Pressure-dependent structures of amorphous red phosphorus and the origin of the first sharp diffraction peaks

Characterizing the nature of medium-range order (MRO) in liquids and disordered solids is important for understanding their structure and transport properties. However, accurately portraying MRO, as manifested by the first sharp diffraction peak (FSDP) in neutron and X-ray scattering measurements, has remained elusive for more than 80 years. Here, using X-ray diffraction of amorphous red phosphorus compressed to 6.30 GPa, supplemented with micro-Raman scattering studies, we build three-dimensional structural models consistent with the diffraction data. We discover that the pressure dependence of the FSDP intensity and line position can be quantitatively accounted for by a characteristic void distribution function, defined in terms of average void size, void spacing and void density. This work provides a template to unambiguously interpret atomic and void-space MRO across a broad range of technologically promising network-forming materials.     

Roles of minor additions in formation and properties of bulk metallic glasses

Bulk metallic glasses (BMGs) are of current interest worldwide in materials science and engineering because of their unique properties. Exploring BMGs materials becomes one of the hottest topics in the materials science field. To date, there is very active worldwide development of new BMGs, and extensive efforts have been carried out to understand and improve the glass-forming ability of metallic materials supported by large government and industry programs in North America, Asia, and Europe. Minor addition or microalloying technique, which has been widely used in other metallurgical fields, plays effective and important roles in formation, crystallization, thermal stability and property improvement of BMGs. This simple approach provides a powerful tool for the BMG-forming alloys development and design. In this paper, we present a comprehensive review of the history and the recent developments of this technique in the field of BMGs. The roles of the minor addition in the formation and the properties of the BMGs and the BMG-based composites will be discussed and summarized within the framework of thermodynamics, kinetics and microstructure. The empirical criteria, or the principles and guidelines for the applications of the technique in BMG field are outlined.     

Dissimilar Friction Stir Welding Between 5083 and 6082 Al Alloys Reinforced With TiC Nanoparticles

Dissimilar friction stir welding between aluminum alloys thick plates reinforced with TiC nanoparticles was conducted. The defect-free welds are characterized by good mechanical mixing between the joined materials as well as by good nanoparticle distribution and further grain refinement in comparison with the unreinforced weld. The local mechanical behavior of the produced metal matrix composites was studied and compared with their bulk counterparts and parent materials. Specifically, the measured mechanical properties in microscale and nanoscale (namely hardness and elastic modulus) are correlated with microstructure and the presence of fillers. The hardness, elastic modulus, ultimate tensile strength, percentage of elongation, and yield values increase with the presence of TiC nanoparticles.     

The potential of Zr-based bulk metallic glasses as biomaterials

Zr-based bulk metallic glasses (BMGs) are a new type of metallic materials with disordered atomic structure that exhibit high strength and high elastic strain, relatively low Young’s modulus, and excellent corrosion resistance and biocompatibility. The combination of these unique properties makes the Zr-based BMGs very promising for biomaterials applications. In this review article, the authors give an overview of the recent progress in the study of biocompatibility of Zr-based BMGs, especially the relevant work that has been done in the metallic glasses group in Huazhong University of Science and Technology (HUST), including the development of Ni-free Zr-based BMGs, the mechanical and wear properties, the bio-corrosion resistance, the in vitro and in vivo biocompatibility and the bioactive surface modification of these newly developed BMGs.     

The potential of Zr-based bulk metallic glasses as biomaterials

Zr-based bulk metallic glasses (BMGs) are a new type of metallic materials with disordered atomic structure that exhibit high strength and high elastic strain, relatively low Young’s modulus, and excellent corrosion resistance and biocompatibility. The combination of these unique properties makes the Zr-based BMGs very promising for biomaterials applications. In this review article, the authors give an overview of the recent progress in the study of biocompatibility of Zr-based BMGs, especially the relevant work that has been done in the metallic glasses group in Huazhong University of Science and Technology (HUST), including the development of Ni-free Zr-based BMGs, the mechanical and wear properties, the bio-corrosion resistance, the in vitro and in vivo biocompatibility and the bioactive surface modification of these newly developed BMGs.     

