Alpha Phase Research Articles

Structural Factors Inducing Cracking of Brass Fittings.

Cu–Zn–Pb brasses are popular materials, from which numerous industrially and commercially used components are fabricated. These alloys are typically subjected to multiple-step processing—involving casting, extrusion, hot forming, and machining—which can introduce various defects to the final product. The present study focuses on the detailed characterization of the structure of a brass fitting—i.e., a pre-shaped medical gas valve, produced by hot die forging—and attempts to assess the factors beyond local cracking occurring during processing. The analyses involved characterization of plastic flow via optical microscopy, and investigations of the phenomena in the vicinity of the crack, for which we used scanning and transmission electron microscopy. Numerical simulation was implemented not only to characterize the plastic flow more in detail, but primarily to investigate the probability of the occurrence of cracking based on the presence of stress. Last, but not least, microhardness in specific locations of the fitting were examined. The results reveal that the cracking occurring in the location with the highest probability of the occurrence of defects was most likely induced by differences in the chemical composition; the location the crack in which developed exhibited local changes not only in chemical composition—which manifested as the presence of brittle precipitates—but also in beta phase depletion. Moreover, as a result of the presence of oxidic precipitates and the hard and brittle alpha phase, the vicinity of the crack exhibited an increase in microhardness, which contributed to local brittleness.

The structural language of crystalline polymers*

AbstractIn sharp contrast with their amorphous counterparts, crystalline polymers provide a wealth of information on their chain conformation, the packing of the chains in unit‐cells, the existence or not of mirror symmetric conformations, on the crystalline lamellae, on the folds linking the crystalline stems, and so forth. Three illustrations of these insights are presented here: (a) the orientation of polymer folds in crystalline lamellae, (b) the complete structural analysis of the alpha phase of isotactic polypropylene by a simple observation of the lamellar morphology, and (c) the so‐called lateral spread following secondary nucleation on the growth faces of a polymer crystal.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Effect of cryogenic treatment on tribological behavior of Ti6Al4V alloy fabricated by selective laser melting

Ti6Al4V fabricated by selective laser melting (SLM) is commonly used as an artificial hip joint. However, the inferior tribological properties fall far short of the requirements for biomedical engineering and is harmful to the patients. In this study, cryogenic treatment (CT) was proposed to solve this problem. CT at −196 °C with a soaking time of both 2 h and 72 h were conducted, and their tribological properties were investigated by microstructure observation, Vickers hardness, and wear tests. The results showed that the microstructure of the as-fabricated and cryogenic treated (CTed) for 2 h are mainly composed of acicular martensite (α′ phase). However, the microstructure is transformed into alpha and beta phases after CT for 72 h. Vickers hardness of the specimens as-fabricated, CTed for 2 h and 72 h are 390.79 HV, 388.03 HV, and 371.34 HV, respectively. CT induces a significant improvement in tribological properties, which is correlated with the microstructure evolution. A similar wear mechanism can be observed for the specimens as-fabricated and CTed for 2 h, which is the co-action of severe abrasive wear, adhesive wear, and fatigue wear. While the specimen CTed for 72 h is slight abrasive wear mechanism.

Nanomechanical Characterization of the Deformation Response of Orthotropic Ti–6Al–4V

The nanoindentation‐induced mechanical deformation response is applied to identify the orthotropic elastic moduli using the Delafargue and Ulm method as well as to validate the asymmetric orthotropic CPB06 nonlinear plasticity model required in simulations of nonuniform macroscopic mechanical response of the Ti–6Al–4V alloy. Scanning electron microscope (SEM) technique allows to select the maximum penetration depth for the indentation in the deformed alpha phase and alpha–beta interphase,αandα/β, respectively. The apparent macromechanical response can be successfully derived from several residual imprints conducted at micro‐ and/or submicrometric length scale and distributed throughout samples of the investigated bulk alloy, as demonstrated by correlation with finite element simulations based on the orthotropic elastoplastic model. The accurate numerical response obtained validates the material model and the Delafargue and Ulm approach, opening a window for next generation identification methods of macromechanical plasticity models with hybrid experimental–numerical method based on instrumented indentation and the use of SEM technique.

