Magnetic properties and microstructural characteristics of bulk Nd–Al–Fe–Co glassy alloys

Bulk Nd–Al–Fe–Co glassy alloys with diameter up to 5 mm were investigated by magnetic measurements, magnetic force microscopy (MFM) and high resolution electron microscopy (HREM) at room temperature. The results from the measurement of vibrating sample magnetometer show that these samples with compositions Nd₆₅Al₁₀Fe_25-xCo_x (x=0–10 at.%) and Nd₆₀Al₁₀Fe₂₀Co₁₀ display hard magnetic properties with H_C of 300 kAm⁻¹, M_S of 10 Am² kg⁻¹, and M_r of 7 Am² kg⁻¹. The MFM measurements of the Nd₆₀Al₁₀Fe₂₀Co₁₀ bulk metallic glass (BMG) reveal the existence of magnetic domains with a period of about 0.36 μm, and the ordered clusters with the averaged size of about 5 nm was observed by the HREM on the sample. The domain structure or cluster is believed to be associated with the appearance of hard-magnetic properties in this alloy system. The existence of the large-size domains demonstrates that magnetic moment of a great deal of ordered atomic clusters in the BMG has been aligned by exchange-coupling.     

Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy

Solute clustering and partitioning behavior in the early crystallization stage of an Fe_73.5Si_13.5B₉Nb₃Cu₁ amorphous alloy have been studied by employing a three-dimensional atom probe (3DAP) and a high resolution electron microscope (HREM). Results from the 3DAP have clearly shown that Cu atom clusters are present in the amorphous state after annealing below the crystallization temperature. The density of these clusters is in the order of 10²⁴/m³, which is comparable to that of the α-Fe grains in the optimum nanocrystalline microstructure. In the early stage of primary crystallization, Cu clusters are in direct contact with the α-Fe nanocrystals, suggesting that the α-Fe primary particles are heterogeneously nucleated at the site of Cu clusters. In the early stage of crystallization, the concentration of Si is lower in the primary crystal than in the amorphous matrix phase, unlike in the late stage of the primary crystallization, where Si partitions into the α-Fe phase with a composition of approximately 20 at.%.     

The formation of a tungsten containing surface layer in a carbon steel by compression plasma flow

Morphology, phase and elemental composition of a carbon steel surface layer after treatment by compression plasma flows containing a dispersed tungsten powder have been investigated in this work. The action of relatively short (∼ 120 μs) and intense (15-20 J/cm² per pulse) plasma pulses resulted in the formation of tungsten containing thin film consisting of clusters with the size of 100-200 nm. Besides, the film formation treatment with a few pulses allowed to alloy a surface steel layer with tungsten. This treatment also led to the formation of Fe₃W₃C and WC carbides in the surface layer.     

Structure of misfit dislocations in niobium–sapphire interfaces and strength of interfacial bonding: an atomistic study

The formation of networks of misfit dislocations is investigated at the (0001)_Al2O36(111)_Nb interface using a recently proposed approach (Vitek et al., Phil. Mag. A, 1995, 71, 1219) which employs a very simple pair-potential to describe interaction between the metal and the substrate that contains the strength of interfacial adhesion as a parameter. The calculations demonstrate how the strength of bonding between the two materials decides both the form of the network and the atomic structure of the cores of these dislocations. At the same time it reveals that diffusion is essential for the formation of the observed triangular network of 1/2〈111〉 dislocations. The calculated structures are then used to investigate related high resolution electron microscope (HREM) images using a multislice technique. In these simulations translational symmetry along the electron beam was not assumed but for each slice of material along the beam different sub-structures were used. This allowed us to investigate fully the effect of the dislocation intersections upon the images of the dislocation cores. Their effect is, indeed, considerable if an intersection is in the region producing the image but if not, the images of the cores of misfit dislocations are affected only marginally and HREM can capture fine details of the core structure. A direct comparison of an experimental observation in Mayer and co-workers (Acta metall. mater., 1992, 40, S217; Scripta metall. mater., 1994, 31, 1097) with the present simulations demonstrates this ability.     

Local atomic structure of Pd–Ni–P bulk metallic glass examined by high-resolution electron microscopy and electron diffraction

Structural fluctuation in a Pd₄₀Ni₄₀P₂₀ bulk metallic glass is investigated by transmission electron microscopy and electron diffraction. Local atomic ordered regions with a fcc-(Pd,Ni) type structure was sharply imaged by a high-resolution electron microscopy (HREM) attached with a Cs-corrector. Interference function for the glassy state was obtained from electron-diffraction intensity profiles using energy-filter and imaging-plate techniques. We used a reverse Monte Carlo (RMC) simulation method to develop a realistic structure model. The model consists of a dense-random-packing structure, in which an fcc ordered region with Pd, Ni, and P atoms was embedded. The structure model is consistent with the diffraction and HREM results. In Voronoi polyhedral analysis of the RMC simulated structure, P-centered (Pd,Ni)-P trigonal prisms are found primarily in the matrix structure embedding the fcc-cluster. Around Pd and Ni atoms deformed-fcc type polyhedra were frequently observed. From these local structural features, nanoscale phase separation was revealed to occur during the glass formation.     

