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Amorphous/crystalline high-entropy-alloy (HEA) composites show great promise as structural materials due to their exceptional mechanical properties. However, there is still a lack of understanding of the dynamic nanoindentation response of HEA composites at the atomic scale. Here, the mechanical behavior of amorphous/crystalline HEA composites under nanoindentation is investigated through a large-scale molecular dynamics simulation and a dislocation-based strength model, in terms of the indentation force, microstructural evolution, stress distribution, shear strain distribution, and surface topography. The results show that the uneven distribution of elements within the crystal leads to a strong heterogeneity of the surface tension during elastic deformation. The severe mismatch of the amorphous/crystalline interface combined with the rapid accumulation of elastic deformation energy causes a significant number of dislocation-based plastic deformation behaviors. The presence of surrounding dislocations inhibits the free slip of dislocations below the indenter, while the amorphous layer prevents the movement or disappearance of dislocations towards the substrate. A thin amorphous layer leads to great indentation force, and causes inconsistent stacking and movement patterns of surface atoms, resulting in local bulges and depressions at the macroscopic level. The increasing thickness of the amorphous layer hinders the extension of shear bands towards the lower part of the substrate. These findings shed light on the mechanical properties of amorphous/crystalline HEA composites and offer insights for the design of high-performance materials. [ABSTRACT FROM AUTHOR]

This study investigated the microstructure and grain orientation of Ni(V)/SAC396/OSP Cu and Ni(V)/SAC105/OSP Cu solder joints during thermal cycling test (TCT). In general, adjusting Ag content in solders influenced the performance of thermal cycling (TC). Nevertheless, it is still crucial to find the optimal condition of Ag and related mechanisms of crack formation during TCT. This study applied FE-SEM to observe the crack propagation and the precipitation of Sn-Ag IMCs in the solders. Moreover, FE-EPMA and EBSD were used to detect the elemental distribution and grain orientation near the cracks, respectively. The locations of crack initiation are strongly related to the CTE mismatch between Si chip and solder alloy. Besides, the large number of AgSn precipitations in the joint with high Ag content efficiently hinder the crack propagation due to the effects of precipitation hardening. It is revealed that the Ni(V)/SAC396/OSP Cu exhibits good thermal stability during thermal cycle test. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution electron microscopy was used to research the age-hardening behaviors in 6005A alloy and the microstructural characterizations of precipitates. It was found that β″, β′ and Q′ precipitates had 12 orientation variants, respectively, the smaller size and a high-density β′′ precipitates existed in alloy for a long time, which played a very important role in controlling an anti-overaged softening behavior in 6005A alloy. Further, by the crystallographic interface and morphology analysis, a main reason was that a 2-dimensions coherency strain-field not only had the high-density β′′ precipitates become the biggest obstacle of dislocations movement, but also made them transform into β′ and Q′ precipitates with more difficulty. Moreover, it was also found that β′ and Q′ precipitates had weaker relatively strain-fields, the larger size and a lower density, which were largely associated with the reduction in hardness that occurred upon overaging. Further, the strengthening of precipitates on alloys could be summarized as: β″>Q′>β′. Based on the structural information, the quantitative Moiré fringes analytical technique was also used to verify the lattice parameters and orientation of precipitates. [Copyright &y& Elsevier]

Abstract: The effects of annealing temperature and duration on the formation of iron silicides prepared by magnetron sputtering (MS) are investigated. The Fe/Si structures were annealed at 1073 K–1223 K for 6–18 hours. The results show that 1143 K is a critical temperature to the formation of - FeSi2 from Fe/Si structures deposited at room temperature. The single phase semiconducting - FeSi2 can be obtained after annealing at 1173 K for 12–18 hours. The SEM images indicate that both annealing temperature and duration have obvious effects on microstructures of the film. When annealing time is more than 15 hours, the crystallinity of - FeSi2 improves and the distribution of grains becomes homogeneous. Most of the grains of - FeSi2 grow toward substrate and embed themselves into Si matrix. [Copyright &y& Elsevier]