Metallic glasses from “alchemy” to pure science: Present and future of design,processing and applications of glassy metals

Metallic glasses, first discovered a half century ago, are currently among the most studied metallic materials. Available in sizes up to several centimeters, with many novel, applicable properties, metallic glasses have also been the focus of research advancing the understanding of liquids and of glasses in general.Metallic glasses (MGs), called also bulk metallic glasses (BMGs) (or glassy metals, amorphous metals, liquid metals) are considered to be the materials of the future. Due to their high strength, metallic glasses have a number of interesting applications, for example as coatings. Metallic glasses can also be corrosion resistant. Metallic glasses, and the crystalline materials derived from them, can have very good resistance to sliding and abrasive wear. Combined with their strength – and now, toughness – this makes them ideal candidates for bio-implants or military applications. Prestigious Journals such as “Nature Materials”, “Nature” frequently publish new findings on these unusual glass materials. Moreover Chinese and Asian scientists have also been showing an interest in the study of metallic glasses.This review paper is far from exhaustive, but tries to cover the areas of interest as it follows: a short history, the local structure of BMGs and the glass forming ability (GFA), BMGs’ properties, the manufacturing and some applications of BMGs and finally, about the future of BMGs as valuable materials.     

Investigation on the mechanical properties and frictional performance of Ni–Cu–Si alloy

The microstructure evolution, mechanical properties and dry sliding behaviour of Ni–30Cu–xSi alloy have been investigated systematically. As the volume fraction of microscale second-phase particles and nanoscale precipitates increases, the hardness, yield strength and ultimate tensile strength of alloy are improved significantly but elongation is reduced. Through confocal laser scanning microscope and atomic force microscope, it is suggested that the wear mode changes from the mixture of abrasive and adhesive wear to single abrasive wear. Owing to the existence of netlike microscale second-phase particles which are more likely to split the matrix, the Ni–30Cu–5.5Si alloy exhibits an abnormal higher wear rate even with the highest hardness. The netlike structure which deteriorates the friction performance should be avoided in wear-resistant materials.     

Design of Zr–Al–Ni–Cu bulk metallic glasses with network structures

A series of Zr–Al–Ni–Cu bulk metallic glasses (BMGs) with network structures were carefully designed and their heterogeneous microstructures were investigated by carefully etching the cross sections of these BMGs. It is found that the heterogeneous microstructures of these BMGs can be uncovered by etching with mixed solution of HF and HNO₃. Some of the studied Zr-based BMGs characterize in micrometer network structure. The cell size of the network structure depends on the composition of the BMG and is related with the fractural mode and mechanical property. The found network structure of the Zr-based BMGs benefits for understanding the unique mechanical properties of the Zr-based BMGs.     

Ultra-high strength Mg-Li based bulk metallic glasses: Preparation and performance research

In this paper, new Mg-Li based bulk metallic glasses (BMGs) are prepared by conventional copper mold injection casting method. The alloys exhibit excellent mechanical properties, such as ultra-high compressive fracture strength (maximal 729 MPa), high Vickers hardness (>2 GPa) and low elastic modulus (∼35 GPa). Compared with the corresponding crystal alloys, the density of the amorphous alloy samples is reduced by about 1.5% due to their free volume. Thus, it is believed that this new BMGs with these outstanding properties will broaden Mg-Li based alloys’ application fields.     

Predicting the structure of screw dislocations in nanoporous materials

Extended microscale crystal defects, including dislocations and stacking faults, can radically alter the properties of technologically important materials. Determining the atomic structure and the influence of defects on properties remains a major experimental and computational challenge. Using a newly developed simulation technique, the structure of the 1/2a <100> screw dislocation in nanoporous zeolite A has been modelled. The predicted channel structure has a spiral form that resembles a nanoscale corkscrew. Our findings suggest that the dislocation will enhance the transport of molecules from the surface to the interior of the crystal while retarding transport parallel to the surface. Crucially, the dislocation creates an activated, locally chiral environment that may have enantioselective applications. These predictions highlight the influence that microscale defects have on the properties of structurally complex materials, in addition to their pivotal role in crystal growth.     

