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• The specific roles of Ti in the heterogeneous nucleation of α-Al grains were summarized, and the formation mechanism of Ti-rich zones in Al–Si alloys was revealed. • The dual effects of Ti on the grain refinement and mechanical properties of Al–Si–Mg alloys are investigated. • Novel Al–TCB alloy refine primary α-Al grains of Al–Si–Mg alloy, from 388.4 μm to 133.1 μm. • Elongation and quality index of Al–Si–Mg alloy are increased by 27.4 % and 20.0 %. Hypoeutectic Al–Si alloys are becoming increasingly popular in automotive and aerospace engineering fields due to their excellent overall performance, and grain refinement is regarded as an important way to improve casting and mechanical properties. Titanium (Ti) is a basic element for grain refinement; thus, a certain amount of Ti is often included in Al–Si alloys. In the present work, the changes in the grain refinement, mechanical, and casting properties of Al–Si alloys with different Ti concentration levels under various grain refinement conditions were systematically investigated. The specific roles of Ti in the heterogeneous nucleation of α-Al grains were summarized, and the formation mechanism of Ti-rich zones in Al–Si alloys was revealed. Excess Ti concentration could not efficiently reduce the grain size of Al–Si alloys and eventually resulted in inferior mechanical and casting qualities; hence, the recommended Ti concentration level for the aluminum alloy grades of A356 and A357 is ≤ 0.1 wt%. Furthermore, an optimized technique for the grain refinement of hypoeutectic Al–Si alloys was presented. A small amount of an Al–TCB master alloy was introduced to achieve the best grain refinement and mechanical properties in a trace Ti environment. The addition of 0.5 wt% of the Al–TCB master alloy at the Ti concentration level of 0.06 wt% increased the ultimate tensile strength, elongation, and quality index of the Al–7Si–0.45Mg alloy to 328.8 ± 5.0 MPa, 14.4 % ± 0.6 %, and 970.7 ± 33.1 MPa, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

• Al-Si alloy microcapsules with flexible ceramic shells with the total thickness below 1 µm were prepared. • The shell flexibility is attributed to its fibers network structure embedded with α-Al 2 O 3 /AlN. • Both high heat storage density and super thermal cycle stability are achieved for the microcapsules. • After 3000 cycles, the thermal performance of the microcapsules reaches an amazing nearly zero loss. Al-Si alloy, a high temperature phase change material, has great potential in thermal management due to its advantages of high heat storage density and thermal conductivity. Microencapsulation of Al-Si alloy is one of the effective techniques to solve high temperature leakage and corrosion. In this paper, commercial Al-10Si alloy micro powders were encapsulated with flexible ceramic shells whose total thickness is below 1 µm by hydrothermal treatment and heat treatment in N 2 atmosphere. The compositions and microstructures were characterized by XRD, SEM and TEM. The shell was composed of AlN fibers network structure embedded with α-Al 2 O 3 /AlN which prevented the alloy from leaking and oxidizing, as well as had excellent thermal stability. The latent heat of microcapsules was 351.8 J g–1 for absorption and 372.7 J g–1 for exothermic. The microcapsules showed near zero thermal performance loss with latent heat storage (LHS)/ release (LHR) was 353.2/ 403.7 J g–1 after 3000 cycles. Compared with the published Al-Si alloy microcapsules, both high heat storage density and super thermal cycle stability were achieved, showing promising development prospects in high temperature thermal management. [Display omitted] [ABSTRACT FROM AUTHOR]

