Eutectic Colonies Research Articles

Relationship between spacing of eutectic colonies and tensile properties of transient directionally solidified Al-Ni eutectic alloy

Eutectic Al-Ni alloys are widely faced as materials to be considered for advanced structural components. Nevertheless, still there is a lack of research on microstructural aspects of the eutectic Al-6.3 wt%Ni alloy and important points need to be addressed, such as, the morphology of the unsteady solidified eutectic, size and distribution of phases under different solidification cooling rates and the effects of the resulting microstructural features on mechanical properties. As such, in the present study, an Al-6.3 wt%Ni alloy is directionally solidified under a wide range of cooling rates and the resulting microstructure is shown to be formed by eutectic colonies. Three general microstructural features characterize the colony: a fine central zone composed of fibrous α-Al + Al3Ni eutectic, a boundary coarse zone formed by lamellar eutectic, and an Al-rich zone delimiting the colony. A quantitative analysis relating solidification thermal parameters to Al3Ni and colony spacings is outlined. Furthermore, the evolution of tensile properties as a function of these spacings was examined, and the highest strength and elongation of 160 MPa and 15%, respectively, are associated with an ultrafine bimodal structure formed by eutectic colonies with 55 μm in spacing containing very fine fibers (300 nm in spacing) and lamellae with 750 nm in spacing.

Microstructure transition and selection of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics from micronscale to nanoscale: The effect of rapid solidification

Microstructure transition and selection of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics with minimal eutectic spacing of 50nm are studied by laser zone melting under rapid solidification rate. When the eutectic spacings are in micronscale, the ‘Chinese script’ structure is the main microstructure. As increasing the solidification rate (≥ 50µm/s), the eutectic colony structure is obviously formed. The irregular-regular structural transition and coexistence are produced around eutectic spacing of 100–200nm. The mechanism of irregular/regular eutectic structure transition and coexistence is revealed by considering the relative kinetic undercooling of each phase. In addition, the strong formation tendency of amorphous/nano-eutectic structure is observed at the surface of as-solidified ceramic drops by rapid liquid-metal quenching.

Simulating the Solidification of Boride–Boride Eutectics

Computer simulations of structurization in boride–boride ceramics are performed using the standard version of phase-field theory. The simulations reproduce the main properties of eutectic structures observed in experiments, i.e., spatial segregation of components, structure formation from an overcooled melt, and complete decomposition of the system when relaxes toward thermodynamic equilibrium. The eutectic colonies formed in the process of directional solidification and the dependence of spatial parameter of the emerging structures on the solidification rate are reproduced as well. The simulations demonstrate that the ordered fibrous structures are formed only in a certain range of solidification rates.

Microstructural Evolution and Mechanical Properties of Ni57Nb33Zr5Co5 Metallic Glass

The Ni57Nb33Zr5Co5 metallic glass is a promising alloy for bipolar plates in proton exchange membrane fuel cells. It is important to know which phase forms in this alloy under different cooling rates in order to understand its influence on the thermal stability and mechanical properties. In this work, melt-spun ribbons and rod samples with 1, 2 and 3 mm diameters were prepared and their phase formation, microstructure and mechanical properties were investigated by X-ray diffraction, differential scanning calorimetry, optical microscopy, scanning electron microscopy and microhardness. It is found that a completely crystalline structure forms in the lower cooling rate samples (2 and 3 mm diameter rods) with the presence of the equilibrium phases Ni3(Nb,Zr) and Nb7Ni6 as primary phases or as a very fine eutectic structure, while a fully glassy structure is attained in the samples with the highest cooling rate (ribbons). For the sample with an intermediate cooling rate (1 mm diameter rod), polymorphically crystals of an unknown metastable phase with spherical morphology precipitate in the glassy matrix with virtually the same composition as the matrix. The 2 mm diameter sample exhibits higher hardness than the other samples, which is attributed to its very fine eutectic colonies.

Study of the effects of Zr addition on the microstructure and properties of Nb-Ti-Si based ultrahigh temperature alloys

The effects of alloying with Zr on the microstructure, mechanical and oxidation properties of Nb-Ti-Si based ultrahigh temperature alloys have been investigated in this study. The microstructures of the all alloys were comprised of primary γ(Nb,X)5Si3 blocks, Nbss and eutectic colonies, and the additions of Zr do not affect the microstructure and phase constituents of Nb-Ti-Si based alloys. Zr improves both the room-temperature toughness and the high-temperature strength. The alloy with addition of 8 at.% Zr shows the highest fracture toughness of 15.01 Mpa·m1/2. The compressive strengths of the alloys are improved to 278.89–293.08 MPa for the Zr-containing alloys when compare with the Zr-free alloy (194.23 MPa). The oxidation resistance of the alloys was also obviously ameliorated with Zr addition, showing a reduced weight gain with the increase of Zr content.