Anomalous glass transition behavior in Cu–Zr–Sn alloy system

We discuss the effect of Sn addition on anomalous glass transition behavior in Cu–Zr bulk-forming metallic glasses. We found that an unusual endothermic reaction in Cu₅₅Zr₄₀Sn₅ ribbon can originate from the growth reaction of quenched-in nuclei from 0.4 nm to 3.7 nm in the supercooled liquid region. This anomalous devitrification may prove useful for the synthesis of a novel composite with uniform atomic/nanometer scale heterogeneity modulated by controlling cooling rate as well as by tailoring alloy composition.     

Mössbauer probe spectroscopy studies of initial stage of Al-Fe mechanical alloying

The initial stage of mechanical alloying (MA) in a binary mixture of powdered Al and ⁵⁷Fe in an atomic ratio of 99: 1 has been studied using X-ray diffraction and ⁵⁷Fe Mössbauer probe spectroscopy. The proposed microscopic model of MA includes the formation of a nanostructured state (～15 nm) in FCC Al, the penetration of Fe atoms across Al grain boundaries, and the formation of isolated Fe atoms and Fe-Al clusters in boundary distorted zones of Al matrix interface similar to the local atomic environment in deformed Al₆Fe and Al₉Fe₂ phases. Based on the published data, it is assumed that with increasing Fe concentration in the initial mixture, and, correspondingly, increasing amount of clusters, distorted FCC interface regions are transformed into amorphous phase.     

The Oxidation of NdFeB Magnets

The oxidation kinetics in air of a commercial NdFeB magnet have been investigated over the temperature range 335–500°C. The oxide microstructure has been characterized by SEM, XRD and cross-sectional TEM. The results show that the external scale formed consists of an outer layer of Fe₂O₃ and an inner layer of Fe₃O₄ but that the principal degradation process is the formation of an extensive zone of internal oxidation. HREM has been used to show that this zone contains NdO particles embedded in an -Fe matrix. These particles are discrete and very small, approximately 2 nm in diameter, and have an amorphous structure. The -Fe matrix has a columnar grain structure with a grain width of approximately 100 nm. It is argued that the high rates of internal oxidation arise because the external-oxide layers are not protective at the oxidation temperature, and oxygen penetrates to the zone front by fast diffusion along the columnar -Fe grain boundaries.     

TEM and DFT investigation of CVD TiN/κ-Al2O3 multilayer coatings

This paper investigates the interfacial structure in hot-wall CVD TiN/κ-Al₂O₃ multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the κ-Al₂O₃ layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/κ-Al₂O₃ coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/κ-Al₂O₃ coatings, are not present. However, the orientation relationships at the TiN/κ-Al₂O₃ interfaces are the same in both multilayers. The HREM investigations show that κ-Al₂O₃ (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, γ-Al₂O₃ can be grown, at least partly stabilized by the cube-on-cube orientation relationship between γ-Al₂O₃ and the underlying TiN. The DFT calculations show very similar adsorption strengths for an O monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fcc site. O adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for κ-Al₂O₃ on TiN(111) than on TiC(111) and a higher stability for κ-Al₂O₃ than for α-Al₂O₃ on both TiC and TiN. This indicates that the observed higher stability of κ-Al₂O₃ on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of κ-Al₂O₃ over α-Al₂O₃ can be partly attributed to the mismatch.     

Nanoscale structural evolution of Al3Sc precipitates in Al(Sc) alloys

Precipitation of the Al₃Sc (L1₂) phase in aluminum alloys, containing 0.1, 0.2 or 0.3 wt% Sc, is studied with conventional transmission and high-resolution (HREM) electron microscopies. The exact morphologies of the Al₃Sc precipitates were determined for the first time by HREM, in Al–0.1 wt% Sc and Al–0.3 wt% Sc alloys. The experimentally determined equilibrium shape of the Al₃Sc precipitates, at 300°C and 0.3 wt% Sc, has 26 facets, which are the 6 {100} (cube), 12 {110} (rhombic dodecahedron), and 8 {111} (octahedron) planes, a Great Rhombicuboctahedron. This equilibrium morphology had been predicted by first principles calculations of the pertinent interfacial energies. The coarsening kinetics obey the (time)^1/3 kinetic law of Lifshitz–Slyozov–Wagner theory and they yield an activation energy for diffusion, 164±9 kJ/mol, that is in agreement with the values obtained from tracer diffusion measurements of Sc in Al and first principles calculations, which implies diffusion-controlled coarsening.     

