Dendritic Spacing Research Articles

Inhibition of stray grains at melt-back region for re-using seed to prepare Ni-based single crystal superalloys

Assisted by the mold preinstalled an alumina tube into seed segment, the influence of the original primary dendrite spacing of seed on the formation of stray grains at melt-back region was investigated during Ni-based single crystal casting using seeding method. The results showed that the interface reaction between the seed and mold as well as the formation of stray grains at the surface of seed were avoid using an alumina tube with surface roughness of 0.35 μm as the mold inner wall. As the original primary dendrite spacing of the seed decreased to less than 201 μm, the morphology of un-melted solid phase in semi-solid zone changed from isolated dendrite stem to the complex network, resulting in the inhibition of the formation of stray grains inside of the seed. The seed with original primary dendrite spacing of 201 μm was also successfully re-used to fabricate single crystal casting.

Coupling in situ synchrotron X-ray radiography and phase-field simulation to study the effect of low cooling rates on dendrite morphology during directional solidification in Mg–Gd alloys

The effect of low cooling rate with a fixed temperature gradient during directional solidification of Mg–Gd alloys is investigated by coupling in situ X-ray synchrotron radiography and phase-field method. Results show that the primary dendrite spacing decreases with the increase of cooling rates, and the relationship between the logarithm of primary dendrite arm spacing and the logarithm of cooling rate is linear. After fitting, the slope of primary dendrite arm spacing is −0.71, while that of phase field simulation is −0.74, which indicates the experimental statistics are basically consistent with that of phase field simulation. The simulation thoroughly studies the kinetic transition process of seaweed-like morphology and the evolution of solute distribution for dendrites, both the solid volume fraction and the velocity of liquid-solid interface increase with the increase of cooling rate. The simulation is in good agreement with experimental results.

Measurement and interrelation of length scale of dendritic microstructures, tensile properties, and machinability of Al-9 wt% Si-(1 wt% Bi) alloys

Al-Si alloys are increasingly being employed in the aerospace industry, satellite bearings, and inertial navigation systems. Hypoeutectic compositions are more attractive because of the low material cost and excellent castability. The addition of alloying elements to these alloys is generally used to modify the morphology of Si particles with a view to meeting mechanical strength requirements. Considering manufacturing requirements, to efficiently predict the quality of the machining process, it is essential to analyze the machinability of the material to be used, which depends on several factors, including microstructural features. In the present study, the effects of solidification cooling rate and Bi addition to Al-Si alloys on microstructural features, tensile properties, and rate of heat generation during necking processes are analyzed in samples of directionally solidified Al-9 wt% Si-(1.0 wt% Bi) alloy castings. The microstructure is composed of α-Al dendrites characterized by measured primary, secondary, and tertiary dendritic arm spacings, with a eutectic mixture of α-Al and Si-(Bi) particles. The addition of Bi is shown to attenuate the sharp tips of Si crystals. The dendritic length scale of the Al-rich matrix is correlated with the solidification cooling rate through experimental equations, and the evolution of the ultimate tensile strength with the primary dendritic spacing is experimentally represented by a single Hall-Petch-type equation for both examined alloys. Considering a same primary dendritic arm spacing, the addition of 1 wt% Bi to the Al-9 wt% Si alloy is shown to have the following effects: similar ultimate tensile strength, the elongation to fracture is improved, higher machinability based on significant decrease in the heating rate.

