Amorphous Volume Fraction Research Articles

Effect of cold-drawn-induced amorphous phase on microstructure and pseudoelasticity properties of nanocrystalline NiTi alloys

Amorphous structures can be induced by cold deformation in the NiTi shape-memory alloys, which transform into nanocrystalline structures after annealing. In this study, the effect of cold-drawn-induced amorphous phase on the microstructure and pseudoelasticity properties of nanocrystalline NiTi alloys was investigated. Ni-49.2 at.%Ti alloy samples were subjected to different cold-drawn deformations. Transmission electron microscopy was used to investigate the microstructure of the samples. Three types of microstructures: twin-related lamellae, nanocrystalline, and amorphous, were observed in the cold-drawn samples. The amorphous volume fractions of the samples were determined using differential scanning calorimetry. The results suggested that the amorphous volume fraction increased with increasing cold-drawn deformation. The pseudoelasticity properties of the samples with different cold-drawn deformations were investigated using tensile tests. Notably, 59.05% cold-drawn deformation, which induced approximately 39.1% volume fraction of the amorphous phase, was the turning point in the performance of the NiTi alloy. The stress hysteresis and upper plateau stress increased with increasing cold-drawn deformation when the cold-drawn deformation was lesser than 59.04%. Furthermore, the difference in stress hysteresis and upper plateau stress was not significant when the cold-drawn deformation was greater than 59.04% because a complete nanocrystalline microstructure could be obtained after annealing when the volume fraction of the amorphous phase reached 39.1% in the NiTi alloy.

Resistance Drift Convergence and Inversion in Amorphous Phase Change Materials

AbstractPhase change materials (PCMs) are key to the development of artificial intelligence technologies such as high‐density memories and neuromorphic computing, thanks to their ability for multi‐level data storage through stepwise resistive encoding. Individual resistance levels are realized by adjusting the crystalline and amorphous volume fraction of the memory cell. However, the amorphous phase exhibits a drift in resistance over time that has so far hindered the commercial implementation of multi‐level storage schemes. In this study, the underlying physical process of resistance drift with the goal of modeling is elucidated that will help minimize and potentially overcome drift in PCM memory devices. Clear evidence is provided that the resistance drift is dominated by glass dynamics. Resistivity convergence and drift inversion for the amorphous chalcogenide Ge15Te85 and the PCM Ge3Sb6Te5 are experimentally demonstrated and these changes are successfully predicted with a glass dynamics model. This new insight into the resistance drift process provides tools for the development of advanced PCM devices.

Laser‐induced heating of porous Ge layers implanted with Ag+ and Cu+ ions

AbstractPorous Ge layers consisting of nanowires were formed by low‐energy high‐dose implantation of monocrystalline c‐Ge substrate with Ag+ and Cu+ ions. The obtained layers were studied by Raman spectroscopy using solid‐state and He‐Ne exciting lasers with wavelength of 532 and 633 nm, respectively. The crystalline volume fraction and local temperature of porous layers at the surface sites of laser probing were determined. To interpret the change in the shape of the spectra under the action of laser radiation, deconvolution into amorphous and nanocrystalline components was carried out. Analysis of the spectral line shape made it possible to estimate the crystalline and amorphous volume fractions. The quantum confinement model for Ge nanocrystallites was taken into account for accurate estimations of volume fractions. These estimates showed that the implanted amorphous layers of all samples were locally crystallized with a solid‐state laser. He‐Ne laser excitation resulted in partial crystallization only for layers obtained by Cu+ ion implantation. The measured ratio of the Stokes to anti‐Stokes components demonstrated that the porous layers obtained by implanting Ag+ ion were not heated by a He‐Ne laser and according to the analysis of spectral lines, the crystalline volume fraction did not appear in these layers during probing. The results obtained are explained by the difference in the penetration depth for various samples of the exciting radiation.

Effect of Oxidation and Crystallization on Pitting Initiation Behavior of Fe-Based Amorphous Coatings

Fe-based amorphous coatings are typically fabricated by high-velocity oxygen-fuel spraying using industrial raw materials. The bonding mode between the coating particles and the corrosion mechanism of the coating in the chloride-rich environment were studied. The results indicate that some fine crystallites such as α-Fe and Fe3C tend to precipitate from the amorphous matrix as the kerosene flow rate increases or the travel speed of spraying gun decreases. Moreover, some precipitates of the (Cr, Fe)2O3 nanocrystal were detected in the metallurgical interfaces of the amorphous coating. The relationship among the amorphous volume fraction, porosity, and spraying parameters, such as the kerosene flow rate and the travel speed of the spray gun, were established. Due to an oxidation effect during spraying process, atomic diffusion, crystallite precipitation and regional depletion of Cr occur in the area along the pre-deposited side near the metallurgical bonding interface, leading to the initiation of pitting. A model of pitting initiation and expansion of Fe-based amorphous coatings is proposed in this paper.

