Dendritic Spacing Research Articles

Slow and rapid cooling of Al–Cu–Si ultrafine eutectic composites: Interplay of cooling rate and microstructure in mechanical properties

Abstract

Transition of solid-liquid interface and tensile properties of CoCrFeNi high-entropy alloys during directional solidification

In order to understand effect of solidification velocity on microstructure evolution and mechanical properties of high entropy alloys (HEAs), CoCrFeNi HEA samples were directionally solidified at different solidification velocities (5–50 μm/s). The results show that all samples were directionally solidified, but the solid-liquid interface and columnar grains are different. Microstructure of them is only composed of FCC solid solution phase. Morphology of solid-liquid interface transforms from cellular, to cellular-dendritic, and then to dendritic with increasing of solidification rate. The criterion of solidification rate for cellular/dendrite transition is calculated as about 26.1 μm/s based on experimental results. The width of columnar grains decreases from 200 μm to 100 μm with solidification rate increasing from 5 μm/s to 50μm/s. Meanwhile, secondary dendrite spacing decreases from 200 μm to 30 μm for dendritic solidified samples (v = 10–50 μm/s). Tensile testing show that the yield strength improves from 233 MPa to 383 MPa, and the ultimate tensile strength improves from 431 MPa to 596 MPa with increasing of solidification rate. Meanwhile, the elongation of all samples is more than 34.3%, but rupture morphology analysis shows that the rupture changes from quasi-cleavage fracture to ductility fracture. The improvement of tensile properties is resulted from grain boundary strengthening of the columnar grain and second dendrite.

Effect of zero, normal and hyper-gravity on columnar dendritic solidification and the columnar-to-equiaxed transition in Neopentylglycol-(D)Camphor alloy

Centrifugal casting is an important method to produce superior-quality cast parts. The microstructure and therefore materials properties strongly depend on the hyper-gravity level. To investigate the effect of gravity level on the solidification behavior, the transparent model alloy Neopentylglycol-(D)Camphor is used, which solidifies like metals. Its optical transparency allows for in-situ observation of columnar and equiaxed crystal growth. The experiments were carried out in a constant temperature gradient with two different cooling rates, and for g-levels ranging from zero-g in microgravity environment up to 10g0 on a centrifuge. It is found that the overall temperature and solutal fields close to the solidification front are not significantly altered by the laminar 3D melt flow in hyper-g environment. As a consequence, the growth velocity and primary spacing of columnar dendrites is nearly independent of the g-level. Increasing the cooling rate allows for nucleation and growth of equiaxed dendrites in the undercooled region ahead of the columnar solid-liquid interface. Here, at high g-levels the nucleation rate increases significantly and an accelerated, cascade-like settling of the growing equiaxed dendrites is observed. As a result, an earlier transition from columnar to equiaxed dendritic growth is detected in hyper-g environment.

Laser additive manufacturing of nano-TiC reinforced Ni-based nanocomposites with tailored microstructure and performance

Laser additive manufacturing has demonstrated a promising capability in the simultaneous formation of high-performance nanocomposites with unique microstructure characteristics. The present work studied the densification, microstructure and mechanical properties of nano-TiC reinforced Inconel 718 composites processed by selective laser melting (SLM) with variation of laser energy linear density (E). It revealed that a fully dense TiC/Inconel 718 part was fabricated at a proper E of 300 J/m. On increasing E from 225 to 300 J/m, the nano-TiC reinforcement experienced severe agglomeration to uniform distribution along the grain boundaries and inside the grains of matrix. The morphologies of nano-particles transferred from irregular polygonal to near-spherical shape. The presence of nano-TiC could also accelerate the refinement of columnar dendrites spacing of γ matrix. A high nanohardness of 4.48 GPa, a low coefficient of friction of 0.36 and resultant low wear rate of 3.83 × 10−4 mm3/N⋅m were obtained at E of 300 J/m, showing a significantly improved mechanical performance compared to the SLM-processed unreinforced nickel-based alloys.