MICROSTRUCTURE FEATURES OF CHROME-NICKEL COATING WELDED WITH FILLER WIRE PL AN-111 WITH A 50% OVERLAP

Purpose . The paper involves investigation of microstructure features of the coating welded with filler wire PL AN-111 with a 50% beads overlap. Methodology. Wear-resistant layer was formed by means of electric arc deposit welding using filler wire PL AN-111 on the plate from steel 09G2S. Deposit welding was conducted under the following parameters: welding current is of 650-750 A; arc voltage is of 30-34 V; welding speed is of 32 m/h. Microstructure was researched with application of optical microscopies “Neophot-21”, “Nikon Eclipse M200” and electron scanning microscopy JEOL JSM-6510 LV. Microhardness of structural constituents was measured with tester FM-300 (Future-Tech) under loading of 10-50 g. Findings . It is shown that the overlap of the beads leads to the formation of inhomogeneous microstructure in the cross section that varies by zones from free-carbide austenite to hypereutectic microstructure with primary chromium carbides. The analysis of the microhardness of the structural constituents in various coating areas was carried out. It was found that hardness of austenite, carbide eutectic and carbides M 7 C 3 varies in coatings in the range of 3 100-3 850 МPа, 4 100-6 800 МPа and 12 100-15 100 МPа, accordingly. Originality . Authors determined that Cr-Ni coating comprises substantially austenitic-carbide eutectic with different density and thickness of carbide fibers within eutectic colonies. Along the border “base/coating” a single-phase austenitic layer lies which turns into a layer with a hypoeutectic structure. In the heat affected zone from beads fusion austenite disintegration with the granular carbides formation was recorded. This leads to decreasing of matrix corrosion resistance due to chromium depletion. Above the zone of beads fusion, the coating has a hypereutectic structure with the presence of large primary chromium carbides. Practical value . It is shown that deposit welding with filler wire PL AN-111 with a 50% overlap of beads results in a coarsening of the structure due to formation of a hypereutectic structure comprising in the presence of large primary chromium carbides with lower microhardness.

Effect of isothermal aging on microstructure, electrical resistivity and damping properties of Sn–Ag–Cu solder

The present study investigates the changes in microstructures and their effects on electrical resistivity, moduli, hardness and damping properties in an environmental-friendly eutectic Sn–3.0Ag–0.5Cu (wt%) solder alloy when exposed to harsh service environments. A thorough microstructural investigation was conducted by scanning electron microscopy with diffraction analysis and transmission electron microscopy. In the as-received Sn–Ag–Cu solder alloy, very-fine needle-shaped Ag3Sn and Cu6Sn5 intermetallic compound (IMC) particles are found to be uniformly dispersed in the eutectic colony. However, after exposing at harsh service environments (e.g., aging temperature at 150 °C for various aging time) these IMC particles were appeared with coarse structure. This coarsening nature of IMC particles degraded the mechanical properties of electronic interconnections. This was confirmed by measuring a range of electrical and mechanical properties that included electrical resistivity, Young’s moduli, shear moduli and microhardness. A comparison between the as-received and isothermal aging solder alloy shows that the electrical resistivity of as-received and 60 days isothermal aged alloys was about 12.5 and 10.0 μΩ cm, respectively. Further, the degradation in shear moduli and hardness was about 27.3 and 25.5%, respectively. However the isothermal aged solder alloy displayed high damping property as compare to the as-received alloy.

Microstructure and Plastic Deformation of the As-Welded Invar Fusion Zones

The as-welded Invar fusion zones were fabricated between cemented carbides and carbon steel using a Fe-Ni Invar interlayer and laser welding method. Three regions in the as-welded Invar fusion zones were defined to compare microstructures, and these were characterized and confirmed by scanning electron microscopy and X-ray diffractometry. The structure and plastic deformation mechanism for initial Invar Fe-Ni alloys and the as-welded Invar fusion zones are discussed. (1) After undergoing high-temperature thermal cycles, the microstructure of the as-welded Invar fusion zones contains γ-(Fe, Ni) solid solution (nickel dissolving in γ-Fe) with a face-centered cubic (fcc) crystal structure and mixed carbides (eutectic colonies, mixed carbides between two adjacent grains). The mixed carbides exhibited larger, coarser eutectic microstructures with a decrease in welding speed and an increase in heat input. (2) The structure of the initial Invar and the as-welded Invar is face-centered cubic γ-(Fe, Ni). (3) The as-welded Invar has a larger plastic deformation than initial Invar with an increase in local strain field and dislocation density. Slip deformation is propagated along the (111) plane. This finding helps us to understand microstructure and the formation of dislocation and plastic deformation when the Invar Fe-Ni alloy undergoes a high-temperature process.

