Eutectic Si Phase Research Articles

Influences of different Zr/Sc ratios on microstructure and mechanical properties of Al-2Si alloy

The influences of different Zr/Sc ratios on the microstructural evolution and mechanical performances of Al-2Si alloy were investigated in this paper. The as-cast samples were characterized and analyzed by the scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) to indicate the refining effect of the different Zr/Sc ratios on the α-Al and Si phases in Al-2Si alloy. The results indicate that the α-Al phases are refined from coarse dendritic structure to relatively fine equiaxed grains when the Zr/Sc ratio is equal to 1:1. Meanwhile, the eutectic Si phase in Al-2Si-0.2Zr-0.2Sc alloy is modified from the coarse plate and needle to fine fiber and partial granular structure, and the average roundness of eutectic Si is decreased by 75% from 16 to 4. With the refinement of α-Al phases and the modification of eutectic Si structure, the ultimate tensile strength (UTS) is improved by 53% and the elongation (EL) is increased by 64% when the Zr/Sc ratio is equal to 1:1. In addition, the UTS decreases and EL is enhanced with the increase of tensile temperature. Moreover, the fracture model of Al-2Si-0.2Zr-0.2Sc alloy in 250 °C changes into the ductile fracture.

Phase Transformations, Microstructural Refinement and Defect Evolution Mechanisms in Al-Si Alloys Under Non-Hydrostatic Diamond Anvil Cell Compression

Non-hydrostatic compression of materials using a diamond anvil cell (DAC) can transform equilibrium microstructure of an alloy to novel, potentially metastable states. In this study, in situ synchrotron X-ray diffraction (XRD) during compression up to 24 GPa and subsequent ex situ, high resolution analytical electron microscopy (AEM) after decompression of an Al-Si alloy provided insight into the crystallographic changes during the compression as well as microstructural refinement, and defect structures caused by such a high pressure compression-decompression process. Pressure resolved in-situ synchrotron XRD was used to detail the phase transformation pathway of the eutectic Si phase in Al-Si alloy, from Si-I → Si-XI → Si-V during compression, and a final transformation predominantly to Si-III after decompression. Using scanning and transmission electron microscopy (S/TEM), site specific analysis of the alloy immediately underneath the anvil contact surface demonstrated a highly complex microstructure. A narrow region of thick amorphous Al oxide interspersed with nanocrystalline grains was found at the top surface. Underneath this Al oxide, while the majority the eutectic Si was transformed into highly-deformed, polycrystalline (PC) Si-III, a complex intermediate layer was discovered at the interface between Al and Si, comprised of a small fraction of Al nanocrystals and a majority of nanocrystalline Si-I. This combination of pressure resolved in-situ synchrotron XRD coupled with subsequent high resolution, electron microscopy resolved the phase transformation as well as non-equilibrium microstructures in a metallic alloy induced by a non-hydrostatic high pressure compression followed by decompression.

Processing and characterisation of modified strain-induced melt activation processed Al–Si alloys

In the present study, the microstructural features and its correlation with mechanical properties of modified strain-induced melt activation (modified SIMA) processed Al- (7, 12, 14 wt-%) Si alloys are investigated. The microstructure of modified SIMA process Al–Si alloys consists of globular α-Al grains, primary Si particles and Al–Si eutectic phase. The casting defects and surface roughness are minimised and the morphology of eutectic silicon is modified after modified SIMA treatment. The mechanical properties of modified SIMA processed samples are increased significantly as compare to as-cast alloy due to reduction of porosity, formation of fine globular microstructu+re and accumulation of high dislocation tangle near primary Si particles.