Dendritic Spacing and Macrosegregation Affecting Microhardness of an Al-Si-Mg Alloy Solidified Under Unsteady State Conditions

The prediction of microstructure morphology is fundamental for the manufacture of metallic components, since the expected levels of mechanical properties will be associated with the final aspects of the microstructure. In this work, an Al-7wt.%Si-3wt.%Mg alloy was directionally solidified in unsteady state conditions in order to investigate the influence of the addition of 3wt.%Mg to an Al-7wt.%Si alloy on the solidification evolution. The microstructure of the examined alloy is shown to be characterized by a more complex arrangement of phases, as compared to that of the Al-7wt.%Si alloy, which includes the binary (α-Al+Mg2Si) and refined ternary (α-Al+Si+Mg2Si+Fe-rich IMC) eutectic mixtures. A higher Vickers hardness profile is shown to be associated with a more refined microstructural arrangement. However, for cooling rates lower than 2K/s the microhardness is shown to increase with the increase in the microstructural spacing, which is shown to occur caused by Si macrosegregation and higher content of free Mg.

Directional solidification casting technology of heavy-duty gas turbine blade with liquid metal cooling (LMC) process

In this work, some important factors such as ceramic shell strength, heat preservation temperature, standing time and withdrawal rate, which influence the formability of directionally solidified large-size blades of heavy-duty gas turbine with the liquid metal cooling (LMC) process, were studied through the method of microstructure analysis combining. The results show that the ceramic shell with medium strength (the high temperature flexural strength is 8 MPa, the flexural strength after thermal shock resistance is 12 MPa and the residual flexural strength is 20 MPa) can prevent the rupture and runout of the blade. The appropriate temperature (1,520 °C for upper region and 1,500 °C for lower region) of the heating furnace can eliminate the wide-angle grain boundary, the deviation of grain and the run-out caused by the shell crack. The holding time after pouring (3-5 min) can promote the growth of competitive grains and avoid a great deviation of columnar grains along the crystal orientation , resulting in a straight and uniform grain structure. In addition, to avoid the formation of wrinkles and to ensure a smooth blade surface, the withdrawal rate should be no greater than the growth rate of grain. It is also found that the dendritic space of the blade decreases with the rise of solidification rate, and increases with the enlarging distance between the solidification position and the chill plate.

Multi-scale simulation of dendrite growth for direct energy deposition of nickel-based superalloys

In this study, a multi-scale model that integrates a macro-scale transient three-dimensional mass and heat transfer model and a micro-scale thin-interface phase-field model is developed to simulate the solidification behavior of the molten pool and the dendritic growth behavior for direct energy deposition process. Both the longitudinal molten pool morphology and the primary dendritic arm spacing (PDAS, Λ) of the columnar dendrites are validated by experiments. As revealed from the macro-scale simulation, the solidification velocity Vs and the temperature gradient G are coupled with an opposite trend, and the cooling rate R decreases along the solid/liquid interface from the top to the bottom of the molten pool. The micro-scale simulation shows that a power-law function (Λ ∝ R−1.31) with a scaling factor much larger than a traditional directional solidification (Λ ∝ R−0.5). Moreover, contrary to Hunt's and Kurz-Fisher's linear models, a circuitous relationship between Λ and G−0.5Vs−0.25 is found to be clustered within a narrow region. The coupled solidification parameters along the solidification interface with an asymptotic increase of G near the bottom of the molten pool contribute to the above PDAS observations.

π-Stacking Interactions of Graphene-Coated Cobalt Magnetic Nanoparticles with Pyrene-Tagged Dendritic Poly(Vinylidene Fluoride).