Dry annealing of radiation-damaged zircon: Single-crystal X-ray and Raman spectroscopy study

Structural reconstitution upon dry thermal annealing of mildly to strongly radiation-damaged, gem-quality zircon from Sri Lanka has been studied by single-crystal X-ray diffraction and Raman spectroscopy. Results of structure refinement of a strongly radiation-damaged zircon (sample GZ5, calculated alpha dose ~4 × 1018 g−1) indicate the existence of an interstitial oxygen site that is sparsely occupied (about 4% of all O atoms). Annealing of this sample at Ta (annealing temperature) = 700 °C has resulted in nearly complete recrystallization of its amorphous volume fraction and significant decrease in the occupation of O-interstitial sites. For all samples studied, annealing up to Ta ≤ 650–700 °C is characterised by preferred recovery of Raman shifts (compared to Raman FWHMs; full width at half band maximum) and extensive contraction of the unit-cell volume, in particular along unit-cell dimension a. This low-T annealing is dominated by epitaxial growth of the crystalline volume fraction at the expense of the amorphous volume fraction, and general recovery of low-energy defects. During annealing at Ta = 700–1400 °C there is preferred recovery of Raman FWHMs (compared to Raman shifts) and only mild unit-cell contraction. High-T annealing is dominated by the recovery of high-energy defects such as recombination of cation Frenkel pairs. Here, unit-cell parameter a shows a remarkable behaviour (namely, mild re-increase at Ta = 700–1150 °C and mild final shrinking at Ta = 1000–1400 °C), which is attributed to enhanced contortion of ZrO8 polyhedrons due to cation repulsion. The combined data set of Raman band and unit-cell parameter presented herein will help analysts to assign Raman spectra of annealed unknowns to certain recovery stages.

Influence of chemical vapor precursor flow rate on the structure and thermal stability of phase-modulated VSiCN/Ag coatings

Ceramic/silver coatings are of interest for tribological and antimicrobial applications. Here, the amorphous phase volume fraction in VSiCN/Ag coatings was varied and the microstructure, mechanical properties, and vacuum annealing behavior of the phase-modulated VSiCN/Ag coatings were compared with columnar VN/Ag coatings. The coatings were deposited by filament-assisted reactive magnetron sputtering in a gas mixture of argon, nitrogen, and hexamethyldisilazane (HMDS). HMDS vapor flow rates were varied to influence coating amorphous phase content (0 at. % ≤ Si ≤ 10 at. %, 0 at. % ≤ C ≤ 20 at. %), and the sputter power supplied to a V58Ag42 target was manipulated to control metal content (3 at. % ≤ Ag ≤ 8 at. %). Silver surface segregation did not occur during coating growth, nor immediately after deposition. VN/Ag coatings exhibited columnar growth structures with >100 nm B1-VNx grains and 10−50 nm silver inclusions, which preferentially decorated the column boundaries. Columnar growth was completely disrupted in the VSiCN/Ag coatings and discrete silver inclusions were not observed within the microstructure. The hardness of all coatings was similar (12.3–16.5 GPa), while the apparent Young's modulus decreased from 247 to 169 GPa with increasing amorphous phase content. Rockwell indentation tests revealed that coating adhesion began to suffer after deposition at higher HMDS flow rates. Silver surface segregation was observed on all coatings after vacuum annealing at 550 °C for 1 h. The size of the surface particles ranged between 50 and 200 nm for the VN/Ag coating but was below 20 nm for the VSiCN/Ag coatings. Silver segregation occurred preferentially at coating growth defects in the columnar VN/Ag coating while the particle distribution was uniform across the surface of the VSiCN/Ag coatings. The results of this study demonstrate that phase modulation of ceramic/silver coatings may enable high-diffusivity microstructural features to be tailored for improved thermal stability and control of silver surface segregation at elevated temperatures.

