Hypoeutectic Compositions Research Articles

Preparation Technology of Al-Fe-V-Si Heat-Resistant Aluminum Alloy with Single Roller Spinning Method

High strength aluminum alloy is mainly traditional alloy hypoeutectic composition, containing the solid solubility of alloy elements atomic fraction is greater than 2, and reached the alloy trengthening effect by the precipitation of intermetallic compounds in the process of aging. But in more than 150°C, the precipitated phases coarsening, the material performance dramatically, thus limiting its use scope. In the preparation process, the planar flow casting process will melt thrown by 35-80um thick slices with, because the cooling speed can reach 105K/s the process of melt, so the microstructure is very fine, after extrusion. Grain size of about 1um, precipitates spherical and uniform distribution, the size of about 50nm, Al12(Fe,V)3Si phase coarsening rate is low, so the alloy has comprehensive mechanical performance and excellent high temperature stability.

Phase selection and microstructure formation in undercooled Co–61.8 at.% Si melts under various containerless processing conditions

The hypoeutectic composition Co–61.8at.% Si was undercooled and solidified using electromagnetic levitation, electromagnetic levitation under a static magnetic field, electrostatic levitation and glass-fluxing. The samples generally showed two thermal events, either separated or continuous depending on undercooling. In situ monitoring of the two thermal events with a high-speed camera revealed a sudden decrease of dendritic growth velocities of primary phases at a critical undercooling of 88K. Scanning electron microscopy studies of the solidified samples showed that the CoSi compound and the CoSi2 compound nucleate as the primary phase for low and high undercoolings, respectively. The microstructure of the samples depends not only on undercooling, but also on the onset temperature or delay time of the second thermal event. Melt convection has no effect on the primary phase selection in undercooled melts, but it has a significant effect on the delay time and therefore on microstructure formation of the samples for high undercoolings.

Fractography as a tool to assess the occurrence of fatigue fractures in complex microstructure structural components

Fractographic assessment of fatigue fractures may be difficult if they occur in metallic components characterized by low ductility complex microstructures. In these cases reconciliation of known fatigue rupture mechanisms with fractographic appearance of fatigue fractured surfaces is challenging. Special techniques coupled with theory development may be necessary. Pearlitic steels or steels with predominant pearlitic microstructures are among the ones that are visited and their fatigue fractures interpreted. Analogously, fatigued Al foundry alloys, with hypoeutectic Al-Si compositions, are also illustrated.

Fractography as a tool to assess the occurrence of fatigue fractures in complex microstructure structural components

Fractographic assessment of fatigue fractures may be difficult if they occur in metallic components characterized by low ductility complex microstructures. In these cases reconciliation of known fatigue rupture mechanisms with fractographic appearance of fatigue fractured surfaces is challenging. Special techniques coupled with theory development may be necessary. Pearlitic steels or steels with predominant pearlitic microstructures are among the ones that are visited and their fatigue fractures interpreted. Analogously, fatigued Al foundry alloys, with hypoeutectic Al-Si compositions, are also illustrated.

Study on in situ Al-Si functionally graded materials produced by traveling magnetic field

AbstractThe aim of this contribution was to investigate the microstructure of in situ Al-Si functionally graded materials produced by traveling magnetic field. The research shows that the composition and associated microstructural feature of Al-Si alloys processed by this method changes from the outer of samples to the inner, respectively, from Al-Si hypereutectic with particles of primary Si to Al-Si eutectic to hypoeutectic composition with a great number of primary Al dendrites. Moreover, the hardness, the wear resistance of samples and the volume fraction of primary Si particles all have obviously gradient characteristics in the samples.

Microstructure and properties of Ni–Ni3Si composites by directional solidification

Ni–Ni3Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni3Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni–Ni3Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni–Ni3Si eutectic composites display regular lamellar eutectic structure at the solidification rate R=6.0–40.0 μm/s and the lamellar spacing is decreased with the increase of the solidification rate. Moreover, the Ni–Ni3Si hypoeutectic composites present lower micro-hardness than pure Ni3Si, which indicate Ni–Ni3Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni–Ni3Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni31Si12 phase in the Ni–Ni3Si eutectic composites.