Neuro-amelioration of cinnamaldehyde in aluminum-induced Alzheimer’s disease rat model

ABSTRACTAluminum (Al) is a neurotoxic substance which has played an important role in the etiology, pathogenesis, and development of amyloid-β (Aβ) plaques. This study was carried out to evaluate the neuroprotective effect of aqueous cinnamon extract against aluminum chloride (AlCl3)-induced Alzheimer’s disease. Forty adult male albino rats, randomly divided into four equal groups. Control group; ACE200 group administered aqueous cinnamon extract (ACE) orally; AlCl3 group received daily intraperitoneal (i.p.) injection of AlCl3 for 60 days to induce neurotoxicity and AlCl3 + ACE200 group received a combination of AlCl3 and ACE in the same dose and route as previous groups. Aluminum administration significantly enhanced the memory impairment and the Aβ formation in the rat model. The cerebellum exhibited a significant reduced number of Purkinje cells, marked decrease in the density of dendritic arborization and prominent perineuronal spaces in the molecular layer. There was loss of dendritic spines, neurofibrillary degeneration, and appearance of neuritic plaques. Concomitant administration of AlCl3 and ACE displayed an observable protection against these changes with progressive improvement in memory and intellectual performance. In conclusion, ACE may play a protective role against formation of amyloid-β plaques in cerebellum.

Microstructure and grain growth direction of SRR99 single-crystal superalloy by selective laser melting

Nickel-based Single Crystal (SX) superalloys have an excellent creep ductility and thermal fatigue resistance at elevated temperatures. But SX structure is difficult to be manufactured under the demand of increasing operating temperature and complex shapes. In this work, the feasibility of manufacturing Nickel-based SX-superalloys by selective laser melting (SLM) was investigated theoretically and experimentally. Firstly, the solidification conditions including temperature field, thermal gradient and solidification speed of the SLMed multi-track samples were calculated by an established finite element model to judge the theoretical feasibility based on the columnar to equiaxed transition (CET). Then, the single-track, triple-track and cuboid samples on a single-crystal SRR99 substrate were deposited to judge the experimental feasibility and explore their grain growth mechanism during SLM. The results showed that CET can be avoided during the whole process of SLM. The unidirectional SRR99 SX structure with a height of 2 mm can be successfully obtained by SLM. On horizontal section, the microstructures can be divided into [001] orientation zone, [010] orientation zone and overlapping zone with a typical secondary dendrite structure. On vertical section, the columnar dendritic structure along the dominant thermal gradient direction is present. The primary dendrite spacing of the SLMed samples (∼1–2 μm) is more than two orders of magnitude lower than that of the cast one (∼320 μm). So, SLM technology possesses the capacity to manufacture SX superalloy. When the deposited height reaches 6 mm, the misorientation angle is about 25° and a crack initiates. Finally, the grain growth mechanism of the SLMed SRR99 single-crystal superalloy was proposed based on the local thermal gradient, the existing well-aligned crystallographic orientation and the preferential grain growth direction.

Effect of Ultra-high Magnesium on SKS51 Liquid Steel Cleanliness and Microstructure

This study emphasizes that ultra-high Mg ([%Mg]>0.03) content is very difficult to be achieved under conventional smelting conditions. The content of Mg in steel has significant influence on the cleanness of molten steel and microstructure. As the content of Mg increases, the content of O and S in steel decreases significantly, with O content as low as 0.0002% and S content as low as 0.0008%. Almost all inclusions in ultra-high Mg steel are magnesium-bearing oxide, sulfide, even carbide. As the content of Mg increases, the number of inclusions in the steel increases and the size decreases. But if too much Mg is added into the steel, the size and number of inclusions will rapid increase. The as-cast secondary dendrite spacing of steel decreased obviously with the increase of Mg content in steel. As the content of Mg increases, the as-cast microstructure changes from lamellar pearlite to granular pearlite. The phase diagram of SKS51 steel was calculated by Thermal-calc software. The calculated results showed that MgC2 was precipitated in the austenite before the austenite was converted into perlite. MgC2 may become the nuclear core, leading to perlite transformation. Suspected MgC2 was found in spheroidized annealing Fe3C core.

