Solidification behaviour of Al–7% Si–0.3% Mg during rotary spray forming

The solidification behaviour of an Al–7% Si–0.3% Mg alloy during rotary spray forming was studied. The ability to form a coating was insensitive to the thermal processing parameters, yielding material exchangess greater than 90%. The level of porosity varied typically between 1.5 and 4.75%. The dendrite arm spacing was evaluated and used to estimate the cooling rates. Typical dendrite secondary arm spacings were of the order of 3 m, 12 m and 25 m, corresponding to cooling rates of 4630 K s^–1, 72 K s^–1 and 8 K s^–1, respectively. The fraction primary precipitation was experimentally determined and the partition coefficient calculated indirectly using the Scheil equation. The partition coefficient is increased during rotary spray forming. This is explained by the presence of trapped vacancies at the solidification front. The vacancies change the solid's free energy and thus change the phase diagram and the partition coefficient. A simplistic analysis of entrapment and condensation of vacancies and their influence on the partition coefficient is made.     

Performance of Si–Ni nanorod as anode for Li-ion batteries

Si–Ni nanorod structures as anode materials for Li ion batteries have been prepared by depositing Si coatings on electrodeposited Ni nanocone arrays using plasma enhanced chemical vapor deposition. The obtained samples were characterized by field emission scanning electron microscopy and X-ray diffraction. The electrochemical performance was evaluated by a galvanostatic battery testing. It is shown that the first discharge capacity is high as 4125 mAh/g with a high first coulombic efficiency of 92% at C/20 rate. A capacity of 3249 mAh/g at C/5 rate is attained with retention of 95.7% after 30 cycles.     

Thermodynamic Properties of Si–Ge and Si–Sn Melts

The mixing enthalpies of Si–Ge and Si–Sn liquid alloys were measured in an isoperibolic calorimeter. The results demonstrate that the formation of Si–Ge melts is accompanied by a small heat release, while the formation of Si–Sn melts is an endothermic process. Calculations of the Si activity in Si–Sn melts by Schroeder's equation indicate large positive deviations from Raoult's law.     

Phase composition,structure, and mechanical properties of arc PVD Mo–Si–Al and Mo–Si–Al–N coatings

Using an arc physical vapor deposition process, we have produced nanostructured Mo–Si–Al coatings with a uniform distribution of equiaxed grains 8–12 nm in size and Mo–Si–Al–N coatings with a multilayer structure and a modulation period from 22 to 25 nm. The former coatings consist of MoSi₂ and Mo and the latter consist of Mo2N and amorphous Si₃N₄ and AlN. The hardness of the Mo–Si–Al–N and Mo–Si–Al coatings is 41 and 18 GPa, respectively; they are similar in resistance to elastic deformation; and the Mo–Si–Al–N coating has a considerably higher resistance to plastic deformation. The coatings have roughly identical coefficients of friction (~0.67–0.69 at 20°C and ~0.52–0.56 at 550°C), but the wear resistance of the Mo–Si–Al–N coating is higher by three and two orders of magnitude at 20 and 550°C, respectively. The coatings of the two systems exhibit good adhesion to the substrate and cohesive fracture. Partial wear of the Mo–Si–Al and Mo–Si–Al–N coatings in the course of scratch testing occurs at indentation loads of 80 and 63 N, respectively.     

Potential of short Si–Ti–C–O fiber-reinforced epoxy matrix composite as electromagnetic wave absorbing material

The effects of fiber electrical properties on electromagnetic wave absorbing potential in short Si–Ti–C–O fiber-dispersed epoxy matrix composites were studied. Six kinds of short Si–Ti–C–O fibers with different respective electrical resistivity were incorporated into an epoxy matrix and the dielectric properties of the composites in a frequency range from 1 MHz to 1 GHz were measured. The penetration depth of electromagnetic wave, which is defined as the distance to reduce 1/e of the incident electromagnetic wave power, is obtained from the measured dielectric properties. It is found that the dielectric properties of the composites are strongly dependent on the electrical resistivity of the fiber: the use of lower electrical resistivity fiber leads to a shorter penetration depth. Independent of the electrical resistivity of fiber, the penetration depth decreases with increase in the frequency. This result demonstrates the potential of the composite as a thin electromagnetic wave absorbing material.

