Rapid Directional Solidification Research Articles

Featuring vertical channel structure through directional solidification method to endow GNP-aerogels with enhanced ultra-electromagnetic wave absorption

Three-dimensional (3D) carbon-based electromagnetic waves absorbing materials (EWAMs) have garnered significant attention due to high electrical conductivity, high dielectric loss, low density, and improved characterization of impedance matching. However, there is a paucity of research on the design of material structures. In this regard, we proposed the graphene nanoplates (GNPs) EWAMs featuring a vertically aligned channel structure through liquid nitrogen rapid directional solidification and freeze-drying techniques. By utilizing methyl cellulose (MC) as a binder to modify the internal ice crystal structure, an exceptional electromagnetic waves (EWs) absorption material exhibiting excellent impedance matching characteristics and demonstrating multiple polarization phenomena were created. Notably, the material showcased superior absorption performance with a minimal reflection loss (RLmin) of −57.20 dB. Furthermore, the pre-solidification slurry possesses remarkable plasticity, facilitating the fabrication of various complex structures for EWAMs during directional solidification. This method provides novel insights for future exploration and fabrication of tuneable frequency band EWAMs.

Experimental and simulation studies on damage mechanisms of tungsten and molybdenum under compressed plasma flow irradiation

The failure mechanism of plasma-facing components (PFCs) under extreme plasma conditions relevant for fusion reactors were investigated. Here, edge-localized mode (ELM)-like transient thermal shock irradiation experiments were performed on tungsten and molybdenum using compressed plasma flow, and combined with thermal–mechanical analysis by means of finite element simulations to discuss the grain structure evolution, cracking behavior and variations of hardness. When ELM-like thermal shock irradiation was sufficient to melt tungsten and molybdenum, a submicron-sized cellular sub-grain structure was created on their surface due to the high temperature gradient of the molten layer under the effect of Bénard–Marangoni instability. Rapid directional solidification from the bottom of the molten layer to the surface induced the formation of columnar grains dominated by the <200> orientation. While the formation of cellular sub-grains increased hardness, the thermal effect of irradiation and the formation of columnar grains led to softening. The high thermal stress induced by the ELM-like thermal shock produced macro-cracks and micro-cracks on the surface of tungsten and only micro-cracks on the surface of molybdenum. Macro-cracks were generated due to the intrinsic brittleness of tungsten. As a result of stress evolution, longitudinal macro-cracks extending perpendicular to the surface experienced transverse transformation within the material. Micro-cracks formed due to the embrittlement of the re-solidification zone, and their width increased with the melting depth. These results help us to understand failure mechanisms in PFCs under extreme operating conditions and are valuable for developing future fusion reactors.

An OpenMP GPU-offload implementation of a non-equilibrium solidification cellular automata model for additive manufacturing

In this paper, performance strategies on GPU-based HPC platforms of a cellular automata (CA) simulation code for non-equilibrium solidification, including nucleation, grain growth, solute partitioning and transport for the metal additive manufacturing (AM) process are investigated using OpenMP 4.5. To accurately report the speed-up for multicore CPUs and GPUs, a rigorous performance analysis employed optimizations appropriate for both CPU-only code (baseline) and GPU offload codes for an isothermal test problem. The performance results on Summit at the Oak Ridge Leadership Computing Facility indicate that using a precomputed list of interface cells significantly decreased the wall-clock time on GPUs. The speedup due to GPU acceleration was evaluated for a full Summit node and measured to be 1.8X when comparing a 6 MPI tasks run with 6 GPUs versus 36 MPI tasks on the CPU only. That speed-up was found to be 7.9X when comparing 6 MPI tasks with 6 GPUs versus the 6 MPI tasks running on the CPU only. Performance measurements showed that system total time is almost constant for runs with more than 96 MPI tasks (or GPUs), indicating that the GPU-accelerated code showed an excellent weak scaling performance.Finally, a rapid directional solidification problem was considered to demonstrate the CA code capability on Summit. It was found that a mesh size of at least 0.05 μm is recommended for the AM-like simulations in order to obtain accurate elongated grain microstructure and elongated subgrain features, which are in qualitative good agreement with experimental data. The results presented in this study indicate that the performance strategies on GPU-based HPC platforms for the CA code are appropriate for novel HPC exascale platforms.