Effect of process conditions on the evolution of microstructure and mechanical properties of AA3003 vacuum furnace brazing joints

In this study, vacuum furnace brazing was used for joining AA3003 alloy with different thicknesses of 0.6 and 3 mm using BALSI-4 filler metal. Firstly, the samples were degreased using water and soap and then cleaned with acetone. The set-up was assembled and pre-heated for two-steps at 235 and 339 °C and subsequently, moved to furnace. Vacuum brazing was conducted at temperature range of 590 to 630 °C for different holding times of 5 and 10 min followed by furnace cooling. After primary visual checking, three samples for each thickness were selected for evaluation of microstructure and mechanical properties. Results indicated that furnace temperature and holding time significantly influenced microstructural evolution and mechanical properties. This effect was attributed to the distribution of Si and its relevant eutectic and pro-eutectic phases. It was deduced that the amount and distribution of such phases together with joint assembling design controlled the mechanical and fracture behavior. Furthermore, it was found that the quantitative amounts of pro-eutectic Si and Al–Si eutectic phases significantly increased the hardness of the joint. Therefore, the joint brazed at 610 °C and dwelling time of 10 min was identified to offer the optimal mechanical properties.

Synergistic Effect of Nd and Ga on the Microstructure and Mechanical Properties of ER4047 Alloy Wires

The synergistic effects of Nd and Ga on the microstructure and mechanical properties of ER4047 alloy wires and the resulting MIG(Melt inert-gas welding) joints were investigated. Experimental results show that the hydrogen content in the liquid and solid alloy decreases rapidly and then increases slowly with the addition of Nd in the range of 0–0.8 wt.%. Under the same conditions, the hydrogen content ratio in the solid and liquid is between 1.9 and 2.1. After adding 0.08–0.2 wt.% Nd, the α-Al dendrites and eutectic Si phases in the ER4047 alloys are refined and modified, respectively; the quality index of the cast rod increases and the weld percent porosity of the MIG joint significantly decreases. However, when 0.8 wt.% Nd is added, although the size of the eutectic Si phase reaches a minimum value, a large number of coarse needle-like Nd-rich phases precipitate in the structure. Moreover, Ga inhibits the precipitation of the Nd-rich phases, thus improving the mechanical properties, especially the elongation of the cast rod. When 0.2 wt.% Nd and 0.05 wt.% Ga are added into the alloy wire, the porosity of the MIG joint reaches the lowest value herein, and the tensile strength and face/back bending angles reach their maximum values.

Effect of Mn Addition on the Mechanical Properties of Al–12.6Si Alloy: Role of Al15(MnFe)3Si2 Intermetallic and Microstructure Modification

Effect of manganese (Mn) addition (0.0, 1.0, 2.0 and 3 wt%) on the microstructural morphology, hardness, tensile properties and fracture behaviour of the gravity cast eutectic Al–12.6Si alloy has been studied through XRD analysis, chemical analysis, optical metallography, FESEM analysis, energy dispersive spectroscopy analysis, hardness test, tensile test and quantitative phase analysis. As-cast Al–12.6Si–0.0Mn alloy has a non-uniformly distributed coarser and irregular shape primary and eutectic silicon particles inside the α-Al phase, and both the Si phase have very sharp corners. Whereas, the 1 wt% Mn added alloy has uniformly distributed fine eutectic and primary Si particles with blunt corners. Further, the addition of 1.0 wt% Mn forms very few (0.26 vol %) irregular shape Al15(MnFe)3Si2 intermetallic phase within the α-Al phase and eutectic Si phase. But, 2.0 wt% and 3 wt% Mn added alloy has an irregular shape coarse plate-like Al15(MnFe)3Si2 intermetallic phase besides the primary and eutectic Si phase. The bulk hardness of the Al–12.6Si alloy is increased with an increase in Mn concentration as the harder Al15(MnFe)3Si2 intermetallic phase forms and both the Si phase morphology modify. The microhardness of the constituent phases also varies with the change in Mn concentration in the alloy. The Mn addition improved the ultimate tensile strength, yield strength, and elongation (%) of the alloy. However, fractographs reveal that the brittle mode of fracture has been increased due to the presence of a higher volume of brittle Al15(MnFe)3Si2 intermetallic in 2.0 and 3.0% Mn alloy. On the other hand, the amount of brittle and cleavage fracture of Si particles decreased, and ductile fracture with dimples formation increased in 1.0 wt%Mn added alloy.

