Dendritic Alloys Research Articles

Study of growth advantage of twinned dendrites in aluminum alloys during Bridgman solidification

Abstract

Effect of additional solute elements (X= Al, Ca, Y, Ba, Sn, Gd and Zn) on crystallographic anisotropy during the dendritic growth of magnesium alloys

Based on the first-principles calculations and crystallographic anisotropy analysis, the effect of additional solute elements on the dendritic growth behavior of binary Mg-X (X = Al, Ca, Y, Ba, Sn, Gd and Zn) alloys are investigated in terms of the orientation-dependent surface energy. The preferred growth direction of the α-Mg dendrite is found to be dependent on the magnitude of the anisotropic surface energy and the difference of crystallographic anisotropy between the matrix Mg and the additional solute X. The most densely packed crystallographic planes are found to be the energetically favorable planes with the minimum surface energy, as exemplified by the {111} plane of Al with fcc (face-centered cubic) structure, the {110} plane of Ba with bcc (body-centered cubic) structure, and the {0001} plane of Y with hcp (hexagonal-close packed) structure. For all additional solute elements studied, Zn exhibits the maximum anisotropy and the according effect on the growth behavior of the α-Mg dendrite is the most significant, which could also be reflected by the complex growth patterns of the Mg-Zn alloy dendrite observed in experiments. Compared with Zn, the crystallographic anisotropy of the other additional solutes is weaker, and their effect on the α-Mg dendrite growth is not as significant as Zn, which is reflected by their similar eighteen-primary branch dendritic morphology in 3D.

Microstructure evolution of directionally solidified NiAl–28Cr–6Mo eutectic alloy under different withdrawal rates

ABSTRACTThe microstructure evolutions of directionally solidified NiAl–28Cr–6Mo eutectic alloy at withdrawal rates of 6–500 µm s−1 and temperature gradients of 250 K cm−1 were studied. The microstructure was composed of lamellar NiAl and Cr(Mo) phase, which had the crystallographic orientations of [111]NiAl∥[111]Cr(Mo) and NiAl∥Cr(Mo) as well as the growth direction of <111>. With increasing withdrawal rate, the growth interface changed from planar to cellular and dendritic, the microstructure also transformed from planar eutectic to two-phase cellular eutectic and dendritic eutectic. The eutectic interlamellar spacing λ decreased according to the relationship of λ = 4.48V−0.40, which indicated that the J–H model was applicable not only for the planar eutectic alloy but also for the cellular and dendritic eutectic alloys.

Atomistic underpinnings for growth direction and pattern formation of hcp magnesium alloy dendrite

The three-dimensional (3D) growth pattern, preferred growth directions and the underlying growth mechanism of magnesium alloy dendrite are investigated via 3D experimental characterization and multiscale mathematical simulations. It is found that the formation of the dendritic microstructure is associated with the magnitude of surface energy anisotropy. The results based on synchrotron X-ray tomography and electroback scattered diffraction techniques show that typical 3D morphology of the α-Mg dendrite exhibits an 18-primary-branch pattern, with six along the <112¯0> basal direction, and the other twelve along the <112¯3> nonbasal direction. The underlying mechanism is investigated by performing relevant atomistic calculations at the ground state and the elevated temperatures in light of density functional theory (DFT) and quasi-harmonic approximation (QHA). The results indicate that the preferred growth direction for the α-Mg dendrite growth is <112¯x> rather than <101¯x>, and the anisotropic surface energy decreases as the temperature increases. Subsequent analysis further confirms that the preferred growth directions of the α-Mg dendrite at different temperatures correspond consistently to those orientations with higher surface energy anisotropy, i.e., the <112¯0> and <112¯3>. Accordingly, the 3D phase-field simulations are performed to investigate the growth behavior of the α-Mg dendrite, with the anisotropic strength determining via DFT-based calculations.