Effects of shear flows on columnar dendritic microstructure during rapid solidification of IN718 alloy: A cellular automaton-lattice Boltzmann modeling study

The growth of columnar dendrites during rapid directional solidification of IN718 alloy in the presence of shear flows is simulated by using a coupled cellular automaton-lattice Boltzmann (CA-LB) model. The developed CA-LB model is benchmarked by the mesh independency of the steady-state dendrite growth velocity and the primary dendrite arm spacing (PDAS) under different cooling rates. The CA-LB model is then applied to simulate the columnar dendrite growth under different shear flows and cooling rates in the conditions of rapid solidification. The values of PDAS under different conditions are determined from the simulation results, and are correlated with the shear flow rate (Ush) and cooling rate (CR). A unified fitting formula is developed to represent the effects of Ush and CR on the evolution of PDAS. The PDAS is found to decrease with CR increasing, but increase with Ush increasing. Moreover, it is revealed that the influence of Ush on PDAS becomes stronger as CR increases.

Microstructure evolution in immiscible alloys during rapid directional solidification

Abstract A model has been developed by taking into account the common action of the nucleation, the diffusional growth, the collisions and coagulations of the minority phase droplets and the spatial phase segregation to describe the microstructure evolution in a rapidly directionally solidified immiscible alloy. Numerical solutions have been performed to investigate the microstructure evolution in a directionally solidified hypermonotectic Al –Pb alloy. It has been shown that an undercooled region appears in front of the solid/liquid interface and the liquid –liquid decomposition takes place there. If the solidification velocity is high enough, a steady state of solidification can be established and a fine dispersion of minority phase particles in a hypermonotectic alloy can be obtained.

Harnessing elastic anisotropy to achieve low-modulus refractory high-entropy alloys for biomedical applications

A high-priority target in the design of new metallic materials for load-bearing implant applications is the reduction of Young’s modulus approximating that of cortical bone in the predominant loading direction. Here, we explore how directionally preferential bulk elastic properties of implant materials are achieved by harnessing elastic anisotropy. Specifically focusing on recently proposed biocompatible refractory high-entropy alloys (RHEAs) in the body-centered cubic structure, we conduct systematic density-functional theory calculations to investigate the single-crystal elastic properties of 21 Ti-containing RHEAs. Our results provide evidence that the valence electron count has a dominant influence on elastic anisotropy and crystal directions of low Young’s modulus and high torsion modulus in the RHEAs. By means of modeling the orientation distribution function for crystallographic texture, we examine the effect of non-random texture on the anisotropic poly-crystalline Young’s modulus and torsion modulus with varying texture sharpness. We adopt fiber textures that can result from rolling and distinct texture orientations that can form during rapid directional solidification. We discuss the potential for lowering Young’s modulus in the RHEAs by using single crystals or textured aggregates. Furthermore, we prepare four of the theoretically considered alloys by arc-melting and report their lattice parameters, quasi-isotropic Young’s moduli, and Wickers hardnesses.

Cellular growth during rapid directional solidification: Insights from quantitative phase field simulations

In this paper, columnar cellular growth with kinetic effects including kinetic undercooling and solute trapping in rapid directional solidification of alloys was investigated by using a recent quantitative phase-field model for rapid solidification. Morphological transition and primary spacing selection with and without kinetic effects were numerically investigated. Numerical results show that doublon structure is an intermediate state in the primary spacing adjustment of cellular arrays. It was found that the inclusions of kinetic effects result in the increase of the solute in the solid phase and the solute enrichment in the interdendritic liquid channel. Moreover, predicted results indicate that the growth directions of the cellular arrays in rapid directional solidification with and without kinetic effects are independent of the Péclet number. Therefore, the kinetic effects play important roles in numerical simulations of the growth pattern selection and solute distribution during rapid solidification. Neglecting them will result in the inaccurately predicted results.

