Crystallization acceleration of amorphous Fe40Ni40P14B6 alloy by fluxing technique

The effect of fluxing on the structure and the crystallization of amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy has been studied. Subjected to fluxing, the incubation time upon isothermal crystallization decreases, whereas, the onset crystallization temperature upon non-isothermal crystallization (with constant heating rate) decreases, and crystallization peaks become less sharp. via structural characterization, it is considered that fluxing promotes relaxation of the system; the atomic structure becomes more similar to the corresponding crystallized phase, thus alleviating the transient effect on nucleation and accelerates the crystallization.     

Solid and liquid thermal expansion and structural observations in the quasicrystalline Cd84Yb16 compound

The structure of single-grain Cd₈₄Yb₁₆ samples aligned along the twofold and fivefold axes has been followed from 300 to 1050?K using high-energy synchrotron X-rays. The quasicrystal phase is stable up to its melting temperature of 914?K and has a large linear thermal expansion of 37.1?ppm?K^?1 over this temperature range. The samples melt congruently over a temperature range of less than 1?K. The liquid is 7% less dense than the solid and, upon cooling from the melt, the quasicrystal phase directly solidifies within a 1?K interval. The amount of undercooling achieved, about 5–25?K, was dependent on the cooling rate. The total scattering function of the liquid is consistent with a dilute liquid Cd structure. These results agree with suggestions that the structure of the liquid must undergo reordering in order to form the solid phases. However, there is no compelling evidence for icosahedral short-range order in the liquid prior to the formation of the quasicrystalline structure.     

Nucleation of the C40-to-C11 b transformation in magnetron-sputtered MoSi 2 thin films

Transmission electron microscopy has been used to reveal the microstructure of metastable C40 MoSi 2 thin films produced by annealing amorphous magnetron-sputtered deposits at 700°C. The films contain nanoscale acicular grains elongated parallel to (0001), with extensive basal faulting. The faults are intrinsic with R ´ 1/3[0001] and correspond to thin slabs of the equilibrium C11 b phase. It is proposed that these faults may act as nuclei for the subsequent transformation from C40 to C11 b by a process akin to discontinuous coarsening.     

Microstructural features of phase transformation in a binary Pd–Si metallic glass

Glassy ribbons of Pd–Si alloys were prepared by a combination of melt spinning and flux treatment. The crystallization behaviour of a Pd₈₁Si₁₉ glassy alloy was studied through isothermal annealing at temperatures ranging lower than the glass-transition temperature T _g to around the onset of crystallization. The evolution of microstructures arising from isothermal annealing was investigated by X-ray diffraction (XRD) and (high-resolution) transmission electron microscopy ((HR)TEM). XRD spectra showed that, after the sample was annealed at a sub-T _g temperature, its first diffraction peak was split into two overlapping broad peaks. TEM analysis revealed the formation of a spherical, particle-like glassy phase embedded in the glassy matrix together with a finely connected network morphology within both. Combining these observations with compositional analysis suggested that phase separation had taken place during sub-T _g annealing. When the glassy alloy was annealed at temperatures higher than T _g, nanocrystalline structures, composed of Pd₃Si and Pd phases plus a Pd₉Si₂ phase with a lamellar structure, was formed.     

Formation of FeNi with L10-ordered structure using high-pressure torsion

FeNi with the L1₀-ordered structure is formed over an astronomical timescale in meteorites. In this study, severe plastic deformation using high-pressure torsion (HPT) is employed for the production of the L1₀ structure in the laboratory. Its formation is confirmed by X-ray diffraction analysis and transmission electron microscopy. Processing of elemental nanopowders by HPT is an effective method for the formation of the L1₀ phase, which is enhanced by the addition of Co to FeNi or annealing after HPT. The formation of the phase must be associated with enhanced diffusion through HPT.     

Formation of ω -Al7Cu2Fe phase during laser processing of quasicrystal-forming Al–Cu–Fe alloy

The formation of an ω-Al₇Cu₂Fe phase during laser cladding of quasicrystal-forming Al₆₅Cu_23.3Fe_11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an α-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations.     