Effect of RE elements on the microstructural and mechanical properties of as-cast and age hardening processed Mg–4Al–2Sn alloy

Abstract In the present article, the effect of Rare Earth elements on the microstructural development and mechanical properties of as cast and age treated Mg–4Al–2Sn (AT42) alloy is studied. Investigation has been conducted by optical and scanning electron microscope, XRD and tensile tests. Analysis of the data showed that alloy's dendrites turn into larger dendritic structure with sharp and narrow arms from equiaxed rosette type by addition of RE elements. In contrast to the base alloy, aging treatment shows a positive effect on the mechanical properties (yield strength, tensile strength and elongation) of AT42 + 1RE alloy mainly because of retention of the thermally stable RE containing intermetallics as strong barriers to grain growth. Also, increase of solute aluminum due to the decomposition of Mg17Al12 along with saturated RE elements led to formation of blocky shape Al2RE in the microstructure during aging which enhanced the mechanical properties. It was found that the best result (yield of 70 MPa, tensile strength of 168 MPa and elongation of 14%) could be achieved by aging the AT42 + 1RE alloy at 443 K (170 °C) for 8 h. However, mechanical properties of AT42 + 1RE alloy starts to decrease after exceeding its optimum aging conditions due to the coarsening of intermetallics.

Effect of thermal anisotropy on binary alloy dendrite growth

A numerical model to study the effect of thermal anisotropy on binary alloy dendrite growth is presented. The model is based on the volume averaged enthalpy method with explicit surface tension anisotropy for crystal orientation. Thermal anisotropy is incorporated using anisotropic thermal conductivity in the energy equation. This is done by splitting the anisotropic conductivity into two parts, equivalent isotropic conductivity and anisotropic departure source term, enabling the use of a conventional isotropic solver to model anisotropic heat transfer. The proposed model is applied to study the effect of thermal anisotropy ratio on tip velocity, aspect ratio and equivalent radius of an equiaxed grain growing in an undercooled binary alloy melt. It is found that the thermal energy stored in the grain during solidification plays an important role in interface evolution, and thus anisotropic conductivity in the solid affects the grain morphology. There is a consistent increase in the aspect ratio of grains with increase in thermal anisotropy ratio, although the grain volume remains almost invariant. Due to unequal growth rates of the perpendicular arms, severe distortion of the solid crystal is seen at higher thermal anisotropy ratios. The model is further extended to study the growth of multiple dendrites in order to simulate microstructure evolution with thermal anisotropy. It is observed that thermal anisotropy significantly affects the grain morphology at low grain density but has a smaller influence at high grain density as compared to other governing factors such as solute transport.

Mechanism of the growth pattern formation and three-dimensional morphological transition of hcp magnesium alloy dendrite

Based on an anisotropy function with the anisotropic strength determined via atomistic calculations, the mechanism of the three-dimensional (3D) growth pattern formation of magnesium alloy dendrite is investigated by performing phase-field simulations with the parallel adaptive-mesh-refinement algorithm. It is found that the 3D morphological transition of the $\ensuremath{\alpha}$-Mg dendrite is dependent on the growth parameters, including the partition coefficient, the anisotropic strength, and the supercooling during solidification. The $\ensuremath{\alpha}$-Mg dendrite exhibits growth tendency along both the basal and nonbasal directions, but the dendritic growth tendency along the basal direction is weaker. Consequently, the 3D morphology of the $\ensuremath{\alpha}$-Mg dendrite would transform from an 18-primary-branch pattern to a 12-primary-branch pattern if the local growth driving force on the basal plane is insufficient. During dendrite growth, the solute concentration increases as the distance from the dendritic nucleus center increases, and reaches the local maximum at the solid/liquid interface, beyond which it decreases before reaching a constant value. The simulation results agree well with those found in experiments and the existing solidification theory.

Electrochemical Reduction of Carbon Dioxide at Alloy Systems: Cu-In and Cu-Bi

The ability to maintain high efficiencies while simultaneously tuning the selectivity of the electrochemical reduction of CO2 (ERC) using low cost electrodes has proven to be one of the greatest obstacles to the widespread commercialization of this technology. In this study, we prepare dendritic copper-indium and copper-bismuth alloys of various compositions and investigate their catalytic activity and selectivity towards the reduction of CO2. The Cu-In electrocatalysts are increasingly dendritic with higher In fraction and, depending on composition, consist of mixed phases of Cu, In and Cu-In intermetallic phases. ERC at these electrodes produces formate at high efficiencies (up to 62% with a 80 at% alloy, -1 V)) while also tuning the CO/H2 ratio to achieve an ideal syngas composition with a 40 at% In alloy, -1 V. The observed product distribution as a function of alloy composition and applied potential is rationalized in terms of the relative adsorption strengths of CO and COOH intermediates at Cu and In sites, and their distinct variation with applied potential may be induced by the differences in electronic structure. We also studied dendritic Cu, Bi, and Cu-Bi alloys with various compositions. The dendritic structures exhibit a high density of defect sites such as grain boundaries and twins, with these defects being increased at alloy dendrites. While the pure metals exhibited high H2 production and relatively high formate formation, the alloys exhibit a lower overall current density and a much higher selectivity towards formate, up to about 90%. Most importantly we show in this work that a combination of a d metal (Cu) with a p-block metal (In, Bi) with intimate contact between the two components is capable to tune the relative adsorption strength of the intermediates, thus steering the product distribution towards formate. The relatively purity of this product would be of interest for the direct feed of a direct formate fuel cell. Differences between In and Bi will be discussed in terms of the affinity for oxygen. This study highlights the opportunities of using alloys to enhance control over the product distribution and suggests that suitable alloys could be promising catalysts for the inexpensive and efficient production of fuels.