This paper reports recent theoretical perspectives and experimental results on microstructural evolution during sintering in terms of the interface structure, which is either rough (atomically disordered) or faceted (atomically ordered). The paper presents theoretical predictions and calculations of grain growth during liquid-phase sintering based on crystal growth theories. It is shown that various types of grain growth behavior, which may be normal, abnormal, or stagnant, can appear as a result of the coupling effects of the maximum driving force for growth and the critical driving force for appreciable growth. The predictions are also shown to be valid in the case of solid-state sintering. A number of experimental observations showing the effect of some critical processing parameters have been found to be in excellent agreement with the predictions. Principles of microstructure development (grain growth control) during sintering are suggested. In addition, the effect of the interface structure on densification is briefly described and discussed. [ABSTRACT FROM AUTHOR]

A microstructural characterisation of the family of N-methyl-N-alkylpyrrolidinium tetrafluoroborate organic salts was carried out by observation of powder surface morphologies with the aim of extending the microstructure-property correlation. Inherent difficulties limiting extensive studies of organic solids by SEM, including volatility under vacuum, charging due to electron beam irradiation, and air-sensitivity were overcome with the use of a Field Emission SEM and cryostage attachment. This technique, providing considerable improvements in image quality at low accelerating voltages, enabled direct observation of complex microstructural features in samples exhibiting high temperature plastic crystalline phases (N,N-dimethylpyrrolidinium tetrafluoroborate [P11BF4]; N-methyl-N-ethylpyrrolidinium tetrafluoroborate [P12BF4]; N-methyl-N-propylpyrrolidinium tetrafluoroborate [P13BF4]). Extensive lattice imperfections including grain boundaries, slip planes and dislocation pits were observed within particles of approximately 200 μm diameter. The N-methyl-N-butylpyrrolidinium tetrafluoroborate (P14BF4) sample in this series revealed columnar single crystals with high aspect ratios. The origin of plastic flow properties is discussed using single crystal and polycrystalline slip observations and a relationship proposed between defect characteristics and transport properties. [ABSTRACT FROM AUTHOR]

The two glass-ceramics studied here derive from the complex system (MgO,ZnO,Li2O)-Al2O3-SiO2 and are obtained by controlled devitrification of the same parent glass. Although they have the same chemical composition, one is a “β-quartz” (or “β-eucryptite”) type while the other one is a “β-spodumene” glass-ceramic. A detailed microstructural analysis of these materials has been performed at different scales by several complementary characterization methods (SEM, TEM, DTA, XRD and FTIR). This extensive study has shown the great microstructure difference (grain distribution, grain size, nature of vitreous and crystalline phases) between these two glass-ceramics obtained from the same parent glass. [ABSTRACT FROM AUTHOR]

2 inch-diameter GaN films with homogeneous thickness distribution have been grown on AlN/Si(111) hetero-structures by pulsed laser deposition (PLD) with laser rastering technique. The surface morphology, crystalline quality, and interfacial property of as-grown GaN films are characterized in detail. By optimizing the laser rastering program, the ~300 nm-thick GaN films grown at 750 °C show a root-mean-square (RMS) thickness inhomogeneity of 3.0%, very smooth surface with a RMS surface roughness of 3.0 nm, full-width at half-maximums (FWHMs) for GaN(0002) and GaN(102) X-ray rocking curves of 0.7° and 0.8°, respectively, and sharp and abrupt AlN/GaN hetero-interfaces. With the increase in the growth temperature from 550 to 850 °C, the surface morphology, crystalline quality, and interfacial property of as-grown ~300 nm-thick GaN films are gradually improved at first and then decreased. Based on the characterizations, the corresponding growth mechanisms of GaN films grown on AlN/Si hetero-structures by PLD with various growth temperatures are hence proposed. This work would be beneficial to understanding the further insight of the GaN films grown on Si(111) substrates by PLD for the application of GaN-based devices. [ABSTRACT FROM AUTHOR]