This study investigates the non-covalent coating of cobalt magnetic nanoparticles (MNPs) involving a graphene surface with pyrene-tagged dendritic poly(vinylidene fluoride) (PVDF). Dendrimers bearing a pyrene moiety were selected to play the role of spacers between the graphene surface of the MNPs and the PVDF chains, the pyrene unit being expected to interact with the surface of the MNPs. The pyrene-tagged dendritic spacer 11 decorated with ten acetylenic units was prepared and fully characterized. Azido-functionalized PVDF chains were then grafted onto each branch of the dendrimer using Huisgen's [3+2] cycloaddition reaction. Next, the association of the resulting pyrene-tagged dendritic PVDF 13 with commercially available Co/C MNPs by π-stacking interactions was studied by fluorescence spectroscopy. Evaluated were the stability of the π-stacking interactions when the temperature increased and the reversibility of the process when the temperature decreased. Also, hybrid MNPs were prepared from pyrene-tagged dendrimers decorated either with acetylenic functions (11) or with PVDF branches (13), and they were characterized by transmission electron microscopy and comparative elemental analysis was carried out with naked MNPs.

Length Scale of the Dendritic Array Tailoring Strength of a 5052 Aluminum Alloy

The present research aims to characterize 5xxx alloy series, considering chemistries based on the commercial 5052 alloy with three Mg contents (2.4wt.%, 2.6wt.% and 3.2wt.%) through transient directional solidification experiments. Very representative incoming impurities to a given twin-roll casting procedure were reached. As such, the Si, Fe, Cu, Mn and Cr-contents in the tested samples typically trend in between the suitable alloying spectrum. Microstructural analyzes were performed using polarized light microscopy of samples taken from various ingot positions. Growth relationships between the secondary dendritic spacing (λ2) and the growth velocity were determined. The 5052 alloys containing higher Mg content may induce a decrease in λ2 for a certain growth velocity. The hardness values measured across the three directionally solidified castings were directly related to the λ2, which can be considered a fundamental variable affecting mechanical strength. For representative conditions vis-a-vis those employed in industry, it was shown that even relatively small changes in Mg content of the 5052 alloy may have some impact on λ2. These results open new ways to predict the final as-cast microstructure characterizing commercial 5052 alloy products, with a view to controlling not only the dendritic growth but also the Mg content during casting operations.

Relationship Between Aluminum-Rich/Intermetallic Phases and Microhardness of a Horizontally Solidified AlSiMgFe Alloy

An experimental study with an A356-AlSiMgFe alloy was developed to evaluate the microhardness performance in the microstructure resulting of an unsteady-state horizontal solidification process. The Al-7wt%Si-0.3wt%Mg-0.15wt%Fe alloy was elaborated and directionally solidified in a water-cooled horizontal solidification device. In order to experimentally determine the cooling and growth rates (VL and TR, respectively), a thermal analysis was also conducted during solidification. Microstructural characterization by optical microscopy, SEM/EDS elemental mapping and microanalysis of the punctual EDS compositions allowed to observe the presence of an Al-rich dendritic phase (Al(α)) with interdendritic phases second composed of an eutectic mixture: Al(α-eutectic) + Si + Al8Mg3FeSi6(π) + Mg2Si(θ). Furthermore, the dendritic microstructure was characterized by measuring the secondary dendritic spacings (λ2) along the horizontally solidified ingot. Higher HV values were observed within the eutectic mixture.

The crystal growth, intercellular spacing and microsegregation of selective laser melted Inconel 718 superalloy

The solidified microstructure of SLM-fabricated IN718 was investigated from macroscopic to microscopic perspective in this study using optical microscope, scanning electron microscope, transmission electron microscopy. Based on the experimental results and theoretical analysis, the solidification behavior, the characteristics of the primary dendrite spacing λ1 and microsegregation were elucidated. In the present SLM-produced IN718, the solidification starts at the molten pool boundaries by the epitaxial growth. The solidification rate is estimated in the range of 0.1–1 m/s. The newly-formed crystals grow into cellular dendrites, the direction of which is either parallel to the original direction or changed by 90°. The primary dendrite spacing λ1 varies over a wide range of 27.6 × 10−2 ∼147.3 × 10−2 μm due to dramatically changeable average cooling rate from 1.1 × 105–1.2 × 107 °C/s. There is a significant microsegregation of Nb in the inter-dendritic region, leading to the formation of Laves phases, the amount of which is about 9.0 ± 1.3%. Meanwhile, a solute trapping of Nb in the dendritic trunk occurs during rapid solidification.