High-resolution characterization of the precipitation behavior of an Al–Zn–Mg–Cu alloy

The metastable particles in an Al–Zn–Mg–Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin η′ plates were formed with a thickness of seven atomic planes parallel to the {111}_Al planes. The five inner planes of the η′ phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the η phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the η′ models previously set forth by other authors. Clear structural similarities between η′ and η were observed, suggesting that drawing distinctions between η′ and η phases may not be necessary or useful.     

A new counter-example to Kelvin's conjecture on minimal surfaces

A survey of the published literature on undercooled metallic and oxide melts suggests that phase selection during solidification can be categorized as nucleation controlled or growth controlled. Common characteristics governing the phase-selection pathway have been identified for various alloy systems. It is recognized that when competing stable and metastable phases share the same crystalline characteristics and have comparable interface kinetic coefficients, the principle of nucleation control applies for primary phase formation in a deeply undercooled melt. However, there can be a difference of two or three orders of magnitude in the interface kinetic coefficients for competing phases, either between an ordered intermetallic compound and a disordered solid solution, or between a crystalline phase with a high level of complexity and a simple crystal. In such cases, the principle of growth control will apply; more specifically, the phase with the faster growth kinetics should be favoured and the competing counterpart with sluggish interface kinetics should be suppressed at high undercoolings. Some simple predictions are suggested on the basis of this principle when considering stable and metastable phase diagrams. The specific conditions under which the present categorization is applicable are outlined. Future work is required to elucidate phase competition under conditions of very rapid solidification.     

Formation of yittrium oxide in cemented carbides

Rare-earth (RE) elements are helpful in improving the mechanical properties of cemented carbides. In this work, the formation of Y₂O₃ in functionally graded cemented carbides is investigated using transmission electron microscopy. The results indicate that Y₂O₃ can stabilize the Co phase with the face-centred cubic structure (α-Co), since it has a cubic structure similar to the high-temperature Co phase. The α-Co phase has a higher plasticity than ε-Co, and this suggests a possible mechanism for the role of RE elements in toughening cemented carbides.     

The crystallographic structure of λ transition-alumina compared with the periodic antiphase boundary structure of a Ga2O3-rich Mg gallate

One of the high-temperature metastable phases found in the MgGa₂O₄–Ga₂O₃ system is the ε_Mg non-stoichiometric Mg gallate. It has a one-dimensional periodic antiphase boundary (PAPB) structure based on the spinel structure. The APBs are parallel to the {3?1?0} planes and the APB vectors are of the 1/4?1 1 0? type when referred to the spinel structure. This generally leads to the existence of 12 monoclinic twin variants in the studied samples. By comparing the X-ray or electronic diffraction patterns of both phases, it will be shown that the recently discovered?λ?transition-alumina has the same type of structure.     

Formation of ductile Al-based metallic glasses without rare-earth elements

Al₇₅Cu₁₇Mg₈ is a eutectic composition according to the ternary phase diagram, which can be quenched into a fully amorphous phase by adding 2-8at.% Ni, but the addition of a similar percentage of Gd failed to form the amorphous phase. The amorphous alloys obtained exhibit two broad diffuse peaks in the X-ray diffraction curves and, correspondingly, two halo rings in the electron diffraction patterns, implying that two types of local atom configuration exist. Thermal analysis of the amorphous alloys indicates that the primary crystallization peak shifts to higher temperatures with increasing Ni content. The occurrence of a nucleation and crystal growth peak during isothermal crystallization reveals the amorphous nature of the quenched ribbon alloys. The quenched amorphous ribbons do not break after bending by 180°. Mechanical testing yielded a tensile strength of 810MPa for (Al₇₅Cu₁₇Mg₈)₉₅Ni₅, and a vein structure, characteristic of amorphous fracture, is apparent in scanning electron micrographs. The different effects of Ni and Gd on the glass formation indicate that the large atomic size of Gd is not critical to the glass formation.     

