Solidification microstructure of as-cast Mg–Zn–Y alloys

Composites consisting of icosahedral (i) phase and ductile α-Mg can be fabricated by controlling the alloy composition. With increasing Mg content, the primary solidification phase changed from the i phase to the α-Mg phase and single eutectic structure can be obtained at a composition of Mg₇₂Zn_23.5Y_3.5. The i phase showed a variation in structural order from the well-ordered icosahedral phase to the 1/1 rhombohedra1 approximant with lattice constants a=27.2 Å and α = 63.43°. The structural change in the i phase can be explained by microdomain formation due to compositional change during solidification. Annealing treatment improves the structural order of the i phase by homogenization. By controlling the alloy composition, a single eutectic structure consisting of a hard i phase and a ductile α-Mg phase could be obtained.     

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185?°C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.     

Substitutional effect of Cr ions on the properties of Mg–Zn ferrite nanoparticles

The effect of Cr³⁺ substitution in Mg–Zn ferrite, with a chemical formula Mg_0.5Zn_0.5Cr_xFe_2−xO₄ (x=0.0–1.0), synthesized by a sol–gel auto-combustion reaction is presented in this paper. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), vibrating sample magnetometry (VSM), and DC resistivity. The XRD pattern revealed that the cubic spinel structure is maintained for the all the compositions. The particle sizes measured from XRD and TEM are in good agreement with each other. The cation distribution suggests that Mg²⁺, Cr³⁺ and Fe³⁺ have strong preference towards octahedral B-site. The theoretical lattice constant and experimental lattice constant match each other very well. The IR analysis supports the presently accepted cation distribution. The saturation magnetization decreases linearly with increasing Cr³⁺ content. Curie temperatures are obtained by the Laoria and AC susceptibility techniques. The dc resistivity has been investigated as a function of temperature and composition.     

Magnetic behavior of Mg–Al–Zn–Fe mixed oxides from precursors layered double hydroxide

Mixed oxides of Mg–Al–Zn–Fe were obtained by calcination of layered double hydroxides (LDH) prepared by coprecipitation reaction with hydrothermal treatment. The structural characterization of precursors and oxides was carried out by X rays diffraction, showing increases of ZnO phase with the increase of the zinc content. Magnetic behavior was studied by vibrating sample magnetometer (VSM) and by a superconducting quantum interference device (SQUID) showing both paramagnetic and super paramagnetic behavior depending on both particles size and composition.     

Characteristic chemical shifts of quasicrystalline Zn–Mg–Zr alloys studied by EELS and SXES

Chemical shifts of the constituent atoms of primitive icosahedral quasicrystal (P-QC), face-centred icosahedral quasicrystal (F-QC) and 1/1-approximant (1/1-AP) of F-QC Zn–Mg–Zr alloys were investigated for the first time using high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Zn M-shell and Mg L-shell excitation EELS spectra of P-QC, F-QC and 1/1-AP alloys, only the quasicrystalline alloys showed a chemical shift towards the larger binding energy side. In Zn-L and Zr-L emission SXES spectra, the P-QC and F-QC alloys showed a chemical shift towards larger binding energy side. The magnitudes of the shifts in the Zn-L emission spectra of the quasicrystalline alloys were almost the same as for ZnO. These results strongly suggest a decrease in valence charge in quasicrystalline states. Therefore, it should be concluded that bonding in quasicrystalline states involves a characteristic increase in covalency compared with bonding in corresponding approximant and standard metal crystals.     