On the mechanism of formation of zinc pack coatings

The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 °C) is endothermic and involves the transformation of α-NH₄Cl to β-NH₄Cl and the NH₄Cl decomposition to NH₃ and HCl. During the second step (at 248.6 °C), which is exothermic, Zn²⁺ salts are formed and especially ZnCl₂. Finally at 264.1 °C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl₂. The as-cast Zn diffuses into the iron lattice forming the gamma (Γ-Fe₁₁Zn₄₀) and delta (δ-FeZn₁₀) phases. Al₂O₃ is not involved in the above-mentioned mechanism and acts only as filler.     

Microstructure and mechanical properties of W-C:H coatings deposited by pulsed reactive magnetron sputtering

Reported are results of microstructure, mechanical and tribological properties studies for thin, amorphous hydrogenated carbon based coatings with tungsten content from 4.7 at.% up to 10.3 at.%. Studied coatings have been deposited by pulsed, reactive magnetron sputtering on substrates under planetary rotation. Resulting coatings, characterized by transmission electron microscopy (TEM) also at high resolution (HREM), show multilayer structure consisting of sub-layers of W-C:H type, with alternately high and low tungsten concentration. Thickness and number of sub-layers depend on rotation speed of planetary substrate holder. An average tungsten concentration decreases with increasing partial pressure of reactive gas (C₂H₂) during deposition. More insight into the microstructure of coatings provided HREM analysis showing crystalline precipitations of about 1-2 nm in size as well as tungsten-rich and tungsten-poor W-C:H sub-layers. Raman spectra confirm presence of amorphous, hydrogenated carbon (a-C:H) phase in the coatings. Microhardness of studied coatings depends on tungsten content and increases from 10.7 GPa to 13.7 GPa, for 5.1 at.% and 10.3 at.% of tungsten content, respectively. The highest cracking resistance and best adhesion (L_c2 = 78 N and HF1) has been achieved for coatings containing 4.9 at.% of tungsten and a sub-layer thickness of 5 nm. Tribological processes occurring in the coating-coating contact zone are dominated by graphitization and oxidation of W-C:H coating. Very low friction coefficient (0.04) and low wear rate seems to be an effect gaseous micro-bearing by tribo-generated carbon oxides and methane as well as hydrogen released from the coating. In the W-C:H-steel contact zone a tribo-layer composed of iron and tungsten oxides mixed with graphite-like products is growing at the surface of steel counterpart. This tribo-layer becomes a barrier restricting direct contact of steel with the coating and thus preventing it from further intense wear.     

Internal structure of clusters of partially coherent nanocrystallites in Cr-Al-N and Cr-Al-Si-N coatings

Nano-sized clusters consisting of strongly preferentially oriented, partially coherent nanocrystallites were observed in Cr-Al-N and Cr-Al-Si-N coatings deposited using cathodic arc evaporation. Microstructure analysis of the coatings, which was done using the combination of X-ray diffraction (XRD) and transmission electron microscopy with high resolution (HRTEM), revealed furthermore stress-free lattice parameters, size and local disorientation of crystallites within the nano-sized clusters in dependence on the aluminium and silicon contents, mean size of these clusters and the kind of structure defects. Within the face-centred cubic (fcc) Cr_{1 − x − y}Al_xSi_yN phase, the stress-free lattice parameter was described by the equation a = (0.41486 − 0.00827 · x + 0.034 · y) nm. The size of individual crystallites decreased from ∼ 11 nm in Cr_0.92Al_0.08N to ∼ 4 nm in Cr_0.24Al_0.65Si_0.10N. These nanocrystallites formed clusters with the mean size between 36 and 56 nm. The mutual disorientation of the partially coherent nanocrystallites forming the clusters increased with increasing aluminium and silicon contents from 0.5° to several degrees. The disorientation of neighbouring nanocrystallites was explained by the presence of screw dislocations and by presence of phase interfaces in coatings containing a single fcc phase and several phases, respectively.     

Formation of nanoparticles and nanorods via UV irradiation of AgNO3 solutions

The synthesis of silver nanoparticles via UV irradiation of AgNO₃ solutions was controlled by using UV–vis absorption spectra and TEM (transmission electron microscope) images. The UV–vis absorption method is good enough for the general control of synthesis process, and TEM images give us information about size of formed species. For investigated solutions of silver nitrate in ethanol and water, we observed formation of large nanoparticles (size about 100 nm) and nanorods (100 nm in length). Moreover, there was effort to confirm evidence of formation of these particles by using TOF mass spectrometer. Due to laser desorption/ionization process there is only evidence of small silver nanoparticles Ag_x, x ≤ 4 (clusters), and variety of silver compounds Ag_xN_yO_z (x ≤ 5, y ≤ 2, z ≤ 3).     

