Eutectic Si Phase Research Articles

Hot deformation behavior of a Sr-modified Al-Si-Mg alloy: Constitutive model and processing maps

Abstract Isothermal compression experiments were conducted to study the hot deformation behaviors of a Sr-modified Al-Si-Mg alloy in the temperature range of 300–420 °C and strain rate range of 0.01–10 s−1. A physically-based model was developed to accurately predict the flow stress. Meanwhile, processing maps were established to optimize hot working parameters. It is found that decreasing the strain rate or increasing the deformation temperature reduces the flow stress. The high activation energy is closely related to the pinning of dislocations from Si-containing dispersoids. Moreover, the deformed grains and the Si-containing dispersoids in the matrix are elongated perpendicular to the compression direction, and incomplete dynamic recrystallization (DRX) is discovered on the elongated boundaries in domain with peak efficiency. The flow instability is mainly attributed to the flow localization, brittle fracture of eutectic Si phase, and formation of adiabatic shear band. The optimum hot working window is 380–420 °C and 0.03–0.28 s−1.

Effects of Ti and La Additions on the Microstructures and Mechanical Properties of B-Refined and Sr-Modified Al–11Si Alloys

The effects of Ti and La additions on the microstructures and mechanical properties of B-refined and Sr-modified Al–11Si alloys were investigated in the present work. The interactions among Ti, La, B and Sr elements were discussed employing microstructure observation, thermal analysis and tensile test, respectively. It was found that the addition of 0.05 wt% B induces a transformation of eutectic Si from finely fibrous to coarsely plate-like morphology in the Al–11Si alloy with 0.02 wt%Sr modification, owing to the poisoning of IIT mechanism, and the eutectic Si grows only with TPRE mechanism. Both titanium and lanthanum can neutralize the co-poisoning effect between Sr and B in the Al–11Si alloy, but the neutralizing effect of La is dependent on the addition sequence. The combinative addition of La and B elements promotes the effective refinement of α-Al grains, but an inhomogeneous modification of eutectic Si phases is also observed, leading to a slightly decrease in the elongation.

Microstructure and mechanical properties of foundry Al-Si-Cu-Hf alloy

Solidification microstructure, precipitation microstructure, and mechanical properties of foundry Al-Si-Cu-Hf alloy were investigated to elucidate the role of Hf during the solidification and precipitation processes. In the as-casting condition, α-Al dendrites, eutectic Si phases, and intermetallic compounds (Al2Cu, the needle-like β-Fe and Chinese-script α-Fe) were observed in the base alloy. However, no β-Fe was detected in the modified alloy, strongly indicating that Hf addition promotes the formation of the α-Fe phase but suppress the formation of the β-Fe phase. Furthermore, Hf addition can result not only in the formation of Hf-containing intermetallic compounds with different morphologies, which were identified as Si2Hf, but also, more importantly, in a significant decrease in the size, number, density, and volume fraction of porosity. After treatment with solution at 520 °C for 20 h, the formation of thermally stable Si2Hf precipitate was observed. After aging treatment at 180 °C up to 72 h, a delayed aging hardening was observed, which may be because Hf addition suppressed the formation of θ’ precipitate. Clearly, Hf addition in Al-Si-Cu alloy can modify the solidification microstructure and the precipitation microstructure, which is believed to be beneficial for mechanical properties. Indeed, a significant increase in elongation by 51.4% without a sacrifice of strength was obtained, compared with the unmodified Al-Si-Cu alloy. This investigation highlights the importance of a well-controlled solidification microstructure and precipitation microstructure for the improvement of mechanical properties, in particular to elongation.

Preparation of Al matrix nanocomposites by diluting the composite granules containing nano-SiCp under ultrasonic vibaration

In this work, an efficient process by diluting the nano-SiCp/Al composite granules in the molten matrix under ultrasonic vibration (UV) was developed to prepare metal matrix nano-composites (MMNCs). Millimeter-sized composite granules with high content of SiC particle (8wt%) were specially fabricated by dry high-energy ball milling (HBM) without process control agent, and then remelted and diluted in molten Al alloy under UV. The MMNCs melt was finally squeeze cast under a squeeze pressure of 200MPa. Microstructure of the composite granules during dry HBM was investigated, and the effect of UV on microstructure and mechanical properties of the MMNCs was discussed. The results indicate that nano-SiC particles are uniformly distributed in the nano-SiCp/Al composite granules, which are covered by vestures of pure Al. During diluting, nano-SiC particles released from the composite granules are quickly dispersed in the molten matrix by UV within 4min. Microstructure of MMNCs is significantly refined under UV and squeeze casting, eutectic Si phase modified to fine islands with an average length of 1.4μm. Tensile strength of the squeeze cast MMNCs with 1wt% of nano-SiC particles is 269MPa, which is improved by 25% compared with the A356 alloy matrix.