In-situ study of precipitates in Al–Zn–Mg–Cu alloys using anomalous small-angle x-ray scattering

In the present work,the precipitate compositions and precipitate amounts of these elements(including the size distribution,volume fraction,and inter-precipitate distance) on the Cu-containing 7000 series aluminum alloys(7150 and 7085 Al alloys),are investigated by anomalous small-angle x-ray scattering(ASAXS) at various energies.The scattering intensity of 7150 alloy with T6 aging treatment decreases as the incident x-ray energy approaches the Zn absorption edge from the lower energy side,while scattering intensity does not show a noticeable energy dependence near the Cu absorption edge.Similar results are observed in the 7085 alloy in an aging process(120℃) by employing in-situ ASAXS measurements,indicating that the precipitate compositions should include Zn element and should not be strongly related to Cu element at the early stage after 10 min.In the aging process,the precipitate particles with an initial average size of ~ 8 ?A increase with aging time at an energy of 9.60 ke V,while the increase with a slower rate is observed at an energy of 9.65 ke V as near the Zn absorption edge.     

Influence of CuO and sintering temperature on the microstructure and magnetic properties of Mg–Cu–Zn ferrites

The synthesis of a series of Mg–Cu–Zn ferrites with the substitution of Cu for Mg has been obtained by solid-state reaction method. Microstuctural and structural analyses were carried out using a scanning electron microscope and X-ray diffraction (XRD), respectively. The lattice parameter is found to increase with increasing copper content. A remarkable densification is observed with the addition of Cu ions in the ferrites. Microstructural analyses indicate that CuO influences the microstructure of the ferrites by the formation of liquid phase during sintering. The grain size significantly increases with increasing copper content. Exaggerated grain growth is observed for the samples of x=0.25–0.35. The initial magnetic permeability (μ′) increases sharply with increasing concentration of Cu ions. This increase in μ′ is explained with the grain growth mechanism and enhanced densification of the ferrites. The resonance frequency of all the samples shifts toward the lower frequency as the permeability increases with Cu content. Sintering temperature T_s also affects the densification, grain growth and initial magnetic permeability of the samples.     

Contributions of different strengthening mechanisms to the shear strength of an extruded Mg–4Zn–0.5Ca alloy

The shear deformation behaviour of an extruded Mg–4Zn–0.5Ca alloy was studied using shear punch testing at room temperature. The extrusion process effectively refined the microstructure, leading to a grain size of 4.6 ± 1.4 μm. Contributions of different strengthening mechanisms to the room temperature shear yield stress, and overall flow stress of the material, were calculated. These mechanisms include dislocation strengthening, grain boundary strengthening, solid solution hardening and strengthening resulting from second-phase particles. Grain boundary strengthening and solid solution hardening made significant contributions to the overall strength of the material, while the contributions of second-phase particles and dislocations were trivial. The observed differences between calculated and experimental strength values were discussed based on the textural softening of the material.     

Modeling the TDFD dissolution of Al–Fe–Mn–Si particles in an Al–4.5Zn–1Mg alloy

Dissolution of large particles in DC-cast 7xxx aluminum alloys is one of the primary objectives of the homogenization process. A mathematical model to describe and predict this complex thermodynamical and kinetical process is of great significance. In this paper, the details of a diffusion-limited dissolution model, based on the thinning, discontinuation and full dissolution (TDFD) mechanism, to predict the dissolution of the Al₁₇(Fe_3.2, Mn_0.8)Si₂ particles is described. The model is capable of predicting the volume fraction and thickness of the particles during homogenization at different temperatures and time intervals. The predicted results are in good agreement with measurements using quantitative X-ray diffraction (QXRD) and quantitative field emission gun-scanning electron microscopy (QSEM). The model predictions of the supersaturation parameter, interface position, interface movement rate of the planar surfaces and the cylindrical edges, and the effect of the occurrence of discontinuities on the dissolution extent are presented.     

Effect of electropulsing treatment on microstructure and tensile fracture behavior of aged Mg–9Al–1Zn alloy strip

The effect of electropulsing treatment (EPT) on the microstructure, mechanical properties, and tensile fracture behavior of aged Mg–9Al–1Zn alloy strip at room temperature was investigated. The results indicated that EPT accelerated the spheroidizing and dissolution of β phase tremendously in the aged Mg–9Al–1Zn alloy strip. The EPT-induced microstructural change resulted in remarkably increasing elongation to failure, remained tensile strength unchanged. A mechanism for rapid spheroidizing and dissolution process of β phase during EPT was proposed based on the reduction of nucleation thermodynamic barrier and enhancement of atomic diffusion. Fracture analysis showed that with increase in frequency of EPT transgranular dimple fracture becomes predominant instead of the quasicleavage fracture.     