A review of Laser Powder Bed Fusion Additive Manufacturing of aluminium alloys: Microstructure and properties

Additive manufacturing (AM) of metallic alloys for structural and functional applications has attracted significant interest in the last two decades as it brings a step change in the philosophy of design and manufacturing. The ability to design and fabricate complex geometries not amenable to conventional manufacturing, and the potential to reduce component weight without compromising performance, is particularly attractive for aerospace and automotive applications. This has culminated in rapid progress in AM with Ti- and Ni-based alloys. In contrast, the development of AM with Al-alloys has been slow, despite their widespread adoption in industry owing to an excellent combination of low density and high strength-to-weight ratio. Research to date has focused on castable and weldable AlSiMg-based alloys (which are less desirable for demanding structural applications), as well as on the development of new AM-specific AlMgSc alloys (based on 5xxx series). However, high strength wrought Al-alloys have typically been unsuitable for AM due to their unfavourable microstructural characteristics under rapid directional solidification conditions. Nevertheless, recent research has shown that there is promise in overcoming the associated challenges. Herein, we present a review of the current status of AM with Al-alloys. We primarily focus on the microstructural characteristics, and on exploring how these influence mechanical properties. The current metallurgical understanding of microstructure and defect formation in Al-alloys during AM is discussed, along with recent promising research exploring various microstructural modification methodologies. Finally, the remaining challenges in the development of AM with high-strength Al-alloys are discussed.

Deterministic modeling of solidification microstructure formation in directed energy deposition fabricated Ti6Al4V

Metal additive manufacturing (MAM) technology is now widely applied in the manufacturing and remanufacturing industry, among which microstructure simulation gradually shows its importance. Traditional solidification microstructure simulation methods all have their merits as well as drawbacks when applied in MAM cases. In this work, a deterministic solidification microstructure model, named “invasion model”, is established to avoid the essential defects of traditional methods. This model focuses on the interaction between the neighboring bi-crystals instead of simulating the growth kinetics of each columnar grain or deriving the field form of variables. Within a bi-crystal system, the grain boundary tilt from thermal gradient direction is understood as a transient invasion behavior of one grain to another, and the competitive grain growth behavior along the buildup process of MAM is a summary of all the invasions in bi-crystal systems. To fill the gaps in the rapid solidification theory, a database recording the anisotropic growth effect under a rapid directional solidification condition was established with the help of an artificial neural network (ANN). Wire feed directed energy deposition (DED) fabricated Ti6Al4V thin-wall samples with full dendritic columnar grains (prior beta) are used as a benchmark to test the validation of the novel simulation model. The grain geometrical structure of reconstructed prior β grains along the build-up direction has a good agreement with the simulation result. The model can also be applied to other cases of MAM or combine with various models to achieve real-time as-solidified crystallographic feature prediction when it meets the scope of application. • Traditional solidification microstructure simulation methods all have their merits and drawbacks when applied in MAM cases. • Invasion model is established to investigate the transient competitive grain growth behavior within bi-crystal systems. • Wire DED Ti6Al4V thin-wall samples are used as a benchmark to test the validation of the novel simulation model.

Adhesion-free Bridgman growth of AlSb

AlSb crystals as a dual-carrier compound semiconductor have a great potential application for the room-temperature detectors, comparative to high-purity Ge and CdZnTe detectors. However, the single crystal growth of AlSb suffers from the high reactivity of Al, which causes strong adheres of the crystal to the crucible walls. In this work, an in-situ synthesis method was developed using a pBN crucible sealed in a quartz crucible. Meanwhile, the reaction of Al and Sb was settled under the melting point with two stages for solid reaction and liquid reaction in turn. The cycles of rapid directional solidification and refusion were implemented for homogenization. Bulk crystals of AlSb with large grain size 5 × 5 mm2 were obtained from the vertical Bridgman method. The results of the XRD and EDS test showed a single zinc blende structure phase with a nearly stoichiometric atomic ratio of 49.7: 50.3 (Al: Sb). AlN was observed between AlSb/BN interface during the crystals with normal overheating synthesis. The sticking mechanism was ascribed to the increase of the adhesion energy with PBN crucible after the production of AlN. The chemical reactivity of Al has been fully inhibited during the Bridgman growth of AlSb.

Influence of Minor Cr Additions on Crystal Growth in Rapidly Solidified Al-20Zn Alloys.