Atomic origami

Here we summarize recent experimental work in the field of atomic origami: the folding of 3D structures from sheets that are just atoms thick. We highlight current techniques for folding at the microscale and provide scaling arguments as to why some approaches work better than others at small sizes. Finally, we point out that for folding structures made from 2D materials, miniaturization can extend another three orders of magnitude: current state of the art devices are microns in size while, as a platform, atomic membranes should be foldable down to the nanoscale. The ability to scale folding structures over a wide range in size could open diverse applications, from microscopic robots to new interfaces with biology.     

Size effect on stability of shear-band propagation in bulk metallic glasses: an overview

Understanding of the size effect on shear banding in bulk metallic glasses (BMGs) is currently the topic of active research but also remains under intense debates. In this article, we provide an overview of the recent research findings from experiments, theoretical modeling, and atomistic/continuum simulations which are intended to advance our knowledge related to the size effect on the stability of shear-band propagation in BMGs. Through the compilation of and comparison among the results reported in the literature, we aim at providing a comprehensive understanding of the underlying mechanisms and a unified physical picture of the size effect on shear-band propagation and its resultant ductility in BMGs.     

In situ microscopy techniques for characterizing the mechanical properties and deformation behavior of two-dimensional (2D) materials

Due to the atomic thickness and planar characteristics, two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are considered to be excellent electronic materials, which endow them with great potential for future device applications. The robust and reliable application of their functional devices requires an in-depth understanding of their mechanical properties and deformation behavior, which is also of fundamental importance in nanomechanics. Considering their exceedingly small sizes and thicknesses, this is a very challenge task. In situ microscopy techniques show great superiority in this respect. This review focuses on the progress in in situ microscopy techniques (including atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)) in characterizing the mechanical properties and deformation behavior of 2D materials. The technical characteristics, advantages, disadvantages, and main research fields of various in situ AFM, SEM, and TEM techniques are analyzed in detail, and the corresponding mechanical scenarios from point to plane are realized, including local indentation, planar stretching, friction sliding between atomic layers and atomic movement mechanisms. By virtue of their complementary advantages, in situ integrated microscopy techniques enable the simultaneous study of various mechanical properties, nanomechanical behavior, and inherent atomic mechanisms of 2D materials. Based on the present research, we look forward to further optimized in situ integrated microscopy techniques with high spatiotemporal atomic resolution that can reveal the dynamic structure-performance correlations and corresponding atomic mechanisms between the physical properties, such as mechanical, electrical, optical, thermal, and magnetic properties of 2D materials and their crystal structures, electronic structures, atomic layers, defect densities and other influencing factors under multifield coupling conditions. This will provide beneficial predictions and guidance for the design, construction and application of 2D material-based mechanoelectronic, piezoelectric, photoelectric, thermoelectric, etc. nanoelectronic devices.     

Estimation of anisotropic properties of nano-structured arrays by modal vibration control at microscale

Nano-structured arrays are engineered to meet the requirements of a variety of applications such as microfilters, sensors, and structural interface due to their unique mechanical characteristics, which cannot be achieved by conventional solid materials. However, it is hard to evaluate the elastic properties of nano-structured arrays owing to the discrete structure, sample size, and availability of suitable techniques. To facilitate this, we develop an advanced three-dimensional microscale vibration testing process. In the test, a specially designed three-dimensional microspecimen with tuned mass is excited by a piezoelectric actuator, and the resonance frequencies are detected by a laser device successfully. The anisotropic elastic moduli of nano-structured array composed of helical nano-springs are identified from a single spectrum. This array shows so strong characteristic anisotropy that the solid one hardly can attain. The microscale testing technique can be extended to other materials and microstructures.     

The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.     

Heteroanionic Materials by Design: Progress Toward Targeted Properties

The burgeoning field of anion engineering in oxide‐based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent‐bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide‐only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion‐engineering principles in heteroanionic materials are reviewed.     

Direct observation of local atomic order in a metallic glass

The determination of the atomic configuration of metallic glasses is a long-standing problem in materials science and solid-state physics. So far, only average structural information derived from diffraction and spectroscopic methods has been obtained. Although various atomic models have been proposed in the past fifty years, a direct observation of the local atomic structure in disordered materials has not been achieved. Here we report local atomic configurations of a metallic glass investigated by nanobeam electron diffraction combined with ab initio molecular dynamics simulation. Distinct diffraction patterns from individual atomic clusters and their assemblies, which have been theoretically predicted as short- and medium-range order, can be experimentally observed. This study provides compelling evidence of the local atomic order in the disordered material and has important implications in understanding the atomic mechanisms of metallic-glass formation and properties.     