Effects of stirring parameters on the rheocast microstructure and mechanical properties of aluminum alloy ADC12

In this study, effects of stirring time and stirring speed on the microstructure of semisolid rheocast (SSR) aluminum alloy ADC12 were examined. The research method was done by gravity casting using a metal mold. The aluminum ADC12 slurry is stirred by a mechanical stirrer (round rod stirrer) at 610OC with a variation of speed 0, 250, 300, 350 rpm for 0, 20, 40, 60 seconds. Furthermore, the aluminum slurry of ADC12 is poured on a metal mold with temperature 600OC. The microstructure characteristics were examined by direct observation using optical microscopy, secondary -Al phase dendrite arm spacing, shape factor, and Si eutectic phase were identified. The mechanical properties were investigated by hardness test and tensile test. A short period of stirring below the liquidus temperature to form a non-dendritic structure. The final morphology of the primary particles after a short period of stirring time has little impact as the stirring time increases. The optimal mechanical properties (hardness and tensile strength) were obtained at 20 seconds of stirring time and 300 rpm of stirring rate.

Investigation of Modification Mechanism of Eutectic Si Phase in Al-7%Si Alloy

Investigation of Modification Mechanism of Eutectic Si Phase in Al-7%Si Alloy

The Influence of Cold Forging Process on the Microstructure, Porosity and Wear Behavior of Spray Formed Al-Si Alloys

The microstructural characteristics, porosity, hardness and wear behavior have been investigated for cold forged spray formed Al-6Si alloy. The Comparison of these characteristics has been done among the without forged samples and uniaxially cold forged samples with 25%, 50% and 75% thickness reduction. The microstructure of spray formed Al-6Si alloy showed the equiaxed grain morphology for the α-Al phase and uniformly dispersed eutectic Si phase at the interparticle boundaries. Uniaxial cold forging decreased the porosity level and thereby increasing the hardness and refining the microstructure with elongation of grains normal to the direction of forging. Dry sliding wear tests were performed on pin on disc machine with 1-5 kg loads for without forged and forged samples. The samples with 75% thickness reduction showed good wear resistance than that of without forged, 25% and 50% thickness reduced samples. The wear behavior is discussed in the light of microstructural evolution during cold forging. The worn surfaces were characterized by SEM-EDS and the debris collected during wear test was characterized by XRD.

Quantitative Evaluation of Eutectic Si Phases Distributions and Effect on Mechanical Properties in JIS AC4CH Aluminum Casting Alloy

Quantitative Evaluation of Eutectic Si Phases Distributions and Effect on Mechanical Properties in JIS AC4CH Aluminum Casting Alloy

Effect of Samarium (Sm) Addition on the Microstructure and Tensile Properties of Al–20%Si Casting Alloy

In the present work, the effect of Sm addition (0.2, 0.4, 0.6, and 0.8 wt%) on the microstructure evolution and tensile properties of as-cast hypereutectic Al–20%Si alloy were investigated. The scanning electron microscopy, X-ray diffraction (XRD), and electron probe microanalysis with wavelength dispersive spectroscopy (WDS) were used to quantitatively characterize the samples. The results indicated that the primary Si phases were significantly refined from coarse polygonal and star-like shape to fine block with smooth edges and corners when the addition of Sm increased from 0 to 0.6%. Furthermore, eutectic Si phases were fully modified from coarse platelet-like/needle-like structure to fine fibrous structure and discrete particles with the addition of 0.6%Sm. However, the primary and eutectic Si phases became coarser when the level of rare earth Sm reached 0.8%. The analysis of XRD and WDS shows that Sm mainly exists in the form of Al2Si2Sm compounds. In addition, the tensile properties of as-cast Al–20%Si alloy with different concentrations of Sm were investigated by tensile test. It was found that the ultimate tensile strength (UTS) and elongation (EL) gradually enhanced with the increasing concentration of Sm from 0 to 0.6% due to decrease of the size and the change of morphology on Si phases. Compared with unmodified Al–20%Si alloy, the UTS and EL values of Al–20%Si alloy with 0.6%Sm were increased by 48.5 and 68.8%, respectively.

