodal decomposition. Thirdly, during SPD, a large number of new grain boundaries (GBs) are formed due to the crystallites refinement. Segregation layers exist at these new GBs. The concentration of the components in GBs differs from that in the bulk solid solution. As a result of the formation of a large number of new GBs, atoms leave the bulk solution and form segregation layers. Thus, the composition of the solid solution in the volume also changes. All these processes make it possible to purposefully influence the composition, structure and useful properties of HEAs, especially for medical applications