Crystallization Of Alloys Research Articles

Electrochemical Oxidative Desorption of Adsorbed Sulfur Species on (111) Surfaces of Single Crystals of Pure Pt and Pt-Based Bimetallic Alloys

Electrochemical Oxidative Desorption of Adsorbed Sulfur Species on (111) Surfaces of Single Crystals of Pure Pt and Pt-Based Bimetallic Alloys

Effects of Machining Parameters on Abrasive Flow Machining of Single Crystal γ-TiAl Alloy Based on Molecular Dynamics

Observing the intricate microstructure changes in abrasive flow machining with traditional experimental methods is difficult. Molecular dynamics simulations are used to look at the process of abrasive flow processing from a microscopic scale in this work. A molecular dynamics model for micro-cutting a single crystal γ-TiAl alloy with a rough surface in a fluid medium environment is constructed, which is more realistic. The evolution of material removal, cutting force, temperature, energy, and dislocation during micro-cutting are analyzed. The impact of cutting depth, abrasive particle sizes, and abrasive material on the micro-cutting process are analyzed. The analysis shows that the smaller cutting depth and abrasive particle sizes are beneficial to obtain a better machining surface, and the cubic boron nitride (CBN) abrasive is an effective substitute material for diamonds. The purpose of this study is to provide unique insights for improving the material removal rate and subsurface quality by adjusting machining parameters in actual abrasive flow precision machining.

Influence of substrate temperature on crystallization of Fe-based amorphous alloy prepared by selective laser melting

Selective laser melting (SLM) has potential to prepare complex shaped amorphous alloy parts , however, the almost inevitable crystallization makes it very difficult to obtain excellent performance parts. Most of studies focus on improving properties by optimizing parameters such as laser power, scanning speed, and scanning strategy . As is well known, the substrate is an important component in SLM devices, which directly supports and contacts the initial powder and melting pool, affecting the absorption and transfer of heat, the formation and cooling of the melting pool, and therefore exerts a significant influence on the quality and microstructure of printed parts. However, there is relatively little research on its influence. It is important and necessary to understand the influence of substrate temperature on crystallization behavior of Fe-based amorphous alloy during SLM process. Molecular dynamics (MD) simulations can provide direct evidence for the evolution of clusters and band pairs, which can help clarify the crystallization mechanism and alleviate the crystallization. In this work, the influence of substrate temperature on the crystallization and evolution of atomic clusters in Fe<sub>50</sub>Cu<sub>25</sub>Ni<sub>25</sub> amorphous alloy during SLM is investigated on an atomic scale, using MD simulation under different substrate temperatures (300–900 K), laser power values (500–800 eV/ps), and scanning speeds (0.1–1.0 nm/ps). The research results show that when the substrate temperature is lower than 750 K, the content of characteristic bond pair 1421 and the corresponding <inline-formula><tex-math id="M2">\begin{document}$ \left\langle{0,{\mathrm{ }}4,{\mathrm{ }}4,{\mathrm{ }}6}\right\rangle $\end{document}</tex-math></inline-formula> cluster increase with the substrate temperature rising, thereby increasing face-centered cubic bond pair and cluster and promoting the crystallization. When the substrate temperature rises to a value close to the glass transition temperature, the evolution of bond pairs and clusters becomes complex, which is influenced by the collaborative and competitive effects, such as the ability to form glass, melting and cooling rate. This work reveals the evolution of atomic clusters and band pairs in the SLM process of Fe-based amorphous alloys, and the initiation of crystal phases at different substrate temperatures, providing new ideas for understanding and regulating crystallization.

Quantitative study of thickness debit effect on the microstructure and elemental segregation characteristics in as-cast and heat-treated nickel-based single crystal superalloys