Percolation transitions in pyrochlore: Radiation-damage and thermally induced structural reorganization

Finite element mechanical modeling is used to follow the evolution of the hardness (H), Young's modulus (E), and Poisson's ratio (ν) during the radiation-damage related crystalline-to-amorphous transition in pyrochlore (average main composition Ca2Nb2O6F). According to the model, two percolation transitions have been identified around 16% and 84% amorphous volume fraction, respectively. In this context, earlier results from thermally induced recrystallization experiments have found to indicate noticeable modifications on the short- and long-range order by passing the percolation thresholds. Both percolation points have found to act as specific kinetic barriers during stepwise annealing induced structural reorganization. As phases with pyrochlore structure have been considered as host structures for the long-term disposal of actinides, it is essential to gain better knowledge of their mechanical behavior under radiation-damage and subsequent temperature treatment. The obtained results validate the used models' robustness in predicting radiation-damage related mechanical modifications, at least for ceramics.

Modelling the effect of intrinsic radiation damage on mechanical properties: The crystalline-to-amorphous transition in zircon

Mechanical modelling using the level-cut Gaussian random field approach has been employed to simulate the effect of radiation induced amorphization on the Young´s modulus, Poisson´s ratio and hardness of zircon (ZrSiO4). A good agreement with previous nanoindentation experiments has been achieved. Two percolation transitions occur at ~16% and ~84% amorphous volume fraction, leading to deviations from linearity in the evolution of the Young´s modulus. Interface regions between crystalline and amorphous areas stabilise the hardness for a considerable amount of amorphous fraction. The modelling approach is promising for predicting the intrinsic radiation damage related evolution of the mechanical properties of various materials.

Long-range lamellar formation in blends of divided-lamellar-forming liquid crystal block copolymers with liquid crystal homopolymers

Well-ordered long-range lamellar microdomain morphology is a characteristic of liquid crystal (LC) block copolymers comprising a middle main-chain type LC block connected to amorphous coil blocks at both ends. However, LC block copolymers at amorphous block volume fractions greater than 0.5 aggregate the LC blocks into laterally short lamellae divided by the amorphous blocks. In this study, it was proven that the divided lamellae can be transformed into long-range ordered lamellae by adding an LC homo-polymer into the divided-lamellar-forming LC block copolymer. The lamellar spacing was found to increase with the molecular weight of the coil block. These blends offer an easier method for preparing large-spaced long-range ordered lamellar structures using LC block copolymers.

Coefficient of thermal expansion and biaxial Young's modulus in Si-rich silicon nitride thin films

The coefficient of thermal expansion (α) and biaxial Young's modulus is determined by comparing the differential thermal stress induced in Si-rich silicon nitride thin films deposited on single-crystal Si and sapphire substrates. The amorphous films are deposited in mixtures of dichlorosilane and ammonia, by low pressure chemical vapor deposition, in a temperature range of 1050–1115 K. Temperature-dependent wafer curvature measurements are performed to determine the differential thermal stress, across a temperature range spanning 300–800 K. Observations indicate that both α and the biaxial modulus decrease as the silicon content in the films increases. The trend of reduction in α is consistent with the relative α values for the limiting-case compositions of cubic-Si3N4 and amorphous Si. The decrease in α is attributed to a reduction in anharmonicity associated with Si–Si bonds relative to Si–N bonds. The biaxial modulus is observed to be proportional to the inverse cube root of the amorphous Si volume fraction. This observation is shown to be consistent with volumetric dilation of basal Si-SiyN4-y tetrahedra (y = 0–4), when substoichiometric films are formed through the substitution of Si for N. Additionally, the intrinsic film stress is observed to vary in proportion to the square of the inverse cube root of the amorphous Si volume fraction, which is shown to be consistent with compositionally induced intrinsic strain variation in Si-rich silicon nitride thin films.

Resistance upset welding of Zr-based bulk metallic glasses

Zr-based bulk metallic glasses represent an emerging important class of structural materials with interesting properties due to its amorphous nature. In this study, the structural and microstructural transformations to the heat-affected zone of a Zr-based bulk metallic glass welded by the resistance upset technology were investigated. X-ray diffraction, optical and scanning electron microscopy were used to characterize the base metal and the heat-affected zone. Vickers microhardness testing was performed to assess the mechanical response of the welded joints. The amorphous phase volume fraction measured along the welded joints shows no difference between the base metal and the heat-affected zone. It was possible to achieve a sound weld with no crystallization of the base metal.