Unsteady-State Horizontal Solidification of an Al–Si–Cu–Fe Alloy: Relationship Between Thermal Parameters and Microstructure with Mechanical Properties/Fracture Feature

Aluminum casting alloys have properties which are of great industrial interest, such as low density, good corrosion resistance, high thermal and electrical conductivities, good combination of mechanical properties, good workability in machining processes and mechanical forming. Currently, these alloys are produced in various systems and dozens of compositions. In this investigation, a mutual interaction of thermal parameters, scale of the dendritic microstructure, intermetallic compounds (IMCs), microhardness and tensile properties/fracture characteristics of a casting Al–7wt%Si–3wt%Cu–0.3wt%Fe alloy was analyzed. Solidification experiments were developed using a furnace that promoted horizontal growth under transient heat flow conditions. Then, growth rate (VL), cooling rate (CR), and local solidification time (tSL) were determined from measured temperature profiles. Secondary dendritic spacings (λ2), Si particles, Fe-rich and Al2Cu intermetallic phases were characterized by optical and SEM microscopy, as well as the area mapping and point-wise EDS microanalysis. Hence, the interrelations involving Vickers microhardness (HV), yield strength (σYS), ultimate tensile strength (σUTS) and elongation (E%) with microstructural features were evaluated by mathematical equations. IMCs as well as morphologies of Si were also analyzed in the fracture regions. In addition, the experimental growth law of λ2 = f(tSL) proposed in this study was compared with a predictive theoretical model reported in the literature for multicomponent alloys. It was observed that areas that tend to grow faster (lowest λ2 values) were associated with the highest σUTS and E% values, while HV and σYS properties were not affected by the thermal and microstructural parameters (CR and λ2). In addition, less extensive cleavage planes accompanied by small dimples in were observed in fractured samples with lower λ2 values.

Evolution of the microstructure and solute distribution of Sn-10wt% Bi alloys during electromagnetic field-assisted directional solidification

The effects of forced flows at different velocities on microstructure and solute distribution during the directional solidification of Sn-10 wt% Bi alloys under a simultaneous imposition of a transverse static magnetic field (TSMF) and an external direct current (DC) have been investigated experimentally and numerically. The experimental results show that the solid-liquid interface will gradually become sloping with the increase of the forced flow velocity when the thermoelectric magnetic convection (TEMC) dominates the forced flow at solidification front. However, the interface will gradually become planar as the flow velocity further increases when the electromagnetic convection (EMC) dominates the forced flow. Moreover, when the flow velocity gradually increases, the primary dendrite spacing decreases from 384 to 105 μm accordingly. The simulation results show that the solute distribution at the two sides of the sample can be significantly changed by the forced flow at solidification front. The rejected solute will be unidirectionally transported to one side of the sample along the TEMC (a low-velocity forced flow), thereby causing the formation of a sloping interface. However, the rejected solute will be returned back along the EMC (a higher-velocity force flow), which results in a planar interface. Furthermore, the solute content at the two sides of the sample under the forced flows at different velocities was measured. The results are in good agreement with the simulation results, which shows that the solute content difference between the two sides of the sample reaches the maximum when a 0.5 T TSMF is applied, while the solute content difference decreases to zero with a simultaneous application of a 0.5 T TSMF and a 1.6 × 105 A/m2 external DC.