• Si atoms diffuse into TiC x and promote the formation of the Ti-rich zone. • The low formation energy of C vacancies facilitates the rapid diffusion of Si. • The wide crystallization range of TiAl x Si y is the key to the TiC x decomposition. • Si remarkably increases the driving force of the decomposition reaction of TiC x. TiC x is an excellent composite strengthening particle and grain refiner for Al alloys. However, the stability of TiC x is poor when solute Si exists in Al alloy melts, which significantly depresses its strengthening and grain refining effects. In this work, the destabilization mechanisms of the TiC x particles in Al-Si alloy melt with a composition of Al-7Si-7.5TiC were explored via experiments, first-principles calculations and thermodynamic calculations. The experimental results show that Si atoms diffuse into TiC x and Ti atoms are released into the Al melt to form a Ti-rich transition zone during the insulation of TiC x in Al-Si melt, and the TiAl y Si z and Al 4 C 3 phases are solidified in the Ti-rich zone and at Ti-rich zone/TiC x interface, respectively. The first principles calculations show that the low formation energy of C vacancies facilitates the rapid diffusion of Si atoms in TiC x , while the doping of Si atoms reduces the energy barrier of diffusion of Ti atoms in TiC x and promotes the formation of Ti-rich zones. The thermodynamic calculations show that the wide crystallization temperature range of the destabilized product TiAl y Si z phase is the key to continuous decomposition of TiC x particles. In addition, the driving force of the main destabilization reaction of TiC x in the Al-Si alloys is about 44 times higher than that in the Al alloys without Si addition. This indicates that the presence of solute Si remarkably promotes the subsequent decomposition process of TiC x in the Al-Si alloy melts. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract In this work, nano-AlN particle (nano-AlN p) microskeletons were introduced into an Al–Si–Cu–Ni–Mg alloy by Al–8AlN master alloy in which the nano-AlN p reinforcements connect with each other to form three-dimensional networks. It is found that these nano-AlN p microskeletons mainly distribute in the binary Al–Si eutectic zones resulting in flaky eutectic Si phases being modified to particulates. Meanwhile, the microskeletons strengthen the matrix synergistically with semi-continuous Ni-rich intermetallics in three dimensions. The tensile mechanical properties, micro-hardness and thermal expansion properties of the alloy at different temperatures are significantly improved. Especially, the ultimate tensile strength (UTS) at 350 °C increases from 85 MPa to 106 MPa, rising by 24.7%, which is ascribed to nano-AlN p microskeletons assisting intermetallics with undertaking mechanical loading, and to the modification of eutectic Si phases to reduce the stress concentration at elevated temperatures. [ABSTRACT FROM AUTHOR]

The Al–Si–V system is assessed to understand the phase equilibria of V-containing (3 d transition metal) Al–Si alloys, which are generally of great importance for technical applications. Four annealed alloys were prepared to study the phase equilibria of Al-rich corner (V ≤ 23.3 at.%, Si ≤ 57.6 at.%) in the temperature range of 500–930 °C. The microstructure and phase constituents of the samples were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS). The existence of ternary phase τ (Al 0.6 Si 1.4 V, TiSi 2 -type) was confirmed with the composition ranging from 17.98 to 18.59 at.% Al and 48.89–49.20 at.% Si. However, it did not equilibrate with fcc(Al) below 868 °C, which was determined by differential scanning calorimetry (DSC). The Si 3 V 5 appeared in Al 58.5 Si 18.3 V 23.2 and Al 64.7 Si 20.2 V 15.1 alloys annealed at 500 °C. It nearly disappeared after 3000 h-annealing and its thermodynamic stability was discussed. According to the measured phase relationships, the thermodynamic description of Al–Si–V system was optimized combing with the enthalpies of formation of binary and ternary compounds obtained by density functional theory (DFT). The Si–V system was modified to obtain the congruent liquidus of Si 3 V 5 and enthalpy of formation of Si 3 V 5 and Si 2 V with experimental data. By comparing the calculated phase equilibria and phase transitions with experimental data, it showed that a good agreement was reached. The description of Al–Si–V system could be used to guide the development of Al–Si alloys. [ABSTRACT FROM AUTHOR]

In this paper, the influence of deformation on the microstructure and low-temperature refining behavior of Al–3.5P master alloy was investigated. The results show that the average size of AlP particles can be reduced obviously from 15.3 μm to about 2.1 μm by deformation. However, it exhibits entirely opposite influence on refining performance when Al–3.5P master alloy was deformed at room temperature and high temperature, respectively. Only when Al–3.5P master alloy was subjected to thermal deformation, can an improvement of low-temperature refining performance be obtained. In this condition, primary Si in A390 alloy can be refined from 137 to 21 μm, making it a potential candidate for die casting production. A mechanism associated with the transformation of particle–matrix interface during deformation has been proposed and further experiment has been designed to validate it. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]