A cellular automaton-lattice Boltzmann method for modeling growth and settlement of the dendrites for Al-4.7%Cu solidification

A coupled model of CA-LBM for simulating the simultaneous growth and motion of equiaxed dendrites in solidifying melt is presented. The model uses the CA method for the growth of the dendrite, the LBM for the momentum, mass, and heat transfer of the domain, the Ladd method for handling the rigid solid–liquid interfaces and the equations of motion for tracking the translational and rotational motion of the solid phase. The simulation of a circular particle between two infinite and parallel walls proved the accuracy of boundary treatment. Two-dimensional computations of dendritic translation and rotation show that the shape preservation conditions are satisfied in this method. Then, the present method is used to calculate the growth behavior of a binary alloy dendrite in the process of its dropping motion in the melt. Initially, the dendrite is standard and only translation happened, but later the dendrite is inclined and simultaneous translational and rotational motion happened. The simulation result shows that during its falling process the dendrite settles and grows in a specific morphology, which differs from that of a dendrite dropping vertically without rotation.

Исследование возможности формирования в сплавах Al расщепляющейся морфологии дендритов в условиях больших концентрационных градиентов

Исследование возможности формирования в сплавах Al расщепляющейся морфологии дендритов в условиях больших концентрационных градиентов

Atomistic Determination of Anisotropic Surface Energy-Associated Growth Patterns of Magnesium Alloy Dendrites.

Because of the existence of anisotropic surface energy with respect to the hexagonal close-packed (hcp) lattice structure, magnesium alloy dendrite prefers to grow along certain crystallographic directions and exhibits a complex growth pattern. To disclose the underlying mechanism behind the three-dimensional (3-D) growth pattern of magnesium alloy dendrite, an anisotropy function was developed in light of the spherical harmonics and experimental findings. Relevant atomistic simulations based on density functional theory were then performed to determine the anisotropic surface energy along different crystallographic directions, and the corresponding anisotropic strength was quantified via the least-square regression. Results of phase field simulations showed that the proposed anisotropy function could satisfactorily describe the 3-D growth pattern of the α-Mg dendrite observed in the experiments. Our investigations shed great insight into understanding the pattern formation of the hcp magnesium alloy dendrite at an atomic level.

Correlation between crystallographic anisotropy and dendritic orientation selection of binary magnesium alloys

Both synchrotron X-ray tomography and EBSD characterization revealed that the preferred growth directions of magnesium alloy dendrite change as the type and amount of solute elements. Such growth behavior was further investigated by evaluating the orientation-dependent surface energy and the subsequent crystallographic anisotropy via ab-initio calculations based on density functional theory and hcp lattice structure. It was found that for most binary magnesium alloys, the preferred growth direction of the α-Mg dendrite in the basal plane is always langle 11bar{2}0rangle , and independent on either the type or concentration of the additional elements. In non-basal planes, however, the preferred growth direction is highly dependent on the solute concentration. In particular, for Mg-Al alloys, this direction changes from langle 11bar{2}3rangle to langle 22bar{4}5rangle as the Al-concentration increased, and for Mg-Zn alloys, this direction changes from langle 11bar{2}3rangle to langle 22bar{4}5rangle or langle 11bar{2}2rangle as the Zn-content varied. Our results provide a better understanding on the dendritic orientation selection and morphology transition of magnesium alloys at the atomic level.