Eutectic colony formation in systems with interfacial energy anisotropy: A phase field study

Instability of a binary eutectic solidification front to morphological perturbations due to rejection of a ternary impurity leads to the formation of eutectic colonies. Whereas, the instability dynamics and the resultant microstructural features are reasonably well understood for isotropic systems, several experimental observations point to the existence of colonies in systems with anisotropic interfaces. In this study, we extend the understanding of eutectic colonies to anisotropic systems, where certain orientations of the solid-liquid or solid-solid interfaces are associated with a lower free energy than the others. Through phase field simulations in 2D and 3D, we have systematically probed the colony formation dynamics and the resulting microstructures, as functions of the pulling velocity and the relative orientation of the equilibrium interfaces with that of the imposed temperature gradient. We find that in 2D, stabler finger spacings are selected with an increase in the magnitude of anisotropy introduced, either in the solid-liquid or in the solid-solid interface. The fingers have a well-defined orientation for the case of anisotropy in the solid-liquid interface, with no fixed orientations for the lamellae constituting the colony. For the case where anisotropy exists in the solid-solid interface, the lamellae tend to orient themselves along the direction of the imposed temperature gradient, with tilted solid-liquid interfaces from the horizontal. The 3D simulations reveal existence of eutectic spirals which might become tilted under certain orientations of the equilibrium interfaces. Our simulations are able to explain several key features observed in our experimental studies of solidification in Ni-Al-Zr alloy.

Influence of Nb on microstructure and mechanical properties of Ti-Sn ultrafine eutectic alloy

In this study, A series of the high strength (T82Sn18)100-xNbx (x=0, 1, 3, 5, and 9 at%) ultrafine eutectic alloys with large plasticity are developed by suction casting method. The Ti82Sn18 binary eutectic alloy consists of a mixture of a hcp Ti3Sn and a α-Ti phases having the plate-like lamellar type duplex structure with micro scaled eutectic colony. From the (T82Sn18)97Nb3, the alloy display structural heterogeneous distribution of ultrafine-scaled phases composed of β-Ti(Nb) solid solution surrounded by alternating plate-like shaped Ti3Sn and α-Ti phases. With increasing Nb content, the volume fraction of β-Ti is continuously increased, which induced improving mechanical properties both strength and plasticity. Especially, (Ti82Sn18)91Nb9 alloy has the outstanding combination of the high strength (σ y ≈1.1 GPa) and large plasticity (e p ≈36%) at room temperature.

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.

Microstructure and segregation behaviour of directionally solidified tungsten bearing 25Cr-35Ni-Nb heat-resistant stainless steels

Two series of heat-resistant 25Cr-35Ni-Nb stainless steels with tungsten addition and Nb/C ratio of ≈2.6 and 3.9 were directionally solidified under two cooling rates of 23–6 °C s−1, respectively. EDS analysis, electron backscatter diffraction (EBSD), differential thermal analysis (DTA) and field emission scanning electron microscopy (FESEM) tests were carried out to show that tungsten as a carbide phase nucleant promoted weak coupled eutectic growth of γ/ M7C3 and decreased the lamellar eutectic colonies. Substantial negative segregation of Ni and Fe and positive segregation of Nb and Cr in tungsten-bearing alloys stimulated the γ/M23C6 reaction and the M23C6 possessed a dendritic morphology. Two morphologies of MC carbides consisted of Widmanstätten and dendritic with a wavy surface were detected depending on the cooling rate and tungsten addition. The partition coefficient of Nb was determined as 0.31 and 0.52 for the alloy with and without tungsten, respectively.