Effect of vacuum degree and T6 treatment on the microstructure and mechanical properties of Al–Si–Cu alloy die castings

This study investigated the effect of T6 treatment on the microstructure and mechanical properties of a die-cast Al–Si–Cu alloy at three vacuum degrees: 1013, 200, and 100 mbar. Corresponding microstructure and mechanical properties were performed. At the as-cast state, the eutectic Si sizes increased unexpectedly with increasing vacuum degree because oxides can act as heterogeneous nucleation sites for the eutectic Si at atmospheric pressure. Also, the increasing vacuum level from 1013 mbar to 100 mbar decreased pore size and porosity, which was the main reason for 12% and 55% increases in tensile strength and elongation, respectively. After T6 treatment, the decreasing pore size and porosity prevented the formation of blister. In addition, numerous θ′ phases were precipitated to strengthen the α-Al grains, evidently improving the tensile strength. The fractography of alloys under 1013 mbar revealed transgranular fractures because the spheroidized eutectic Si reduced the stress concentration at the grain boundaries. In contrast, the fractography of the alloys under 200 and 100 mbar showed coexistence of transgranular fractures and intergranular fractures due to the eutectic Si phase with coarse morphology presented at the grain boundaries.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of a Composite Made of Al-Si-Cu-Mg Aluminum Alloy Reinforced with SiC Particles

In this paper, the effect of heat treatment (solution treatment and artificial aging) on the microstructure and properties of as-cast Al5Si1Cu0.5Mg aluminum alloy and its composite reinforced with 1.5 wt.% SiC particles was studied. The results showed that at 520 °C the optimal solution time for the aluminum alloy and its composite is 9 h and 6 h, respectively. After solution treatment, the microstructure of these two materials consists of a uniform distribution of nearly spherical eutectic Si and skeletal γ phase, furthermore, the composite eutectic Si phase is smaller and γ phase is more dispersed. After artificial aging at 175 °C for 6 h, the microstructure of the composite is more dispersed and finer than that of the aluminum alloy on the whole and Al2Cu is precipitated. After heat treatment, the microhardness, ultimate tensile strength, and elongation of the aluminum alloy and its composite are higher than those of the as-casts. At the same time, the morphology of tensile fracture surface changes very much from a large area of cleavage plane to a large number of dimples and the tearing ridges become thicker for both the aluminum alloy and its composite.

Microstructure evolution of A380 aluminum alloy during rheological process under applied pressure

In squeeze casting process, the essence of mold-filling and feeding is rheological solidification of the alloy melt under pressure. Microstructure evolution is inevitable in this process, which affects the mold-filling and feeding in turn. In this work, the Archimedes spiral sample prepared by indirect squeeze casting was applied to investigate the microstructure evolution during the rheological process under pressure. The results showed that the primary α-Al phase was transformed from fine rosette-like or granular structure to coarse platelet-like structure with the increase of spiral length. However, the primary α-Al grain size of the starting point had a slight growing trend compared with that at the position of 140 mm away from the starting point. The volume fraction of the primary α-Al phase increased from 45.57% to 70.35% along the spiral length direction, demonstrating that the experimental pressure improved the rheological ability of the alloy melt to some extent. Furthermore, the eutectic Si phase was varied from a fine granular or wormlike structure to a coarse platelet-like or needle-like structure, and the dispersion of eutectic Si particles was also varied along the spiral length direction. This microstructure evolution was mainly owing to the comprehensive action of rheological solidification and pressurized solidification. More specifically, the microstructure evolution strongly depended on the pressure and rheological velocity during the rheological process, however, the effect of rheological distance was relatively small.