Chemomechanics of complex materials: challenges and opportunities in predictive kinetic timescales

What do nanoscopic biomolecular complexes between the cells that line our blood vessels have in common with the microscopic silicate glass fiber optics that line our communication highways, or with the macroscopic steel rails that line our bridges? To be sure, these are diverse materials which have been developed and studied for years by distinct experimental and computational research communities. However, the macroscopic functional properties of each of these structurally complex materials pivots on a strong yet poorly understood interplay between applied mechanical states and local chemical reaction kinetics. As is the case for many multiscale material phenomena, this chemomechanical coupling can be abstracted through computational modeling and simulation to identify key unit processes of mechanically altered chemical reactions. In the modeling community, challenges in predicting the kinetics of such structurally complex materials are often attributed to the so-called rough energy landscape, though rigorous connection between this simple picture and observable properties is possible for only the simplest of structures and transition states. By recognizing the common effects of mechanical force on rare atomistic events ranging from molecular unbinding to hydrolytic atomic bond rupture, we can develop perspectives and tools to address the challenges of predicting macroscopic kinetic consequences in complex materials characterized by rough energy landscapes. Here, we discuss the effects of mechanical force on chemical reactivity for specific complex materials of interest, and indicate how such validated computational analysis can enable predictive design of complex materials in reactive environments.     

Nanocomposite Hard Coatings: Deposition Issues and Validation of their Mechanical Properties

The limitations of conventional coatings due to inferior hardness or poor oxidation stability can be overcome by nanocomposite hard coatings such as nc‐TiN/a‐SiN_x, which consists of nanocrystalline TiN and a non‐crystalline tissue phase of SiN_x which are mutually immiscible. The properties of nanocomposite coatings, especially their increased hardness, can be explained by their nanostructure, which leads to a maximum hardness at typically 80 atomic percent of the crystalline phase. We show that enhanced hardness can only be attained when the silicon nitride phase is sufficiently nitrided. The accurate and reliable measurement of the hardness and elastic modulus requires the use of appropriate nanoindentation equipment and a careful tip correction with periodical validation. It is shown that for a correct hardness determination of a few microns thick nanocomposite coatings, an indentation depth of 100 nm is sufficient. The maximum hardness of our nc‐TiN/a‐SiN_x coatings deposited by a hybrid UBM/arc‐PVD process is about 40 GPa. This value represents a global hardness value, due to the nanocomposite structure there may be a local hardness variation of about ±10 %.     

Search for superhard materials: Nanocrystalline composites with hardness exceeding 50 GPa

The search for superhard materials with Vickers hardness H ≥ 40 GPa (about 4000 kg/mm²) concentrates mainly on polycrystalline diamond, cubic c-BN and C₃N₄ or the substoichiometric CN_x. This approach is based on the theoretical strength which is proportional to the bulk modulus. However, the practically achievable strength (and hardness) of engineering materials is two to four orders of magnitude smaller because their failure occurs due to flaws, and it is determined by their microstructure. Therefore, an alternative approach deals with the design of materials with an appropriate microstructure, such as heterostructures.Recently, we have developed new superhard nanocrystalline composites $\text{nc-Me}{}_{\mathrm{n}}\text{N}\text{a-Si}{}_3\text{N}{}_4$ (Me = Ti, W; V,…). These materials consist of ≤ 4 nm small nanocrystals of a hard transition metal nitride embedded into < 1 nm thin matrix of amorphous silicon nitride. Unlike pure nanocrystalline metals and the heterostructures which show softening when the crystallite size or lattice period decreases below 5–6 nm, the hardness of our composites strongly increases with decreasing crystallite size in that range and approaches the hardness of diamond. In this paper we shall briefly summarize the concept for the design of these materials and experimental results achieved so far. New results to be reported concern the surprising structural stability of these composites and a discussion of the possible origin of the superhardness.