Growth pattern and orientation selection of magnesium alloy dendrite: From 3-D experimental characterization to theoretical atomistic simulation

Based on synchrotron X-ray tomography and EBSD techniques, it was found that magnesium alloy dendrite exhibits a typical 3-D growth pattern with eighteen-primary branches, of which six grow along 〈112¯0〉 in the basal plane and the other twelve along 〈112¯3〉 in non-basal planes. The underlying mechanism behind such growth pattern was investigated by determining the surface energy related anisotropy along different crystallographic orientations via atomistic simulation in light of density functional theory and hcp lattice structure. Results showed that the surface energy exhibits an orientation-dependent behavior, and these specific crystallographic planes perpendicular to which lay the preferred growth directions of α-Mg dendrite always exhibit higher surface energy. Accordingly, the orientation selection of α-Mg dendrite growth was confirmed to be 〈112¯0〉 and 〈112¯3〉, which agreed well with the experimental findings. A geometrical model was developed to illustrate the 3-D growth pattern of the α-Mg dendrite based on its outer contour shape at equilibrium energy state.

Phase-field simulation of tip splitting in dendritic growth of Fe-C alloy

Two types of dendrite tip splitting including dendrite orientation transition and twinned-like dendrites in Fe-C alloys were investigated by phase-field method. In equiaxed growth, the possible dendrite growth directions and the effect of supersaturation on tip splitting were discussed; the dendrite orientation transition was observed, and it was found that the orientation regions of anisotropy parameters were reduced from three to two with increasing the supersaturation, which was due to the effect of interfacial anisotropy controlled by the solute in front of S/L interface changing with the increase of supersaturation. In directional solidification, it was found that the twinned-like dendrites were formed with the fixed anisotropy couples and no seaweed dendrites were observed; these were concluded from the results of competition between process anisotropy and inherent anisotropy. The formation process of twinned-like dendrite was investigated by tip splitting phenomenon, which was related to the changes of dendrite tips growth velocity. Then, the critical speed of tips splitting and solute concentration of twinned-like dendrites were investigated, and a new type of microsegregation in Fe-C alloys was proposed to supplement the dendrite growth theories.

The spatial and temporal distribution of dendrite fragmentation in solidifying Al-Cu alloys under different conditions

The directional columnar solidification of Al-Cu alloy dendrites was studied in-situ by synchrotron X-ray radiography to investigate the spatial and temporal distribution of dendrite fragmentation, without and with an applied pulsed electromagnetic field (PEMF) and in different solidification directions. The differing arrangements had a strong and reproducible influence on fragmentation behaviour that was rationalised using a model of solute-driven re-melting of dendrite arms in microstructural regions of high vulnerability. Although absolute rates of fragmentation varied, all the fragmentation distributions had a characteristic peak in fragmentation rate at a distance of 500–800 μm into the mushy zone. Consistent with the need for solute-rich liquid flow for dendrite root re-melting, this peak was explained to occur under conditions where there was both comparatively steep liquid solute concentration gradients, measured directly from the radiographs, and relatively high permeability of the dendrite network to liquid flow.

Effect of different solute additions on dendrite morphology and orientation selection in cast binary magnesium alloys

In solidification, dendritic morphology was observed to change accordingly if either the type or quantity of the additional element was modified. To gain insight into this phenomenon, the 3D dendrite morphology of different binary magnesium alloys, including MgAl, MgBa, MgCa and MgZn alloys was characterized using synchrotron X-ray tomography and electron backscattered diffraction. Results showed that for most Mg-based alloys, the dendrite exhibited a typical 18-branch morphology with preferred growth orientations along 〈112¯0〉 and 〈112¯3〉, whereas for MgZn alloys, the dendrite morphology would change from the 18-branch pattern to 12-branch if the Zn content increased, i.e. the so-called dendrite orientation transition (DOT) took place. This DOT behaviour of the Mg alloy dendrite was then successfully modelled using the 3D phase field method by changing the magnitude of related parameters in the specially formulated anisotropy function based on spherical harmonics.