Effect of Ca modification on the elemental composition, microstructure and tensile properties of Al-7Si-0.3Mg alloy

The effect of Ca addition on the elemental composition, microstructure, Brinell hardness and tensile properties of Al-7Si-0.3Mg alloy were investigated. The residual content of Ca in the alloy linearly increased with the amount of Ca added to the melt. The optimal microstructure and properties were obtained by adding 0.06wt% Ca to Al-7Si-0.3Mg alloy. The secondary dendrite arm spacing (SDAS) of the primary α phase decreased from 44.41 μm to 19.4 μm, and the eutectic Si changed from coarse plates to fine coral. The length of the Fe-rich phase (β-Al5FeSi) decreased from 30.2 μm to 3.8 um, and the Brinell hardness can reach to 66.9. The ultimate tensile strength, yield strength, and elongation of the resulting alloy increased from 159.5 MPa, 79 MPa, and 2.5% to 212 MPa, 86.5 MPa, and 4.5%, respectively. The addition of Ca can effectively refine the primary α phase and modify the eutectic Si phase, likely because Ca enrichment at the front of the solid-liquid interface led to undercooling of the alloy, reduced the growth rate of the primary α phase, and refined the grain size. Also, it could increase the latent heat of crystallization, undercooling, and the nucleation rate of eutectic Si, which was beneficial to the improvement of the morphology of eutectic Si.

Effect of in-situ formation of AlP on solidification of hypereutectic Al–Si alloy

Abstract The effects of in-situ AlP formation on the solidification process of an Al-20 %Si alloy were studied by examining the change law of the solidification microstructures after the addition of Cu–P alloy to the melt and stirring. During the early stage of stirring, the eutectic Si was clearly refined and Si mainly existed in a point-like shape. During the middle stage of stirring, the typical eutectic microstructure disappeared, and the microstructure consisted of fine primary Si uniformly distributed in the α matrix. Many AlP precipitates with sizes near the critical nucleation radius of primary Si were formed after the Cu–P alloy was dissolved in the Al-20 %Si alloy and stirred. These AlP precipitates became the nuclei of the primary and eutectic Si. The Si-poor layers around the primary Si provided the conditions necessary for the precipitation of the primary α phases when the primary Si grew. For finer primary Si, the distance between the primary Si was reduced, and lower temperature facilitated the formation of a complete α loop or continuous α phases around the primary Si. During the eutectic reaction, the ultrafine AlP caused the eutectic Si phases to nucleate alone and grow; the eutectic α phases grew based on the primary α phases, which resulted in the formation of divorced eutectic microstructures.

The Relationship between Residual Amount of Sr and Morphology of Eutectic Si Phase in A356 Alloy.

This paper studied the relationship between the residual amount of Sr and the morphology of the eutectic Si phase in A356 obtained through different modification treatment processes; additionally, the cooling rates of molds were studied. The eutectic Si phase revealed a satisfactory modification effect at residual Sr amounts above 0.01 wt % in A356 alloys cast using an iron mould. Complete modification of the eutectic Si phase could be achieved at a Sr additive amount 0.03 wt % in an A356 melt. The addition of higher amounts of Sr (~0.04–0.06 wt %) did not improve the modification effect. With the addition of 0.06 wt % Sr into A356 alloy melt and holding at 750 °C, the anti-fading capacity of Sr modification effect could be sustained for 120 minutes. More Sr is needed to obtain a good modification of eutectic Si for an A356 alloy cast using a sand mold.

Study on Solidification Process of ZL114A Wheel Casting

In this paper, three different solidification process design schemes of the ZL114A wheel casting are discussed, in according with the comparative analysis of metallurgical quality statistics, a solidification order from bottom to top and from outside to inside is attained by adjusting the chiller material and reducing the pouring temperature, which meets the HB963-2005IIcasting standards. The average tensile strength, yield strength and elongation of the T6 state are 340MPa, 260MPa and 6.5% respectively after the T6 heat treatment and the low temperature stabilization process. The average grain size of the primary α-Al matrix is about 86μm after adding 0.11%Sb element, the morphology of the binary and ternary eutectic Si phases that distributed nearby the grain boundary are spherical and elliptical, the fracture mechanism of T6 state shows a typical dimple fracture.

