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.     

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.     

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.     

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.     

Growth direction and Si alloying affecting directionally solidified structures of Al–Cu–Si alloys

The roles of growth direction and Si content on the columnar/equiaxed transition and on dendritic spacings of Al–Cu–Si alloys still remain as an open field to be studied. In the present investigation, Al–6 wt-%Cu–4 wt-%Si and Al–6 wt-%Cu alloys were directionally solidified upwards and horizontally under transient heat flow conditions. The experimental results include tip growth rate and cooling rates, optical microscopy, scanning electron microscopy energy dispersive spectrometry and dendrite arm spacings. It was found that silicon alloying contributes to significant refinement of primary/secondary dendritic spacings for the upward configuration as compared with corresponding results of the horizontal growth. Experimental growth laws are proposed, and the effects of the presence/absence of solutal convection in both growth directions are discussed.     

Morphologies of Si phase and La-rich phase in as-cast hypereutectic Al–Si–xLa alloys

In the present study, the diversified morphologies of Si phase and La-rich phase in as-casted hypereutectic Al–Si–xLa alloys are presented and investigated. The morphological features were examined using conventional optical microscopy and SEM for observations conducted on the optical samples and deep-etched samples, respectively. The results show that primary Si crystals show several morphologies, such as feathery, star-shaped, faceted polygonal, platelet and so on. There are three types of fivefolded Si crystals existing in the present study, fivefold symmetry as radial growth alone: thin-branched, coarse-branched and well-defined star-shaped growing from the preferred growth from the tips of branches. The eutectic Si in unmodified Al–Si alloys appears only in fibrous morphology, while discrete and interconnected coral and rodlike eutectic Si particles were observed in alloys with the addition of La. The La-rich phase also grows into a variety of morphologies, such as needlelike, broken rodlike in pores, spherical, and flat platelet. In optical microscopy, La-rich phase is observed to envelope some small polygonal Si crystals.     

Structural and chemical stability of Ta–Si–N thin film between Si and Cu

Thermal stability and barrier performance of reactively sputter deposited Ta–Si–N thin films between Si and Cu were investigated. RF powers of Ta and Si targets were fixed and various N₂/Ar flow ratios were adopted to change the amount of nitrogen in Ta–Si–N thin films. The structure of the films are amorphous and the resistivity increases with nitrogen content. After annealing of Si/Ta–Si–N(300 Å)/Cu(1000 Å) structures in Ar–H₂ (10%) ambient, sheet resistance measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) were employed to characterize barrier performance. Cu₃Si and tantalum silicide phase are formed at the same temperature, and the interdiffusion of Si and Cu occurs through the local defect sites. In all characterization techniques, nitrogen in the film appears to play an important role in thermal stability and resistance against Cu diffusion. A 300 Å thick Ta₄₃Si₄N₅₃ barrier shows the excellent barrier property to suppress the formation of Cu₃Si phase up to 800°C.     

The electrochemical properties of melt-spun Al–Si–Cu alloys

Melt spinning was used to prepare Al_75−XSi₂₅Cu_X (X = 1, 4, 7, 10 mol%) alloy anode materials for lithium-ion batteries. A metastable supersaturated solid solution of Si and Cu in fcc-Al, α-Si and Al₂Cu co-existed in the alloys. Nano-scaled α-Al grains, as the matrix, formed in the as-quenched ribbons. The Al₇₄Si₂₅Cu₁ and Al₇₁Si₂₅Cu₄ anodes exhibited initial discharge specific capacities of 1539 mAh g⁻¹, 1324 mAh g⁻¹ and reversible capacities above 472 mAh g⁻¹, 508 mAh g⁻¹ at the 20th cycle, respectively. The specific capacities reduced as the increase of the Cu content. AlLi intermetallic compound was detected in the lithiated alloys. It is concluded that the lithiation mechanism of the Al–Si-based alloys can be affected by the third component. The structural evolution and volume variation can be mitigated due to the formation of non-equilibrium state and the co-existence of nano-scaled α-Al, α-Si, and Al₂Cu for the present alloys.     

