In situ and real-time analysis of TGZM phenomena by synchrotron X-ray radiography

The development of brilliant third-generation synchrotron X-ray sources, together with advances in X-ray optics and detectors, has provided timely efficient tools for in-depth understanding of physical phenomena in a broad spectrum of situations. Synchrotron X-ray radiography enables in situ and real-time observation of microstructure evolution, i.e. a direct access to dynamical phenomena which could not be anticipated from post-mortem analysis. Dedicated experiments are carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) in Al-based alloys to study the dynamics of temperature gradient zone melting (TGZM) phenomenon. TGZM occurs when a liquid–solid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. The thorough characterisation of both the initial solid during the thermal stabilisation phase prior to solidification (static TGZM) in Al–3.5 wt% Ni alloy and the dendritic microstructure in the later stage of solidification in Al–7.0 wt% Si alloy is performed. Based on experimental observations, quantitative data (in particular liquid-migration velocity) are measured and a very good agreement is found with theoretical analysis.     

Eutectic structures in the Ni–Co–Cr–Al system obtained by plasma spraying and by Bridgman growth

Formation of a regular fibrous two-phase microstructure was found in low-pressure plasma spray (LPPS) deposited Ni–Co–Cr–Al–Y coatings by transmission electron microscopy and analyzed by energy dispersive spectroscopy. The structure is compared to aligned lamellar three-phase structures of an Ni–Co–Cr–Al alloy obtained on Bridgman growth under slow unidirectional solidification (UDS) conditions. The composition of the Ni–Co–Cr–Al alloy for UDS experiments has been identified by DTA. The conditions for the formation of both the LPPS and the UDS structures are discussed.     

Effect of a vertical magnetic field on the dendrite morphology during Bridgman crystal growth of Al–4.5 wt% Cu

The effect of a vertical high magnetic field (up to 10 T) on the dendrite morphology has been investigated during Bridgman growth of Al–4.5 wt%Cu alloys experimentally. It is found that the field causes disorder in dendrites and their tilt in orientation. Along with the increase of the magnetic field and decrease of the growth velocity, the dendrites became broken and orientated in 1 1 1 along the direction of solidification instead of 1 0 0. The field also enlarged the primary dendrite spacing and promoted the branching of the dendrites to form high-order arms. Above phenomena are attributed to the thermoelectromagnetic convection effect and orientation caused by the high magnetic field.     

High-velocity banding structure in the laser-resolidified hypoperitectic Ti47Al53 alloy

Stability analysis of a growing solid/liquid interface is the fundamental concept of modern solidification theory. Here, serial laser rapid solidification experiments were performed on a hypoperitectic Ti₄₇Al₅₃ alloy to explore the dendritic growth behavior near the limit of high-velocity absolute stability. SEM and TEM techniques were carried out to investigate the microstructure and identify the phase composition. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly and high-velocity banding structure was firstly detected in Ti–Al peritectic alloys. The high-velocity banding structures are parallel to the solid/liquid interface (normal to the growth direction) and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In light bands with cellular growth mode, all dislocation assembles are parallel to the growth direction and forms the cell boundaries, while all dislocation distributes randomly in dark bands. The determined growth velocity range for the appearance of high-velocity banding structures is about 0.51.1 m s⁻¹ according to the rapid solidification experiments, and the origin of the banding agrees well with the prediction of the CGZK phenomenological model (Acta Metal. Mater. 40 (1992) 983).     

Bi20TiO32 nanocones prepared from Bi–Ti–O mixture by metalorganic decomposition method

Bi₂₀TiO₃₂ in the form of nanocones are reported for the first time, which have been found during the formation of Bi₂Ti₂O₇ nanocrystals. Bi₂₀TiO₃₂ nanocones were prepared by metalorganic decomposition technique. From X-ray patterns, it was found that Bi₂₀TiO₃₂ is a metastable phase, and can transform gradually into Bi₂Ti₂O₇ phase with the annealing time increasing at a temperature of 550°C. The image of field emission scanning electron microscopy shows that the lengths of the nanocones are up to several micrometers and the diameters of cusps range from 20 to 200 nm. The studies of transmission electron microscopy show that the nanocones are crystalline Bi₂₀TiO₃₂. The growth mechanism of Bi₂₀TiO₃₂ nanocones has been proposed, which is similar to the vapor–liquid–solid growth mechanism.     