Strain engineering in novel α-SbP binary material with tensile-robust and compress-sensitive band structures

Antimony phosphorus in the alpha phase (α-SbP) is a novel two-dimensional binary material with suitable band gap and environmental stability, it has attracted attention for its promising applications in the field of optoelectronics. In this work, the electronic and optical properties of α-SbP monolayer under biaxial strain have been studied by first principles calculation method based on density functional theory. It is found that when the tensile strain is applied, the band gap value of monolayer SbP increases firstly and then decreases under tensile strain, but the whole band gap value fluctuated is controlled within 0.08 eV. However, in the process of applying compressive strain, the band gap value decreases linearly with the strain value, and semiconductor-metal transition occurs at about −5% strain value. This is due to the difference in coupling between Sb and P atoms. Its optical properties under different strains are analyzed in detail, and it is found that the optical properties of SbP changed little under tensile strain, the light absorption peak is mainly concentrated in the ultraviolet region. In contrast, the compressive strain significantly adjust the optical properties of SbP. The innovation of this work is that the optoelectronic properties of SbP are studied by means of biaxial strain regulation. We find that the optoelectronic properties of SbP are robust to tensile strain. On the other hand, compressive strain has a significant effect on the photoelectric properties of SbP, and semiconductor-metal phase transition occurs. The research work in this paper provides theoretical support for the applications of monolayer α-SbP as a robust tensile and flexible compression material in optoelectronic nanodevices.

Solid state phase transformation kinetics in Zr-base alloys

We present a kinetic model for solid state phase transformation (alpha rightleftharpoons beta) of common zirconium alloys used as fuel cladding material in light water reactors. The model computes the relative amounts of beta or alpha phase fraction as a function of time or temperature in the alloys. The model accounts for the influence of excess oxygen (due to oxidation) and hydrogen concentration (due to hydrogen pickup) on phase transformation kinetics. Two variants of the model denoted by A and B are presented. Model A is suitable for simulation of laboratory experiments in which the heating/cooling rate is constant and is prescribed. Model B is more generic. We compare the results of our model computations, for both A and B variants, with accessible experimental data reported in the literature covering heating/cooling rates of up to 100 K/s. The results of our comparison are satisfactory, especially for model A. Our model B is intended for implementation in fuel rod behavior computer programs, applicable to a reactor accident situation, in which the Zr-based fuel cladding may go through alpha rightleftharpoons beta phase transformation.

Abnormal phase discontinuity of alpha- and theta-frequency oscillations in schizophrenia

BackgroundSchizophrenia patients have abnormal electroencephalographic (EEG) power over multiple frequency bands, even at rest, though the primary neural generators and spatiotemporal dynamics of these abnormalities are largely unknown. Disturbances in the precise synchronization of oscillations within and across cortical sources may underlie abnormal resting-state EEG activity in schizophrenia patients. MethodsA novel assessment method was applied to identify the independent contributing sources of resting-state EEG and assess the phase discontinuity in schizophrenia patients (N = 148) and healthy subjects (N = 143). ResultsA network of 11 primary contributing sources of scalp EEG was identified in both groups. Schizophrenia patients showed abnormal elevations of EEG power in the temporal region in the theta, beta, and gamma-bands, as well as the posterior cingulate gyrus in the delta, theta, alpha, and beta-bands. The higher theta-band power in the middle temporal gyrus was significantly correlated with verbal memory impairment in patients. The peak frequency of alpha was lower in patients in the cingulate and temporal regions. Furthermore, patients showed a higher rate of alpha phase discontinuity in the temporal region as well as a lower rate of theta phase discontinuity in the temporal and posterior cingulate regions. ConclusionsAbnormal rates of phase discontinuity of alpha- and theta-band, abnormal elevations of EEG power in multiple bands, and a lower peak frequency of alpha were identified in schizophrenia patients at rest. Clarification of the mechanistic substrates of abnormal phase discontinuity may clarify core pathophysiologic abnormalities of schizophrenia and contribute to the development of novel biomarkers for therapeutic interventions.

Effect of the detonation-spraying mode on the tribological properties of NiCr-Al2O3 coatings

The article presents the study results of detonation spraying parameters on the phase composition and tribological properties of NiCr-Al2O3 powder coatings. The spraying was carried out at a ratio of the acetylene-oxygen mixture O2 /C2H2=1.856. The detonation barrel filling volume with an explosive gas mixture varied from 30% to 68%. It is determined that the phase composition of the NiCr-Al2O3 coatings varies depending on the degree of detonation barrel filling. With an increase in the detonation barrel’s filling volume, the intensity of the NiCr diffraction peaks is decreased, and the intensity of the a-Al2O3 reflexes is increased, which indicates an increase in the content of the Al2O3 phase. When low filling volume, there is determined a low coating density and uneven roughness. The tribological test results showed that with an increase in the detonation barrel filling volume, there is a decrease in the wear volume, which confirms the increase in the coatings wear resistance. Determined that the lowest friction coefficient was recorded in the sample obtained at the barrel filling volume 68%. The coatings’ high wear resistance is associated with an increase in the alpha phase volume fraction of a-Al2O3 and coatings density.

