Volume Fraction Of Eutectic Phase Research Articles

Effects of antimony on eutectic silicon and fracture mode of A357 alloy at different cooling rates

AbstractThe effects of antimony on eutectic silicon and fracture mode of A357 alloy were investigated at slow and fast cooling rates by scanning electron microscopy, transmission electron microscopy, polarizing microscope, differential scanning calorimetry, optical microscope and tensile testing. The slow cooling rate is 0.16 °C/s, and the fast cooling rate is 3.20 °C/s. The results show that with antimony addition, the eutectic silicon of A357 alloy is modified. Antimony does not modify eutectic silicon of A357 alloy by increasing the number of twins. Under the same cooling rate condition, with antimony addition, the volume fraction of eutectic phases increases, and when studying the modification of eutectic silicon of some modifiers, we also need to study the eutectic phases including eutectic silicon and eutectic α‐aluminum. With antimony addition, tensile strength and elongation of A357 alloy increase, especially under fast cooling rate conditions. With the increase of elongation, the fracture mode of A357 alloy changes from I to I+II to I+II+III.

Effect of Gd content on hot-tearing susceptibility of Mg-6Zn-xGd casting alloys

The hot-tearing susceptibility (HTS) of Mg-6Zn-xGd (x=0.5, 1, 2, 3, 4, 6) alloys was evaluated using a constrained rod casting (CRC) method. The results show that the HTS curve follows a typical “Λ” shape with the increase of Gd content. The Mg-6Zn-2Gd alloy has the highest while Mg-6Zn-6Gd alloy has the lowest HTS value. The hot-tearing behavior characteristics of Mg-6Zn-xGd alloys were further studied through a multifunctional hot-tearing test device. According to the dendrite contact point inferred from the stress curve, the Clyne-Davies criterion was modified and found to be accurate in predicting the HTS of Mg-6Zn-xGd alloys. Microstructure observation reveals that the grain size and the volume of eutectic liquid are the two key factors affecting HTS of Mg-6Zn-xGd alloys. The large grain with columnar structure can easily promote initiation and propagation of hot-tearing due to the poor feeding and coordinating deformation capability, which have a harmful effect on HTS. A higher volume fraction of eutectic phase can refill the cracking and provide continuous feeding channels by dendrite bridge and thicker liquid film, thus increase the hot-tearing resistance.

Effect of lanthanum content on microstructure and mechanical properties of Al–5Mg–2Si-0.6Mn alloy in squeeze casting

The effect of Lanthanum content on the microstructure and mechanical properties of Al–5Mg–2Si-0.6Mn alloy in squeeze casting was studied. The microstructure of the alloy was characterized by scanning electron microscope (SEM) and X-ray diffractometer (XRD). The mechanical properties of the alloy were tested by tensile testing machine. The results show that the rare earth Lanthanum has a significant effect on the secondary dendrite spacing and the volume fraction of α-Al + Mg2Si eutectic phase, both of which increase at first and then decrease with the increase of Lanthanum. When the content of Lanthanum is 0.05 wt.%, the maximum volume fraction of eutectic phase is 45.09%. When the content of Lanthanum is 0.05 wt.%, the minimum secondary dendrite spacing is 25.85 μm. When the addition of Lanthanum is more than 0.2 wt.%, the bulk LaAlSi phase is formed in the alloy, which is not coherent with the matrix, which reduces the mechanical properties of the alloy. When the addition of Lanthanum is 0.1 wt.%, the microstructure and mechanical properties of the alloy are the best. The ultimate tensile strength, yield strength and elongation of the alloy reached the maximum, which were 274 MPa, 197.2 MPa and 5.7%, respectively.

Nanoparticle promoted solution treatment by reducing segregation in AA7034

The zinc-rich aluminum alloy AA7034 offers the highest tensile strength among all commercial aluminum alloys. However, the commercial AA7034 alloy was produced by Rapid Solidification. At a slow cooling rate, the high-alloying of AA7034 produces coarse secondary phases, causing severe segregation at interdendritic regions, and difficulties in later solution treatment. Nano-treating, by introducing a small volume fraction of nanoparticles, was applied to AA7034 alloy to successfully modify the microstructure and promote solution treatment at a slow cooling rate of 0.25 K/s. Primary phases were refined and the volume fraction of eutectic phases was modified. A modified local solute redistribution equation (LSRE) was applied to show how nanoparticles change the volume fraction of the eutectic phase by altering the viscosity of the liquid and the permeability of the coherent solid network in the terminal stage of solidification. In particular, by adding 1.0 to 2.0 vol percent of nanoparticles, the eutectic phases were not only refined and much more homogeneously distributed, but also their volume fractions were reduced by 24.5% and 30.1% respectively after down-hill casting. The modified microstructures enabled by nanoparticles allowed an accelerated solution treatment as well. The dissolution of the refined and distributed secondary phases was more complete during solution treatment. The nano-treated AA7034 also exhibited high strength and reasonable ductility after extrusion and heat treatment. The nano-treating approach has great potential to make conventional casting applicable to the difficult-to-cast/heat-treatable alloys through refined microstructure and promoted solution treatment.