Microstructure Characteristics of Fe-xCr-C Hardfacing Alloys

A series of Fe-xCr-C hardfacing alloys were successfully fabricated on a substrate of 20g steel by gas tungsten arc welding (GTAW) process. These claddings were disigned to observe hypoeutectic, near-eutectic, and hypereutectic structures with various Cr3C2 and M7C3 carbides at room temperature. According to X-ray diffraction(XRD) and optical microscopy (OM), the hypoeutectic and eutectic composites include the Fe-Cr solid solution(α+A) and Cr3C2 carbides. Hypereutectic structures consist of α+A+M+M7C3 respectively. The cladding of hypereutectic microstructure with great amounts of primary M7C3 carbides had the highest hardness of the all conditions.

Mechanical Spectroscopy Investigation of Liquid Pb-Bi Alloys

Liquid Pb–Bi eutectic alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system. The alloy has been characterized in liquid state from melting (125 °C) to 650 °C by mechanical spectroscopy. Experiments have been carried out using hollow reeds of austenitic stainless steel filled with the Pb-Bi alloy and sealed at the extremities. From 350 °C to 520 °C modulus shows a remarkable drop accompanied by a broad internal friction maximum. In the same temperature range radial distribution functions, determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The anelastic phenomena have been attributed to a structural re-arrangement of liquid metal. For comparison, other alloys of the Pb-Bi system with hypo-eutectic composition have been investigated.

Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9wt% and 0.67wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low ΔK levels up to a mid-ΔK of ∼7MPa√m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of ΔK, the crack increasingly seeks out Si and intermetallic particles up to a ΔK of ∼9MPa√m after which the crack preferentially propagates through intermetallic particles in the 0.67wt%Si alloy or Si and intermetallics in the 6.9wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-ΔK. However, crack closure appears unimportant at high ΔK due to the large crack openings and evidenced by the fast crack growth rates observed.

Tough Hypoeutectic Zr-Based Bulk Metallic Glasses

Significant softening of Zr-based bulk metallic glasses (BMGs) can be seen in a hypoeutectic Zr-enriched composition, which brings about very high toughness (for the Zr55Cu35Al10 and Zr60Cu30Al10 BMGs) and tensile plasticity at room temperature (for the Zr70Ni16Cu6Al8 BMG). The unique features of such BMGs include the formation of multiple shear bands and harmonic alternating movements that can immediately accommodate concentrated stresses and avoid accidental catastrophic fracture.

Microstructure Evolution of Atomized Al-0.61 wt pct Fe and Al-1.90 wt pct Fe Alloys

The microstructure evolution of impulse atomized powders of Al-0.61 wt pct and Al-1.90 wt pct Fe compositions have been investigated with a scanning electron microscope, transmission electron microscope, neutron diffraction, and backscattering electron diffraction (EBSD). Both hypoeutectic and hypereutectic compositions demonstrated similar macrostructure (i.e., primary α-Al dendrites/cells with eutectic Al-Fe intermetallics decorated at the dendritic/cellular walls). Selected area electron diffraction (SAED) analysis and SAED pattern simulation identified the eutectic Al-Fe intermetallic as AlmFe (m = 4.0-4.4). This is verified by neutron diffraction analysis. Cubic texture was observed by EBSD on the droplets with dendritic growth direction close to 〈111〉. The possible reasons are discussed.