Effect of block copolymer morphology on crystallization and water transport

The effect of block copolymer morphology on polymer crystallization and water sorption was investigated using two amphiphilic block copolymers with minority hydrophilic phase. Such polymers have many potential membrane applications, including CO2 capture and water purification. A model block copolymer chemistry comprising polystyrene and poly(ethylene oxide) (PEO) is used in this work. Both block copolymers have strongly phase-separated order, one with lamellar morphology and another with cylindrical morphology. In addition to the block copolymer structure, the PEO phase is semicrystalline at room temperature. The block copolymer structure has dramatic consequences for the degree of PEO crystallinity even in the absence of water. Furthermore, the crystallinity is affected by water sorption. PEO melting and degree of crystallinity were determined as a function of water content. Water solubility was measured between 35 and 75 °C. Interestingly, water sorption properties are dictated almost exclusively by the amorphous PEO volume fraction, being low in the semicrystalline state (below 55 °C) and higher in the amorphous state (above 55 °C). At 55 °C, crystallite dissolution is observed with increasing water solubility. Water diffusion coefficient was also measured; it decreases monotonically with increasing water content. This concentration dependence of diffusion is explained by water clustering, which increases with increasing water content. In addition to block copolymer morphology playing a major role in PEO crystallinity, it has a significant effect on water diffusion in the amorphous polymers and is of practical importance as investigations at elevated temperature are not possible with homopolymer PEO.

The interfacial restructuring to amorphous: A new adhesion mechanism of cold-sprayed coatings

A new plausible adhesion mechanism of cold-sprayed coatings on hard, non-deformable substrates was proposed. Interface analysis of both the heavy (Cu) and light (Al) metal coatings on the ceramic substrates such as AlN and ZrO2 showed that the amorphous cascade volume fractions were created on impact. Furthermore, these amorphous volume fractions provided the channels for quick and effective atomic level intermixing to yield high bond strength in the cold spray process. The important parameters to determine the total amorphous cascade volume were relative hardness, mechanical deformability and density of colliding particles. Therefore, the final bonding strength would be predicted based on the fit between these parameters for coating and substrate materials.

TIG Cladding + Laser Remelting of ZrAlNiCu Amorphous Coating

Abstract The present investigation focused on a new amorphous coating preparing technique which combined the advantages of both TIG arc and laser beam. After Zr 63.8 Ni 17.2 Al 11.4 Cu 7.6 powder bed was cladded on carbon steel surface by TIG arc, the coating was remelted using a CO 2 laser. The microstructure, the phase composition, the microhardness and the corrosion resistance of the present coating were analyzed. TIG arc helps the coating achieve a good bonding with substrate and a homogenous solidification. While laser beam contributes to a quick cooling rate to form an amorphous structure. The coating microstructure is complex, consisting of crystalline phase and amorphous phase. The XRD demonstrates a higher amorphous volume fraction of the present coating compared with traditional laser cladding. The present coating is 1330 MPa higher in microhardness, 0.07 V higher in corrosion potential and more than 10 times lower in corrosion current than traditional laser cladding.

Effect of the remelting scanning speed on the amorphous forming ability of Ni-based alloy using laser cladding plus a laser remelting process

Amorphous coatings exhibit prominent properties, including high hardness and good wear resistance properties. In this study, Ni-based amorphous composite coatings with a composition of (Ni0.6Fe0.4)68B18Si10Nb4 were prepared on a mild steel substrate via high power diode laser cladding plus laser remelting processes. The microstructure of the coating was studied using X-ray diffraction (XRD) and a scanning electron microscope (SEM). The effect of the remelting scanning speed on the amorphous forming ability of the Ni-based alloy was studied systematically by experimental and numerical simulation methods. The results indicated that there was no amorphous phase formed when the scanning speed was 4m/min and 5m/min. The amorphous phase was observed when the remelting scanning speed was 6m/min. The amorphous phase volume fraction increased with an increase in the remelting scanning speed. The fraction was found to be approximately 64% when the remelting speed was 8m/min. The simulation results indicate that the increase in the remelting scanning speed may lead to lower residence time and higher cooling rate of the melted pool. As a result, high remelting scanning speed can be concluded to be a necessary condition for the formation of the amorphous phase when using the laser cladding plus laser remelting process.

Self-Assembly of Flexible–Semiflexible–Flexible Triblock Copolymers

The self-assembly of ABA triblock copolymers comprising flexible amorphous poly(ethyl methacrylate) (PEMA, A block) and semiflexible liquid crystal (LC) polyester (B block) was investigated for amorphous block volume fractions (φ) ranging from 20% to 80%. Two copolymer series with different LC block molecular weights (Mn,LC) were examined. At φ < 55%, block copolymers with Mn,LC = 11 600 formed lamellar microdomains in which LC segments mostly extended along the lamella normal but folded to fit in the lamellae. When φ was augmented, the LC segment fold (Nfold) increased to enhance the interface between the LC and amorphous segments and counterbalanced the increase in amorphous lamella thickness by reducing the LC lamella thickness. When φ > 68%, the LC lamellae were divided into cubes, transforming into spheres in the PEMA matrix. When Mn,LC decreased to 6300, the copolymers showed similar morphology. However, the lamellae adopted zigzag configurations showing a greater tilting angle between the lamella normal to the LC chain axis with increasing φ. Thus, the LC and PEMA segments enhanced their interface by mutually sliding along the chain axis instead of increasing Nfold.