Tailoring of Processing Parameters, Dendritic Microstructure, Si/Intermetallic Particles and Microhardness in As-cast and Heat-Treated Samples of Al7Si0.3Mg Alloy

In this work, solidification and T6-heat treatment experiments were performed with the Al7Si0.3Mg alloy in order to investigate the effect of processes parameters on the microstructure and microhardness (HV). A directional solidification device was used, and the solidification thermal parameters investigated were the growth and cooling rates (VL and TR). The heat treatment applied was the T6 (T6-HT), under the following conditions: solution treatment for 3 h at 520????????2 ??C, followed by quenching in warm water (70????????20 ??C), aging for 1, 2, 3 and 4 h at 155????????2 ??C and air-cooling. The microstructure observed in both as-cast and T6-heat treated samples is composed of a primary phase consisting of an Al-rich dendritic network and secondary eutectic phases, located within the interdendritic regions, formed by Si and Mg2Si particles and Fe-intermetallic phases. The dendritic microstructure was characterized by secondary dendritic spacing (??2) and, for both investigated samples, a single mathematical expression was proposed on the ??2 dependence with the position in the ingot. Spheroidized-like eutectic Si particles have been found in both analyzed samples for finer microstructures. Elements quantitative and qualitative microanalysis by SEM/EDS as well as HV measurement at the dendritic and interdendritic regions attest to the effectiveness of the T6-heat treatment. It is highlighted in this work that mathematical expressions have been proposed to characterize the HV dependence on aging time.

Upward Unsteady-State Solidification of Dilute Al–Nb Alloys: Microstructure Characterization, Microhardness, Dynamic Modulus of Elasticity, Damping, and XRD Analyses

Aluminium alloys form many important structural components, and the addition of alloying elements contributes to the improvement of properties and characteristics. The objective of this work is to study the influence of thermal variables on the microstructure, present phases, microhardness, dynamic modulus of elasticity, and damping frequency in unidirectional solidification experiments, which were performed in situ during the manufacturing of Al–0.8 Nb and Al–1.2 Nb (wt.%) alloys. Experimental laws for the primary (λ1) and secondary (λ2) dendritic spacings for each alloy were given as a function of thermal variables. For Al–0.8%wt Nb, λ1 = 600.1( T ˙ )−1.85 and λ2 = 186.1(VL)−3.62; and for Al–1.2%wt Nb, λ1 = 133.6( T ˙ )−1.85 and λ2 = 55.6(VL)−3.62. Moreover, experimental growth laws that correlate the dendritic spacings are proposed. An increase in dendritic spacing influences the solidification kinetics observed, indicating that metal/mold interface distance or an increase in Nb content lowers the liquidus isotherm velocity (VL) and the cooling rate (Ṫ). There is also a small increase in the microhardness, dynamic modulus of elasticity, and damping frequency in relation to the composition of the alloy and the microstructure.

Phase field simulation of dendrite sidebranching during directional solidification of Al-Si alloy

In this paper, the phase field method is used to study the multi control factors during directional solidification sidebranching growth of Al-Si alloy. The effects of primary dendritic spacing, pulling velocity, temperature gradient and thermal noise on the evolution of sidebranch are studied. As the primary dendritic spacing increases, the number of secondary branches symmetrically growing on both sides of the primary dendrites increases and the secondary branches are more developed; The increase of the pulling velocity and the temperature gradient can increase the instability of the sidebranch and the degree of solute enrichment in the secondary branches gap. Sidebranch remelting and coarsening will occur when the pulling velocity and temperature gradient are large; Without changing the tip morphology and steady-state behavior of the dendrite, appropriate thermal noise can trigger the sidebranching and make the sidebranch on both sides of the dendrite asymmetrically growth.

Photochemical Amplifier Based on Self-Immolative Dendritic Spacers.

A self-immolative dendritic structure was synthesized. It is based on phenol derivatives with three hydroxymethyl arms at both ortho and para positions of the core unit, potentially releasing up to 27 leaving groups in a third-generation dendrimer. The triggering event is the photolysis of a photosentive ortho-nitrobenzyl group. In doing so, we expected to transform a weak chemical or photochemical input into a large chemical output, which fulfills the definition of a molecular amplifier. Such dendrimers could find application as an indicator, a drug-delivery vector, or a solubilizing agent. The prepared dendrimer indeed released up to 27 leaving groups upon photolysis at 360 nm.