X-ray Computed Microtomography Characterization of Ti6Al4V/CoCrMo Biomedical Composite Fabricated by Semi-solid Sintering

The semi-solid sintering of a novel Ti6Al4V/CoCrMo biomedical composite was investigated for the first time by semi-in situ X-ray computed microtomography. Composite was fabricated by dry mixing 15 vol% CoCrMo reinforcing particles with Ti6Al4V particles as matrix. The mixture was poured into a 1 mm diameter quartz capillary. 3D images of the same sample were acquired before and after sintering at different temperatures between 1050 and 1130 °C. Qualitative and quantitative analysis of the 3D images was carried out to observe the evolution of the microstructure. Formation and evolution with temperature of eutectic phase were monitored. It was found that the eutectic reaction started between Ti6Al4V and CoCrMo particles at 1050 °C, first in the contact area and then the liquid of eutectic phase was distributed over the surface of Ti6Al4V particles and filled the interparticle pores. Size and sphericity of Ti6Al4V and CoCrMo particles diminished as the volume fraction of eutectic phase increased. After sintering at 1130 °C most of CoCrMo particles disappeared because they react with Ti6Al4V to form the Ti2Co phase. Porosity is reduced 15% mainly by reducing the smaller pores that can be filled by the eutectic phase. Thus, it was concluded that semi-in situ X-ray computed microtomography is an effective tool to investigate the semi-solid state sintering and to analyze the microstructure evolution and densification mechanisms.

Effect of Zn addition on the microstructures and mechanical behaviors of As-cast Mg-2.5Y-1Ce-0.5Mn alloy

The effect of Zn addition on the microstructures and mechanical behaviors of as-cast Mg-2.5Y-1Ce-0.5Mn alloy was investigated. Microstructure observation demonstrated that with the addition of 1wt%, 3wt% and 5wt% Zn, the ternary phases of LPSO phase, LPSO phase +W-phase and W-phase +T-phase (Mg-Zn-Ce) are precipitated orderly, and the volume fraction of eutectic phases increases. The results of tensile tests demonstrated that with increasing Zn addition, the yield strength of as-cast alloys increases continuously while the ultimate tensile strength and elongation increase nonlinearly. Based on the analysis of microstructure, nanoindentation results and deformation surfaces, it found that the is increased by the increased volume fraction of hard ternary phases. The largest in 1wt% Zn alloy is contributed from the LPSO phase with an excellent plastic accommodation while the insufficient accommodated role of LPSO phase and the easily broken W-phase deteriorate the ductility of 3wt% Zn alloy. The hard and brittle T-phase also damages the ductility, while the fine grains contribute to the moderate elongation in 5wt% Zn alloy. The mainly arises from a sustainable increase of strain hardening ability after yielding that associated with both the high yielding point and excellent ductility.

Rapid Solidification and Non-equilibrium Phase Constitution in Laser Powder Bed Fusion (LPBF) of AlSi10Mg Alloy: Analysis of Nano-precipitates, Eutectic Phases, and Hardness Evolution

The non-equilibrium phase evolution during laser powder bed fusion (LPBF) of AlSi10Mg alloy is thoroughly characterized and analyzed by means of advanced electron microscopy and analytical simulation of rapid solidification phenomenon. The evolution of microstructural strengthening agents such as inter-cellular eutectic phase and intra-cellular precipitates is presented in correspondence with the local variation of cellular/dendritic solidification patterns within a typical melt pool. The eutectic phase exhibits two different morphologies: lamellar and fibrous. As with the cell size variation, the overall volume fraction of eutectic phase and the lamella spacing is shown to gradually decrease by moving away from the melt-pool boundary (MPB), i.e., through crossing over from a coarse to a fine cellular zone. The eutectic-free regions within the α-Al cells contain a large number density of nano-sized precipitates that are predominantly Si-rich and are either fully or semi-coherent with the Al matrix. The formation of nano-precipitates is linked to the increased (non-equilibrium) solubility limits of α-Al cells due to the rapid solidification effect. For the first time, we identify such nano-precipitates with non-equilibrium crystal structures and morphologies: “Spheres” and “Ellipsoids” with Face Centered Cubic (FCC), and “Plates” and “Needles” with a Diamond Cubic (DC) superlattice structure that emerges from within the Al matrix. The microstructure in the heat-affected zone (HAZ) right underneath the MPB exhibits an absence of cell boundaries and eutectic phases while consisting primarily of large Si-rich and Mg-rich precipitates. Finally, the local variation of nano-hardness across a solidified melt pool is shown to correlate well with the corresponding profile of microstructural refinement, i.e., exhibiting a minimum at the HAZ and a peak at around the melt-pool centerline. The findings here can significantly advance the state of knowledge for the strengthening behavior in an as-built LPBF-processed AlSi10Mg alloy.