Multiple carrier transport in high-quality α-Sn films grown on CdTe (001) by molecular beam epitaxy

The epitaxial growth of tin in an alpha phase (α-Sn) is desired for its topological properties. In this study, we have successfully grown a series of α-Sn films on CdTe (001) substrates by molecular beam epitaxy with different thicknesses. A (2 × 1) surface reconstruction of CdTe is obtained due to efficient cleaning by atomic hydrogen, which favors the α-Sn growth. The high quality of the α-Sn films has been confirmed by x-ray diffraction, atomic force microscopy, etc. Thickness and temperature-dependent electrical transport properties have been studied. All the samples show a p-type transport at room temperature, but transitions in transport type are observed at lower temperatures. These transport behaviors can be well explained by a three-band model, and a phase diagram illustrating the transport behaviors in α-Sn is presented.

Prestimulus alpha power but not phase influences visual discrimination of long-duration visual stimuli.

Occipital oscillations in the alpha band are closely related to visual perception and attention. In multiple studies, increased alpha power has been shown to reduce detection rates of hard-to-detect visual stimuli. Recent studies explain this finding by a shift in perceptual bias. Moreover, the phase of alpha oscillations prior to stimulus onset appears to be critical for the detection of visual stimuli. This is explained by a shift in cortical excitability over the course of each alpha cycle. However, prior studies often used short presentation times of visual stimuli at the perceptual threshold. Here, we use longer presentation times to elucidate whether the same mechanisms hold for the perception of salient but challenging visual stimuli presented for up to 1,500ms. To this end, we presented participants with hard to distinguish but salient upright or tilted Gaussian gratings in a two-alternative forced choice task, while recording occipital electroencephalographic activity. Previous reports link alpha power to stimulus detection hit rates, and we found that low prestimulus power at the individual alpha frequency relates to higher perceptual accuracy. Contrary to recent findings, we neither found an influence of alpha power on criterion, nor an influence of alpha phase on perception or response speed. We argue that longer presentation times might attenuate a possible response bias, and increased excitability might sharpen the discrimination ability, thereby leading to increased perceptual accuracy and unaffected response criterion.

Formability and microstructure of TC4 titanium alloy hollow shafts formed by cross-wedge rolling with a mandrel

The formability and microstructure of TC4 titanium alloy hollow shafts formed by cross-wedge rolling (CWR) are being investigated to ensure that products manufactured for utilisation in the aviation sector are lightweight. The flow behaviour of the TC4 alloy was studied via isothermal hot compression tests. The constitutive equations in different phase regions were then established and applied to a finite element (FE) model to study the effect of process parameters on the ellipticity of the TC4 alloy hollow shafts formed by CWR. Corresponding CWR experiments were conducted to validate the FE model; further, the microstructure of the TC4 alloy hollow shafts was investigated. The results demonstrate that forming angle, stretching angle and area reduction considerably affect the ellipticity of the TC4 alloy hollow shafts by varying the contact area between a die and a workpiece. The ellipticity evidently increases as the relative wall thickness decreases, as the flattening deformation increases. An increase in the deformation temperature will result in a decrease in the deformation resistance of the TC4 alloy and an increase in the ellipticity. Moreover, the effect of the deformation temperature, area reduction and wall thickness of the workpiece on the microstructure of the TC4 alloy hollow shafts formed by CWR was investigated. The degree of kink or globularisation of the strip alpha phase increases with the above parameters. The volume fraction of the beta phase increases with the deformation temperature. The microstructure is typically equiaxed when the deformation temperature is 950 °C.

Synthesis and Electrochemical Properties of Nanocrystalline Nickel Molybdate

We have obtained nanocrystalline hydrate and alpha phase of nickel molybdate by a hydrothermal technique. On the basis of the obtained cyclic voltammetry data, we have evaluated the contribution of faradaic and non-faradaic processes to the total capacitance of molybdates under study. It was found that the specific capacitance of hydrate NiMoO4·H2O is 621 F/g at a scan rate of 1 mV / s and the specific capacitance of the α-NiMoO4 is 281 F/g. Cathodes for hybrid supercapacitors were formed on the basis of the obtained nickel molybdates. As a result of electrochemical studies, it was found that the specific capacitance of hybrid supercapacitor based on NiMoO4·H2O/C was 256 F/g at the current of 0.2 A/g, while the specific energy was 80 W h/kg and specific power – 304 W/kg and these results are higher below in the α-NiMoO4 /C-based hybrid supercapacitor.