Influence of static magnetic field on microstructure and mechanical behavior of selective laser melted AlSi10Mg alloy

In this work, the influence of a static magnetic field on the microstructure and mechanical behavior of AlSi10Mg alloy was studied. Our findings show that the applied magnetic field results in increase of the relative density and decrease of cellular dendrite spacing in the processed material. Moreover, the fraction of grains with a columnar morphology decreases and the fraction of equiaxed grains increases with increasing magnetic field intensity. As a result, the AlSi10Mg alloys fabricated via selective laser melted (SLM) with a superimposed magnetic field exhibited both a high ultimate tensile strength and ductility, which are superior to the AlSi10Mg alloys using identical process parameters without magnetic field. Furthermore, the influence of a static magnetic field on the melt pool scale and mushy zone scale was analyzed and simulated numerically. Our results suggest that the decrease pores density may be attributed to magnetic damping of convection and the volume force imposed on the cellular dendrite reaches 105 N/m3, which is sufficient to fracture the columnar grains and refine the cellular dendrite spacing. The present study provides novel insight into the potential of using a superimposed magnetic field during SLM processing and the associated benefits in terms of materials performance.

Primary spacing selection and phase transformation in directional solidified Cu-21%Ce alloy

In this study, a hyper-peritectic Cu-21 wt% Ce alloy was prepared by directional solidification method at various withdrawal rates of 100 μm s−1, 25 μm s−1, 6 μm s−1, 5 μm s−1, 4 μm s−1 and 2 μm s−1. The microstructure evolution was investigated by optical microscope. The phase constitutions were analysed by the x-ray diffraction, differential scanning calorimetry and transmission electron microscope. Further, the minimum spanning tree method was selected to investigate the spacing characters of dendritic arrays. The experimental results demonstrate that primary phase microstructure transforms from dendritic to island structure with decreasing the withdrawal rates, and the maximum volume fraction of the primary phase reached 45.16% at the withdrawal rate of 6 μm s−1. It was found that when reducing the withdrawal rate from 100 μm s−1 to 2 μm s−1, the primary dendrite spacing increased first and then decreased, with a maximum value of 144.64 μm presenting at 6 μm s−1. The phase constitutions in the directional solidified samples were confirmed to be Cu6Ce, Cu4Ce and Cu4Ce + Cu2Ce eutectics. Moreover, the maximum compressive force was determined to be 10.3 kN and the maximum compressive strain was 3.73% respectively, corresponding to the maximum primary phase volume fraction at a withdrawal rate of 6 μm s−1. Directional solidification method has been proved to be an efficient way to improve the mechanical properties of hyper-peritectic Cu-21%Ce alloy.

Characterization of plasma rotating electrode atomized Nb-Ti based alloy powder

Nb-Ti based alloy powder was prepared by plasma rotating electrode process. The relationship between microstructure, particle size and cooling rate was investigated. Phase constituents and surface state of the powder were characterized. The results indicated that particle size of the Nb-Ti based alloy powder ranged from 30 to 165 μm. Most of the atomized Nb-Ti based alloy powder particles had a smooth surface and near-perfect spherical shape. The atomized powder was found to be single β phase with dendritic, cellular or microcrystalline structure. The reduction in powder size and the increase in cooling rate had transformed the solidification morphologies of the PREP powder from dendrite in major to a cellular structure. The secondary dendritic spacing was measured to be 1.32 ∼ 2.71 μm, and the corresponding cooling rates were estimated to be in the range of 4.50 × 104 ∼ 3.75 × 105 K·s−1. Oxygen contamination on the surface of the powder existed in the form of Nb2O5 and TiO2, and the thickness of the oxide layer was about 8 nm.

