Effect of Cerium and Magnesium on Surface Microcracks of Al–20Si Alloys Induced by High-Current Pulsed Electron Beam

The effect of Ce and Mg on surface microcracks of Al–20Si alloys induced via high-current pulsed electron beam (HCPEB) was studied. Mg was revealed to refine the primary Si phase in the pristine microstructure by forming a Mg2Si phase, leading to the suppression of microcrack propagation within the brittle phase after HCPEB irradiation. The incorporation of Ce into the Al–Si–Mg alloys further refined the primary Si phase and reduced the local stress concentration in the brittle phase induced by HCPEB irradiation. Ultimately, the surface microcracks were observed to be eliminated by the synergistic effects between the two elements. For Al–20Si–5Mg–0.7Ce alloys, Ce demonstrated a homogeneous distribution in the Al matrix on the HCPEB-irradiated alloy surface, while the Mg and Si exhibited a certain degree of aggregation in the Mg2Si phase. Metastable structures were formed on the HCPEB-irradiated alloy surface, including the nano-primary silicon phase, nano-cellular aluminium structure, and nano-Mg2Si phase. Compared with alloy specimens containing Mg, the Al–20Si–5Mg–0.7Ce alloy specimens exhibited an excellent anticorrosion property after HCPEB irradiation mainly due to the combined effects of the grain refinement and microcrack elimination.

Calculation Based on the Formation of Mg2Si and Its Effect on the Microstructure and Properties of Al–Si Alloys

In order to investigate the effect of Mg2Si formation on the microstructure and properties of an Al−Si alloy, the critical point of a hypereutectic Al−17Si−4Cu−Mg alloy was calculated by Pandat software. The calculation results of the equilibrium phase diagram show that the critical point for Mg2Si phase formation for the alloy was obtained when the Mg content was 2.2%. The contents of 0.5 wt.% Mg and 2.5 wt.% Mg were selected as the research object. The content of Mg increased from 0.5 wt.% to 2.5 wt.%, the eutectic Si in the matrix was reduced, and the Chinese character-like Mg2Si phase appeared in the microstructure. In the peak ageing state, in addition to θ” and Q’ phases that were mainly precipitated, there was also needle-like β” precipitation in the 2.5 wt.% Mg content alloy. Larger precipitates were found in 2.5 wt.% content alloys, mainly due to the promotion of the solid solution having the aggregation and segregation of more solute elements in the matrix. The tensile strength, elongation, and hardness of hypereutectic Al−17Si−4Cu−0.5Mg alloy under peak ageing were 331 MPa, 3.11%, and 152.1 HB, respectively. The tensile strength and the elongation decreased while the hardness increased with the 2.5 wt.% Mg content, which is due to the formation of hard and brittle Mg2Si and Al8FeMg3Si, which has a splitting effect on the matrix.

Effect of Semi-Solid Processing on the Microstructure and Mechanical Properties of Aluminum Alloy Chips with Eutectic Mg2Si Intermetallics

This study reports the microstructural changes and mechanical properties of high-strength aluminum alloy chips prepared in the semi-solid state at different temperatures, pressures, and holding times. In semi-solid processes, these processing parameters must be optimized because they affect the microstructure and mechanical properties of the chips. In microstructural analysis, these parameters clearly influenced the spheroidization of the aluminum matrix. The aluminum matrix was uniformly spheroidized after semi-solid processing, and the densities of the final samples increased with the holding time. After 30 min holding time at a given temperature, the density approached the theoretical density, but the compressive strength of the samples seriously deteriorated. Meanwhile, fracture surface investigation revealed a deformed Mg2Si phase, which is formed through a eutectic reaction. The strength of this phase significantly decreased after increasing the holding time of the semi-solid processing from 10 to 30 min. Therefore, deformation of the Mg2Si phase caused by diffusion of aluminum into this phase can be a key factor for the decrease in the mechanical properties of samples fabricated with 30 min holding time.