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Effect of inclusions on the tensile properties of Al–7% Si–0.35% Mg (A356.2) aluminium casting alloy

The present study was performed on an A356.2 alloy. Two types of initial materials were used, i.e. fresh and recycled. A total of 13 operations representing those normally applied in aluminium foundries were simulated under dry atmospheric conditions (humidity 15%–20%). The molten metal was cast into test bars which were T6 tempered prior to tensile testing. The results show that holding the liquid metal for a long time, i.e. 72 h at 735°C leads to sedimentation of most inclusions towards the bottom of the melting crucible. However, a change in the surrounding humidity may cause absorption of hydrogen and, hence, a large amount of porosity. Degassing using dry argon injected into the liquid metal through a rotary impeller (speed 160 r.p.m) appears to be the best technique for inclusion removal. The efficiency of this process is significantly improved when it is coupled with filtration using ceramic foam filters (10 and 20 p.p.i). A linear relationship between alloy ductility and logarithm of percentage inclusions has been established. Owing to decohesion between the inclusions/oxide films and the surrounding matrix, cracks are easily initiated at their interfaces, leading to unpredicted failure. © 1998 Chapman & Hall     

Structure,Composition, and Properties of Arc PVD Mo–Si–Al–Ti–Ni–N Coatings

Using an arc physical vapor deposition process, we have produced nanostructured Mo–Si–Al–Ti–Ni–N coatings with a multilayer architecture formed by Mo₂N, AlN–Si₃N₄, and TiN–Ni and a crystallite size on the order of 6–10 nm. We have studied the physicomechanical properties of the coatings and their functional characteristics: wear resistance, adhesion to their substrates, and heat resistance. According to high-temperature (550°C) wear testing and air oxidation (600°C) results, the coatings studied here are wearand heat-resistant under appropriate temperature conditions. Their properties are compared to those of Mo–Si–Al–N coatings.     

High-pressure phase transitions and structure of Al–20 at % Si hypereutectic alloy

Phase transformations of an Al–20 at % Si high-silicon hypereutectic alloy have been studied by differential barothermal analysis at temperatures of up to 800°C in argon compressed to 100 MPa. High pressure has been shown to raise the melting point of the alloy by 5°C during heating and to lower the eutectic solidification temperature by 5°C during cooling in comparison with the canonical phase diagram of the Al–Si system. At a temperature of 553°C, heating and cooling lead to silicon dissolution and decomposition of the aluminum-based solid solution, respectively. After high-pressure solidification, the silicon particles in the alloy have a bimodal size distribution. Quantitative porosity characteristics in the alloy after a barothermal scanning cycle are similar to those in the as-prepared alloy. The lattice parameters of the silicon and aluminum remain unchanged. The microhardness of the aluminum matrix of the alloy corresponds to that of pure aluminum.     

Effects of Si and Cu contents on grain size of Al–Si–Cu alloys

The influence of the silicon and copper contents on the grain size of high-purity Al–Si, Al–Cu, and Al–Si–Cu alloys was investigated. In the Al–Si alloys, a poisoning effect was observed and a poor correlation between the grain size and growth restriction factor was obtained. A possible cause of the poisoning effect in these alloys is the formation of a TiSi₂ monolayer on the particles acting as nucleation sites or another poisoning mechanism not associated with TiSi₂ phase formation. In the Al–Cu alloys, a good correlation between the grain size and growth restriction factor was found, whereas in the Al–Si–Cu alloys, the correlation between these two parameters was inferior.     