Cross hatched structure of polyethylene spherulites

We apply electron microscopy to evaluate the structure of spherulites in films of linear low density polyethylene (LLDPE). Improvements in this imaging technique allow us to show that the spherulites consist of two crossing patterns of lamellar stacks; one type of stack is composed of the usual radial lamellae while the second type of lamellae stacks is tangential. Radial lamellae are thicker and have a helical twist while tangential lamellae do not twist but rather create parallel stacks. The observed structure is akin to the structure frequently encountered in spherulites of polypropylene and shows the same dependence on crystallization temperature. At the same time the formation mechanism must be different from widely discussed epitaxial overgrowth in polypropylene because the polyethylene orthorhombic crystal lattice does not provide the same matching of lattice parameters as alpha phase of polypropylene.

Pre-stimulus alpha-band power and phase fluctuations originate from different neural sources and exert distinct impact on stimulus-evoked responses.

Ongoing oscillatory neural activity before stimulus onset influences subsequent visual perception. Specifically, both the power and the phase of oscillations in the alpha-frequency band (9-13Hz) have been reported to predict the detection of visual stimuli. Up to now, the functional mechanisms underlying pre-stimulus power and phase effects on upcoming visual percepts are debated. Here, we used magnetoencephalography recordings together with a near-threshold visual detection task to investigate the neural generators of pre-stimulus power and phase and their impact on subsequent visual-evoked responses. Pre-stimulus alpha-band power and phase opposition effects were found consistent with previous reports. Source localization suggested clearly distinct neural generators for these pre-stimulus effects: Power effects were mainly found in occipital-temporal regions, whereas phase effects also involved prefrontal areas. In order to be functionally relevant, the pre-stimulus correlates should influence post-stimulus processing. Using a trial-sorting approach, we observed that only pre-stimulus power modulated the Hits versus Misses difference in the evoked response, a well-established post-stimulus neural correlate of near-threshold perception, such that trials with stronger pre-stimulus power effect showed greater post-stimulus difference. By contrast, no influence of pre-stimulus phase effects were found. In sum, our study shows distinct generators for two pre-stimulus neural patterns predicting visual perception, and that only alpha power impacts the post-stimulus correlate of conscious access. This underlines the functional relevance of prestimulus alpha power on perceptual awareness, while questioning the role of alpha phase.

Microstructural and Mechanical Analysis of High-Performance Parts Produced by Hybrid Additive Manufacturing of Powder LMD on Forged Base Components

Ti-6Al-4V is used as a high-performance material in many industries (mainly automotive and aerospace, but also the medical industry) and traditionally produced by hot forging, with subsequent extensive post-processing and machining, leading to a material yield far from 100 % [1]. New production chains, such as additive manufacturing, enable the near net shape production of high-performance parts, however, still with long production cycles and high manufacturing costs, especially for larger parts [2]. Therefore, an efficient and feasible production is often limited to low quantities and/or small pieces. In the present study, we propose a hybrid manufacturing route, combining additive laser metal deposition (powder LMD) on hot forged base components, enhancing material efficiency, but still enabling the production of industrial quantities. Primary investigations on the microstructure and mechanical properties of the material show results similar to conventional hot forged material, but reduce the number of processing steps and increase the material yield.In more detail, the relationship between the primary beta grain size and the secondary alpha phase characteristics was investigated and moreover, related to the cooling history of the material. Furthermore, the influence of the microstructure and phase characteristics on the mechanical properties of the material was analyzed. For the determination of the primary beta grain size, the programming language MATLAB as well as its integrated open-source toolbox MTEX were used, where a GUI has been developed for the reconstruction of the primary beta grain orientations and sizes from recorded EBSD data of the secondary alpha (Ti) phase, using the Burger’s orientation relationship (BOR, [3-7]).

Evolution of δ ferrite in a CF3 cast stainless steel upon neutron irradiation to 3, 5, 10, 20, and 40 dpa