Effect of cooling rate on grain refining behavior of Mg-4Y-3Nd-1.5Al alloy

The effect of cooling rate on grain refining behavior of Mg-4Y-3Nd-1.5Al alloy was carried out by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and x-ray diffraction (XRD) analysis. The different cooling rates were obtained by using step-sample method. The experimental results show that the grain size of Mg-4Y-3Nd-1.5Al alloy decreases with increasing cooling rate. The volume fraction of eutectic phases increases with increasing cooling rate. The Al2RE particles are effective heterogeneous nucleation sites as α-Mg. The refining effect on Mg-4Y-3Nd-1.5Al alloy is affected by the number and size of Al2RE particles. The number of increases and the average particle size of Al2RE phase decreases with increasing cooling rate.

Thermal stability of Al–Fe–Ni alloy at high temperatures

The Al–Fe–Ni eutectic alloy was produced by gravity cast, and the thermal stability of this eutectic alloy was studied for the first time in this work. The phase composition was characterized by X-ray diffraction, and the morphology evolution of eutectic phase was analyzed by differential scanning calorimeter and scanning electron microscopy. According to the microstructure evolution of the eutectic phase exposed to different temperatures, three stages can be divided to distinguish coarsening behaviors of the eutectic phase, including the stable stage, transient coarsening stage and fully coarsening stage. In each stage, the coarsening behavior of eutectic phase was studied. Firstly, it was found that the stable stage can be maintained up to at least 673K. Subsequently, the eutectic phase should go through coarsening and spheroidization in the transient coarsening stage. Among this stage, both the shape change and diffusion factors have played the vital roles in affecting the coarsening rate and spheroidization process. Finally, in the fully coarsening stage, the volume fraction of eutectic phase was deemed to be the main reason accounted for the difference between the results given in literature and this work.

Effects of aging treatment on microstructure and tribological properties of nickel-based high-temperature self-lubrication wear resistant composite coatings by laser cladding

The NiCr/Cr3C2–WS2 high-temperature self-lubrication wear resistant composite coatings were fabricated on substrate of a hot-rolled AISI304 austenitic stainless steel by laser cladding. The high-temperature phase stability of the composite coatings was evaluated by aging at 600 °C for 10 h, 30 h, 50 h, and the microstructures of the as-laser clad and aged coatings were examined by means of XRD, SEM, EDS, respectively. The sliding wear resistance of the as-laser clad and aged coatings was evaluated at 600 °C. The results show that NiCr/Cr3C2–WS2 composite coating has excellent high-temperature phase stability, the γ-(Fe,Ni)/Cr7C3 eutectic phases, Cr7C3 and (Cr,W)C hard phases, CrS/WS2 mixed solid lubricant phases all existed in the as-laser clad and aged coatings. The volume fraction of eutectic phases decreased gradually with the increasing of aged time due to their dissolution. The microhardness of the aged coating decreased slightly after aging the coating 50 h at 600 °C due to the dissolution of the eutectic phases and notable breaking or granulation of the Cr7C3 hard phase, but the tribological properties were not significantly affected by aging treatment.

Effects of T6 Age Hardening on Microstructure and Mechanical Properties of Al-Si-Cu Cast Aluminium Alloy

Effects of T6 artificial aging heat treatment on microstructure, microhardness and ultimate tensile strength of Al-4.93 wt% Si-3.47 wt% Cu alloy were investigated. The T6 age hardening treatment consists of solution treatment at 500±5°C for 8 hours followed by quenching into hot water at 80°C and artificial aging at 150, 170, 200 and 230°C for 1-48 hours followed by quenching into hot water. Microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). XRD and SEM revealed that the microstructure in the as-cast condition consists of primary dendritic α-Al, acicular-plate and globular forms of eutectic Si and intermetallic phases including globular Al2Cu and a flake-shape Al5FeSi. By T6 aging hardening, some intermetallics were dissolved and spheroidized. The volume fraction of eutectic phases in the as-cast, solution-treated, and solution-treated plus aging at 170°C for 24 hours is 17%, 12% and 10%, respectively. TEM results showed that precipitates in under-aging condition at 170° C for 6 hours are in the form of disc shape with the diameter in the range of 7-20 nm. At peak aging at 170°C for 24 hours, thin-plate precipitates with about 3-10 nm in thickness and 20-100 nm in length were found, lengthening to about 30-200 nm at longer aging time. The microhardness and ultimate tensile strength were increased from 71 HV0.05 and 227 MPa in the as-cast condition up to 140 HV0.05 and 400 MPa after solution treatment plus aging at 170°C for 24 hours, and decreased at prolong aging time.

The relationship between effective entropy change and volume fraction of the eutectic phases in eutectic microstructures

The characterization of eutectic microstructures is related to the effective entropy change and the volume fraction of eutectic phases. For eutectic growth, both parameters can be inter-associated as long as solidification occurs at small Peclet numbers. There is no determinate relationship between the volume fraction and the effective entropy change of either eutectic phases, and the volume fraction is only related to the ratio of the effective entropy change of the two phases. By comparing with well-known eutectic systems, the results indicate that the classification depends on rather the volume fraction than the effective entropy change. In addition, there exists a certain relationship between the eutectic arrangement and the symmetric characteristic of phase diagram.