Atom probe tomography study of the decomposition of a bulk metallic glass

The microstructural evolution of a Pd₄₀Ni₄₀P₂₀ bulk metallic glass that was isothermally annealed at 260 °C for 14 h, and then aged at 340 °C for times up to 1280 min has been studied. Differential scanning calorimetry (DSC) curves of the aged samples show an endothermic peak at approximately 370 °C in addition to the ubiquitous glass transition. The endothermic peak appears after 20 min aging and disappears after 320 min aging. The corresponding X-ray diffraction (XRD) data show no Bragg peaks that could indicate the formation of a crystalline phase. Near-atomic-resolution atom probe tomography (APT) was used to study changes in the atomic spatial distributions as a function of aging time. The chemical environment around each of the atomic species, and the tendencies for solute clustering and chemical short range ordering, were determined from statistical analysis of the APT data. Clustering and possible phase separation are identified by APT after only 20 min aging at 340 °C, which correlates with the appearance of the peak in the DSC signal. Crystallization is apparent in the APT and XRD data after aging for 320 min. The study suggests that the amorphous Pd₄₀Ni₄₀P₂₀ annealed at a temperature 40 °C above T_g phase separates into two or more amorphous phases. The endothermic peak in the DSC trace is produced by the dissolution of the phase separation.     

Upconversion luminescence properties of Er<Superscript>3+</Superscript> doped GeS<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>-KCl chalcohalide glasses

Er3+ ions doped chalcohalide glasses with the composition of 56GeS2-24Ga2S3-20KCl were fabricated by a melt-quenching method.Under 800 nm laser excitation,strong green emissions centered at 525 nm and 550 nm and weak red emission centered at 660 nm were observed,which were assigned to 2H11/2→4I15/2,4S3/2→4I15/2,and 4F9/2→4I15/2 transitions,respectively.The intensity reached maximum when the Er3+ ions concentration was 0.1 mol%.The possible upconversion luminescence mechanism was proposed from the discussion...     

XRD and HREM studies from the decomposition of icosahedral AlCuFe single-phase by high-energy ball milling

In this investigation the Al₆₄Cu₂₄Fe₁₂ alloy was melted in an induction furnace and solidified under normal casting conditions. In order to obtain the icosahedral phase (i-phase) in a single-phase region, the as-cast sample was subject to a heat treatment at 700 °C under argon atmosphere. Subsequently, the i-phase was milled for different times in order to evaluate phase stability under heavy deformation. X-ray diffraction (XRD) and high-resolution electron microscopy (HREM) analysis were conducted to the structural characterization of ball-milled powders. XRD results indicated a reduction in quasicrystal size during mechanical ball milling to about 30 h. HREM analysis revealed the presence of aperiodic nano-domains, for example, with apparent fivefold symmetry axis. Therefore, the i-phase remains stable over the first 30 h of ball-milling time. However, among 30-50 h of mechanical milling the i-phase transforms progressively into β-cubic phase.     

Microstructure of Oxide Scales Formed on Ti--48Al--8Cr--2Ag Alloy in Air at 900--1000°C

The oxidation of Ti--48Al--8Cr--2Ag (at. %) was studied for 50 h at 900 and 1000°C. The micro structure of the oxide scales was characterized by means of scan ning-elec tron microscopy (SEM) and high-resolution elec tron microscopy (HREM). The results showed that the mass gain of the alloy at 1000°C was lower than that at 900°C. At 1000°C, a continuous and densescale of quite pure Al₂ O₃ formed, but at 900°C, a complex scale with an outer layer of Al₂O₃ + TiO₂ and an layer of inner Al₂O₃. A Laves phase and a Z-phase were found at the scale/substrate interface, which might be a factor beneficial to the formation of Al₂O₃     

Magnetite nanoparticles by organic-phase synthetic route for carbon nanotube growth

In the present study, monodisperse Fe₃O₄ nanoparticles with diameters ranging from 10 nm to 25 nm were synthesized using a simple organic-phase synthetic route and these monodispersed nanoparticles were then used as catalyst for seed growth of carbon nanotube. Fe₃O₄ nanoparticles were reduced to iron nanoparticles assembly by Argon mixed with 5% Hydrogen gas at 400 °C and then it was examined by atomic force microscopy (AFM), thermomechanical analysis (TMA) and powder diffraction X-ray spectroscopic techniques. XRD indicates that iron clusters are bcc in nature and AFM image shows that the iron nanoparticles assemblies are 50–65 nm in size. To control the agglomeration of iron nanoparticles, nanoporous hybrid support material of Al₂O₃ and SiO₂ was used. However, this matrix also fails to stop the agglomeration of iron nanoparticles mainly due to the inhomogeneous distribution of pore diameters. TMA analysis of iron clusters shows a temperature-dependent morphology, therefore, the CNTs growth temperature critically ascertain the nature and structure of CNTs.