Planar faults in MoSi2 single crystals deformed at high temperatures

Abstract

Dislocation structure and planar faults have been examined in MoSi₂ single crystals deformed at high temperatures. Pure stacking faults were found in a crystal deformed at 900°C. The formation of the stacking fault is closely related to the phase stability of the C11_b and C40 ordered structures. Profuse stacking faults with increasing deformation temperature assist the ductility improvement of the MoSi₂ above about 1200°C. The critical resolved shear stress for {110}(331) and {013}(331) slip is determined in the temperature range 1000 to 1500°C.     

Microstructure and mechanical behavior of nanostructured composite Cu60Fe40 alloy

In this study, bulk nanostructured composite Cu₆₀Fe₄₀ alloy is prepared by a combustion synthesis technique. The prepared Cu₆₀Fe₄₀ alloy consists of Cu(Fe) solid solution and Fe(Cu) solid solution phases. The large-scaled compositional segregation in the Cu-rich and Fe-rich phases is not observed, respectively. A few micron-sized dendrite (Fe(Cu) solid solution) is embedded into the nanostructured matrix (Cu(Fe) solid solution). The grain size of the matrix is in the range 50–300?nm. The yield and fracture strength of the Cu₆₀Fe₄₀ alloy are 540 and 1050?MPa, respectively, and the fracture strain obtained from the compression test is about 20.9%. The Cu₆₀Fe₄₀ alloy displays notable work hardening in the compressive deformation.     

Microstructural characterization of the M23C6 carbide in a long-term aged Ni-based superalloy

The microstructural features of M₂₃C₆ carbide in a long-term aged heat- and corrosion-resistant Ni-based superalloy have been investigated in detail using various kinds of transmission electron microscope (TEM) techniques. It is found that TEM contrast, which is related to structural and chemical inhomogeneities inside the grains, always exists in the interior of grains in the alloy. The structure of these inhomogeneous regions has been determined to be the same as that of the γ′ and t-M₂₃C₆ phases, where t-M₂₃C₆ indicates a transitional and metastable phase. Although possessing the same structure as the M₂₃C₆ phase, the chemical composition of the t-M₂₃C₆ is different from that of the M₂₃C₆ phase. Compared with M₂₃C₆, t-M₂₃C₆ is richer in Ni, Co, Al and Ti but poorer in W, Mo and Cr. This phenomenon of structural and chemical inhomogeneity demonstrates that pristine M₂₃C₆ carbide (p-M₂₃C₆) precipitated in standard heat-treated samples is unstable. Therefore, upon long-term ageing treatment, Ni, Co, Al and Ti may locally enrich inside the p-M₂₃C₆ phase, finally forming the γ′ phase, which can be described by the decomposition reaction p-M₂₃C₆ → M₂₃C₆ + γ′.     

ω-phase formation in a rapidly solidified Cr-40 at.% Al alloy

The diffuse ω structure has been identified by electron diffraction for the first time in a C11_b matrix of the melt-spun Cr-40 at.% Al alloy ribbon. The C11_b matrix consists of nanometre-scale ordered domains produced by a long-period chemical ordering in the precursor A2-B2 structure. Each C11 _b unit cell in the Cr-40 at.% Al alloy is based on three cubic B2 cells along the c axis and contains two antiphase boundaries. The structure of the diffuse ω is consistent with that observed by De Fontaine. The relative stability of the crystalline ω structure has been predicted with respect to bcc-type precursor phases as a function of the displacement parameter z from calculation of the cohesive energy of the ω phase using the full-potential linear muffin-tin orbital method. The results show that the three-dimensional crystalline ω structure is stable with respect to bcc-type precursor phases in the Cr-Al system and reveals the physical background as to why the ω structure in Cr-40 at% Al is diffuse in nature.     

