Identification and quantification of microstructures formed during nanocrystallization of amorphous (Fe, Co)-Nb-(Si, B) systems

The effect of addition of Si and variation of the Fe/Co ratio on the evolution of the nanostructure was studied in a modification of the Fe–Nb–B system. The entire system (Fe, Co)₇₃Nb₇(Si, B)₂₀ was prepared in an amorphous state by rapid quenching using the planar flow casting method over a wide range of Fe/Co atomic ratios, ranging from 0 to 1. Nanocrystallization was investigated by evolution of the electrical resistivity with time and temperature. The microstructural analysis was performed using transmission electron microscopy as well as electron and X‐ray diffraction. The results from microscopy observations were used to determine the distribution of grain size, which in these alloys attain very small dimensions of ～5–8 nm. New algorithms of microscope image analysis were used for grain size determination, crucial for quantifying the microprocesses controlling nucleation and growth from the amorphous rapidly quenched phase.     

Diagnosis of the structural state of bulk nano- and submicrocrystalline magnetically soft materials

Different structural states, namely microcrystalline, submicrocrystalline, nanocrystalline (each with a different set of grain sizes) and amorphous ones, in magnetically soft materials intended for fabricating sensitive elements in high-tech equipment are considered. The effects of the structural state of Fe₅Co₇₀Si₁₅B₁₀, Fe₆₀Co₂₀Si₅B₁₅, Co_81.5Mo_9.5Zr₉, and Fe_{73.5 − x} Co _x Cu1Nb₃Si_13.5B ₉ (x = 0, 10, 20, 30 at.%) amorphous alloys on their magnetic characteristics in different nanocrystallization conditions are analyzed. The correlation of the peculiarities of the fine structure and the grain sizes in the alloys under study with their magnetic parameters and informative parameters of the Barkhausen effect is shown.     

Pecularities of nanocrystal formation in rapidly quenched (FeCo)MoCuB amorphous alloys

The effect of the substitution of Fe by Co on the enhancement of glass‐forming ability limits and subsequent nanocrystallization was studied in a rapidly quenched amorphous system (Fe_xCo_y)₇₉Mo₈Cu₁B₁₂ for y/x ranging from 0 to 1. The effect of Cu on nanocrystallization was investigated by comparison with Cu‐free amorphous Fe₈₀Mo₈B₁₂. Systems partially crystallized at the surface layer were prepared for y/x = 0 using different quenching conditions. The effect of heat treatment of master alloys used for ribbon casting was also assessed. The microstructure and surface/bulk crystallization effects were analysed using transmission electron microscopy and electron and X‐ray diffraction in relation to the expected enhancement of high‐temperature soft magnetic properties, drastically reduced grain sizes (～5 nm) and Co content. Unusual surface phenomena were observed, indicating the origin of possible nucleation sites for preferential crystallization in samples with low Co content.     

New mechanism for field-induced anisotropy of cobalt-rich amorphous alloys

It is generally assumed that amorphous magnetic alloys respond to field annealing by a process of local directional ordering which leaves the amorphous structure intact. We have made a comparative microstructural study of field-annealed Co_95-xFe₅(BSi)x amorphous alloys using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with thin sections parallel to the ribbon surface. Field annealing response was measured from anisotropy in low-field hysteresis loops. These alloys show appreciable surface crystallization for annealing as much as 80 K below the bulk crystallization temperature. The surface crystallization proceeds by a known mechanism (selective oxidation) to which we have added a more detailed understanding. Three steps are involved: (1) formation of an amorphous borosilicate surface oxide layer during annealing; (2) depletion of glass stabilizing elements (boron and silicon) from the underlying amorphous metal substrate; (3) primary crystallization of the destabilized, metal-enriched subsurface layer to an fee or hep cobalt-rich phase. Striking differences in the microstructural morphology were revealed for different glass former ratios B/Si. For high B/Si ratios, the surface crystallites are predominantly fee Co and show a high density of oxygen faults. For low B/Si ratios, the surface crystallites are predominantly hep Co and almost free of faults. Response to field annealing is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystallites. Electron diffraction and microprobe analysis indicate that the surface oxide in silicon-rich alloys is a dense silica glass which appears to be an effective diffusion barrier to oxygen. The surface oxide in boron-rich alloys is a more porous oxide richer in B₂O₃. These observations appear to be related to those from perminvar alloys where oxygen was found to be necessary for field annealing to be effective.     