Micromechanical model for fracture toughness prediction in Al–Zn–Mg–Cu alloy forgings

An attempt has been made to model the plane-strain fracture toughness, K _Ic, in Al–Zn–Mg–Cu alloy forgings subjected to overageing. The proposed model, based on the multiple micromechanisms, reveals the quantitative relations between fracture toughness, fraction of all fracture modes and microstructural parameters associated with multiscale-sized second-phase particles and precipitate-free zones. The new model is validated by the present quantitative data of microstructural and fractographic analysis performed along with mechanical tests on hot-forged plates in T73 condition. The relevant parameters changed by the compositional variations were determined in two orientations. It was found that the predicted K _Ic values represent the tendency of fracture toughness change well. The new model provides better agreement for the case of dominant transgranular fracture mode.     

Sol–gel auto-combustion synthesis,structural and enhanced magnetic properties of Ni substituted nanocrystalline Mg–Zn spinel ferrite

Nanocrystalline arrays of Ni²⁺ substituted Mg–Zn spinel ferrite having a generic formula Mg_0.7−xNi_xZn_0.3Fe₂O₄ (x=0.0, 0.2, 0.4 and 0.6) were successfully synthesized by sol–gel auto-combustion technique. The fuel used in the synthesis process was citric acid and the metal nitrate-to-citric acid ratio was taken as 1:3. The phase, crystal structure and morphology of Mg–Ni–Zn ferrites were investigated by X-ray diffraction, scanning electron microscopy, and Fourier transformer infrared spectroscopy techniques. The lattice constant, crystallite size, porosity and cation distribution were determined from the X-ray diffraction data method. The FTIR spectroscopy is used to deduce the structural investigation and redistribution of cations between octahedral and tetrahedral sites of Mg–Ni–Zn spinel structured material. Morphological investigation suggests the formation of grain growth as the Ni²⁺ content x increases. The saturation magnetization and magneton number were determined from hysteresis loop technique. The saturation magnetization increases with increasing Ni²⁺ concentration ‘x’ in Mg–Zn ferrite.     

Effect of growth rate and Mg content on dendrite tip characteristics of Al–Cu–Mg ternary alloys

An experimental analysis is presented to correlate the secondary dendrite arm spacing λ ₂ and dendrite tip radius R with growth rate V and Mg content C _0-Mg of Al–Cu–Mg ternary alloys. Under constant temperature gradient G (4.84±0.13 K mm⁻¹), a series of directional solidification experiments were performed at five different growth rates V (16.7–83.3 μm/s) and five different Mg contents C _0-Mg in Al–5 wt.% Cu–(0.5–5) wt.% Mg alloys. Solid–liquid interface was investigated from the longitudinal sections of the quenched samples, and λ ₂ and R were measured on the dendrite tips. The dependencies of λ ₂ and R on V and C _0-Mg were determined. The experimental results showed that the values of λ ₂ and R decrease as V and C _0-Mg increase at a constant G. The present exponent values related to V are found to be slightly lower than the values of the theoretical models and previous experimental works; however, C _0-Mg exponent values are found to be much lower than the theoretical models and previous experimental works. The ratio of the secondary dendrite arm spacing to the dendrite tip radius is 2.09±0.15, in good agreement with the scaling law. At a constant C _0-Mg, the values of VR ² were found to slightly increase with the ascending V. However, as C _0-Mg increases, the values of VR ² decrease.     

