Amorphous Bands Research Articles

Delaminated structure of Zn-doped TiO2 coatings induced by ultrasound-auxiliary micro-arc oxidation: the influence of ultrasonic power

In this study, two representative ultrasonic vibration (UV) methods were employed to fabricate Zn-doped TiO2 micro-arc oxidation (MAO) ceramic coatings with enhanced corrosion resistance on pure Ti substrate. On the one hand, the surface morphology and chemistry of the coatings are characterized as partially crystallized hydroxyapatite nanoparticles and amorphous bands rich in Ca, P, Zn, and O. On the other hand, the MAO-modified coatings exhibit much greater corrosion resistance than the bare Ti surfaces, showing a reduction in corrosion current density (icorr) by at least an order of magnitude. However, a key finding is that a lower UV power setting of 360 V (named as the TZ-optimized protocols here) surpasses the higher 900 V (named as the TZ-powerful protocols) in terms of corrosion resistance (i.e. icorr =∼1.47×10−8 A/cm2 vs ∼7.85×10−7 A/cm2, respectively) as well as adhesion strength to the substrate (the critical load of ∼6.4 N vs ∼2.9 N, respectively), which can be attributed to the delaminated structure formed at higher UV powers. This work is expected to remind the potential side effect of such delamination in ceramic coatings produced by UV-assisted MAO processes on Ti-based alloys.

Oxidation resistance mechanism of copper wire regulated by nano-palladium coating

The microstructure characteristics of palladium coating are the key to the oxidation resistance of palladium-coated copper wire. In this paper, the microstructure characteristics of palladium coating thickness, nanocrystalline size and amorphous band width were controlled by adjusting the process parameters of micro-area coating. The relationship between microstructure characteristics of palladium coating and oxidation resistance was studied. The results show that the wire has good oxidation resistance when the deposition temperature is 400℃ and the copper wire/mold clearances are 2.5 μm. The surface of the copper wire has a palladium coating with a thickness of 74 nm, and the palladium particles are composed of amorphous encapsulated nanocrystals. The relationship between the thickness of palladium coating and the copper wire/mould clearances is y=0.012x+44, and the increase of palladium coating thickness can inhibit the migration of copper ions. The increase in the size of the nanocrystalline and the width of the amorphous band has a certain inhibitory effect on the lattice or grain boundary diffusion of copper to the palladium coating. The multi-stage distorted nanotwins in the amorphous palladium coating can reduce the strain difference at the palladium-copper bonding interface, reduce the diffusion driving force of copper outward, and thus improve the oxidation resistance of palladium-coated copper wires.

Effect of rare earth element Y on the crack propagation behavior of Mg alloys: A molecular dynamics simulation

The effect of the distribution and concentration of rare earth (RE) element Y, crack orientation, temperature, and strain rate on the crack propagation behavior of the Mg alloys is investigated by molecular dynamics/Monte Carlo simulations. The results show that the RE element Y tends to form locally short-range order structures in the Mg alloys, and the introduction of the RE element Y can enhance the fracture toughness of the Mg alloys. The results indicate that with the increase of Y concentration, the crack propagation mode of the Mg alloys shifts from a mode, dominated by the dislocation emissions at the crack tip and the crack cleavage propagation to a mode dominated by the solid-state amorphization and the slip of amorphous bands. In addition, the results show that the faster the strain rate, the slower the crack propagation speed, and the crack propagation resistance increases with increasing temperature.

Friction Mechanism of Sulfur-Containing Lubricant Additives Confined between Fe(100) Substrates.

Sulfur-containing lubricant additives can chemically react with metal surfaces under extreme conditions, such as high temperature and high pressure, forming protective films on the surfaces. However, the formation mechanisms and the friction-reducing and antiwear properties of these films remain unclear. In this study, we investigated the friction process of sulfur-containing additives confined between two iron surfaces using reactive molecular dynamics simulations. Our research revealed that in systems with a higher S/C ratio, an iron sulfide layer formed on the iron surfaces with Fe-S-Fe bridging bonds at the interface, resulting in relatively smaller friction and wear even under higher loads. However, in systems with lower S/C ratios, the presence of numerous interfacial Fe-Cn-Fe bridging bonds, caused by the hydrocarbon radicals released from the additives, led to the formation of thick amorphous shearing bands at the interface between the two substrates. In this case, the distributed sulfur atoms also exhibited some effect in reducing the shear resistance of the amorphous shearing bands due to the weak strength of S-Fe bonds compared to the strength of C-Fe bonds. These atomic-level insights help understand the antiwear characteristics of sulfur-containing lubricant additives confined between iron substrates.