Morphological and magnetic response of copper-substituted nickel ferrite nanoparticles

Ferrite nanoparticles are interesting materials owing to their unique physical and chemical properties. The metal-doped ferrites have well-defined structures and magnetic response, such as high permeability for a specific frequency range. In this study, copper-substituted nickel ferrite (Ni_1?xCu_xFe₂O₄) nanoparticles with a compositional range of 0?≤?x?≤?0.3 were synthesised through a co-precipitation technique. Metal chlorides were used as precursors and NaOH as a precipitating agent for the growth of ferrite nanoparticles. To minimise the internal stresses and maximise the magnetic response, ferrite nanoparticles were annealed in a furnace at 700°C for 6 h. The structural and magnetic response of Ni_1?xCu_xFe₂O₄ ferrite with different values of x were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM) and X-ray Diffraction (XRD) techniques. XRD analysis confirmed the formation of cubic spinel structure of single phase for all the compositions. The lattice constant decreased with increase in the value of x. FT-IR study showed two main metal oxygen bonds in the range 500–700 cm^?1 confirming the formation of a single-phase cubic inverse structure of Cu-substituted Ni ferrite. VSM results revealed the formation of ferrimagnetic nanoparticles. The optical and magnetic response of the ferrite nanoparticles changed with Cu content.     

Effect of local chemistry,structure and length scale of heterogeneities on the mechanical properties of a Ti45Cu40Ni7.5Zr5Sn2.5 bulk metallic glass

We report on distinct variations in local chemistry, structure and length scale of heterogeneous regions in Ti₄₅Cu₄₀Ni_7.5Zr₅Sn_2.5 fully glass rods of different diameters, i.e. rods subjected to different cooling rates. The present investigations indicate that the mechanical properties of the Ti₄₅Cu₄₀Ni_7.5Zr₅Sn_2.5 bulk metallic glass can be modified within a wide range of strength and plastic deformability by controlling the scale of the heterogeneous regions in the glass through appropriate variation of the cooling rate applied for solidification.     

A first-principles study of the formation of θ′′ phase in Al–Cu alloys

The θ′′-Al₃Cu phase plays an important role in the precipitation process of Al–Cu alloys. This phase has a sandwich structure—every two {200}_Cu layers are separated by three {200}_Al layers. To analyse the formation mechanism of this structure, the elastic strain energy of the {200}_Cu and {200}_Al layers, and the chemical bonding energy that reflects the interaction between the electrons in Cu and neighbouring Al atoms are calculated and analysed by first-principles calculations, projected density of states and Bader analysis. Our computation results reveal that this sandwich structure is energetically preferred in the competition of elastic strain and chemical bonding energies. To minimise the elastic strain energy of {200}_Al and {200}_Cu layers, the {200}_Cu layers prefer being apart from each other, whereas the chemical bonding energy favours the opposite arrangement because the intermetallic bond between Al and Cu atoms may form through p-d hybridization.     

Identification of a new tetragonal phase in the Nb-Ti-Al system

During the course of investigation of the phase equilibria in the Nb-Ti-Al system, a new phase, designated theta, was observed in three Nb-Ti-40at.%Al alloys following ageing at low temperatures (less than 1000 C). Using X-ray diffraction and transmission electron microscopy techniques, this phase was determined to have a bct structure, with lattice parameters a₀ = 5.106 A and c₀ = 28.168A, a space group of I 4₁/amd (number 141) and composition near 45at.%Nb25at.%Ti-30at.%Al. An orientation relationship between the new tetragonal phase and the gamma-TiAl phase of [101]_gamma//[120]_theta and (111)_gamma//(001)_theta was observed within the microstructure.     

Site preference and alloying effect of tungsten in the μ phase of Co7Mo6

The electronic structures of the μ phase of Co₇Mo₆ were investigated by means of first-principles calculation based on the density functional theory. The calculated equilibrium lattice parameters of Co₇Mo₆ are in good agreement with the experimental results. The results of calculation reveal that the addition of tungsten (W) promotes the stability of the μ phase of Co₇Mo₆ and W tends to participate in its formation. Through analysing the calculated electronic structure (partial density of states and charge density difference), it can be noted that a strong interaction exists due to d–d hybridization. Also, the majority of the atoms may contribute their valence electrons to the formation of metallic bonds in binary and ternary systems. Furthermore, the calculated results show that the doped W atom prefers to occupy the Co site in Co₇Mo₆ μ phase.