It has been discovered quite recently that Icosahedral Short-Range Order (ISRO) of atoms in the liquid phase of metallic alloys surrounding some trace elements added to the melt can influence both the nucleation and growth of the primary phase. In this work, Al-20wt.%Zn alloys without and with 0.1 wt.% Cr additions have been processed using a free-falling droplet technique. This technique allows to undercool the liquid droplet during its fall and thus to have rapid directional solidification conditions when it collides a copper-cooled substrate. Under such rapid solidification conditions, microstructural and EBSD analyses have shown that, under such rapid solidification conditions, Cr addition is responsible for the nucleation and growth of feathery grains (or twinned dendrites). This morphology specific to aluminum alloys has been discovered more than seventy years ago without a clear identification of its origin. The angular analysis between twinned dendrites indicates a behavior similar to those of the propagation of topological defects, through an ISRO-induced stacking fault mechanism.

Nonlinear Dynamics of the Formation of a Periodic Superstructure of Impurity Bands during Rapid Directional Solidification of Binary Alloys

An analytical description is proposed for the nonlinear dynamics of formation of regular impurity superstructures during a rapid solidification of binary alloys supplemented by a numerical model calculation. It is shown that there is a stable limit cycle, and spatial profiles of the impurity concentration in a solid product are calculated. The inclusion of a finiteness of the impurity atom jump velocity is found to lead to very substantial decrease in the cycle sizes and a change in the profiles of impurity superstructures.

Strong shear-flow modulation of instabilities in rapid directional solidification

We examine the effect of a strong shear flow on morphological instabilities that occur in the directional solidification of a dilute, binary alloy when the interface departs from local thermal equilibrium in a frozen-temperature, one-sided model. In particular, the flow velocity U∞ is much larger than the rate of solidification V and the Schmidt number is arbitrary. In contrast to solidification processes under small or no flow, for which both a cellular and an oscillatory mode of instability appear, the liquid-solid interface under flows of large magnitude is susceptible to a single mode of instability. All experiments on banding occur in the overlap region between these modes under no flow and since these two primary modes coalesce into one time-dependent, spatially-dependent mode, there is no mechanism for the production of bands anymore in the large-flow regime. No experiments have been conducted to date on this rapid solidification under flow and this suggests that such experiments would show no bands. The flow significantly stabilizes and selects higher wavenumbers for the preferred form of instability in comparison to systems involving small or no flow. The introduction of a strong flow provides an effective mechanism to eliminate instabilities at high solidification rates. However, stability thresholds at low solidification rates remain mainly indifferent to the presence of flow.

Strong Shear-Flow Modulation of Instabilities in Rapid Directional Solidification

We examine the effect of a strong shear flow on morphological instabilities that occur in the directional solidification of a dilute, binary alloy when the interface departs from local thermal equilibrium in a frozen-temperature, one-sided model. In particular, the flow velocity U∞ is much larger than the rate of solidification V and the Schmidt number is arbitrary. In contrast to solidification processes under small or no flow, for which both a cellular and an oscillatory mode of instability appear, the liquid-solid interface under flows of large magnitude is susceptible to a single mode of instability. All experiments on banding occur in the overlap region between these modes under no flow and since these two primary modes coalesce into one time-dependent, spatially-dependent mode, there is no mechanism for the production of bands anymore in the large-flow regime. No experiments have been conducted to date on this rapid solidification under flow and this suggests that such experiments would show no bands. The flow significantly stabilizes and selects higher wavenumbers for the preferred form of instability in comparison to systems involving small or no flow. The introduction of a strong flow provides an effective mechanism to eliminate instabilities at high solidification rates. However, stability thresholds at low solidification rates remain mainly indifferent to the presence of flow.

Instabilities in rapid directional solidification under weak flow.

We examine a rapidly solidifying binary alloy under directional solidification with nonequilibrium interfacial thermodynamics viz. the segregation coefficient and the liquidus slope are speed dependent and attachment-kinetic effects are present. Both of these effects alone give rise to (steady) cellular instabilities, mode S, and a pulsatile instability, mode P. We examine how weak imposed boundary-layer flow of magnitude |V| affects these instabilities. For small |V|, mode S becomes a traveling and the flow stabilizes (destabilizes) the interface for small (large) surface energies. For small |V|, mode P has a critical wave number that shifts from zero to nonzero giving spatial structure. The flow promotes this instability and the frequencies of the complex conjugate pairs each increase (decrease) with flow for large (small) wave numbers. These results are obtained by regular perturbation theory in powers of V far from the point where the neutral curves cross, but requires a modified expansion in powers of V^{1/3} near the crossing. A uniform composite expansion is then obtained valid for all small |V|.