Effect of Zinc Content on Microstructural Evolution and Electrification-Fusion-Induced Failure Mechanism of Sn-<I>x</I>Zn Alloys

Microstructural features of Sn-xZn alloys with varying Zn content of 7, 9, 20, 30 mass% on the electrification-fusion phenomenon were investigated in this study. Experimental results showed that the critical fusion current density (CFCD) of Sn-xZn alloys increased with increasing Zn content. The enrichment of Zn-rich phase was the main factor in the improvement of electrical conductivity and the required electrical current density for triggering microstructural evolution for the hypereutectic Sn-30Zn alloy was much higher than the hypoeutectic Sn-7Zn alloy. There is an obvious difference in the increase rate of CFCD from the hypoeutectic composition (Sn-7Zn) to the eutectic composition (Sn-9Zn) due to the microstructural evolution with increasing Zn content. Through the in-situ examination of microstructural evolution during electrificationfusion tests, the initial site of electrification-fusion-induced failure was significantly emerged from the Sn/Zn eutectic phase for both the hypoeutectic composition (Sn-7Zn) and the hypereutectic composition (Sn-30Zn). The fusion behavior of Sn-7Zn was dominated by double massive fusion regions on Sn/Zn eutectic phase and � -Sn phase, whereas the fusion behavior of Sn-30Zn was dominated by massive fusion regions only on Sn/Zn eutectic phase. [doi:10.2320/matertrans.M2010259]

Investigation of the solidification behavior of Al 2O 3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al 2O 3/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al 2O 3/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al 2O 3/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al 2O 3/YAG binary eutectic and fine Al 2O 3/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al 2O 3/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.

Electrochemical corrosion parameters of as-cast Al–Fe alloys in a NaCl solution

The aim of this study is to evaluate the general electrochemical corrosion resistance of Al–Fe alloys in the range of hypoeutectic compositions, Al–0.5 wt.% Fe and Al–1.5 wt.% Fe alloy. EIS plots, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters in a 0.5 M NaCl solution at 25 °C. It is shown that for an Al–0.5 wt.% Fe alloy, coarse cells tend to improve the corrosion resistance mainly due to the reduction in cellular boundaries and for an Al–1.5 wt.% Fe alloy, an opposite trend has been detected.

Characterization of the Intermetallic Compounds Formed during Hot Dipping of Electrical Steel in a Hypo-Eutectic Al-Si Bath

In order to improve the magnetic properties of electrical steels, it may be desirable to increase the Si and/or Al content of the steel. A possible and alternative route to realize this is through the application of an Al-Si-rich coating on the steel substrate using a hot dipping process, followed by a diffusion annealing treatment. Previously, a series of compositions were used for dipping, namely: pure Al, Al + 10wt% Si (hypo-eutectic composition) and Al + 25wt% Si (hypereutectic composition). After these dipping experiments, and the subsequent evaluation of the coating and its formed intermetallic phases, the use of a hypo-eutectic Al-Si-bath was recommended for further investigation, because of certain advantages: i.e. hypo-eutectic concentrations allow lower dipping temperatures and reduce the formation of ordered Fe-Si-structures that cause brittleness in the coating and substrate. The present work reports on the results obtained on materials that were hot dipped in a hypo-eutectic Al-Si bath. An Al + 1wt%Si bath was used to coat electrical steel substrates with different silicon contents with dipping times, varying between 0 to 20 seconds, after a preheating of the samples to a temperature of 700°C. A thorough characterization of the formed intermetallics was made by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). Three different compounds were identified as Fe2Al5, FeAl3 and a nearly pure Al phase.

Heat-Treatment Effects on the Microstructure and Tensile Properties of Powder Metallurgy Ti-6Al-4V Alloys Modified with Boron

The Ti-6Al-4V (Ti-64) alloys modified with two levels of boron (1B and 1.7B (wt pct)) representing hypoeutectic and hypereutectic compositions, produced via a prealloyed powder metallurgy approach, were subjected to various standard heat treatments of Ti-64 to study the microstructural evolution and its influence on tensile properties. Boron-modified Ti-64 (Ti-64B) alloys exhibited differences in microstructural response to heat treatment compared to that of Ti-64 due to variations in constituent phase fractions and the influence of TiB on the beta-to-alpha phase transformation kinetics. The tensile elastic modulus of Ti-64B alloys increased nearly linearly with the boron content (or TiB volume fraction) and the increase could be satisfactorily predicted with an isostrain rule of mixtures (ROMs) and the Halpin–Tsai model. The Ti-64-1B possessed a good combination of tensile strength (1200 to1370 MPa) and ductility (10 to 13 pct), while Ti-64-1.7B exhibited high strength (1300 to 1695 MPa) and modest ductility (2 to 3.5 pct). Coarse primary TiB particles present in Ti-64-1.7B were found to initiate premature failure. Strength modeling revealed that load sharing by the micron-sized TiB whiskers provides the major contribution for the increase in yield strength.