Hardness Regulation of Fe-Based Amorphous Composite Coatings by Laser Remelting

A Fe-based amorphous composite coating was deposited on a carbon steel substrate by arc spraying, and remelted with different laser energies by the Nd: YAG laser cladding system, in order to improve the mechanical properties of the coatings. The microstructure and microhardness of the composite coatings were investigated. The variation of harndness was measured as a function of the modified layer depth, which indicates that the laser remelting improves the bonding strength and hardness. Increasing the laser power, the quality of coating gets better, but the amorphous volume fraction decreases. It is obtained that the optimal laser electric current for the coating of 280 μm thickness is about 300 A, in which the remelted coating with medium energy densities has the highest average Vickers hardness of 741. Through the volume fraction change of the nanocrytals, the hardness of the composite coating is regulated by the laser power input, which amplified the application fields of the amorphous coatings.

Silicon nanoparticle size-dependent open circuit voltage in an organic–inorganic hybrid solar cell

We have incorporated silicon nanoparticles (Si-nps) into organic–inorganic hybrid solar cells in place of the chalcogenide nanocrystals that are commonly employed in such devices. Poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PCBM) were employed as hole and electron transport layers, respectively. We used transmission electron microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy to fully characterize the Si-nps and relate their characteristics to the performance of the hybrid solar cells. We show that the open circuit voltage (VOC) was largely dependent on the size and amorphous volume fraction of Si-nps. Our findings imply that the amorphous phase and small size of Si-nps produce band gap widening that increases the VOC when coupled with PCBM as acceptor. The maximum VOC was up to 0.634 V in a hybrid device with 5.7 nm Si-nps.

Shadowed off-axis production of Ge nanoparticles in Ar gas atmosphere by pulsed laser deposition: Morphological, structural and charge trapping properties

In this work, a novel customized shadowed off-axis deposition set-up is used to perform an original study of Ge nanoparticles (NPs) formation in an inert Ar gas atmosphere by pulsed laser deposition at room temperature varying systematically the background Ar gas pressure (Pbase(Ar)), target–substrate distance (d) and laser repetition rate (f). The influence of these parameters on the final NPs size distributions is investigated and a fairly uniform droplets-free and non-agglomerated NPs distribution with average height 〈h〉=2.8±0.6nm is obtained for optimized experimental conditions (Pbase(Ar)=1mbar; d=3cm; f=10Hz) with a fine control in the NPs density (from 3.2×109cm−2 to 1.1×1011cm−2). The crystalline quality of as-deposited NPs investigations demonstrate a strong dependence with the Ar gas pressure and a crystalline to amorphous phase volume fraction χc>50% is found for Pbase(Ar)=2mbar. The NPs functionality for charge trapping applications has been successfully demonstrated by capacitance–voltage (C–V) electrical measurements.

Soft magnetic composites manufactured by warm co-extrusion of bulk metallic glass and steel powders

Soft magnetic composites of Fe-based bulk metallic glass and low-alloy steel have been manufactured by warm co-extrusion of precursor powders at temperatures within the supercooled liquid region of the glass. Composites were manufactured with amorphous volume fractions of 75%, 67%, and 100%. Full consolidation of the constituent powders was observed with the bulk metallic glass remaining substantially amorphous. The composite electrical resistivity was observed to be anisotropic with a resistivity of 79 μΩ cm measured transverse to the extrusion axis in a sample with 75% amorphous volume fraction. A 0-3 connectivity pattern with the low-resistivity steel phase embedded in a 3-dimensionally connected high-resistivity bulk metallic glass phase was observed with scanning electron microscopy. This confirms that the flow characteristics of the bulk metallic glass and the steel powders were comparable during extrusion at these temperatures. The saturation magnetization of 1.3 T was consistent with the volume weighted average of the saturation magnetization of the two phases. A relatively high quasistatic coercivity of 8 Oe was measured and is likely due to slight crystallization of the bulk metallic glass as well as domain wall pinning at prior particle boundaries. Careful control of the thermal environment during the extrusion process is required to minimize glass crystallization and achieve the desired balance of magnetic and electrical properties.