Defect Susceptibility of Tensile Properties to Microporosity Variation in As-Cast Al–xSi Alloys

The defect susceptibility of tensile properties to microporosity variation in as-cast Al–xSi (x = 2, 5, 8, 11) binary alloys was investigated in relation to the effective void area fraction, which includes the area fraction and the size distribution of the pre-existing micro-voids and damaged eutectic Si particles. The area fraction of eutectic Si particles and Si colonies increases with increasing nominal Si content, and the typical morphology of micro-voids changes from shrinkage hole type to blow hole type as the Si content increases. The tensile properties of Al–xSi alloys depend on microstructural characteristics, including the nominal area fraction of eutectic Si particles and eutectic Si colonies, the width of the pro-eutectic Al matrix, and the variation in microporosity. The defect susceptibility of ultimate tensile strength and elongation is estimated by the nominal area fraction of eutectic Si colonies rather than the nominal Si content. The relative contribution of the eutectic Si particles to the effective void area fraction can be described using a modified constitutive model that may confirm that the damage evolution of the eutectic Si particles influences the tensile properties of Al–xSi alloys.

Effect of eutectic Si particles on the defect susceptibility of tensile properties to microporosity variation in Al-xSi binary alloys

The variability in the defect susceptibility of tensile properties to microporosity variation in as-cast Al-xSi binary alloys was investigated in terms of the effective void area fraction, which includes the area fraction and size distribution of micro-voids and eutectic Si particles. The test samples were prepared in the form of Al-xSi (x=2,5,8,11) alloys with 0.02wt%Sr modification using gravity-casting process. The tensile properties of Al-xSi alloys fundamentally depend on microstructural changes, with a remarkable deviation caused by microporosity variation. The defect susceptibility of the ultimate tensile strength and elongation is practically varied with the nominal area fraction of eutectic Si colonies, rather than the real fraction of eutectic Si particles. The damage evolution of Si particles practically influences the tensile properties of Al-xSi alloys, together with the variation of the area fraction of pre-existing micro-voids. Additionally, the relative contributions of microporosity and eutectic Si colonies to the variability in the defect susceptibility of tensile properties to an effective void area fraction can be definitively described by considering the incoherency parameter and the plastic constraint factor of both micro-components and by using a modified constitutive model. The tensile properties of Al-xSi alloy in the high fraction of eutectic Si colonies are practically degraded by the increase of microporosity, even though the tensile properties in the low fraction of eutectic Si colonies are insensitive to the microporosity variation.

Study of the solidification structure of compacted graphite cast iron

This investigation focuses on the study of the solidification mechanism of compacted graphite cast iron (CGI). The solidification macrostructure was revealed in cast samples using a special technique known as direct austempering after solidification (DAAS). The microstructure was revealed by colour etching. The results were compared with earlier investigations of the solidification of spheroidal (SGI) and lamellar (LGI) graphite irons, and show that, similarly to other free graphite cast irons, the solidification of CGI is dominated by the presence of relatively large grains of austenite that can be observed with bare eyes. The CGI cast samples show a typical ingot structure, containing columnar and equiaxed grains, with a narrow columnar to equiaxed transition. The microstructure analysis showed that a dendritic substructure and a large number of eutectic colonies form the grains. Microsegregation is located inside the grains, mostly between secondary dendrite arms. The results indicate that the growth mechanism during solidification of CGI resembles that of LGI, but not the mechanism of SGI.

A New Approach for Refining Carbide Dimensions in M42 Super Hard High-speed Steel

Obtaining small carbides is crucial but difficult for high-speed steels. A new approach for refining carbide dimensions in M42 super hard high-speed steel by increasing cooling rate and spheroidizing treatment was proposed. The morphologies and properties of eutectic carbides formed at different cooling rates were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and differential scanning calorimeter (DSC). The results show that eutectic carbides change from a lamellar shape into a curved-rod shape as cooling rate increases. Despite different morphologies, the two carbides are both of M2C type with a hexagonal close-packed structure and display a single crystal orientation in one eutectic colony. The morphology of M2C mainly depends on the growing process of eutectic carbides, which is strongly influenced by cooling rate. Compared with lamellar carbides, M2C carbides with curved-rod shapes are less stable, and decompose into M6C and MC at lower temperatures. They are more inclined to spheroidize during heating, which ultimately and distinguishably refines the carbide dimensions. As small carbides are much easier to dissolve into matrices during austenization, the process described herein improves the supersaturation of alloying elements in martensite, which leads to an increment of hardness in M42 steel.

