Thermally Stable Nanotwins: New Heights for Cu Mechanics.

Nanocrystalline and nanotwinned materials achieve exceptional strengths through small grain sizes. Due to large areas of crystal interfaces, they are highly susceptible to grain growth and creep deformation, even at ambient temperatures. Here, ultrahigh strength nanotwinned copper microstructures have been stabilized against high temperature exposure while largely retaining electrical conductivity. By incorporating less than 1 vol% insoluble tungsten nanoparticles by a novel hybrid deposition method, both the ease of formation and the high temperature stability of nanotwins are dramatically enhanced up to at least 400 °C. By avoiding grain coarsening, improved high temperature creep properties arise as the coherent twin boundaries are poor diffusion paths, while some size-based nanotwin strengthening is retained. Such microstructures hold promise for more robust microchip interconnects and stronger electric motor components.

Combinatorial Study of Phase Composition, Microstructure and Mechanical Behavior of Co-Cr-Fe-Ni Nanocrystalline Film Processed by Multiple-Beam-Sputtering Physical Vapor Deposition.

A combinatorial Co-Cr-Fe-Ni compositional complex alloy (CCA) thin film disk with a thickness of 1 µm and a diameter of 10 cm was processed by multiple-beam-sputtering physical vapor deposition (PVD) using four pure metal sources. The chemical composition of the four constituent elements varied between 4 and 64 at.% in the film, depending on the distance from the four PVD sources. The crystal structure, the crystallite size, the density of lattice defects (e.g., dislocations and twin faults) and the crystallographic texture were studied as a function of the chemical composition. It was found that in a wide range of elemental concentrations a face-centered cubic (fcc) structure with {111} crystallographic texture formed during PVD. Considering the equilibrium phase diagrams, it can be concluded that mostly the phase composition of the PVD layer is far from the equilibrium. Body-centered cubic (bcc) and hexagonal-close packed (hcp) structures formed only in the parts of the film close to Co-Fe and Co-Cr sources, respectively. A nanocrystalline microstructure with the grain size of 10-20 nm was developed in the whole layer, irrespective of the chemical composition. Transmission electron microscopy indicated a columnar growth of the film during PVD. The density of as-grown dislocations and twin faults was very high, as obtained by synchrotron X-ray diffraction peak profile analysis. The nanohardness and the elastic modulus were determined by indentation for the different chemical compositions on the combinatorial PVD film. This study is the continuation of a former research published recently in Nagy et al., Materials 14 (2021) 3357. In the previous work, only the fcc part of the sample was investigated. In the present paper, the study was extended to the bcc, hcp and multiphase regions.

Microstructure, Hardness, and Elastic Modulus of a Multibeam-Sputtered Nanocrystalline Co-Cr-Fe-Ni Compositional Complex Alloy Film.

A nanocrystalline Co-Cr-Ni-Fe compositional complex alloy (CCA) film with a thickness of about 1 micron was produced by a multiple-beam-sputtering physical vapor deposition (PVD) technique. The main advantage of this novel method is that it does not require alloy targets, but rather uses commercially pure metal sources. Another benefit of the application of this technique is that it produces compositional gradient samples on a disk surface with a wide range of elemental concentrations, enabling combinatorial analysis of CCA films. In this study, the variation of the phase composition, the microstructure (crystallite size and defect density), and the mechanical performance (hardness and elastic modulus) as a function of the chemical composition was studied in a combinatorial Co-Cr-Ni-Fe thin film sample that was produced on a surface of a disk with a diameter of about 10 cm. The spatial variation of the crystallite size and the density of lattice defects (e.g., dislocations and twin faults) were investigated by X-ray diffraction line profile analysis performed on the patterns taken by synchrotron radiation. The hardness and the elastic modulus were measured by the nanoindentation technique. It was found that a single-phase face-centered cubic (fcc) structure was formed for a wide range of chemical compositions. The microstructure was nanocrystalline with a crystallite size of 10-27 nm and contained a high lattice defect density. The hardness and the elastic modulus values measured for very different compositions were in the ranges of 8.4-11.8 and 182-239 GPa, respectively.

Molecular layer deposited alucone thin films from long-chain organic precursors: from brittle to ductile mechanical characteristics.

Molecular layer deposition (MLD) is a strongly emerging thin-film technique for deposition of ultra-thin inorganic-organic hybrid ("metalcone") coatings directly from the gas phase, even on complex three-dimensional surfaces. In particular alucones (Al-based hybrids) have been found interesting e.g. for Li-ion battery and gas-barrier applications owing to the promise for enhanced mechanical performance provided by the organic fragments in the materials' structure. However, the metalcones based on short/small organic fragments are relatively brittle from the mechanical perspective. Here, we demonstrate an efficient approach for tailoring mechanical properties of MLD-fabricated hybrid inorganic-organic thin films through control over the organic precursor chain length. The proof-of-concept data is presented for alucones prepared using trimethyl aluminum together with 1,6-hexanediol or 1,10-decanediol as the precursors. Tensile testing coupled with in situ optical microscopy reveals a gradual increase in stretchability with the increasing chain length, such that the crack onset strain value of 9.9 ± 0.2% is obtained for the 1,10-decanediol-based 100 nm-thick film. Through the demonstration of substantially suppressed crack propagation-as a sign of brittle-to-ductile transition-and the decrease in the elastic modulus value down to 4.6 ± 2.1 GPa, the mechanical performance of the alucone family is extended to the polymeric regime. The substantial increase in the mechanical performance within the metalcone material family makes the results particularly interesting for high-capacity high-volume-change battery electrodes requiring mechanically highly robust coatings.

Comparison study of silicon carbide coatings produced at different deposition conditions with use of high temperature nanoindentation.

The elastic modulus and hardness of different silicon carbide (SiC) coatings in tristructural-isotropic (TRISO) fuel particles were measured by in situ high temperature nanoindentation up to 500 °C. Three samples fabricated by different research institutions were compared. Due to varied fabrication parameters the samples exhibited different grain sizes and one contained some visible porosity. However, irrespective of the microstructural features in each case the hardness was found to be very similar in the three coatings around 35 GPa at room temperature. Compared with the significantly coarser grained bulk CVD SiC, the drop in hardness with temperature was less pronounced for TRISO particles, suggesting that the presence of grain boundaries impeded plastic deformation. The elastic modulus differed for the three TRISO coatings with room temperature values ranging from 340 to 400 GPa. With increasing measurement temperature the elastic modulus showed a continuous decrease.