Synthesis of calcium carbonate polymorphs in the presence of polyacrylic acid

Calcium carbonate precipitates are prepared from a solution of CaCl₂ and K₂CO₃ in the presence of polyacrilic acid. The effect of polyacrilic acid incorporation in the [25–80 °C] temperature range on crystal morphologies and CaCO₃ precipitated polymorph concentrations are investigated using scanning electron microscopy and X-ray diffraction quantitative microstructural and phase analysis. Large changes in morphology and phase proportions are observed in the presence of polyacrylic acid, which strongly depend on the solution temperature. While crystallization of vaterite is favoured in the presence of polyacrilic acid up to 50 °C, it is largely destabilized at higher temperatures. Our process also enables the elaboration of particles in the range 10–20 nm.     

Preparation of zirconia mullite flakes via a plasma rapid solidification process using starting materials derived from a sol–gel technique

Flakes of zirconia–mullite with different zirconia contents varying from 3 to 24 wt% were produced from sol–gel derived raw materials via a plasma melting method followed by a rapid solidification process using a rotating copper roll. The morphology, phase constitution and microstructure development of the as-prepared flakes and of the flakes after various heat treatments were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). It was found that the starting materials could be transformed from the initial irregular-shaped powders into flakes which consisted of mullite, zirconia phase, a relatively large amount of glassy phase and pores. Using TEM, it was shown that the crystalline phases consisted of zirconia polymorphs and mullite. The glassy phases consisted of Al₂O₃–SiO₂ solid solution supersaturated with zirconia. Firing at 1500 °C or 1700 °C resulted in full crystallisation of the flakes and a fairly homogeneous distribution of zirconia particles in terms of size and shape dispersed in the mullite matrix, which contained both intra-granular and inter-granular precipitates. The microstructural characteristics of the flakes may provide promising physical properties for applications in high temperature thermal insulation materials.     

Physical properties of Zn‐ 22 wt.% Al‐ x wt.% Ce melt spun eutectoid alloy

Zn‐ 22 wt.% Al (Zn ‐ 40 Al in atomic%) eutectoid alloys with different Cerium (Ce) contents of 0, 1, 2, and 6 (in wt.%), 0.35, 0.70 and 2.1 (in atomic%) were rapidly solidified by melt spun technique. The effects of high cooling rate and alloying element (Ce) on microstructure of the studied alloys were analyzed by X‐ray diffractometry (XRD), scanning electron microscopy (SEM) and electrical resistance measurements. The results showed that the dendrites as well as grains size were refined by the additions of Ce. The main phases in melt spun alloys were α‐Al and η‐Zn, in addition to intermetallic CeZn₅ and Al₄Ce. Additional metastable intermetallic Al_0.71Zn_0.29 phase has been observed only for melt spun alloy of 6 wt.% Ce content. XRD peaks of melt spun alloys demonstrated a considerable broadening with percentage of Ce due to the grain refinement and lattice distortion. Moreover, increase of Ce content results in a decrease of Al lattice constant which could be related to formation of supersaturated solid solution of Zn and/or Ce in α‐Al. Crystallite size of all phases were in the range of nanometer scale which reflects the role of the high cooling rate and the existence of Ce as alloying element for producing nanocrystalline structure. Resistance measurements of melt spun alloys show that the relative resistance rate for the alloys of higher Ce content relaxed faster to lower value than that of lower Ce content. Electrical resistance and microstructure exhibit strongly Ce content dependence.     

Crystallization behavior of Ce70−xAl10Cu20Cox (x = 0, 1, 3, 5 at.%) amorphous alloys

The effect of Co addition (substituting for Ce) on crystallization behavior of Ce₇₀Al₁₀Cu₂₀ amorphous alloys has been investigated using X-ray diffraction (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The Co addition has an obvious effect on topological short-range ordering of Ce–Al–Cu–(Co) amorphous alloys. Moreover, the Co addition can slightly improve the thermal stability of Ce–Al–Cu based amorphous alloys. The 1 and 3 at.% Co additions do not obviously change the crystallization behavior of the Ce–Al–Cu–(Co) amorphous alloys, and the final crystallization products are FCC–CeAlCu(Co)O. However, the 5 at.% Co addition can alter the crystallization behavior of the Ce₇₀Al₁₀Cu₂₀ amorphous alloys. Proper content of Co can effectively suppress the formation of oxide phases during annealing of the Ce–Al–Cu–(Co) amorphous alloys.     