Amorphous shear band formation in crystalline Si-anodes governs lithiation and capacity fading in Li-ion batteries

The cycling stability of Li-ion batteries is commonly attributed to the formation of the solid electrolyte interphase (SEI) layer, which is generated on the active material surface during electrochemical reactions in battery operation. Silicon experiences large volume changes upon the Li-insertion and extraction, leading to the amorphization of the silicon-interface due to the permeation of the Li-ions into the silicon. Here, we discover how generated non-hydrostatic strain upon electrochemical cycling further triggers dislocation and eventually shear band formation within the crystalline silicon core. The latter boosts the non-uniform lithiation at the silicon interface affecting the SEI reformation process and ultimately the capacity. Our findings are based on a comprehensive multiscale structural and chemical experimental characterization, complemented by molecular dynamics modelling. This approach highlights the importance of considering electrochemical, microstructural and mechanical mechanisms, offering a strategy for developing improved anode materials with enhanced cycling stability and reduced capacity loss.

Increase in interplanar distances and formation of amorphous shear bands in deformed diamond

Amorphous transformation-deformation bands have been observed in diamond under cyclic loading (at planetary ball milling) near the graphite-diamond equilibrium line at temperatures below the Debye temperature. The maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K, respectively. These bands add to the set of plastic deformation mechanisms in diamond. TEM methods and conventional X-rays were used to investigate the mechanisms of plastic deformation of diamond. The observed mechanisms include amorphous deformation bands, bond breaking, twin formation, and phase transitions of diamond-lonsdaleite and diamond-onions. The increased interplanar distances provide evidence of the plastic deformation mechanism due to bond breaking. The distances of 0.222 and 0.255 nm are significantly larger than the initial value of 0.206 nm, which is characteristic of d111 diamond. This increase in distance cannot be attributed to lattice expansion caused by point defects, but rather to the breakage of interatomic bonds.

The nucleation and growth mechanism of solid-state amorphization and diffusion behavior at the W–Cu interface

The W–Cu materials hold vast potential for applications in electronic information, nuclear energy, and aerospace sectors. Here, we report a new occurrence of solid-state amorphization on the Cu side near W–Cu interface. A potential function with accuracy close to density functional theory (DFT) is constructed using machine learning, while the atomic mechanism of solid amorphous nucleation and growth is unveiled through a combination of in-situ transmission electron microscopy (TEM) and molecular dynamics (MD) simulations. Our findings indicate that the Cu near W–Cu interface experiences an amorphous phase transition at 400 °C. This amorphous nucleation is linked to the stress coupling between the W–Cu interface and dislocations within Cu. The lattice distortion arising from dislocations, combined with interfacial stress, results in lattice twisting, leading to the formation of dislocation pileup, HCP and disordered structures. The results of the in-situ TEM show that the dislocation stacking and HCP structures exist for a short period of time, and these structures quickly turn into disordered structures, eventually forming an amorphous band with a width of about 8 nm at the W–Cu interface. However, the amorphous structure is unstable. As temperature rises, the amorphous structure undergoes recrystallization into an ordered structure. Furthermore, we investigated the atomic diffusion behavior of W–Cu. Simulation results reveal that the defect in W significantly impacts diffusion. In summary, our study provides theoretical support for the nucleation mechanism of solid-state amorphization, the understanding of interfacial stress-strain, and the application of W–Cu materials.

Amorphous shear band formation in elemental β-boron

We report the formation of shear-induced amorphization in elemental β‑boron (β-B) using transmission electron microscopy (TEM) and density function theory (DFT) simulations. TEM observations beneath the residual indentation of β-B revealed intragranular nanoscale shear bands along 1¯11¯ and 111 crystallographic planes, which are preceded by nucleation of dislocations. Strain map analyses of the shear band indicated that compressive strain assists in the process of bond breaking and shear strain is involved in the disassembly of icosahedral clusters by forming amorphization within a unit cell of β-B. Further DFT simulations confirmed that the deconstruction of triply fused icosahedral (B28) along 11¯0111slip system in β-B106 has the lowest shear strength leading to the formation of a kink followed by amorphization, which is in line with our experimental observations. This study illustrates the structural evolution of shear amorphization in β-B and provides the basis to improve their mechanical properties through modification of internal structure.

