Microstructure evolution during the solutionizing course of Al-12 wt%Si-4 wt%Cu-1.2 wt%Mn alloy was characterized by XRD and TEM-SAD (selected area diffraction), and the contribution of dispersoid Mn-rich precipitates to high temperature strength is discussed qualitatively. Fine and dispersoid Al20Cu2Mn3 (orthorhombic structure, a=2.34 nm, b=1.32 nm, c=0.75 nm) and Al15Mn3Si2 (body center cubic structure, a=2.259 nm) particles are found to precipitate during the solutionizing course. Both the solutionizing temperature and time have an important influence on the precipitation behavior of these particles. The number density of the precipitates first increased and then decreased with the solutionizing temperature. It decreased monotonically with the solutionizing time. The variation trend of the size of precipitates with temperature and time was opposite to that of the number density. The precipitation of these dispersoid Al20Cu2Mn3 and Al15Mn3Si2 particles has a considerable influence on the high temperature strength.

Microstructure evolution of Al-0.2wt.%Mg-0.36wt.%Si-0.3wt.%Ce alloy at three different homogenization temperatures for 6 h was observed by optical microscopy, scanning electron microscopy, and x-ray diffraction. Conductivity and tensile properties of the alloy were tested. Results indicate that homogenization temperature has little effect on the macro-segregation and grain size of the as-cast Al-Mg-Si-Ce alloy; however, it has a significant effect on the conductivity. The conductivity is first improved to a maximum value of about 57.3% IACS with homogenization temperature increasing to 560 °C (2.7% higher than that of the as-cast sample), and then decreased when the temperature continuously increasing. The main contributor is considered to be vacancy concentration, which is directly related to the lattice distortion, thus affects the conductivity. The studied homogenization temperatures almost make no difference among the tensile properties of the alloy. The best homogenization temperature for Al-0.2wt.%Mg-0.36wt.%Si-0.3wt.%Ce alloy is 560 °C with the highest conductivity and no decrement of strength.

Two Pd-loading routes and three Pd-precursor matters were adopted to prepare Pd/(Ce,Y)O2/γ-Al2O3/cordierite monolithic catalyst. The surface active species on the catalyst were characterized by XPS, and its catalytic activity for methane combustion was tested, and the dynamics of the catalytic combustion reaction was also discussed. Pd-loading route and Pd-precursor mass have a significant influence on the catalytic activity and surface active species. The sol dipping method is more advanced than the aqueous solution impregnating method. PN-sol catalyst, by sol dipping combined with Pd(NO3)2-precursor, has the best catalytic activity. The physical reason is the unique active Pd phase coexisting with active PdO phase on the surface, and thus the Pd3d5/2 binding energy of surface species and apparent activation energy of combustion reaction are considerably decreased. The catalytic activity index, Pd3d5/2 binding energy and apparent activation energy are highly tied each other with exponential relations.

Al–Si eutectic growth mechanism was investigated in a directionally-solidified Al–13 wt% Si alloy with different strontium (Sr) and magnesium (Mg) additions, growth velocities and temperature gradients. Macro- and micro-scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying Al–Si eutectic front and the final morphology of eutectic grains in the solidified Al–13 wt% Si alloys. By varying (tailoring) these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition (CET) of eutectic structure, which can be interpreted on the basis of Hunt's Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.

•The content of Si has a significant effect on properties of Al–Si alloys.•Dynamic recrystallization of Al matrix can take place during hot extrusion.•Dynamic recrystallization becomes more matured with increasing content of Si.•Near eutectic Al–Si alloys can be hot-extruded to profiles with high performance.Microstructure and mechanical properties of as-cast and as-extruded Al–Si–Mg alloys with different Si content are investigated by tensile test, microstructure observation. High density of Si particles in the Al alloys can induce dynamic recrystallization during hot extrusion and it becomes more matured with an increase in the density of Si particles. The tensile strength of as-cast and as-extruded alloys can be improved with the increase of Si content and hot extrusion make the elongation of alloys increase dramatically. Considerable grain refining effect caused by recrystallization occurred during hot extrusion of S2 (equivalently commercial A356 alloy) and S3 (near eutectic alloy) alloys plays an important role in the improvement of elongation. A good combination of strength and elongation for the as-extruded S3 alloy indicates that near eutectic Al–Si alloys can be hot-extruded to produce aluminum profiles with high performance.

Mechanical properties of near eutectic Al–12.5 wt%Si–0.6 wt%Mg alloys with and without addition of V were tested. Results show that addition of 0.1 wt% V only has a minor influence on the tensile properties of near eutectic Al–Si–Mg alloy in as-cast and as-homogenized conditions. However, it can significantly improve the tensile properties in as-extruded condition. The YS is enhanced nearly 50% and the elongation can reach 14%. Three factors that may contribute to the enhancement of YS were analyzed by EBSD, XRD and TEM investigation, i.e. Δσgb (the strengthening from (sub-) grain boundaries), M (Taylor factor) and τtot (critical resolved shear stress). Results show that the higher τtot in the alloy with V addition is the main contributor to the higher YS. According to the TEM observation and EDX analysis, the fine precipitates contributed to the higher τtot in 4# alloy are quaternary AlFeVSi phases.