Electrical resistivity of liquid Sb–Zn alloys

Using the DC four-probe method, temperature dependence of the electrical resistivity (ρ???T) of Sb_{100? x} Zn _x (x?=?25,?40,?50,?57,?61,?80 at%) alloys was investigated in the temperature range of 500–860°C. The results showed that resistivity of each liquid alloy decreased non-linearly with temperature increasing above their liquidus (T _L) until reaching critical temperature, at which the resistivity–temperature coefficients dρ/dT–T converts from negative into positive. The phenomena of liquid phase transformation might relate with the dissociation of covalent bonds, chemical orders and associations in Sb–Zn melts.     

Crystal and Magnetic Structures of Granular Powder Spinel Mn–Zn and Ni–Zn Ferrites

Some specific features of the crystal and magnetic structures of granular powder spinel-like ferrites Mn_0.160Mg_0.404Zn_0.448Fe₂O₄, Mn_0.676Zn_0.227Fe_0.09Fe₂O₄, Mn_0.5792Zn_0.2597Fe_0.1612Fe₂O₄, and Ni_0.32Zn_0.68Fe₂O₄ have been studied by neutron diffraction. It has been established that the crystal structure of all the studied compounds has a cubic symmetry with space group Fd\(\bar 3\)m. Ferrimagnetic ordering is observed in all the studied structures. Based on the experimental data, the unit cell parameters and interatomic bond lengths of the studied compounds are determined alongside with the distribution of cations between octahedral and tetrahedral crystallographic positions in their cubic crystal structure. Corresponding average magnetic moments are calculated for different positions in their cubic structure. Some structural mechanisms of the formation of magnetic properties depending on the level of doping and the size of powder grains are discussed.     

Synthesis and performance of Zn–Ni–P thin films

The electroplating of Zn–Ni–P thin film alloys from a sulfate bath containing phosphoric and phosphorous acid was investigated. The bath composition and the deposition parameters were optimized through Hull cell experiments, and the optimum experimental conditions were determined(p H = 2, temperature = 298–313 K, zinc sulfate concentration =30 g·L-1, EDTA concentration = 15 g·L-1, and current density = 1.0–2.0 A·dm-2). The SEM analysis of the coating deposited from the optimum bath revealed fine-grained deposits of the alloy in the presence of EDTA. Optical microscopy analysis indicated an electrodeposited thin film with uniform thickness and good adhesion to the steel substrate. The good adherence of the coatings was also demonstrated by the scratch tests that were performed, with a maximum determined value of 25 N for the critical load. Corrosion resistance tests revealed good protection of the steel substrate by the obtained Zn–Ni–P coatings, with values up to 85.89% for samples with Ni contents higher than 76%. The surface analysis of the thin film samples before and after corrosion was performed by X-ray photoelectron spectroscopy(XPS).     

Surface tension modelling of liquid Cd–Sn–Zn alloys

The thermodynamic model in conjunction with Butler equation and the geometric models were used for the surface tension calculation of Cd–Sn–Zn liquid alloys. Good agreement was found between the experimental data for limiting binaries and model calculations performed with Butler model. In the case of ternary alloys, the surface tension variation with Cd content is better reproduced in the case of alloys lying on vertical sections defined by high Sn to Zn molar fraction ratio. The calculated surface tension is in relatively good agreement with the available experimental data. In addition, the surface segregation of liquid ternary Cd–Sn–Zn and constituent binaries has also been calculated.     

Frequency dependence of the spin glass freezing temperatures in icosahedral R–Mg–Zn (R = rare earth) quasicrystals

We present ac susceptibility measurements with the frequency spanning three orders of magnitude on single grain, icosahedral R–Mg–Zn (R = rare earth) quasicrystals. The freezing temperature in Gd-based, Heisenberg spin glasses in this family increases by ～2% with a frequency increase from 10?Hz to 10?kHz, whereas the freezing temperature in the non-Heisenberg members of the family is significantly more responsive to the frequency change (by 16–22%), suggesting that an additional magnetic anisotropy distribution in the non-Heisenberg spin glasses causes changes in the low frequency magnetic dynamics.