Phase field study of spacing evolution during wire and laser additive manufacturing under transient conditions

Understanding the dynamic evolution of primary dendritic spacing in the laser melt pool is significant from a technological viewpoint because primary spacing is one of the foremost parameters that control the final mechanical properties of additive manufactured products. In this work, a multi-scale computational framework that couples FEM and a developed quantitative phase field method is employed to simulate the evolution of microstructure and primary spacing of a nickel-based superalloy during wire and laser additive manufacturing (WLAM) solidification. Transient conditions in the laser melt pool are considered in which both temperature gradient G and solidification speed VP are made time-dependent. Through the use of this model, the dendritic morphology, tip velocity and spacing evolution during the solidification are investigated to provide the relationship between the laser processing parameters and the final spacing. Moreover, we attempted to clarify the intrinsic mechanism of spacing adjustment under different laser processing parameters from a novel perspective. This work provides meaningful understanding of spacing evolution in nickel-based superalloy and demonstrates the potential of controlling the complex microstructure morphologies and final primary spacing during wire and laser additive manufacturing process.

Cellular-to-Dendritic and Dendritic-to-Cellular Morphological Transitions in a Ternary Al-Mg-Si Alloy

The study is focused on the influence of solidification thermal parameters upon the evolution of the microstructure (either cells or dendrites) of an Al-3wt%Mg-1wt%Si ternary alloy. It is well known that the application properties of metallic alloys will greatly depend on the final morphology of the microstructure. As a consequence, various studies have been carried out in order to determine the ranges of cooling rates associated with dendritic-cellular transitions in multicomponent alloys. In the present research work, directional solidification experiments were conducted using either a Bridgman (steady-state) device or another device that allows the solidification under transient conditions (unsteady-state). Thus, a broad range of cooling rates (Ṫ), varying from 0.003K/s to 40K/s could be achieved. This led to the identification of a complete series of cellular/dendritic/cellular transitions. For low cooling rate experiments, low cooling rate cells to dendrites transition happens. Moreover, at a high cooling rate, a novel transition from dendrites to high cooling rate cells could be observed for the Al-3wt%Mg-1wt%Si alloy. Additionally, cell spacing λC and primary dendritic spacing λ1 are related to the cooling rate by power function growth laws characterized by the same exponent (-0.55) for both steady-state and unsteady-state solidification conditions.

Revealing influence mechanism of a transverse static magnetic field on the refinement of primary dendrite spacing during directional solidification

The refinement mechanism of primary dendrite spacing caused by a transverse static magnetic field during directional solidification was proposed and verified experimentally in this paper. According to the Kurz-Fisher model, this magnetic field-induced refinement is related to the temperature gradient and solute redistribution in the mushy zone during directional solidification. Therefore, the effect of the magnetic field on the temperature gradient was investigated and the solute redistribution induced by the thermoelectric magnetic convection was measured by Rank sort method. The experimental results indicate that the refinement of primary dendrite spacing can be attributed to the solute redistribution rather than the change of temperature gradient.

An innovation for microstructural modification and mechanical improvement of TiAl alloy via electric current application

In this article, microstructural evolution during the solidification of Ti-48Al-2Cr-2Nb with current density, as well as the formation mechanisms, are discussed, along with the impacts on microhardness and hot compression properties. The applied electric current promotes the solidification from the α primary phase to a largely β solidification in Ti-48Al-2Cr-2Nb. With an increase in supercooling, the solidification process have a tendency to change from an α-led primary phase to (α + β)-led primary phase. The primary dendrites, grain size, and lamellar spacing show a tendency to decrease first before increasing with increasing current density. Microhardness and high-temperature yield strength increase with a decrease in primary dendrite spacing, grain size, and lamellar spacing. Correlations between primary dendrite spacing, lamellar spacing, microhardness, yield strength, and current density are described by a fitting formula. An increase of α2 phase, due to the application of electric current, results in improved microhardness. The yield strength of Ti-48Al-2Cr-2Nb alloy increases linearly with microhardness. Yield stress increases with a decrease in microstructure parameters, in accordance with the Hall–Petch equation. The predominant modification mechanism with electric current application for TiAl solidification is the variation of supercooling and temperature gradients ahead of the mush zone due to Joule heating.