In this work, a comparison study on corrosion behavior of extruded near eutectic Al–12.3%Si–0.26%Mg and 6063 alloys has been carried out by mass loss test in 4% H2SO4 aqueous solution in the open air and potentiodynamic polarization test in 3.5 wt.% NaCl aqueous solution. Results indicate that the corrosion resistance of the near eutectic Al–Si–Mg alloy is less than that of 6063 alloy. Macro/microscopy and scanning electron microscopy results clearly show the difference of the corrosion progress of these two alloys in 4% H2SO4 aqueous solution. The corrosion type of 6063 alloy is pitting corrosion. The Mg2Si and AlFeSi particles and surface defects act as nucleation sites for pitting, and the amount and distribution of them have a significant effect on the pitting behavior. For the near eutectic alloy, there are two types of corrosion cells. One is between the extruded primary α-Al and the eutectic, the other is between the eutectic Al and eutectic Si particles. Combination of these two types of corrosion cells leads to a lower corrosion resistance, a higher mass loss of the near eutectic alloy compared with 6063 alloy, and the formation of the paralleling corroded grooves.

The effect of solidification velocity on nucleation and growth of porosity in a directionally solidified A356 alloy was investigated in situ using microfocus x-ray imaging and directional solidification technology. Increasing solidification velocity produces high hydrogen solute segregation in the solidification front, leading to a considerable reduction of nucleation temperature for porosity formation. When the solidification velocity increases from 0.1 mm/s to 0.2 mm/s, the average nucleation temperature decreases from 634.0°C to 614.6°C. The increased solidification velocity also increases the rate and fluctuation of pore growth. As a result, the average pore size in the fast solidified specimen is smaller than that with a slow velocity.

The formation mechanism of irregular shape porosity in hypoeutectic aluminum silicon alloy (A356) was investigated by X-ray real time observation on porosity evolution during solidification and re-melting. Porosity in the hypoeutectic aluminum A356 alloy with high hydrogen content (>0.3 mL/100 g Al) first forms in the liquid as small spherical gas bubbles, then expands along with the pressure drop in the mushy zone due to shrinkage and lack of feeding, and finally deforms into irregular morphology by the impingement of aluminum dendrite network. Degassing is a key to eliminate porosity in aluminum alloy castings.

Effect of multi-pass cold-rolling on the mechanical properties and microstructure of a near-eutectic Al-12%Si-0.2%Mg casting alloy was investigated. Optical microscopy, SEM, and TEM were employed to resolve the as-rolled microstructure, and the microstructure of samples after aging treatment. It has been found that Brinell hardness increases considerably with rolling reduction ratio; and further annealing leads to a remarkable drop in hardness. Two mechanisms, namely precipitation hardening and recovery softening, were found to develop simultaneously in the subsequent aging treatment following cold rolling. In contrast, recovery softening dominated the aging of cold-rolled specimen with prior intermediate annealing. Tensile properties were also performed to measure the effect of cold rolling and subsequent aging treatment.

Eutectic solidification in near-eutectic Al-13 wt pet Si casting alloys and the effect of trace addition of boron or strontium on it have been investigated using thermal analysis and microstructural characterization. In unmodified alloy, dual eutectic structure has been observed. The coarse eutectic (dendrite-like AI+ coarse Si flakes) is formed above the equilibrium temperature of eutectic (Al+Si) reaction (577°C). The coarse eutectic (CE) grains nucleate from the primary silicon particles formed earlier due to local enrichment of silicon solute and grow in a divorced mode between the dendritic AI phase and large silicon flakes. The fine eutectic (FE) grains nucleate later on other potential sites activated by melt undercooling and grow in coupled-growing mode with the silicon crystals as fine flakes. The formation of the FE grains is favored in the alloys containing boron because of a great number of potential nucleation sites being added from boron-containing particles. Addition of strontium to the alloys restrains completely the formation of primary silicon particles and hence limits the nucleation of the CE. This is because the eutectic point has moved far enough to make the alloy, at this composition (AI-13 wt pet Si), hypo-eutectic. Local cooling rate during solidification has an important influence on competition formation of these two eutectics.

Combined addition of strontium and boron led to a much greater decrease in the eutectic grain sizes in near-eutectic Al–Si alloys than that caused by addition of strontium alone. As expected, an increased cooling rate during solidification refines the eutectic grains. The mechanism of this refinement in the presence of strontium and boron is related to the combined effect of increased undercooling of the eutectic reaction and the effective number density of eutectic nuclei.

The as-cast microstructures of near-eutectic Al–Si alloys with the combined addition of strontium and boron were observed to highlight the interaction of strontium and boron. A mutual poisoning effect is found when the contents of Sr and B go beyond a certain limit. The effect is proposed to be due to the formation of a (Sr,B) compound.

The effect of strontium on the crystallization of Mg2Si phase in Al–11.6%Si–0.4%Mg alloys was investigated with optical microscope and SEM. In the partially modified alloys, the Mg2Si phase grows as network or bamboo-shoot shape. However, very few and fine Mg2Si particles are isolated at the boundaries of the eutectic cells in the fully modified alloys. Addition of strontium restrains the crystallization of Mg2Si phase. The Mg2Si phase in Al–Si–Mg casting alloy is thought to nucleate on the surfaces of the eutectic silicon flakes. This hypothesis was analyzed from the point of view of the lattice match between the Mg2Si and Si crystals. The restraining effect of strontium is thought to be related to the combination of two effects of strontium, the increase of the amount of dendritic α phase and the change of the surface characteristic of silicon crystal caused by the addition of strontium.