Reducing blade wall thickness has become essential with advancements in blade cooling technologies such as air film cooling, impact cooling, and transient cooling. Heat dissipation is improved by this reduction, which significantly extends the lifespan of the blades. The impact of thickness variations on the microstructure and elemental segregation of as-cast and heat-treated nickel-based single crystal superalloys is investigated in this study. Thin-walled specimens of four different thicknesses (1.513mm, 2.370mm, 3.060mm, and 5.790mm) were directly cast using a shell mold equipped with varying cavity thicknesses. The microstructure and elemental segregation of these samples were quantitatively analyzed using optical microscope (OM), scanning electron microscope (SEM), and energy-dispersive spectroscopy (EDS). This research not only involves thin-walled specimens of various thicknesses but also includes specimens of the same thickness positioned at different distances from the casting core. Experimental data demonstrate that the size of the γ′ phase follows the log-normal distribution, and its circularity conforms to the four-parameter Dagum distribution. In the as-cast specimens, increases of the thickness from 1.513mm to 5.790mm led to 33.7% increase in the average primary dendrite arm spacing, 86% increase in the average percentage of the eutectic structure, 44.2% increase in the average γ′ phase size, and 8% decrease in the average circularity of the γ′ phase. The degree of elemental segregation also increases, particularly for elements such as W and Re that exhibit significant increases in segregation as the sample thickness increases. The differences in casting thickness lead to initial state differences. Approximately 95% of the eutectic structures are eliminated during heat treatment, but the efficacy of the process is constrained by differences in the initial state of the alloy. The greater the initial state heterogeneity, the more stringent the required heat treatment time and conditions, and the more limited the effectiveness of the heat treatment. The final microstructure and elemental distribution are the results of the coupled effects of the initial state and heat treatment. In the heat-treated samples, as the thickness increases from 1.513mm to 5.790mm, the average primary dendrite arm spacing increases by 33.5%, the average percentage of eutectic structures increases by 275%, the average size of the strengthening phase (γ′ phase) increases by 66.7%, and the average circularity of the γ′ phase decreases by 33%. The variations in microstructural parameters for specimens at different distances from the casting core also show regular patterns. Key findings indicate that decrease in casting thickness significantly affects γ' phase characteristics, elemental segregation, and microstructural uniformity. These findings can provide valuable guidance for optimizing the design and manufacturing processes of the nickel- based single crystal alloys.

Research on the Structural and Magnetic Phase Transitions of CeMn2Ge2 Alloy.

Magnetic phase transitions play crucial roles in various material applications, including sensors, actuators, information storage, magnetic refrigeration, and so on. Typically, these magnetic phase transitions exhibit discontinuous first-order phase transitions. When a material undergoes a magnetic phase transition, it often exhibits simultaneous changes in both its crystal and electronic structures. However, the coupling relationship between the crystal structure and electronic structure during these phase transitions has not been well studied. This lack of understanding hinders our ability to integrate macroscopic physical phenomena with microscopic crystal and electronic structures. In this paper, we prepared single crystal and polycrystalline CeMn2Ge2 alloy, which has been extensively studied in recent years as a material of skyrmions. The relationships between the magnetic phase transition and the crystal structure of CeMn2Ge2 were investigated through magnetic measurements, variable-temperature X-ray diffraction (XRD), and experimental electron density analysis via the maximum entropy method (MEM). The results indicate that the antiferromagnetic phase transition at TN = 415 K is characterized by an increase in the intralayer Mn-Mn bond and a decrease in the Ge-Ge bond. More importantly, the ferromagnetic transition at TC = 315 K can be divided into two stages: the first stage involves the anisotropic transformation of Mn, and the second stage involves the electron enhancement of Mn. The combination of phase transition features and transport properties indicates strong anisotropy in CeMn2Ge2. Notably, our work reveals a coupling between a material's physical properties, crystal structure, and electronic structure. Our study offers a new approach for determining the origin of magnetic phase transitions and the causes of their physical properties in materials at the electronic level.

Mechanical behavior of supersonic fine particle bombardment single crystal γ-TiAl alloys based on atomistic simulation: effects of velocity and crystal plane

Abstract The crystal orientation significantly affects the plastic deformation and mechanical properties of γ-TiAl alloys. However, there are few studies on the mechanical behavior of supersonic fine particle bombardment (SFPB) of single-crystal γ-TiAl alloys impacted with different crystal planes and velocities at the nanoscale. And the characterization of the mechanical properties in the presence of dislocation defects after impact is lacked. Therefore, in this paper, molecular dynamics is used to simulate the impact response and post-impact mechanical response of SFPB single-crystal γ-TiAl alloy. The results show that: at a certain impact velocity, the different ways of inducing dislocation slip due to the number of slip system initiations under crystal anisotropy lead to a smaller depth of subsurface damage on the (001) crystallographic plane compared to the other crystallographic planes; moreover, the yield strength and Young’s modulus of the (111) and (1 1 ¯ 0) crystallographic planes are the largest after the impact, respectively. Under the same crystal plane, the subsurface damage depths of all crystal planes decrease with the increase of impact velocity; the yield strength of the (111) crystal plane of γ-TiAl alloy gradually increases, the yield strength of the (001) and (1 1 ¯ 0) crystal planes decreases, and the modulus of elasticity of the (001) and (111) crystal planes after impact is less sensitive to velocity. The angle between the two slip surfaces formed by the L-C dislocation during impact is 70.24°, while the angle between the two slip surfaces formed by the Hirth dislocation is 109.76°. This will provide atomic-scale deformation details of the plastic deformation and mechanical response of single-crystal γ-TiAl alloys bombarded by supersonic fine particles.