Analysis of eutectic silicon modification during solidification of Al-6Si using in-situ neutron diffraction

Aluminum-silicon alloys contain coarse, acicular Si plates that are detrimental to material properties. However, trace additions of Sr have been found to modify the eutectic Si to have a fine, fibrous structure, which improves elongation, strength, and thermal conductivity. In this study, in-situ neutron diffraction was conducted during the solidification of unmodified and Sr-modified binary Al-6 wt.% Si to obtain a novel view of the modification mechanism. Neutron diffraction intensity data was collected at temperatures ranging from 660 °C to 200 °C and was integrated to create fraction solid curves for the individual Al and Si phases in each alloy. Scanning electron microscopy revealed that the Sr-modified alloy microstructure contained many micro-sized Al-Si-Sr intermetallics scattered about the eutectic regions at the Al-Si interfaces, and a few Si flakes close to some β-(Al,Si,Fe) intermetallics, suggesting important interactions between both the Fe-bearing phases and Sr in the solidification of eutectic Si. Comparing the isolated phase evolution temperatures for each alloy revealed in-situ for the first time that Sr suppresses the nucleation temperatures of both the eutectic Al and eutectic Si phases by several degrees. Accordingly, the eutectic reaction was observed to evolve at a higher primary Al solid fraction with Sr modification.

Microstructural evolution and mechanical properties of Al-7Si-0.3Mg alloys with erbium additions

The mechanical properties of hypoeutectic Al-Si foundry alloys are generally controlled by their microstructures. The present study was performed to determine the influence of erbium (Er) in Al-7Si-0.3Mg alloys on the microstructures and mechanical properties. The evolution of microstructure was studied by thermal analysis and interrupted solidification technique. Optical microscopy, scanning electron microscope, and energy-dispersive X-ray were employed to characterize the alloy microstructures. The results demonstrated that the addition of Er significantly refined both the secondary dendrite arm spacing (SDAS) and the primary α-Al grain. Simultaneously, Er modified eutectic Si phase from coarse acicular plate-like structures into fine fibrous features. We observed that Er additions strongly suppress the nucleation of eutectic Si due to the preferential formation of ErP instead of AlP. Er additions change the eutectic growth mode from nucleation and growth on AlP particles to the propagation of a defined eutectic front from the mold walls. Moreover, Er addition that was cast for different holding times indicated that there was no fading effect on the SDAS, primary α-Al grain size, and eutectic Si morphology. Importantly, the tensile properties of the Er-modified alloys at as-cast and T6 conditions can be enhanced due to simultaneous refinement both the SDAS and the α-Al grains and modification of the eutectic Si.

Strength and strain hardening of a selective laser melted AlSi10Mg alloy

In this paper, we report that a selective laser melted (SLM) AlSi10Mg alloy has unique hierarchical microstructures. There are micro-sized Al grains and transgranular ultrafine cellular structures around primary Al, and the cell boundary is made up of alternate eutectic Si and eutectic Al phases. The SLM AlSi10Mg alloy has both high tensile strength and high strain-hardening capability, which are superior to Al-Si alloys fabricated by conventional powder metallurgy and cast methods. Post-tensile high-resolution microscopy analyses provide further evidence supporting Orowan looping as a strengthening mechanism of the SLM Al-Si alloy.

急冷凝固したAl-11%Si-2.5%Cu合金の共晶Si相形態の変化と機械的性質への影響

急冷凝固したAl-11%Si-2.5%Cu合金の共晶Si相形態の変化と機械的性質への影響

Al-7%Si合金の共晶凝固における共晶Si相の先行と形態遷移

Al-7%Si合金の共晶凝固における共晶Si相の先行と形態遷移

Effect of Rare Earth Er on Microstructure and Mechanical Properties of Cast Al-Si-Mg Alloy

It is well known that Al-Si-Mg alloy can form Chinese script eutectic Mg2Si and large blocky primary Mg2Si phases. In this paper, the microstructures and mechanical properties of cast Al-18Si-10Mg-xEr (x=0, 0.3, 0.5, 0.8 and 1.0 wt.%) alloy were investigated. The as-cast samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) with wavelength dispersive spectroscopic (WDS). The results show that the morphology of primary Mg2Si changed from large block or Chinese script-like to fine block with increasing of Er contents from 0.3% to 1.0%., and the morphology of eutectic Si phases transformed into fine coral-like fibrous structure and the size of eutectic Si dramatically decrease. The mechanical properties were researched by tester with different concentration of rare earth Er. It was found that the ultimate tensile strength and hardness increased by 32.1% and 21.4%, respectively.

Effect of combinative addition of Ti and Sr on modification of AA4043 welding wire and mechanical properties of AA6082 welded by TIG welding

To improve the mechanical properties of AA6082 weld welded by tungsten inert gas welding using AA4043 welding wire, the effect of addition of Ti and/or Sr on continuous cast and rolled AA4043 welding wire was investigated. Experimental results indicated that Ti and Sr are excellent modifiers, which improve the microstructure of the AA4043 welding wire and enhance the mechanical properties of the AA6082 weld. It was found that the combinative addition of Ti and Sr can effectively modify both the α(Al) dendrites and eutectic Si phases compared with individual addition of Ti or Sr. In addition, Ti and/or Sr also changed the microstructure of the AA6082 weld. The tensile strength of the AA6082 weld reached the maximum value when 0.08% Ti and 0.025% Sr were added simultaneously. These results indicate that the combinative addition of Ti and Sr can be an effective composite modifier.