Solidification path of single-crystal nickel-base superalloys with minor carbon additions under laser rapid directional solidification conditions

The solidification path of single-crystal nickel-base superalloys containing minor carbon was investigated under various laser rapid directional solidification (LRDS) conditions. By controlling the solidification rate, LRDS processing can provide the evidence whether some diffusion-controlled phase transformations occur because such transformations will be suppressed under high cooling rates. Results show that the solidification path and final solidification microstructure depend upon the cooling rate; the microstructure without γ-γ′ eutectic can be obtained as long as the cooling rate is high enough. A peritectic transformation in carbon-containing single-crystal superalloys was first experimentally verified by controlling the cooling rate during LRDS processing.

A new microsegregation model for rapid solidification multicomponent alloys and its application to single-crystal nickel-base superalloys of laser rapid directional solidification

The microsegregation in single-crystal nickel-base superalloys, which may result in many undesired precipitated phases, can be effectively reduced by rapid solidification. A new microsegregation model for rapid solidification multicomponent alloys was developed in this paper to predict the microsegregation in the single-crystal superalloys of laser rapid directional solidification and study the influence of processing parameters on the microsegregation behavior. To verify the predicted accuracy of the present model, the concentration profiles and microsegregation ranges under different processing conditions were examined and compared with those calculated by different models. Compared with previous models, this multicomponent model not only has definite physical meaning but also takes nonequilibrium solidification and dendritic tip undercooling into account. Therefore, it is suitable for a wide range of solidification conditions and can obtain better predicted results for complex multicomponent alloys. Moreover, although the microsegregation behavior depends primarily on the solidification velocity, a high temperature gradient also contributes to reducing the microsegregation. Consequently, the microsegregation can be effectively controlled by using a set of processing parameters that can obtain high temperature gradient and solidification velocity. Such a model contributes to the control of the microsegregation while determining relevant processing windows for the preparation of superalloy components using a controlled single-crystal laser processing.

Prediction of primary dendritic arm spacing during laser rapid directional solidification of single-crystal nickel-base superalloys

Primary dendritic arm spacing (PDAS) is an important microstructure feature in the nickel-base single-crystal (SX) superalloys by laser rapid directional solidification (LRDS). A combined numerical model was developed in this paper to investigate the influence of laser processing parameters on the PDAS. This model consists of (1) the theoretical PDAS models which relate PDAS to the solidification conditions and (2) the heat-flux calculations of laser processing which provide the solidification conditions as a function of the processing parameters. It is therefore able to immediately relate the PDAS to the processing parameters and obtain corresponding processing–microstructure maps. To verify the predicted accuracy, the PDAS values calculated by different theoretical models were compared with those produced under different processing conditions. Results show that the PDAS firstly decreases and then increases at a lower laser power whereas it decreases with increasing scanning velocity at a higher laser power, leading to nose-shape contour lines. The predicted accuracy depends on appropriate selections of material thermo-physical properties. These processing–microstructure maps can accurately capture the trends of the PDAS variation with the processing parameters and contribute to the control and optimization of dendritic microstructure while determining relevant processing windows for controlled SX laser processing.

Complex banded structures in directional solidification processes

A combination of theory and numerical simulation is used to investigate impurity superstructures that form in rapid directional solidification (RDS) processes in the presence of a temperature gradient and a pulling velocity with an oscillatory component. Based on a capillary wave model, we show that the RDS processes are associated with a rich morphology of banded structures, including frequency locking and the transition to chaos.

Room temperature mechanical properties and high temperature oxidation resistance of a high Cr containing Nb–Si based alloy

Abstract A high Cr containing Nb–Si based alloy (Nb–12Si–24Ti–10Cr–2Al–2Hf) was prepared by non-equilibrium rapid directional solidification, and then was heat treated at 1400 °C for 10 h. The microstructure was greatly improved by these processes. Big block-like Nb 5 Si 3 was avoided and vermicular one as a strengthening phase uniformly distributed in the toughening Nb SS matrix. Exciting fracture toughness (17.9 MPa m 1/2 ) and tensile strength (771 MPa) were achieved for this high Cr containing Nb–Si based alloy. Oxidation behaviors of this alloy at 1200 °C and 1250 °C were also studied.