Structure and Barkhausen effect parameters of amorphous alloys after various heat treatments

The influence of the structural state of amorphous magnetically soft Fe₆₀Co₂₀Si₅B₁₅ and Co_81.5Mo_9.5Zr₉ alloys on the magnetic properties and informative parameters of the Barkhausen effect is studied. The structure of amorphous ribbons is investigated using transmission electron microscopy. The results obtained allow the conclusion that the study of the Barkhausen effect provides magnetic and structural information about an alloy. The parameters of the Barkhausen effect are shown to correlate with features of the fine structure of the investigated devitrifying amorphous alloys.Translated from Defektoskopiya, Vol. 40, No. 9, 2004, pp. 63–68.Original Russian Text Copyright © 2004 by Noskova, Shulika, Lavrentev, Potapov, Korzunin.     

Crystalline phases found in rapidly quenched Ni–Nb–Zr alloys

The properties of metallic alloys can be significantly improved by developing non‐equilibrium phases in the microstructures through rapid solidification techniques, thus the characterisation of these unusual structures is extremely important. In this research, the microstructures of three rapidly quenched alloys, namely Ni_65.2Nb_33.8Zr_1.0, Ni_54.8Nb_31.1Zr_14.1 and Ni_54.8Nb_21.6Zr_23.6 (at. %) were investigated in greater detail in order to determine the structures and compositions of their crystalline phases. These crystalline phases were characterised using a combination of scanning electron microscopy, energy dispersive x‐ray spectroscopy, x‐ray diffraction and transmission electron microscopy techniques. The phases were compared to the crystalline structures reported in the literature. Our results indicate some agreement with the Ni–Nb phase diagram and an isothermal section of the Ni–Nb–Zr phase diagram; however, it is detected zirconium solubility in the Ni₃Nb phase, as well as, the absence of expected crystalline phases.     

The structure and mechanical properties of amorphous and nanocrystalline Fe–Si–B alloys

This article describes the results of investigations of the microstructure of failure surfaces and the mechanism of deformation of an amorphous Fe₈₀Si₁₁B₉ alloy, nanocrystallized with the use of Nd:YAG pulsed laser heating. The research included ‘in situ’ tensile testing in a scanning electron microscope. Mechanical properties were also measured on an Instron‐type machine for the amorphous and nanocrystalline alloys. The mechanical tensile tests performed on the amorphous and nanocrystalline samples showed a ductile fracture surface with very high fracture stress. Detailed observations of the flow deformation and fractures revealed the relationship between the quenched‐in crystalline and mechanical behaviour.     

Properties and structures of Co-based metallic glasses

A fundamental study on Co-based metallic glasses has been conducted. The study focused on understanding the changes of the properties and structures of an Fe-B-Si glass as a function of Co, Co-Ni, Co-Mn, and Co-Ni-Mo additions. The separate addition of Co, Co-Ni, Co-Mn, and Co-Ni-Mo elements was successful in such a way that four new metallic glasses were produced. The compositions of the new glasses are Fe₆₆Co₁₈B₁₅Si₁, Co₆₆Fe₄B₁₄Si₁₅, Co₇₆Fe₂Mn₄B₁₂Si₆, and Co₆₉Fe₄Mo₂B₁₂Si₁₂. Consequently, an evaluation of the physical and magnetic properties was determined. Furthermore, the internal and surface structures of the glasses have been characterized by a transmission electron microscope (TEM), and a scanning tunneling microscope (STM), respectively. A comparison between the internal and surface structures of the glasses was carried out on both amorphous and crystalline forms. As a result, a correlation between the properties and structures of the glasses is established.     