Effects of sintering aids on the densification of Mo–Si–B alloys

The effects of seven sintering aids (0.5?at.% Ni, Co, Fe, Cr, Zr, Nb, and Pd) on the densification of Mo–Si–B alloys of six different compositions (Mo, Mo–0.2Si, Mo–0.2Si–0.02B, Mo–2.5Si–2.5B, Mo–7Si–5B, and Mo–8.9Si–7.7B?at.%) are systematically investigated. It was found that Ni, Co, and Fe are effective in enhancing densification of Mo–Si–B alloys, and Ni is the most effective sintering aid. This study supports a previously proposed hypothesis that activated sintering results from enhanced mass transport in the sintering-aid-induced quasi-liquid intergranular films (a type of grain boundary complexion). The relative effectiveness of these sintering aids can be rationalized by analyzing several key thermodynamic parameters that control the stability of premelting-like grain boundary complexions. Future studies are needed to develop interfacial thermodynamic models and methods for computing “grain boundary complexion (phase) diagrams” for multicomponent systems, which can be a useful component for the “Materials Genome” project that will enable better predictions of the activated sintering and other materials phenomena.     

Enhancement of mechanical properties of Al–Mg–Si alloys by means of manganese dispersoids

Abstract

The effects of Mn dispersoids on the enhancement of mechanical properties in Al–Mg–Si(–Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding Mn to Al–Mg–Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0·05 to 0·5 μm are formed by the use of proper heat treatments. The as extruded alloys containing 1·0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al–0·69Mg–0·79Si–0·48Cu–0·27Zn–0·37Mn–0·3Cr– 0·11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more tortuous crack path. This result can be explained by the increasing energy absorption through crack deflections and tortuous crack paths by the Mn dispersoids.     

Effect of casting and homogenizing treatment conditions on the formation of Al–Fe–Si intermetallic compounds in 6063 Al–Mg–Si alloys

The effect of casting and homogenizing treatment conditions on the formation of several Al–Fe–Si intermetallic compounds in 6063 aluminum alloy was investigated using X-ray diffraction and transmission electron microscopy (TEM). The four kinds of alloys containing 0.1 to 0.5 mass% Fe were melted and then cooled at three different cooling rates ranging from 0.06 to 50 K/s, following the homogenization at 858 K for 54 ks and 2400 ks. The Al–Fe–Si compound particles were extracted from the alloy ingots using the thermal phenol method. The as-cast 0.1 mass% Fe ingot obtained at the casting cooling rate of 0.06 K/s had a largest amount of the phase among the ingots investigated. When this ingot was homogenized at 858 K for 54 ks and 2400 ks, the amount of the phase decreased, while that of the phase increased. On the other hand, the as-cast 0.5 mass% Fe ingot obtained at the casting cooling rate of 50 K/s had the largest amount of the phase among the ingots investigated. When this ingot was homogenized at 858 K for 54 ks, a large amount of the phase remained. However, the homogenization at 858 K for 2400 ks resulted in the transformation of the phase to the phase. The main phase in the as-cast 0.2 mass% Fe ingot obtained at the casting cooling rate of 5 K/s, close to the industrial cooling rates, was the phase. The phase gradually decreased, and the relative amounts of the and phases increased during homogenization at 858 K for 54 ks. Furthermore, almost all of the Al–Fe–Si intermetallic compounds were transformed into the phase in the ingots homogenized at 858 K for 2400 ks.     

Effect of Porosity on Thermal Conductivity of Al–Si–Fe–X Alloy Powder Compacts

The thermal conductivity and thermal diffusivity of hot- and cold-pressed Al–17Si–5Fe–3.5Cu–1.1Mg–0.6Zr (mass%) alloy powder compacts were investigated as a function of the porosity volume fraction. Samples with a very low degree of porosity were produced by hot-pressing air atomized alloy powder with a particle size of 45–100 m. The same powder was used to produce highly porous compacts by cold compaction using a manual press. The thermal diffusivity of the powder compacts was measured using a sinusoidal modulation photopyroelectric technique in a configuration that is similar to the laser flash method. The thermal diffusivity of the material decreases by a factor of about 13 with an increasing porosity of 25 vol% and a factor of about 300 at 60 vol % porosity. Since the calculated specific heat (weighted average of mass specific heat values of major alloy compounds) is much less porosity dependent, the porosity dependence of the thermal conductivity is similar to the thermal diffusivity and decreases exponentially with increasing porosity. Microstructural characterization of high porosity samples prepared by cold compaction indicated that the distribution of pores is not uniform over the cross-section. An interconnecting network of open and closed pores in the form of channels created pockets of porosity,clearpage 2.3pc which are largely responsible for a drastic reduction of thermal conductivity 4pc with increasing porosity.     

Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.