Effects of ultrasonic cavitation on microstructure and surface properties of Mn–Cu alloy

In the presented study, the effects of ultrasonic vibration on microstructure and surface properties of Mn–Cu alloy were investigated. At the early stage of ultrasonic cavitation treatment, grains of Mn–Cu alloy are refined, nanocrystals and amorphous bands are produced. As the ultrasonic cavitation treatment proceeds, the face-centered cubic structure is transformed into the body-centered tetragonal structure, and the martensitic phase transformation arises. The transformation is accompanied by plastic deformation and the formation of dislocations. The generation of martensite is caused by the stress and strain. Due to grain refinement, dislocations, and generation of martensite, the surface hardness of Mn–Cu alloy is enhanced, and the friction coefficient decreases. The resistance to wear is thereby improved.

Micro-mechanisms underlying enhanced fatigue life of additively manufactured 316L stainless steel with a gradient heterogeneous microstructure

The fatigue life of additively manufactured 316L stainless steel (SS) in an aqueous corrosive medium is shown to improve markedly when the surfaces were processed by surface mechanical attrition treatment (SMAT). SMAT induced surface nano-crystallization owing to severe plastic deformation and yielded a gradient microstructure beneath the surface of additively manufactured SS 316L, which increases the resistance to fatigue crack initiation and propagation. During cyclic deformation, dislocations are generated and accumulated in the cell boundary, resulting in the evolution of persistent slip bands (PSBs) at the surface of the as-manufactured specimen. Continuous intrusions and extrusions of PSBs provide crack initiation sites due to strain localization at the surface of the as-manufactured specimen, whereas the crack initiation region moved from the surface to the sub-surface layer after SMAT. Mainly, the mutual crossing of shear bands or twin boundaries accommodates most of the plastic deformation and provides crack nucleation sites at subsurface regions in this case. To elucidate the fatigue failure mechanism associated with fatigue loading (interactions of stacking faults, slip bands, and deformation twins), electron back-scattered diffraction and transmission electron microscopy analysis of the fatigue-failed alloy after SMAT was performed at different distances adjacent to the fatigue-failed tip. The interfaces between dislocation forest and twin-twin boundaries hinder dislocation motion and disrupt stress concentration. In addition, amorphous bands were formed at the interfaces between twin and shear bands which co-deform with the matrix during dynamic loading and act as a natural sink for dislocations, delaying decohesion and commencement of fracture. The changes in the crack initiation site and fatigue micro-mechanism cause considerable changes in fatigue life. This work provides insight into the failure mechanisms of additively manufactured SS for designing surface modification techniques to enhance the fatigue life of the parts post-printing.

Amorphous shear bands in crystalline materials as drivers of plasticity.

Traditionally, the formation of amorphous shear bands in crystalline materials has been undesirable, because shear bands can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently were shear bands found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here we have discovered trends in materials properties that determine when amorphous shear bands will form and whether they will drive plasticity or lead to fracture. We have identified the materials systems that exhibit shear-band deformation, and by varying the composition, we were able to switch from ductile to brittle behaviour. Our findings are based on a combination of experimental characterization and atomistic simulations, and they provide a potential strategy for increasing the toughness of nominally brittle materials.

Enhanced multi-scale nanomechanical deformation behavior in reactive plasma sprayed Fe-based amorphous/TiNx composite coating

Creep performance plays a pivotal role in evaluating the stability of materials during application. In particular, creep in amorphous alloys exhibits nonlinear and outburst characteristics. This work attempted to strengthen the Fe-based amorphous coating under loading by introducing TiNx ceramic material with high stability to the amorphous interlamellar. The results revealed that the creep resistance and indentation size effect of the composite coating were improved, compared with the monolithic Fe-based amorphous coating. The creep curves of both coatings can be well fitted by the Kelvin-Voigt formula composed of two relaxation processes. Moreover, the diffusion-mediated creep ratio of the composite coating was higher than that of the amorphous coating. And the composite coating had less free volume. Accordingly, more shear transition regions were activated synergistically during the deformation process, so the atom rearrangement effect was enhanced, which was well proved by the time relaxation spectrum and deformation characteristics. The expansion caused by excessive free volume in the amorphous shear band region caused plastic flow and strain softening. While the less free volume in the composite coating improved this situation, showing a less obvious indentation size effect. Ceramic phase doping appears to be a very effective and promising way to wield the plastic flow, accordingly to modify the amorphous coating.