High-resolution transmission electron microscopy of inorganic materials in cellular and topological random systems

The structure of amorphous Se films, in the topological random system, has been studied by the computer-simulation, electron diffraction (ED), and high-resolution transmission electron microscopy (HRTEM). As an example of HRTEM of the cellular random system, our recent investigation on Ba-ferrites is reviewed beforehand. In HRTEM images of spin-glass BaTi_2–xSn_xFe₄O₁₁ (x < 0.6), magnetic clusters or clusters of FeO₆-octahedra surrounded by TiO₆-octahedra have been found. The structure of BaSn₂Fe₄O₁₁ (x = 2) has been determined. The ordering of Sn (Ti) and Fe ions increases with increasing x, which interprets the change from the spin-glass state to nonspin-glass state at x = 0.6. It is shown that a cluster of polarized lattice ions is detectable in images of polar-glass Ba_xK_2–xFe_xTi_6–xO₁₃ (x ≥ 1.2). The structure models of amorphous Se films forming on a substrate have been constructed on a computer, and their radial distribution function (RDF) and HRTEM images have been calculated. Experimental RDF and HRTEM images have been obtained from vacuum-deposited amorphous Se films and are compared with the calculated ones. It is concluded that in the as-deposited films most of Se molecules may be composed of Se atoms as small as three and that by the electron-beam irradiation the molecules link to form spiral chains in amorphous state and then arrange to have the hexagonal crystal structure.     

High resolution electron microscope observations of microstructures in A15 type Nb3X superconductors

Microstructures of superconducting Nb₃X(A15) compounds are studied by means of high resolution electron microscopy. Structure images are obtained with the 400kV high resolution electron microscope for both the annealed and ion-irradiated crystals. The images obtained from the annealed crystals show bright contrast patterns similar to the projections of Nb and X atoms comprising the A15 structure. Defects observed in the annealed crystal irradiated with 40 kV Nb⁺ ions show characteristic features in contrast. The dependence of image contrast on the element X and the atomic structures of the defects are discussed by comparison with the calculated images.     

Heating effect of electron beam bombardment

The effect of heat in a target bombarded by an electron beam with certain energy was studied. A physical model was proposed according to the pattern of electron scattering, separately for a bulk specimen in electron probe microanalysis and a thin-film specimen in transmission electron microscopy. Then formulas of rising temperature were deduced for these two events. The results of a melting experiment of Bi-coating on glass and crystallizing of amorphous Fe₇₈Si₁₂B₁₀ film under electron beam bombardment confirmed the proposed model.     

Wear performance of C–W2B5 composite sliding against bearing steel

The C–W₂B₅ composite was fabricated by reaction hot-pressing of B₄C and WC powders. The effect of applied load on the friction and wear behavior was investigated by the block-on-ring test in air. In particular, the composition and microstructural characterization of the mechanically mixed layers (MML) and wear debris formed during sliding wear were studied using scanning electron microscopy (SEM) with EDX and X-ray diffraction (XRD). The results revealed that the friction coefficients and wear rates of the composite increased with increasing testing load. The MML and wear debris generated from the sliding system had similar microstructural features and were composed of a mixture of ultrafine grained structures which mainly consisted of W₂B₅, Fe and Fe₂O₃. The formation mechanism of the MML and their influences on wear mechanism of the multiphase material were also studied on the basis of the microstructural observations.     

Influence of Heat Treatment on Microstructure and Sliding Wear of Thermally Sprayed Fe-Based Metallic Glass Coatings

Fe₆₂Ni₃Cr₄Mo₂W₃Si₆B₁₇C₃ amorphous coatings were thermally sprayed by a high velocity oxygen fuel spraying system (DJ-2700) and heat-treated at the temperatures ranges from 873 to 1,173 K in vacuum for 1 h. Differential scanning calorimetry, X-ray diffraction (XRD), and scanning electron microscopy were used to study the microstructural characteristics of the coatings. Vickers hardness tester was used to measure the hardness of the coatings. At the same time, the sliding wear behavior of the coatings was evaluated in a reciprocating ball-on-disk system. Within the resolution of XRD, amorphous structure without apparent crystalline phases was obtained in the as-sprayed coating. The heat treatments above 873 K led to the crystallization of amorphous phase. With the increase of heat treatment temperature, diffusion and sintering could occur between the layers of the coatings. The highest microhardness was obtained in the coating heat-treated at 973 K. When wear tested at a relative low load of 2 N, a direct correlation between the hardness and wear resistance of the coatings seems to be reasonable. However, at relative high loads, the wear resistance of the coatings is dependent on the resistance to crack initiation and growth between the layers rather than the hardness.     