The microstructural evolution of delta ferrite in a CF3 cast stainless steel irradiated to 3, 5, 10, 20, and 40 dpa with two dose rates was studied with atom probe tomography (APT) and transmission electron microscopy. Spinodal decomposition and G-phase precipitates induced by neutron irradiation and thermal aging were quantified systematically. The neutron irradiation significantly enhances the spinodal decomposition in delta ferrite as both the spinodal wavelength (i.e. a characteristic repeat distance) and amplitude (i.e. magnitude of elemental concentration fluctuation) increase after irradiation. The wavelength only varies slightly when the dose increases from 5 to 40 dpa, while the amplitude increases dramatically from 10 to 20 dpa and starts to saturate before 20 dpa. The dose rate and irradiation temperature also have a notable effect on the spinodal wavelength and amplitude. The study shows that higher dose rate promotes a larger wavelength at given irradiation doses. Regarding the G-phase precipitates, a slightly lower irradiation temperature would result in a smaller mean size of G-phase precipitates, while a lower irradiation dose rate would lead to a larger G-phase precipitates at a given temperature and dose. Overall, the spinodal decomposition and G-phase precipitates in the delta ferrite continue to evolve with the increasing dose beyond 10 dpa. This study confirms that the formation of G-phase precipitates at the interdomain region between alpha and alpha-prime is facilitated by the Si and Mo atoms rejected from Fe rich alpha phase and the Ni and Mn atoms rejected from Cr rich alpha-prime phase. Neutron irradiation plays a dominant role in the ferrite instability, and the effect of prior thermal aging at 400°C for 10,000 hours is negligible as the dose is 3 dpa and above.

Investigation Toiler Weld Blank of SSM 2024 Aluminum Alloys by Friction Stir Welding Joint

The objective of this research was to investigate variables that affected the adhesion of the welded joints for TWBs with FSW in the SSM 2024 aluminum alloys. The results clearly showed that there was a good adhesion of the samples in every experiment. Longitudinal voids can also been found on the top surface of some variables. The average size of cross-links structure of Al2Cu phase (β phase) in SZ, broken into smaller pieces, was around 4 to 7 μm and evenly mixed with alpha phase (α phase). The effect of fracture of Al2Cu phase promoting better mechanical properties. The appropriate tensile strength from the experiment came from variables at the work angle at 15o, rotation speed at 2720 rpm, and welding speed 60 mm/min which provided maximum tensile strength value at 205.0 MPa and the maximum average hardness at 132.4 HV, higher than the weld region. The assessment results showed that the differences in welding variables resulted in different mechanical properties.

Dysmorphic neurons as cellular source for phase-amplitude coupling in Focal Cortical Dysplasia Type II

ObjectiveReliable localization of the epileptogenic zone is necessary for successful epilepsy surgery. Neurophysiological biomarkers include ictal onsets and interictal spikes. Furthermore, the epileptic network shows oscillations with potential localization value and pathomechanistic implications. The cellular origin of such markers in invasive EEG in vivo remains to be clarified. MethodsIn the presented pilot study, surgical brain samples and invasive EEG recordings of seven patients with surgically treated Focal Cortical Dysplasia (FCD) type II were coregistered using a novel protocol. Dysmorphic neurons and balloon cells were immunohistochemically quantified. Evaluated markers included seizure onset, spikes, and oscillatory activity in delta, theta, gamma and ripple frequency bands, as well as sample entropy and phase-amplitude coupling between delta, theta, alpha and beta phase and gamma amplitude. ResultsCorrelations between histopathology and neurophysiology provided evidence for a contribution of dysmorphic neurons to interictal spikes, fast gamma activity and ripples. Furthermore, seizure onset and phase-amplitude coupling in areas with dysmorphic neurons suggests preserved connectivity is related to seizure initiation. Balloon cells showed no association. ConclusionsPhase-amplitude coupling, spikes, fast gamma and ripples are related to the density of dysmorphic neurons and localize the seizure onset zone. SignificanceThe results of our pilot study provide a new powerful tool to address the cellular source of abnormal neurophysiology signals to leverage current and novel biomarkers for the localization of epileptic activity in the human brain.

Novel α + β Zr Alloys with Enhanced Strength.

Low-alloyed zirconium alloys are widely used in nuclear applications due to their low neutron absorption cross-section. These alloys, however, suffer from limited strength. Well-established guidelines for the development of Ti alloys were applied to design new two-phase ternary Zr alloys with improved mechanical properties. Zr-4Sn-4Nb and Zr-8Sn-4Nb alloys have been manufactured by vacuum arc melting, thermo-mechanically processed by annealing, forging, and aging to various microstructural conditions and thoroughly characterized. Detailed Scanning electron microscopy (SEM) analysis showed that the microstructural response of the alloys is rather similar to alpha + beta Ti alloys. Duplex microstructure containing primary alpha phase particles surrounded by lamellar alpha + beta microstructure can be achieved by thermal processing. Mechanical properties strongly depend on the previous treatment. Ultimate tensile strength exceeding 700 MPa was achieved exceeding the strength of commercial Zr alloys for nuclear applications by more than 50%. Such an improvement in strength more than compensates for the increased neutron absorption cross-section. This study aims to exploit the potential of alpha + beta Zr alloys for nuclear applications.