Influence of Si content on thermal expansion characteristic of Al–Si alloys

ABSTRACT

The objective of this study was to find out how Si precipitation affects the linear thermal expansion behaviors of Al–Si alloys with various Si content. These Al–Si alloys were manufactured by gravity casting using 99.98?wt-% pure Al and 98.5?wt-% Si pellets. Solution treatment was carried out at 530°C for 10 h for each specimen. As-quenched specimens were subjected to microstructure observation and linear thermal expansion analysis. Si precipitation and additional linear thermal expansion occurred at lower temperature when Si content was increased to 9.5?wt-%. The activation energy of Si precipitation was also lower when Si content was higher. Eutectic Si reduced diffusion distance and acted as a nucleation site of dissolved Si atoms in the Al matrix during aging. These Si phases decreased Si precipitation temperature and activation energy of Si precipitation.     

Influence of ion energies on the structure,composition, and properties of multilayer Ti–Al–Si–N ion-plasma-deposited coatings

It is established that the energy of deposited particles influences the structure, composition, and properties of multilayer nitride coatings consisting of alternating layers of nanocrystalline TiN and amorphous Si₃N₄ phases with inclusions of nanocrystalline hexagonal AlN formed at energies of titanium, aluminum, and silicon ions exceeding ~317 × 10^–19, 267 × 10^–19, and 230 × 10^–19 J, respectively. As the energy of titanium ions bombarding the substrate increases above ~512 × 10^–19 J, the phase transition from disordered TiN _x to Ti₃N₂ and the appearance of 2- to 3-nm-thick sublayers in 15-nm-thick nanocrystalline TiN _x layers take place in the coating. The maximum hardness of such coatings reaches a level of ~54 GPa.

     

Effect of Si on the Glass Formation and Magnetic Properties of High-Iron Fe–Zr–Si Alloys

High-iron Fe–Zr–Si amorphous ribbons were fabricated through the melt-spun technique. Then, the effects of Si content on the glass-forming ability and magnetic properties of Fe_90?xZr₁₀Si_x (x =?1, 2, 3, 4, 5, 10) alloys were investigated. Results showed that the amorphous structure only formed in an alloy composition of 3 at.% Si. Moreover, α-Fe(Si) and Fe₃Zr phase appeared gradually when Si was added. Fe₈₇Zr₁₀Si₃ alloy is a unique amorphous structure in Fe_90?xZr₁₀Si_x ribbons. The peak temperatures of the two crystallization stages were 464 and 600 ^°C. The saturation magnetization (M_s) values of the alloys ranged from 91.2 to 132.3 emu/g, and all had an initial increase before decreasing and their coercivity (H_c) values increased with increased Si content. The Fe₈₇Zr₁₀Si₃ amorphous alloy exhibited a low H_c value of approximately 39.1 A/m, which shows good magnetic properties in the as-quenched state. After annealing, the M_s of the amorphous sample considerably improved, particularly reaching 165.3 emu/g at 600 ^°C.     

Effect of Si target sputtering power on diffusion barrier properties of Ta–Si–N thin films

Abstract

Ta–Si–N thin films and Cu/Ta–Si–N thin films were deposited on p type Si(111) substrates by magnetron reactive sputtering. Then the films were characterised by four point probe sheet resistance measurement, AFM, SEM and XRD respectively. According to the XRD results, the authors found that the crystallisation of Ta nitrides in Ta–Si–N/Si thin films is suppressed effectively when fabricated by a high Si target sputtering power. As the Si target power varies, the failure temperature of Cu/Ta–Si–N/Si is changed. The sample fabricated by the Si target power of 200 W fails after 800°C rapid thermal annealing and it has the highest failure temperature. The investigation of failure mechanism shows that Cu atoms diffuse through grain boundaries or amorphous structure of the Ta–Si–N barrier, and react with Si to form Cu–Si phase. And it causes the failure of the barrier.     

Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.     