Solidification behavior and morphological evolution in laser surface forming of AlCoCrCuFeNi multi-layer high-entropy alloy coatings on AZ91D

The AlCoCrCuFeNi high-entropy alloy (HEA) coatings after one, two and three layers were laser cladded on the AZ91D substrate. Towards the coating interface, the substrate is composed of Mg dendrites and interdendritic Mg + Q eutectics. The formed coatings can be categorized into a low narrow composite region and a top thick dense HEA region. The dominated microstructures in the dense HEA region are columnar dendrites. Nevertheless, the average primary spacing of columnar dendrites became smaller when the coating grew thicker mainly due to the temperature gradient decreasing slowly. Moreover, the planar morphology at re-melted boundary of each clad layer is absence because of the local conditions being suitable for epitaxial growth. Additionally, it is found that the columnar to equiaxed transition (CET) had occurred in the top area of the three-layer HEA coating. This evolution of the microstructure can be explained by the CET theory combined with the sluggish diffusion kinetics and the abnormal variation of the thermal conductivity with the temperature of the AlCoCrCuFeNi HEA. Additionally, the problem associated with dilution by Mg can be completely suppressed by the HEA coating after three layers.

Effect of cross-section-change induced advective flow on the primary dendrite array morphology of hypoeutectic Pb-Sb alloys during directional solidification

The morphology and distribution of primary dendrites have been examined in Pb-2.2, 5.8 and 10.8 wt. pct. Sb alloy samples directionally solidified (DSed) in ampoules shaped like an hour-glass to examine the influence of cross-section change induced advective flow on the cellular/dendritic interface. This sample design increases the advective flow of the melt towards the array tips, as the liquid-solid interface enters the neck of the ampoule, and then decreases it as the interface exits the neck. The warm solute-rich melt flowing towards the growth front suppresses the extent of side-branching, decreases the primary dendrite spacing, and increases the primary dendrite trunk diameter as observed in the Pb-5.8 and 10.8 Sb alloys. The flow appears to suppress the formation of cells. A cellular interface growing in the Pb-2.2Sb alloy became planar as the solidification front entered the neck, becoming cellular again as it exited the neck.

Multiscale Modeling and Simulation of Directional Solidification Process of Ni-Based Superalloy Turbine Blade Casting

Ni-based superalloy turbine blades have become indispensable structural parts in modern gas engines. An understanding of the solidification behavior and microstructure formation in directional solidified turbine blades is necessary for improving their high-temperature performance. The multiscale simulation model was developed to simulate the directional solidification process of superalloy turbine blades. The 3D cellular automaton-finite difference (CA-FD) method was used to calculate heat transfer and grain growth on the macroscopic scale, while the phase-field method was developed to simulate dendrite growth on the microscopic scale. Firstly, the evolution of temperature field of an aero-engine blade and a large industrial gas turbine blade was studied under high-rate solidification (HRS) and liquid-metal cooling (LMC) solidification processes. The varying withdrawal velocity was applied to change the curved mushy zone to a flat shape. Secondly, the grain growth in the aero-engine blade was simulated, and the grain structures in the starter block part and the spiral selector part in the HRS process were compared with those in the LMC process. The simulated grain structures were generally in agreement with experimental results. Finally, the dendrite growth in the typical HRS and LMC solidification process was investigated and the simulation results were compared with the experimental results in terms of dendrite morphology and primary dendritic spacing.

316L Stainless Steel Manufactured by Selective Laser Melting and Its Biocompatibility with or without Hydroxyapatite Coating

To fabricate metallic 316L/HA (hydroxyapatite) materials which meet the requirements of an implant’s mechanical properties and bioactivity for its function as human bone replacement, selective laser melting (SLM) has been employed in this study to prepare a 316L stainless steel matrix, which was subsequently covered with a hydroxyapatite (HA) coating using the sol-gel method. High density (98.9%) as-printed parts were prepared using a laser power of 230 W and a scanning speed of 800 mm/s. Austenite and residual acicular ferrite existed in the microstructure of the as-printed 316L stainless steel, and the sub-grain was uniform, whose primary dendrite spacing was around 0.35 μm. The as-printed 316L stainless steel showed the highest Vickers hardness, elastic modulus, and tensile strength at ~ (~ means about; same applies below unless stated otherwise) 247 HV, ~214.2 GPa, and ~730 MPa, respectively. The elongation corresponding to the highest tensile strength was ~38.8%. The 316L/HA structure, measured by the Relative Growth Rate (RGR) value, exhibited no cell cytotoxicity, and presented better biocompatibility than the uncoated as-printed and as-cast 316L samples.