Evolution of shear amorphization in superhard cubic silicon carbide

Shear amorphization induced by high stress in cubic silicon carbide (β-SiC) has not yet been experimentally observed till-to-date. In this letter, we report the detailed evolution of stress-induced amorphization in superhard β-SiC using transmission electron microscopy (TEM) and molecular dynamics (MD) simulations. From experimental observations using TEM, shearing by stacking faults along (111) crystallographic plane triggers the onset of amorphization. Further, an increase in the stress results in highly localized strains within the amorphous band led by the coalescence of vicinity dislocations. Based on the MD simulations, it has been verified that the existence of stacking faults decreases the amount of shear stress and shear strain necessary to trigger amorphization compared to the pristine β-SiC, which is in line with experimental observations. This study underscores a detailed understanding of the amorphization process in superhard materials through which stress is alleviated in high-stress environments.

An atomistic study of plastic deformation of SmCo5 by amorphous shear bands

Recently, SmCo5 was found to be capable of forming amorphous shear bands to induce dislocation-free plastic deformation at large strains. The formation of shear band is energetically more favorable compared to cracking and the small density variation in shear bands is not likely to induce high local stress that assists crack opening. However, the related mechanism of crystalline-to-amorphous transition during shear-band formation at the atomic level remains unclear. In this paper, the shear deformation of SmCo5 is investigated by molecular dynamics simulations and we discuss the behavior of shear-band formation by exploring the atomic packing in the interfacial zone between crystal matrix and shear band. The stress-strain curves of SmCo5 show anisotropy with respect to formation of amorphous shear band. The analysis of atomic packing indicates that the behavior of Sm is critical to the atomistic amorphization path and thus accounts for the mechanical anisotropy of the material. Temperature calculations show that the material is not subject to shear melting during deformation that would otherwise lead to embrittlement.

Fused Filament Fabrication and Computer Numerical Control Milling in Cultural Heritage Conservation.

This paper reports a comparison between the advantages and disadvantages of fused filament fabrication (FFF) and computer numerical control (CNC) milling, when applied to a specific case of conservation of cultural heritage: the reproduction of four missing columns of a 17th-century tabernacle. To make the replica prototypes, European pine wood (the original material) was used for CNC milling, while polyethylene terephthalate glycol (PETG) was used for FFF printing. Neat materials were chemically and structurally characterized (FTIR, XRD, DSC, contact angle measurement, colorimetry, and bending tests) before and after artificial aging, in order to study their durability. The comparison showed that although both materials are subject to a decrease in crystallinity (an increase in amorphous bands in XRD diffractograms) and mechanical performance with aging, these characteristics are less evident in PETG (E = 1.13 ± 0.01 GPa and σ = 60.20 ± 2.11 MPa after aging), which retains water repellent (ca = 95.96 ± 5.56°) and colorimetric (∆E = 2.6) properties. Furthermore, the increase in flexural strain (%) in pine wood, from 3.71 ± 0.03% to 4.11 ± 0.02%, makes it not suitable for purpose. Both techniques were then used to produce the same column, showing that for this specific application CNC milling is quicker than FFF, but, at the same time, it is also much more expensive and produces a huge amount of waste material compared to FFF printing. Based on these results, it was assessed that FFF is more suitable for the replication of the specific column. For this reason, only the 3D-printed PETG column was used for the subsequent conservative restoration.

Depth-dependent recovery of thermal conductivity after recrystallization of amorphous silicon

The depth-dependent recovery of silicon thermal conductivity was achieved after the recrystallization of silicon that had been partially amorphized due to ion implantation. Transmission electron microscopy revealed nanoscale amorphous pockets throughout a structurally distorted band of crystalline material. The minimum thermal conductivity of as-implanted composite material was 2.46 W m−1 K−1 and was found to be uniform through the partially amorphized region. X-ray diffraction measurements reveal 60% strain recovery of the crystalline regions after annealing at 450 °C for 30 min and almost full strain recovery and complete recrystallization after annealing at 700 °C for 30 min. In addition to strain recovery, the amorphous band thickness reduced from 240 to 180 nm after the 450 °C step with nanoscale recrystallization within the amorphous band. A novel depth-dependent thermal conductivity measurement technique correlated thermal conductivity with the structural changes, where, upon annealing, the low thermal conductivity region decreases with the distorted layer thickness reduction and the transformed material shows bulk-like thermal conductivity. Full recovery of bulk-like thermal conductivity in silicon was achieved after annealing at 700 °C for 30 min. After the 700 °C anneal, extended defects remain at the implant projected range, but not elsewhere in the layer. Previous results showed that high point-defect density led to reduced thermal conductivity, but here, we show that point defects can either reform into the lattice or evolve into extended defects, such as dislocation loops, and these very localized, low-density defects do not have a significant deleterious impact on thermal conductivity in silicon.