Effect of combinative addition of mischmetal and titanium on the microstructure and mechanical properties of hypoeutectic Al-Si alloys used for brazing and/or welding consumables

Effects of mischmetal (RE) and/or Ti modifier on the microstructure including α-Al dendrites, eutectic Si phases and other secondary phases of Al-Si brazing and/or welding alloys were investigated by differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM). The DSC results showed that an addition of RE decreased the eutectic temperature and caused supercooling, promoting the nucleation of eutectic Si crystals. In addition, the maximum temperature of the first endothermic peak varied with the different RE contents, which had a good correlation with the microstructural modification of the eutectic Si phase. The α-Al dendrites were well refined by increasing the cooling rate or adding 0.08 wt.% of Ti. When 0.05 wt.% RE was added to the Al-5Si-0.08Ti alloy, the morphology of eutectic Si phase was transformed from coarse platelet to fine fibers and the mechanical properties of the resulting welding rod were well improved. Whereas, when excess RE was added, a large number of β-Fe phases appeared and the aspect ratios of β-Fe phases increased. The morphologies and chemical components of two kinds of RE-containing intermetallic compounds (IMCs) were also discussed.

Effects of the Addition of Mg on the Microstructure and Mechanical Properties of Hypoeutectic Al–7%Si Alloy

It is well known that the mechanical properties of hypoeutectic Al–Si alloys are influenced by the size, morphology and distribution of primary α-Al dendrites, eutectic Si crystals and other intermetallics. In the present work, the effect of Mg addition on the size and morphology of the primary α-Al dendrites and eutectic Si phases in Al–7%Si alloy has been investigated. The microstructure of as-cast samples with different Mg content was characterized by scanning electron microscopy. Microstructural analyses demonstrated that primary α-Al was significantly refined from coarse dendrites to fine equiaxed grain with increasing the amounts of Mg. In addition, the eutectic Si structure was modified from coarse needle-like/platelet-like structure to a fine coral-like fibrous structure. The mechanical properties were investigated by tensile test with various concentrations of Mg. It was found that the ultimate tensile strength improved by 61% with an increase in Mg content. However, the ductility initially increased by 80% with an increase in Mg content from 0.5 to 1.5%, then it decreased with further increasing Mg content up to 2.0 and 2.5%. It is due to precipitation of coarse Chinese script Mg2Si phase that leads to the increase in strength and the decrease in ductility.

Influence of Strontium Addition on Microstructure and Mechanical Properties of an Al–10Si–5Cu Alloy

The microstructure and mechanical properties of Al–10Si–5Cu cast alloys with micro-addition of alloying elements (V, Cr and Ni) were studied before and after strontium addition. Samples were examined using the X-Ray diffraction, the optical microscope, the scanning electron microscope and the energy dispersive spectrometer. The results indicated that the α-Al matrix, eutectic Si phase and Al2Cu phase were the main constituent phases of Al–10Si–5Cu alloys before or after strontium addition. Strontium addition affected the refining of the α-Al grains and transforming the configuration of interdendritic phases. The un-modified alloy showed a brittle nature because of existing brittle and aggregated AlSiMnFe phases. Contributing to the alteration of microstructure in strontium modified alloy, the strength and elongation of the alloy were improved. In addition, the fracture mechanism and crack propagation process were investigated in both the alloys.

Microstructural mechanisms and advanced characterization of long and small fatigue crack growth in cast A356-T61 aluminum alloys

Fatigue crack growth-based design is a significant modern engineering consideration for the transportation sector, and its implementation requires accurate characterization and understanding of crack propagation mechanisms with respect to microstructure. To support this goal, long and small fatigue crack growth studies were conducted on widely used A356-T61 cast aluminum alloys in various microstructural conditions. Microstructural variations were created through processing and chemistry means in order to systematically investigate the individual and combined effects of the materials’ characteristic microstructural features on fatigue crack growth at all growth stages. Crack growth mechanisms and failure mode transitions are identified with respect to the eutectic Si morphology/distribution and grain structure by fractographic techniques and electron backscatter diffraction. Crack-microstructure interactions were investigated in depth across all crack sizes, and the respective roles of microstructural features were identified experimentally and further corroborated by numerical models. It is concluded that the eutectic Si phase enhances the alloys’ fatigue crack growth resistance in early growth stages (by transferring stresses off of the α-Al matrix), and progressively decreases due to damage localization. In later growth stages, the eutectic Si phase becomes increasingly detrimental to fatigue crack growth resistance because of its inherently low debonding strength and brittle fracture, as evidenced by the crack selectively following eutectic Si colonies.