Effect of a premelted film on the dynamics of particle–solidification front interactions

Numerical simulations are performed to study the interaction of a solidification front with an embedded particle. A sharp-interface method is used to track both the phase boundary and the particle. The solidification front dynamics is fully coupled with particle motion. The main objective of the paper is to distinguish the role played by the premelted layer between the solidification front and the particle in determining conditions for particle engulfment. Results are obtained by assuming a premelted layer exists in the gap between the particle and the solidification front and compared to those assuming no premelted layer. In the absence of a premelted layer, arbitrary cut-off values for particle-front gap thickness need to be invoked in order to define the critical velocity for which the pushing–engulfment transition occurs. When a premelted layer is assumed to exist, the prediction of the critical velocity is determined solely from the dynamics of the coupled front–particle interaction. In addition, model predictions for the critical velocity based on a steady-state heat transfer analysis are shown to differ from that when the full dynamics of the phase boundary are taken into account.     

Self-catalytic branch growth of SnO2 nanowire junctions

Multiple branched SnO₂ nanowire junctions have been synthesized by thermal evaporation of SnO powder. Their nanostructures were studied by transmission electron microscopy and field emission scanning electron microcopy. It was observed that Sn nanoparticles generated from decomposition of the SnO powder acted as self-catalysts to control the SnO₂ nanojunction growth. Orthorhombic SnO₂ was found as a dominate phase in nanojunction growth instead of rutile structure. The branches and stems of nanojunctions were found to be an epitaxial growth by electron diffraction analysis and high-resolution electron microscopy observation. The growth directions of the branched SnO₂ nanojunctions were along the orthorhombic [1 1 0] and . A self-catalytic vapor–liquid–solid growth mechanism is proposed to describe the growth process of the branched SnO₂ nanowire junctions.     

The effect of solute on ultrasonic grain refinement of magnesium alloys

An experimental study has been conducted to assess the structural refinement of magnesium and its alloys by ultrasonic irradiation during solidification. It is shown that (i) ultrasonic irradiation leads to significant refinement only in the presence of adequate solute, which is alloy dependent; (ii) the attendant grain density increases linearly with increase in solute content at a given irradiation level; (iii) increasing the solute content at a low irradiation level above the cavitation threshold is more effective than substantially increasing the irradiation intensity; and (iv) the difference in the grain size between two ultrasonicated magnesium alloys is mainly determined by the solute content rather than the irradiation intensity. In view of these, the effect of ultrasonic irradiation on solute redistribution in a solidifying magnesium alloy seems rather limited even at a substantial intensity level such as 1700 W cm⁻². The implications of these findings are discussed and a mechanism is proposed to account for the experimental observations.     

Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets

Single crystalline Ce-doped ZnO hexagonal nanoplatelets are successfully synthesized. Zinc acetate, cerium nitrate, potassium hydroxide and poly vinyl alcohol were mixed together and transferred to a 100 mL Teflon-lined stainless steel autoclave kept at 150 °C for 24 h. The obtained precipitant is calcined at 600 °C. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanoplatelets have edge length 25 nm and thickness 11 nm. EDX result showed that the amount of Ce in the product is about 15%. Photoluminescence of these doped hexagonal nanoplatelets exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by Ce-doping. The growth mechanism of the doped hexagonal nanoplatelets was also discussed.     

Ion implantation effects on the microhardness and microstructure of GaN

We study the effect of N⁺ and O⁺ implantation on the microhardness and the microstructure of epitaxially grown GaN. The microhardness is measured using a Knoop diamond indenter while information on the effect of implantation on the surface morphology, microstructure and electronic structure is provided by atomic force microscopy, cross-section transmission electron microscopy and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It is demonstrated that implantation increases the surface microhardness. A possible mechanism for the surface hardening effect is based on the formation of N interstitials that pin the dislocations and prohibit the plastic deformation. In addition to the hardening effect, the implantation induced N interstitials introduce a characteristic resonance in the NEXAFS spectra, at 1.4 eV below the absorption edge.     