Analysis of the influence of deformation in microstructure formation during solidification of Pb-2.5% Sb alloy

In industrial casting operations, the possibility of implantation of a diversity of operational conditions has as consequence the generation of a wide range of solidification structures, affecting the mechanical properties and the quality of the final product. Due to the importance of the study of thin slabs solidification, for this work is used a system that allows the physical simulation of initial stages of continuous casting of thin slabs for laboratory experiments under several solidification conditions. Experiments were carried out using Pb-2.5% Sb alloy that presents low melting temperature and characteristic structure. For the Pb-2.5% Sb alloy thin slabs were obtained in cooled mold without reduction and with 10 and 20 mm reductions. It was found that the degree of deformation applied tends to increase the equiaxed region by the breakage of the dendrites as well as the possibility of the formation of equiaxed dendritic structures. It was observed that the deformation of this alloy directly influences the secondary dendrite spacing, tending to decrease it.

Near-eutectic Zn-Mg alloys: Interrelations of solidification thermal parameters, microstructure length scale and tensile/corrosion properties

Zn-Mg alloys are considered to have potential application in bone implants, since both metals are biocompatible and have biodegradable characteristics. Adding Mg to Zn can boost mechanical and corrosion resistances. However, the literature is very limited on quantifying the interrelation of solidification parameters, microstructural features and mechanical/corrosion properties of Zn-Mg alloys. The present study examines the interrelations of alloy Mg content, macrosegregation effects, morphology and scale of the matrix and eutectic phases, nature of intermetallics and tensile and corrosion properties of near-eutectic Zn-Mg alloys. The alloys samples are obtained by unsteady-state directional solidification resulting in a wide range of solidification thermal parameters and microstructures. We examine microstructural features of both dendritic and complex regular eutectic phases. It is shown that the eutectic exhibits a bimodal pattern with neighboring areas of coarse and fine lamellae. Experimental growth laws relating the primary, secondary and eutectic spacings to the solidification cooling rate and growth rate are proposed. Hall-Petch type equations are derived expressing tensile strength and elongation to dendritic and eutectic spacings. Electrochemical parameters determined by polarization curves during corrosion tests and SEM analyses of corroded areas have shown that the alloy having an essentially eutectic microstructure is associated with better corrosion resistance.

Sn-0.5Cu(-x)Al Solder Alloys: Microstructure-Related Aspects and Tensile Properties Responses

In this study, experiments were conducted to analyze the effect of 0.05 and 0.1 wt.% Al additions during the unsteady-state growth of the Sn-0.5wt.%Cu solder alloy. Various as-solidified specimens of each alloy were selected so that tensile tests could also be performed. Microstructural aspects such as the dimensions of primary, λ1, and secondary, λ2, dendritic arrays, and intermetallic compounds (IMCs) morphologies were comparatively assessed for the three tested compositions, that is, Sn-0.5wt.%Cu, Sn-0.5wt.%Cu-0.05wt.%Al, and Sn-0.5wt.%Cu-0.1wt.%Al alloys. Al addition affected neither the primary dendritic spacing nor the types of morphologies identified for the Cu6Sn5 IMC, which was found to be either globular or fibrous regardless of the alloy considered. Secondary dendrite arm spacing was found to be enlarged and the eutectic fraction was reduced with an increase in the Al-content. Tensile properties remained unaffected with the addition of Al, except for the improvement in ductility of up to 40% when compared to the Sn-0.5wt.%Cu alloy without Al trace. A smaller λ2 in size was demonstrated to be the prime microstructure parameter associated with the beneficial effect on the strength of the Sn-0.5wt.%Cu(-x)Al alloys.