Structure and composition analysis of silicon aluminium oxynitride polytypes by combined use of structure imaging and microanalysis

The relationship between the chemical composition and the crystal structure of the 12H and the 15R polytypes of silicon aluminium oxynitride (sialon) is studied by combined techniques of crystal structure imaging and spectroscopic microanalysis using a new 400 kV high-resolution analytical electron microscope. The chemical compositions of 12H and 15R are determined to be SiAl₅O₂N₅ and SiAl₄O₂N₄, respectively, from quantitative analysis with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy. The space groups are determined uniquely to be non-centrosymmetric P6₃mc and R3m for the 12H and the 15R, respectively, by observing symmetries appearing in both convergent beam electron diffraction patterns and structure images. The structure images observed are interpreted on the basis of calculated images for proposed structure models, in which the structures consist of stacking sequences of the octahedral and the tetrahedral layers. The structure models are confirmed by calculation of the X-ray diffraction pattern.     

Quantification of the modulated structures in TiPdCr alloys

Ti₅₀Pd_(50-x)Cr_xβ₂ alloys (B2, CsCl-type crystal structure) have been investigated as part of an overall study of the systematics of phase transformation behaviour and also for the commercial applicability of the shape memory effect at elevated temperatures. The phase transformations are unique in the sense that electron diffraction and high-resolution transmission electron microscopy of Ti₅₀Pd_(50-x)Cr_x alloys with x between 6 and 10 at.% reveal {110}_B2 modulated structures with periodicities between 3 and 4·5 planes that vary linearly with composition. Satellites along certain <110>* lying at incommensurate positions are observed in electron diffraction patterns and are consistent with transverse lattice displacements. High-resolution transmission electron microscopy (HRTEM) imaging usually reveals sinusoidal displacements; however, a few images appear ‘banded’ at three- and four-plane intervals suggesting the possibility that three- and four-plane commensurate packets combine to give the overall incommensurate satellite spacing. With the use of a newly developed technique for quantifying atomic displacements from HRTEM images, modulated phases in Ti₅₀Pd₄₃Cr₇ and Ti₅₀Pd₄₂Cr₈ alloys, where atoms are shifted from the cubic lattice sites by a transverse lattice displacement wave, have been studied. The real space displacements of the modulated structures were determined and compared to truly incommensurate and varying three- and four-plane wave packet combinations by examination of the goodness of fit as assessed by the correlation coefficient for a least-squares fit. In the cases of the Ti₅₀Pd₄₃Cr₇ and Ti₅₀Pd₄₂Cr₈ alloys, the incommensurate sine wave exhibited much better matching than the commensurate packets, indicating that these modulations are probably incommensurate.     

Diffraction effect of x-rays on the accuracy of quantitative analysis of crystals

In order to understand an apparent discrepancy of concentration of constituent elements in the quantitative analysis of thin KCl crystals with an energy dispersive X-ray analyzer, which has been partly explained by fluorescent X-ray excitation, more detailed experiments were carried out on binary alkali halides and tricomponent amorphous material by changing the azimuthal angle. Intensity ratios of X-rays from the constituent elements in KCl and KBr single crystals varied considerably according to the rotation of the specimens. The observed differences between maximum and minimum intensity ratios were 15% for a thin KCl crystal about 100 nm thick and 20% for a bulk KCl crystal, which corresponded to 4.1% and 4.5% differences in atomic concentration, respectively. It was ascertained that for amorphous materials such as Co₈₇Zr₅Nb₈, such a variation of the intensity ratio of X-rays was not observed. It is thus proved that the variations of the intensity ratios of X-rays from the constituent elements with azimuthal angle for KCl and KBr are attributed to the diffraction effect of characteristic X-rays generated in crystalline specimens. This effect, which is the same as that in the production of Kossel patterns, is one of the essential factors limiting the accuracy of the analysis.     