Impression creep characterization of cast Al–7Si–0.3Mg alloy

The creep behavior of the as cast A356 (Al–7Si–0.3Mg) alloy is investigated by the impression creep test with a flat-ended cylindrical punch over a wide range of stress between 280 and 630 MPa and at the temperature range of 493 to 553 K. The tilt casting method is used to fabricate the cast A356 alloy. The microstructure of the alloy is characterized by the dendritic primary α-Al, the eutectic Si phase and the iron-rich intermetallics (α-AlFeSi and β-AlFeSi). The stress exponent is found to be in the range of 6.1 to 7.6 and the activation energy is determined in the range of 134 to 152 kJ mol−1. It is found that the self-diffusion climb controlled creep is the operative creep mechanism. No evidence of the threshold stress is observed for the present alloy.

Effect of (Pr+Ce) Additions on Microstructure and Mechanical Properties of AlSi5Cu1Mg Alloy

The microstructure and mechanical properties of AlSi5Cu1Mg alloy with (Pr+Ce) addition were investigated by optical microscopy (OM), energy dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The results demonstrated that the rare earth (Pr+Ce) addition refined the grain. The long needle-like eutectic Si phases turned to granual. The secondary dendrite arm spacing (SADS) of the primary α-Al phase with the AlSi5Cu1Mg+0.6 wt.% (Pr+Ce) alloy reached the minimum value, which decreased by 50.2%. The mean length and the aspect ratio of the eutectic Si decreased by 78.8% and 67.4%. The ultimate tensile strength (UTS), the microhardness, and the breaking elongation of the AlSi5Cu1Mg+0.6 wt.% (Pr+Ce) alloy reached a maximum, and increased by 21.5%, 21.7%, and 8.0% compared to the AlSi5Cu1Mg alloy. The fracture examinations manifested in cleaved surfaces and brittle fracture areas, which were seen from the AlSi5Cu1Mg+0.6 wt.% (Pr+Ce) alloy. The number of dimples slightly increased.

Effect of Transition Metals Addition on the Microstructure and Incipient Melting of 354-Based Alloys

Thermal analysis of various 354 alloy melts was carried out to determine the sequence of reactions and phases formed during solidification under close-to-equilibrium cooling conditions. The main reactions observed comprised formation of the α-Al dendritic network at 598 °C followed by precipitation of the Al–Si eutectic and post-eutectic β-Al5FeSi phase at 560 °C; Mg2Si phase and transformation of the β-phase into π-Al8Mg3FeSi6 phase at 540 °C and 525 °C; and lastly, precipitation of Al2Cu and Q-Al5Mg8Cu2Si6 at 498 °C and 488 °C. As a result of the low solidification rate of the thermal analysis castings, all Zr-containing alloys are located in the L + Al3Zr region of the Al–Zr phase diagram during the melting stage. Three main reactions are detected with the addition of Ni, i.e., the formation of AlFeNi, AlCuNi and AlSiNiZr phases. Larger sizes of AlFeNi and AlCuNi phase particles were observed in higher Ni content alloys of 3.6 wt%. Mn addition helps in reducing the detrimental effects of the β-iron phase by replacing it with the less-detrimental Chinese script α-Al15(Fe,Mn)3Si2 phase and sludge particles. With the use of the multi-step solution treatment—involving higher solution temperatures and longer durations, an increased amount of incipient melting is likely to occur.

The relationship between the thermoelectric power and resultant solidification microstructures of Al-Si melt under electromagnetic field

In this paper, the thermoelectric power of hypo/hyper-eutectic Al-Si alloys melt induced by electromagnetic fields has been researched, as well as the resultant microstructures evolution corresponding to different stages of thermoelectric power. The results show that the thermoelectric power of Al-Si alloy melt is strongly sensitive to electromagnetic field, showing the sharp increment. Then thermoelectric power gradually gets back to the initial value after turning off the electromagnetic field. As for the resultant microstructures during the recovery stage of the thermoelectric power, TEM are used to capture the results that smaller Si particles grow up from the combination of Si particles after treatment. SEM observation and statistical processing data results about phase size/distribution show the recovery of eutectic Si phases, as well as the α-Al phases, indicating the pseudo growth of Si and Al clusters. Further, a recovery trend of primary Si phases also indicates the pseudo growth of Si clusters. Pseudo growth of clusters is verified by both the thermoelectric power and the resultant solidification microstructures. The recovery solidification microstructures correspond well to the thermoelectric power decrement after treatment.