Dendritic and eutectic growth of Sn–0.5 wt.%Cu solders with low alloying Al levels

The dependences of microstructures on the solidification thermal parameters of Sn–0.5 wt.%Cu, Sn–0.5 wt.%Cu–0.05 wt.%Al, and Sn–0.5 wt.%Cu–0.1 wt.%Al alloys are examined. Ranges of sizes and morphologies of microstructural phases have been quantitatively assessed due to the broad spectra of cooling rates and thermal gradients associated with the experiments. Various types of growth relations are proposed to represent the microstructural progresses. Al addition does not change either the matrix primary dendritic spacing or the spacing between Cu6Sn5 particles. However, it is shown that the secondary dendrite arm spacing, λ2, is greatly affected, increasing with the increase in the Al content. Both globular-type and fibrous-type morphologies typify the Cu6Sn5 intermetallics located in interdendritic zones.

Microstructure and Obdurability Improvement Mechanisms of the La‐Microalloyed H13 Steel

Rare earth microalloying is one of the most important methods to improve the mechanical properties of die steels. This paper investigates the relationships among addition of La, microstructure, and mechanical properties of H13 steel. The influences of following factors on obdurability improvement of La‐microalloyed H13 steel, such as grain refinement, deoxidization and desulphurization, grain boundary. and precipitations, have been studied. The results indicate that with the increase of La content in the range of 0–0.45 wt%, the primary dendritic spacing and average grain size of as‐cast samples decrease at the beginning and then increase. The tensile strength, yield strengthand impact toughnessof hot‐forged samples after quenching and tempering treatments increase at the beginning and then decrease with the increase of La content. The mechanical test results indicate that with 0.03 wt% La content, the obdurability of hot–forged H13 steel after quenching and tempering treatments increases obviously. As compared with La‐free H13 steel, the and of hot‐forged H13 steel containing 0.03 wt% La increase by 5.0% and 4.6%, increases by 67.8% from 29.0 to 48.7 J·cm−2. La‐microalloyed effect is attributed to grain refinement, La2O2S precipitations and dislocation density of H13 steel.

Microstructure and macro properties of Al2O3 ceramics prepared by laser engineered net shaping

Laser engineered net shaping (LENS) technology, due to its ability in melting and solidification of neat ceramic materials, exhibits great potential for direct fabricating net-shaped ceramic structures. However, due to limitation of sample size, reports on the macro properties of fabricated ceramic structures are still extremely lacking. In the present work, large-sized Al2O3 ceramic samples were prepared using LENS technology to perform microstructural analysis and macro properties testing. Phase composition, grain characteristics and chemical purity of prepared samples were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence (XRF). Macro properties of large-sized samples, including the flexural strength and compressive strength, were given for the first time after being tested using a mechanical tester. Results show that microstructure of prepared Al2O3 ceramics presents typical solidification characteristics and consists of grains that grow along the deposition height direction. Primary dendrite spacing of the Al2O3 grains is about 60 µm. The phase is mainly composed of high-temperature-stable α-Al2O3 and a relative density of 99.5% is obtained. No new impurities are introduced during the depositing process. Element type and element content of fabricated samples are highly consistent with those of original powder. Property test results show that, under current process conditions, the flexural strength and compressive strength of fabricated samples reaches the level of traditional sintering method, reaching the maximum of 350 MPa and 625 MPa respectively.