Strong, Ductile Magnesium-Zinc Nanocomposites

Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesium-zinc alloys resulting in strong, ductile, and castable materials. By ultrasonically dispersing a small amount (less than 2 vol pct) of SiC nanoparticles, both the strength and ductility exhibit marked enhancement in the final casting. This unusual ductility enhancement is the result of the nanoparticles altering the selection of intermetallic phases. Using transmission electron microscopy (TEM), the MgZn2 phase was discovered among SiC nanoparticle clusters in hypoeutectic compositions. Differential thermal analysis showed that the MgZn2 formation resulted in elimination of other intermetallics in the Mg-4Zn nanocomposite and reduced their formation in Mg-6Zn and Mg-8Zn nanocomposites.

Microstructure and mechanical properties of a directionally solidified Al2O3/Y3Al5O12/ZrO2 hypoeutectic in situ composite

Directionally solidified ternary Al2O3/Y3Al5O12(YAG)/ZrO2 hypoeutectic rod composites were successfully fabricated by the laser zone remelting technique. The microstructure and mechanical properties of the composite were investigated. The microstructure presented a complex three-dimensional network structure consisting of fine Al2O3 (41vol.%) and YAG (49vol.%) phases, with smaller ZrO2 (10vol.%) phases partially distributed at the Al2O3/YAG interfaces. The irregular growth behavior in the hypoeutectic was revealed. The hardness and fracture toughness at ambient temperature were measured to be 17.3GPa and 5.2MPam1/2, respectively. The toughness enhancement in comparison with previous binary Al2O3/YAG composites was mainly attributed to the refined microstructure, and crack deflection, branching and bridging. Moreover, the residual stresses, generated by different thermal expansion coefficients of the component phases, also importantly contributed to the improved toughness. Correlations between the addition of the third component ZrO2 and the microstructure and properties were discussed as well.

FeCo-based multiphase composites with high strength and large plastic deformation

A family of FeCo-based multiphase composites with a microstructure consisting of nano-lamellar phase strengthened α-(Fe,Co) dendritic cores surrounded by a network of reinforcement phases of ultrafine eutectics was produced by copper mold casting. The hypoeutectic composites exhibit a high yield stress, which is up to 7 times higher than the equiatomic FeCo alloy, and plastic deformation up to 18% during compressive test. Multiscale o-(Fe,Co) 3(B,C) reinforcement phases are responsible for the remarkable improvement of strength, and α-(Fe,Co) dendrites play a key role to inhibit the propagation of microcracks sourced from the eutectics. Furthermore, a fracture model for explaining the relationship between fracture strain and morphologic characteristics of the composites is presented.

Predicting the Glass-Forming-Ability of Alloys by Molecular Dynamics Simulation: A Working Example of Ti–Co Bulk Metallic Glasses

Predicting the glass-forming-ability (GFA) of alloys before experimental trials has long been a pursuit in the exploration of bulk metallic glasses (BMGs). Researchers can determine GFA criteria, such as reduced glass-transition temperature (Trg) and gamma factor concluded only from experimental data in prepared amorphous metals, but not beforehand. This study predicts the GFA of binary TixCo100-x system, where x = 60, 70, 77 and 84 (hyper-eutectic, eutectic to hypo-eutectic compositions) by calculating their Trg using molecular dynamics simulation with modified tight-binding potentials. The same compositions were also melted and injection-cast into 1 mm rods. Experiment tests confirm that the compositions Ti60Co40 and Ti70Co30, having calculated Trg> 0.5, were able to produce BMGs. Both calculation and differential thermal analysis indicate that Ti–Co BMGs have exceptionally high glass-transition temperatures.