Ion-induced transformations of a W–Si interface

Tungsten silicide formation in multilayer tungsten/silicon structure was investigated. The W–Si multilayers were deposited on thermally oxidized silicon wafers using the dual-target magnetron sputtering. Deposition of the whole stack of sublayers was carried out without breaking vacuum in order to eliminate contamination or oxidation of the interfaces between sublayers. Samples were annealed in the RTA furnace at temperatures ranging from 700 °C up to 1050 °C. Some of the structures were irradiated with argon ion beam before annealing. Reactions between sublayers were studied using SEM imaging of cross-sectional cleavages and by X-ray diffraction analysis. Influence of the irradiation with argon ion beam on structural transformations was investigated using RBS analysis. It has been found that tungsten silicide formation depends on the deposition sequence. The reaction was more effective on interfaces between silicon layer deposited on tungsten then on interface between tungsten deposited on silicon. Ion beam mixing experiment showed that ion–target interaction promotes formation of the WSi₂ phase.     

Dry sliding wear of eutectic Al–Si

The wear of as-cast eutectic Al–Si was studied using pin-on-disk tribotests in two different environments, air and dry argon. The counterface in all tests was yttria-stabilized zirconia. It was found that wear of the Al–Si was reduced by about 60% by the removal of oxygen from the test environment. The zirconia counterfaces showed measurable wear after tests performed in air, while there was very little wear of the zirconia for tests conducted under argon. The near-surface regions of the Al–Si pins were examined using a transmission electron microscope (TEM), using specimens produced by focussed ion beam milling. The specimens that had been worn in air were characterized by a near-surface mechanically mixed layer containing a considerable amount of both aluminum oxide and zirconium oxide—the aluminum oxide particles had evidently acted as abrasive agents to remove material from the zirconia counterface. In contrast, TEM analysis of the Al–Si tested in argon showed little zirconium oxide in the near-surface regions.     

Preparation of a mixed Mg–Si cellosolvate

We have studied reactions of metallic magnesium with ethyl cellosolve and a mixture of ethyl cellosolve and tetraethyl orthosilicate and demonstrated the possibility of preparing a mixed magnesium–silicon cellosolvate that can be used as a precursor for the synthesis of forsterite and enstatite.     

Analysis of cellular microstructures in Al–Fe–Si alloy

Abstract

Experiments have been carried out on a commercial aluminium alloy rolled to foils with various high rolling reductions. Attention was focused on the formation of cells during deformation. Microstructures were examined in the SEM using electron backscatter diffraction (EBSD) and oriented imaging microscopy techniques. A treatment of the results obtained with EBSD has been used to determine the discrete parameters of the individual grains in the aggregate. The local driving pressures were calculated enabling the estimation of grain growth velocities dR/dt. The evolution of texture could be predicted and successfully compared with the experimental results of an annealing treatment.     

Microstructure and solidification behavior of Cu–Ni–Si alloys

The microstructure and solidification behavior of Cu–Ni–Si alloys with four different Cu contents was studied systematically under near-equilibrium solidification conditions. The microstructures of these Cu–Ni–Si alloys were characterized by SEM and the phase composition was identified by XRD analysis. The phase transition during the solidification process was studied by DTA under an Ar atmosphere. The results show that the microstructure and solidification behavior is closely related to the composition of Cu–Ni–Si alloys. The microstructure of Cu–Ni–Si alloys with higher than 40% Cu content consists of primary phase α-Cu(Ni, Si) and eutectic phase (β₁-Ni₃Si + α-Cu(Ni,Si).When the Cu content is about 40%, only the eutectic phase (β₁-Ni₃Si + α-Cu(Ni,Si)) is present. DTA analysis shows there are three phase transitions during every cooling cycle of alloys with higher than 40% Cu content, but only one for 40% Cu content. Cu–Ni–Si alloy with 40% Cu solidifies by a eutectic reaction, but Cu–Ni–Si alloys with higher than 40% Cu content solidify as a hypoeutectic reaction.