Influences of Heat Treatment on the Thermal Diffusivity and Corrosion Characterization of Al-Mg-Si alloy

The effect of the heat treatment on the Mg2Si phase in Al-Mg-Si alloy was investigated by a laser flash apparatus (LFA), Differential scanning calorimetry (DSC) and corrosion test. The alloy samples were solution treated at 590 oC for a half hour followed by warm water quenching, and then aged in air at 180, 200 and 240 oC for 5 hours. The results showed that the corrosion resistance of the solid solution treated sample was more improved than the as cast sample. Aging treatment also helped increase corrosion resistance at room temperature. It is thought that the fine Mg2Si precipitation phase on the grain had a more positive effect on improving corrosion resistance than crystallization of the Mg2Si phase on the grain boundaries. Corrosion rate also decreased with increasing aging treatment. The corrosion rate of AT240 was reduced to 1.16 MPY compared with the AT180 test piece, which had a corrosion rate of 3.79 MRY. The solution treated sample also showed lower thermal diffusivity than the aged samples. The thermal diffusivity increased as the solute concentration of Mg and Si in the a-Al matrix rapidly decreased during aging treatment. On the other hand, the thermal diffusivity of the aged samples, in which precipitation was completed by the aging process, decreased as the temperature rose. The thermal conductivities of all samples were similar near 250 oC when the β'' phase and β' precipitation was completed.

Microstructural Improvement of Eutectic Al + Mg2Si Phases on Al–Zn–Si–Mg Cast Alloy with TiB2 Particles Additions

In this study, the effects of adding TiB2 particles to eutectic Al + Mg2Si phases in aluminum alloys were analyzed. The eutectic Al + Mg2Si phases were modified effectively when a large amount of TiB2 was added, and changes in the shape, size, and distribution of the eutectic Al + Mg2Si phases were confirmed using a polarizing microscope and FE-SEM. The crystal structure of the TiB2 particles and Mg2Si phases were analyzed using HR-TEM, and the analysis confirmed that the TiB2 particles can act as heterogeneous nucleation sites. This paper intends to clarify the principle of phase modification of the eutectic Al + Mg2Si phases by TiB2 particles and proposes a new mechanism to improve Mg2Si phase modification when TiB2 particles are added.

Effect of La Addition on Solidification Behavior and Phase Composition of Cast Al-Mg-Si Alloy

The current study focusses on the phase composition, solidification path, and microstructure evaluation of gravity cast Al-4Mg-0.5Si-xLa aluminum alloy, where x = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La. A computational CalPhaD approach implemented in Thermo-Calc software and scanning electron microscopy technique equipped with electron microprobe analysis (EMPA) was employed to assess its above-mentioned characteristics. The thermodynamic analysis showed that the equilibrium solidification path of La-containing Al-Mg-Si alloys consists of only binary phases LaSi2 and Mg2Si precipitation along with α-Al from the liquid and further solid-state transformation of this mixture into α-Al + Al11La3 + Mg2Si + Al3Mg2 composition. Scheil–Gulliver simulation showed a similar solidification pathway but was accompanied by an increase in the solidification range (from ~55 °C to 210 °C). Furthermore, microstructural observations were congruent with the calculated fraction of phases at 560 °C and related to α-Al + LaSi2 + Mg2Si three-phase region in terms of formation of La-rich phase having both eliminating effect on the eutectic Mg2Si phase. Quantitative EMPA analysis and elemental mapping revealed that the La-rich phase included Al, La, and Si and may be described as Al2LaSi2 phase. This phase shows a visible modifying effect on the eutectic Mg2Si phase, likely due to absorbing on the liquid/solid interface.

Microstructure and Mechanical Properties of Hypo- and Hypereutectic Cast Mg/Mg2Si Composites

In this paper, the microstructure and mechanical properties of two magnesium matrix composites—a hypoeutectic with 1.9 wt% Mg2Si phase and a hypereutectic with 19 wt% Mg2Si compound—were analyzed. The investigated materials were prepared using the gravity casting method. Microstructure analyses of the fabricated composites were carried out by XRD and light microscopy. The tensile and compression strength as well as yield strength of the composites were examined in both uniaxial tensile and compression tests. The microstructure of the hypoeutectic composite was in agreement with the phase diagram and composed of primary Mg dendrites and an Mg–Mg2Si eutectic mixture. For the hypereutectic composite, besides the primary Mg2Si phase and eutectic mixture, additional magnesium dendrites surrounding the Mg2Si compound were observed due to nonequilibrium solidification conditions. The composites exhibited a rise in the examined mechanical properties with an increase in the Mg2Si weight fraction and also a higher tensile and compression strength in comparison to the pure magnesium matrix (cast in the same conditions). Additionally, analyses of fracture surfaces of the composites carried out using scanning electron microscopy (SEM + EDX) are presented.