Simulation of liquid channel of Fe-C alloy directional solidification by phase-field method

In directional solidification, two characteristic parameters determine the dendritic growth: the thermal gradient and the pulling velocity. To achieve the suitable microstructure and improve the performance of casting, they are usually used to resize the pulling velocity or temperature gradient in directional solidification process. The structures obtained under different directional solidification conditions, and their associated properties both have been hot research points. It is difficult to observe the microstructure, which is usually on a micrometer scale, directly in experiment, and the phase-field method becomes a strong tool to understand the dendrite growth pattern. We mainly study the liquid channel formed after Fe-C alloy dendrite tip splitting under the specific condition of directional solidification and analyze the influence on liquid channel of pulling velocity in this paper. We choose the fixed thermal gradient G =20 K/mm which is on the order of the experimental value, and pulling velocity VP no more than 10 mm/s to keep the cooling rate in the range of low speed in dendrite growth, so that the interface kinetic effect can be neglected. Recent experimental results show the different interfacial energies in various compositions of Al-Zn alloy and Fe-C alloy, then we can investigate a series of directional solidification microstructures with fixed alloy Fe-0.5 wt.%C composition at different interfacial energies in our simulations. We find that the liquid channel is formed as a result of anisotropy competition between system and materials, the length and C concentration of liquid channel increase with the pulling velocity increasing, while the diameter of liquid channel is constant. It is interesting to find that there is a minimum of pulling velocity almost equal to 1 mm/s, the tip will not split and no liquid channel forms in the following steps either when the velocity is smaller than the minimum. We also compare the segregation caused by solute enrichment in liquid channel and solute segregation between dendrite arms in a series of simulations: the former is more serious than the latter. Then we point out the way to reduce the segregation caused by liquid phase channel by reducing the pulling velocity properly. It will be more practical to couple the flow field with other external field, such as magnetic field, in the simulation.

Three-dimensional mesoscopic modeling of equiaxed dendritic solidification of a binary alloy

The mesoscopic envelope model is a recent multiscale model that is intended to bridge the gap between purely microscopic and macroscopic approaches for the study of dendritic solidification. It consists of the description of a dendritic grain by an envelope that links the active dendrite branches. The envelope growth is deduced from an analytical microscopic model of the dendrite tip growth kinetics matched to the numerical solution of the mesoscopic solute concentration field in the vicinity of the envelope. The branched dendritic structure inside the envelope is described in a volume-averaged sense by phase fractions and averaged solute concentrations. We present a careful quantitative analysis of the influence of numerical and model parameters on the accuracy of the model predictions. We further perform a validation study through comparisons of 3D simulations to experimental scaling laws giving the shape and the internal solid fraction of freely growing binary alloy dendrites and to analytical solutions for the primary dendrite tip speed. We provide generally valid guidelines for the calibration of the mesoscopic model, enabling reliable control of the accuracy of model predictions over a wide range of undercoolings. The model is applied to simulate strong solutal interactions in large ensembles of equiaxed grains. The potential for mesoscopic simulations to provide refined modeling of microstructures in volume-averaged macroscopic models via scale bridging is demonstrated.

The role of elevated temperature exposure on structural evolution and fatigue strength of eutectic AlSi12 alloys

Pistons of internal combustion (IC) engines are typically subjected during operation to high cycle fatigue loading at elevated temperatures (up to 350°C). The materials typically used for piston production are eutectic Al–Si alloys for their excellent fluidity and suitable mechanical properties. Results of a fatigue testing program of eutectic Al–Si alloys at room temperature and at elevated temperatures (250°C, 300°C and 350°C) performed with the aim to support piston design and material optimization are reported in this paper. Specimens were extracted from piston crowns and tested in a rotating bending test machine. The fatigue strength at 107cycles was quantified by a staircase approach.To investigate the strengthening mechanism based on the formation of precipitates (Guinier–Preston (GP) zones) during decomposition of a metastable supersaturated solid solution, a structural investigation of one of the alloys before and after fatigue testing at elevated temperatures was conducted. Metallography, color etching, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to elucidate the following aspects: (1) the structural features of the alloy (dendrites of α-phase, primary Si particle size and distribution, morphology and distribution of intermetallic phases); (2) the changes of the strengthening mechanism (GP zones) with elevated temperature exposure.

GPU phase-field lattice Boltzmann simulations of growth and motion of a binary alloy dendrite

A GPU code has been developed for a phase-field lattice Boltzmann (PFLB) method, which can simulate the dendritic growth with motion of solids in a dilute binary alloy melt. The GPU accelerated PFLB method has been implemented using CUDA C. The equiaxed dendritic growth in a shear flow and settling condition have been simulated by the developed GPU code. It has been confirmed that the PFLB simulations were efficiently accelerated by introducing the GPU computation. The characteristic dendrite morphologies which depend on the melt flow and the motion of the dendrite could also be confirmed by the simulations.