Electron and light microscopy studies on the domain structures of Zn3B7O13Cl, Zn3B7O13Br and Zn3B7O13I ferroic boracites

The domain structures of Zn₃B₇O₁₃Cl, Zn₃B₇O₁₃Br and Zn₃B₇O₁₃I boracite single crystals were studied by means of polarized light in conjunction with electron microscopy. Single crystals of the three compositions were grown by chemical transport reactions in closed quartz ampoules, at a temperature of 900 °C and were examined by polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For both PLM and SEM, the same as‐grown samples were used without having to resort to metallization of the crystal faces. For TEM the single crystals were crushed and mounted on holey carbon films. Comparative electron microscope images were useful for revealing the domain structure of these ferroelectric/ferroelastic materials previously observed between the crossed polars of an optical microscope. X‐ray diffraction analysis of the pulverized crystals was performed for this triad of halogen boracites containing zinc as a common metal.     

Transmission electron microscopy study of crystallization in Fe-Si–B–Cr–C amorphous alloy

The crystallization behaviour of Fe₇₂Si_9.1B_14.8Cr_2.2C_1.9 alloy was studied. Three forms of the examined alloy were studied: water‐atomized powder, as‐spun ribbon and fully crystallized ribbon. Transmission electron microscopy studies of the examined alloy in the form of powder revealed partial crystallization of Fe₂B. The as‐spun ribbon was found to be fully amorphous, and no evidence of any crystalline phases was detected. Formation of metastable phases in the fully devitrificated ribbon was observed. The examined alloy, heated to above its crystallization temperature, consisted of α‐Fe₃(Si), Fe₂₃(C,B)₆, Fe₃B and Fe₂B crystalline phases.     

Dry-sliding Tribological Properties of Nano-Eutectic Fe<Subscript>83</Subscript>B<Subscript>17</Subscript> Alloy

This paper reports the tribological performance of the nano-eutectic Fe₈₃B₁₇ alloy under dry sliding against Si₃N₄ ceramic ball in ambient environment with varying applied loads and sliding speeds. Worn surfaces of the nano-eutectic Fe₈₃B₁₇ alloy were examined with a scanning electron microscope (SEM) and an X-ray energy dispersive spectroscope (EDS). The wear debris of the samples were also analyzed by X-ray diffractometer (XRD). The wear rate of the nano-eutectic Fe₈₃B₁₇ alloy was of the magnitude of 10⁻⁴ mm³/m, which was lower than that of the coarse grained Fe₈₃B₁₇ alloy. The friction coefficient of the nano-eutectic Fe₈₃B₁₇ alloy was almost the same as that of the coarse grained Fe₈₃B₁₇ alloy. The Fe₂SiO₄ oxide layer was formed on the worn surface of the nano-eutectic Fe₈₃B₁₇ alloy. However, on the worn surface of the coarse grained Fe₈₃B₁₇ alloy was found only a little Fe₂SiO₄. These results demonstrated that the nanostructure improved the wear resistance of the Fe₈₃B₁₇ alloy, but did not significantly affect the friction coefficient. The wear mechanism of the nano-eutectic Fe₈₃B₁₇ alloy was delamination abrasion mainly.     

Electron crystallography at atomic resolution: The structure of the odd-chain paraffin n-tritriacontane

The crystal structure of the odd-chain paraffin, n-tritriacontane, nC₃₃H₆₈, is determined directly by using low-dose electron microscope images and electron diffraction intensity data from epitaxially grown microcrystals. Phases of the most intense “polyethylene” reflections are determined from triplet structure-invariant relationships often used in X-ray crystallography. Low-dose electron microscopic images provide phases of the low-angle “lamellar” reflections and these can be used with one-dimensional structure-invariant relationships to determine other phases on the 00? reciprocal row. The phase set is sufficient to calculate an electrostatic potential map which is directly interpretable as a structure image at atomic resolution.