Microstructural evolution of as-cast Th–U alloys

Microstructural evolution of as-cast (arc-melted) microstructures of selected compositions of Th–U alloys is investigated. Characterizations are done using XRD, optical microscopy, SEM, TEM and hardness measurement. Microstructures vary from single phase to biphasic with increase in uranium content in thorium. Uranium is present as spheroids (<1 μm) or islands (∼7 μm) in high thorium (<10.7 wt.% U) alloys. With further increase in uranium content it forms an interconnected network of uranium rich phase and gets interspersed with similar network of thorium rich phase. High uranium Th–U alloys has dendrites of thorium dispersed in uranium rich phase. Grain size refinement of thorium rich phase is effective up to ≤10.7 wt.% uranium addition. Hardness of Th–U alloys increases with increase in uranium content. Comparatively lower hardness observed in Th–96U is due to presence of softer eutectic colonies in the microstructures. High thorium alloys have potentials for being utilized as dispersion type fuel with fissile uranium spread in thorium matrix.

Eutectic crystallization during fracture of Zr–Cu–Co–Al metallic glass

We report a eutectic-type crystallization phenomenon during the fracture of a Zr-based bulk metallic glass. The eutectic crystalline colonies are found to have an average visible diameter of 390±190nm. This process gives a direct evidence for the deformation-induced crystallization caused by temperature rise, which can reach as high as 1000K. Meanwhile, the occurrence of eutectic crystallization is found to be closely correlated with the plasticity of bulk metallic glass.

Isothermal Aging of Low-Ag SAC with Al Addition

Zinc sulfide (ZnS) thin films have been successfully deposited via spray pyrolysis using an aqueous solution of thiourea and zinc Amongst the candidates of lead-free solders, eutectic or near eutectic Sn-Ag-Cu or SAC has been found to be the most suitable candidate to replace the traditional tin-lead (Sn-Pb) solder. However, due to the higher cost of SAC (expensive Ag), research have now been directed to find a lower cost lead-free solder with comparable or better performance to the Sn-Pb or SAC, i.e low Ag SAC or Sn-Cu solders. The solder alloys developed in this work were SAC 0305 (0.3 wt% Ag, 0.5 wt% Cu and bal. Sn) and SAC0305 with 1 wt% Al and 2 wt% Al. The addition of Al is expected to refine the β-Sn grains and contribute to higher solder strength. Characterization of the solder alloys focused on the bulk solder microstructure, intermetallic compound (IMC) evaluation and wettability of solder alloys in reflowed and aged conditions. Reflow temperature was 270°C while aging was done isothermally in normal atmosphere for 100, 200 and 500hours at 100°C and 150°C. Microstructure of bulk solder and the IMC formed at interface between solder and Cu substrate were observed using SEM equipped with EDX. Wettability of solder was evaluated via wetting force and time, and the wetting angle between solder and the Cu substrate. A low Ag content resulted in fine Ag3Sn IMC as compared to much larger needle Ag3Sn observed in the commercial SAC 305 (Ag 3.0 wt%). With Al addition, SEM result showed finer β-Sn dendrites with fine Ag3Sn and Cu6Sn5 distributed within the eutectic colony of the bulk solder, and also thinner IMC layer in reflowed samples. Isothermally aged samples on the other hand, showed thicker IMC layer for Al added solder samples. This could be attributed to the finer β-Sn dendrites in the bulk solder providing more diffusion path and increased the thickness of IMC layer. The IMC formed at the interface between solder and the copper substrate was identified as Cu6Sn5 in reflowed samples, and both Cu6Sn5 and Cu3Sn when aged. Wettability of the solder alloys decreased with the addition of Al.

Microstructure transformation from irregular eutectic to complex regular eutectic in directionally solidified Al2O3/GdAlO3/ZrO2 ceramics by laser floating zone melting

Directionally solidified Al2O3/GdAlO3(GAP)/ZrO2(GdSZ) ternary eutectic ceramics have been grown by laser floating zone melting. The microstructure evolution rule is investigated as a function of growth rate. The as-solidified samples present high density (>99%), smooth surface and high diameter stability free of pores and cracks. Increasing the growth rate from 4 to 100μm/s, the microstructures undergo a transformation from “Chinese script” irregular eutectic pattern to eutectic colony and to complex regular eutectic patterns (lamellae or rods). The average interphase spacing of the “Chinese script” microstructure presents an inverse square relationship according to λ=10.46×ν−1/2, where the average interphase spacing λ has the dimension of μm and growth rate ν is in μm/s. While the interphase spacing of the complex regular microstructure is finer. The mechanisms of the microstructure evolution and the effects of ZrO2 phase on the morphology and domain size of microstructure are discussed in detail.