Study on nanometer cutting mechanism of single crystal silicon at different temperatures

Thermal-assisted single point diamond turning technology is currently one of the most important methods for realizing nano-scale machining. However, the nano-chip formation mechanism as the core of the nano-cutting mechanism is still unclear, which limits the development of this advanced technology. In this study, based on the molecular dynamics simulations, the nano cutting behaviour of single crystal silicon at different temperatures were studied. The cutting force, crystal structure, particle motion characteristics, stress-strain distribution and temperature distribution during chip formation process were systematically analyzed. The underlying mechanism of nano-chip formation in single crystal silicon was summarized. The results showed that the chip formation mechanism can be divided into two types: crack and shear. At low temperatures, chips are formed by crack propagation. The concentrated distribution of high stress triaxiality and local high temperatures at the crack tip are the main causes of crack propagation. With an increase in temperature, the chip formation mode gradually changed from crack to shear. The highly resolved shear stress distributed along the slip plane is the main reason for the expansion of the amorphous bands. In addition, it's found that higher quality machined surfaces can be obtained through chip formation process by shear than by crack.

Superhardness in nanotwinned boron carbide: a molecular dynamics study.

Boron carbide ceramics are often considered ideal materials for lightweight bulletproof armor, but their anomalous brittle failure at hypervelocity impact limits their use. Recent experiments have reported that nanotwins are ubiquitous in boron carbide and that nanotwinned samples are harder than the twin-free boron carbide, but although the strengthening effect of nanotwins on metals and alloys is well-established, their role in boron carbide ceramics is not well understood. In this study, we used classical molecular dynamics simulations to investigate how nanoscale twins affect the mechanical properties of boron carbide ceramics. Our classical molecular dynamics results show that introducing nanotwins in boron carbide can increase the shear strength limit by 19.72%, reduce the number of amorphized atoms, and narrow the width of the amorphous shear band. Under indentation load, nanotwins can also increase the compressive shear strength limit of boron carbide by 15.97% and change the crystal formation direction and region of the amorphous shear band. These findings suggest that twin boundaries can hinder the expansion of the amorphous shear band and provide a new design idea for improving the impact resistance of boron carbide ceramics and avoiding their abnormal brittle failure.

Rapid amorphization of CrMnFeCoNi high-entropy alloy under ultrasonic vibrations

Due to the distinct design concept, high-entropy alloys (HEAs) exhibit unusual properties and lead an emerging new field. In this work, we show that a typical face-centered cubic crystalline phase CoCrFeNiMn HEA can be readily transformed into the amorphous phase under the ultrasonic vibration treatment (UVT) at a frequency of 20000 Hz. The nanoscale hierarchical features include twins, stacking faults bands, hexagonal-close packed phase bands and even amorphous bands can be obviously identified in samples treated by different UVT energies. The dominant mechanism of ultrasonic vibration-induced amorphization is that the grain refinement promotes the formation of amorphous phases when the defect density at the grain boundaries reaches a critical level. In addition, the mechanical instability is easily induced by ultrasonic vibration at high strain rate to generate amorphous phase inside the grains. As a consequence of UVT, the HEA samples revealed significant mechanical performance improvement owing to the microstructure evolution especially the generation of amorphous phase, such as yielding strength and hardness. This rapid amorphization process provides not only a candidate strengthening mechanism for HEA, but also a novel approach to unveil the pending crystal-amorphous transition problem.

Transport mechanisms in hyperdoped silicon solar cells

According to intermediate band (IB) theory, it is possible to increase the efficiency of a solar cell by boosting its ability to absorb low-energy photons. In this study, we used a hyperdoped semiconductor approach for this theory to create a proof of concept of different silicon-based IB solar cells. Preliminary results show an increase in the external quantum efficiency (EQE) in the silicon sub-bandgap region. This result points to sub-bandgap absorption in silicon having not only a direct application in solar cells but also in other areas such as infrared photodetectors. To establish the transport mechanisms in the hyperdoped semiconductors within a solar cell, we measured the J–V characteristic at different temperatures. We carried out the measurements in both dark and illuminated conditions. To explain the behavior of the measurements, we proposed a new model with three elements for the IB solar cell. This model is similar to the classic two-diodes solar cell model but it is necessary to include a new limiting current element in series with one of the diodes. The proposed model is also compatible with an impurity band formation within silicon bandgap. At high temperatures, the distance between the IB and the n-type amorphous silicon conduction band is close enough and both bands are contacted. As the temperature decreases, the distance between the bands increases and therefore this process becomes more limiting.