Study on as-cast microstructures and solidification process of Mg-Zn-Y Alloys

The microstructures and phases of as-cast Mg-Zn-Y alloys were investigated by means of scanning electron microscopy (SEM), energy-dispersive spectrum (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) in this study. Stable icosahedral Mg₃₀Zn₆₀Y₁₀ quasicrystals were obtained using common casting technique. With increasing Y content, the contents of quasicrystals increased, the distribution of quasicrystals was improved, and the morphologies of quasicrystal and other microstructures of Mg-Zn-Y alloys showed different characteristics. In addition, the solidification process of Mg-Zn-Y alloys was analyzed. Icosahedral Mg₃₀Zn₆₀Y₁₀ quasicrystalline phase is easy to nucleate in slow cooling.     

Influence of thermal stabilization on the solute concentration of the melt in directional solidification

Experiments on Al–25 at%Ni peritectic alloy consisting of melting followed by thermal stabilization ranging from 0 to 2 h were carried out in a Bridgman-type furnace. Temperature distribution, microstructure evolution and solute concentration in the mushy zone are characterized. An analytical model is proposed to evaluate the Ni concentration of the melt after thermal stabilization. Effect of temperature gradient and volume fraction of liquid phase in the mushy zone on the Ni concentration of the melt is discussed. The steady state Ni concentration of the melt is inappropriately below the initial Ni concentration of the sample. The deviation increases with decreasing temperature gradient. Finally, the influence of thermal stabilization on the solute concentration of the melt is discussed based on a comparison of Al–Ni peritectic alloys with Al–Ni hyper-eutectic alloys and Al–Cu hypo-eutectic alloys.     

Influence of ambient gases on the morphology and photoluminescence of ZnO nanostructures synthesized with nickel oxide catalyst

The morphology and luminescence properties of ZnO nanowires synthesized using NiO catalyst in a chemical vapor deposition system under different growth ambient have been studied. ZnO nanostructures were prepared in nitrogen, ammonia and hydrogen ambient and characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence. Growth in nitrogen ambient yields ZnO nanoneedles while growth with ammonia and hydrogen ambient ends up with ZnO nanowires. Presence of the Ni tip at the end in either morphology indicated the involvement of vapor–liquid–solid growth mechanism. Enhanced green emission in ZnO nanowires implies the presence of a high density of oxygen vacancies. Influence of the ambient gases on the morphology and optical properties of ZnO nanostructures is discussed.     

Crystal growth of Co46Ni27Ga27 ferromagnetic shape memory alloys with large single-variant set

The present work proposes a directional solidification method based on liquid melt cooling (LMC) technique to prepare large grain with single-variant set in Co–Ni–Ga alloys. The competitive growth from equaixed grains to steady columnar crystals with 1 1 0 orientation along the axis was observed. The directionally solidified rod has a uniform chemical composition. It can be also found that the unidirectional lamellar martensitic variants were well aligned in a whole grain, forming a single-variant state. Furthermore, the needle-like Ni₃Ga-type γ′ precipitates were formed in alloy with lower growth velocity, and it exhibited the complicated microstructural evolution. At the lowermost part of rod-like crystal, a large number of precipitates were dispersed both in grain interiors and at boundaries but its amount decreased when the columnar crystals were formed and gradually increased again from bottom up to top in the whole rod.     

Effect of Co substitution for Ni on the microstructure and magnetic properties of (Fe, Ni)-based amorphous alloys produced by melt spinning

Transmission electron microscopy with selected area electron diffraction, and X-ray diffraction were applied to study the effect of Co substitution for Ni in (Fe, Ni)-based amorphous alloys. The investigation was performed to obtain information on correlations between microstructure and magnetic properties of the (Fe, Ni, Co)-based amorphous alloys. The examination of the microstructure reveals that there are small crystallized free surface regions because the actual quenching rate is distributed inhomogeneously over the cross-section of the ribbon. Since in the free surface region, the solidification rate is lower, a spontaneous annealing process occurs at the top surface of the ribbon. The crystallization degree of the free surface region is higher for alloy ribbons that contain Co up to a 15 at.% concentration. Magnetic domains pattern are sensitive to the surface crystallization and Co content of the (Fe, Ni)-based alloy ribbons. Fine-scaled stripe domains were evidenced on the free ribbon surface while on contact surface stress domain pattern appeared. With the increase of the Co content, the domain width became small and long stripes appeared. The striped domains are responsible for an increased coercivity of the ribbons. However, there is a critical Co content (x_Co = 10) for which spontaneous narrow stripe domains are no longer more energetically favourable for ribbons with specific magnetic applications.