Effects of Si phase refinement on the plasma electrolytic oxidation of eutectic Al-Si alloy

The plasma electrolytic oxidation (PEO) process of high-silicon aluminum alloys is hard to be started and the properties of the PEO coating is also not satisfactory, which is caused by the existence of coarse eutectic or primary silicon (Si) phase in the substrate. In this study, the Si phase in the eutectic Al-Si alloy was firstly controlled to a much smaller size by 0.1 wt % strontium (Sr) modification treatment. Subsequently, PEO was carried out on the Sr-modified Al-Si alloy. It was shown that as the eutectic Si phase in the substrate getting much smaller and more homogeneous, PEO was much easier to be started. The PEO coating was thicker and more uniform at the initial discharge stage compared with that on the un-modified eutectic Al-Si alloy. The influence of Si refinement was less pronounced after certain time of spark discharge oxidation. After 30 min treatment, the PEO coating was composed of γ-Al2O3, α-Al2O3, mullite and some amorphous phase. The PEO coating on Sr-modified Al-Si alloy showed a more compacted structure and better anticorrosion property compared with that on the un-modified eutectic Al-Si alloy.

Microstructure and mechanical properties of A356 alloy with yttrium addition processed by hot extrusion

The effects of the rare earth element yttrium (Y) and hot extrusion on the microstructure and mechanical properties of A356 alloy were investigated by mechanical properties testing and microstructure observation. The results indicate that the addition of Y improves the microstructure of the as-cast alloy. The distribution of primary α-Al is uniform and orderly. The long needle-like eutectic Si phases and β-Fe phases turn to strips and short rods. When the content of Y increases to 0.2 wt%, the mean diameter of α-Al (40.3 μm) and the aspect ratio of the eutectic Si phase (2.3) reach the minimum values, which are 68.9% and 86.1% lower, respectively, than that of the alloy without Y addition. Under extrusion stress, the shape of the eutectic Si phase is changed from long rod-like to near grain-like after solution treatment. The size of the eutectic Si phase is significantly reduced. The needle-like β-Fe phases are squeezed and broken. The mechanical properties of the as-extruded alloy are significantly improved compared to the as-cast alloy. When the rare earth content is 0.2 wt%, the ultimate tensile strength, hardness and elongation of the alloy reach the maximum values, which are 328.2 MPa, 110.4 HV and 21.3%, respectively, and increase by 42.01%, 37.71% and 481.91%, respectively, in comparison to the as-cast alloy without Y addition.

Microstructure and mechanical properties of in-situ nano γ-Al2O3p/A356 aluminum matrix composite

In this study, the nanometer in-situ γ-Al2O3 particles reinforced aluminum matrix composites was successfully fabricated by the A356 aluminum alloy and Co3O4 powder at 850 °C by means of in-situ reaction in the high temperature melt. The effects of γ-Al2O3 particle and cobalt element on the microstructure and properties of the composites were investigated in detail. The endogenous γ-Al2O3 and cobalt could modify the coarse plate like eutectic Si into fine fibrous like and refine the second dendrite arm spacing (SDAS) of the grains simultaneously. When the content of cobalt was lower than 0.57% and content of γ-Al2O3particles was lower than 0.6 vol%, the particles distributed uniformly in the grain, however, when the particulate amount was up to 0.8 vol%, the particles would distribute at the grains boundaries or form as a rodlike phase. The grain size was sharply refined to 200 μm when the particulate content was 0.8 vol%. It was found that the ultimate tensile strength (UTS), elongation of the 0.6 vol% γ-Al2O3/A356 composites had arrived at the maximum that had enhanced by 43.6% and 85.7% in comparison to the matrix respectively, which were attributed to the homogeneous distribution of the nanoparticles, the refinement of the SDAS of the α-Al grains and the modification of the eutectic Si phases.