Experimental study on deformation mechanism around indentation of GH4169 alloy

The GH4169 alloy is widely used in engineering structures due to its excellent high-temperature strength, corrosion resistance, and mechanical properties. However, local stresses during plastic deformation significantly impact its macroscopic mechanical performance. This study investigates the residual deformation fields and plastic deformation mechanisms around local indentations using digital image correlation (DIC), electron backscatter diffraction (EBSD), and nanoindentation. The results show that the indentation rate affects the accumulation of geometrically necessary dislocations (GNDs). Increasing the loading rate from 2 mN/s to 20 mN/s leads to a 32 % increase in GNDs. When the loading strain rate increases from 0.01 s−1 to 0.2 s−1, GNDs increase by 43 %. Moreover, indentations initiate more than two slip systems in the alloy crystals, with slip trace lines consistent with theoretical predictions. The plastic deformation field distribution around the indentations, obtained using DIC, reveals the influence range of slip generation due to indentations to be around 10 μm.

Bulk-to-surface segregation measurements of Cu and S in a Ni-Cu(S) ternary system with Auger electron spectroscopy

The bulk-to-surface segregation measurement of Cu and S in Ni0.813Cu0.187 alloy crystals was investigated by using Auger electron spectroscopy (AES) to monitor the surface concentration of Cu and S during the constant temperature segregation measurement. The constant temperatures were carried out in the temperature range 770 K to 860 K at a 30 K interval. The results show that the initially both Cu and S segregated to the surface and the first segregation rate of Cu is higher than that of S. Once the Cu reached the maximum surface coverage, it started to desegregate until Cu was replaced by S. The segregation measured profile data with a constant temperature heating method were fitted by the modified Fick’s model extracted the bulk diffusion coefficient as DCu in Ni = 5.8 × 10−14exp(−160.1 kJ/RT) m2/s, DS in Ni = 1.2 × 10−2exp(–222.1 kJ/RT) m2/s, The segregation parameters obtained in this work were compared well with experimental data values obtained from published elsewhere.

A first-principle study on the band structure of GePb alloys

Abstract Single crystal GePb alloys have been considered as potential direct bandgap materials for optoelectronics application. In this work, density-functional theory calculations were performed to investigate the crystalline and electronic structures of the GePb alloys. The lattice constants of the unstrained GePb alloys are found positively deviating from Vegard’s law with a bowing coefficient of 0.587 Å. GePb has a higher Poisson’s ratios than GeSn with a similar alloying concentration. With the increasing Pb concentration x in Ge1−x Pb x , a new alloying energy level brought by Pb appears at the bottom of the conduction band and continuously decreases. The new energy level is constructed to a new valley as compared to the initial Γ valley and the new energy level is acquiring its higher spectra weights with increasing Pb concentration. An indirect-to-direct bandgap transition occurs with a Pb concentration of 3.3%. The effective masses of holes and electrons in the GePb Γ valley are calculated to decrease with the increasing Pb concentration, while the effective masses of the electrons in the L valley only change slightly. The small effective masses of the electrons in the Γ valley are favorable for high-speed GePb device application.

Damage Characteristics and Mechanical Properties of DD6 Single‐Crystal Alloy Based on Crystal Plasticity Theory under Uniaxial Tensile Conditions

Herein, the mechanical behavior and damage characteristics of DD6 nickel‐based single crystal alloy under different crystal orientation conditions are studied based on the theory of crystal plasticity coupled with damage constitutive equations using the crystal plasticity finite‐element method. The results indicate that the crystal plastic finite‐element model coupled with damage constitutive equations can well describe the entire deformation process from elasticity to damage. Under the reference orientation condition, the damage starts from the center position inside the test specimen and extends to the surrounding areas. The stress value in the damaged area decreases gradually from the periphery to the center of the test specimen, and an external stress reduction zone is formed. The crystal exhibits the best elasticity properties in the [0, 1, 1] orientation and the best plasticity properties in the [0, tan22.5°, 1] orientation. As the rotation angle of crystal orientation increases, the damage zones of X‐Z, Z‐Y, and X‐Y sections all exhibit clockwise rotation characteristics.

Bayesian optimization of 7-component (AlVCrFeCoNiMo) single crystal alloy’s compositional space to optimize elasto-plastic properties from molecular dynamics simulations

Abstract Exploring the vast compositional space of high-entropy alloys (HEAs) promises materials with superior mechanical properties much needed in industrial applications. We demonstrate on the 7-component alloy AlVCrFeCoNiMo system with randomly ordered atoms that this exploration of the compositional space can be accelerated by combining molecular dynamics simulations with Bayesian optimization. Our algorithm is tested on maximizing the shear modulus, resulting in pure Mo, an unsurprising result based on Mo’s large density. Maximizing the yield stress results in Co-, Cr- and Ni-based alloys with the optimal composition varying depending on the presence of defects within the crystal. Finally, we optimize the plastic behaviour by aiming for high stresses while minimizing the deformation fluctuations, and find that a predominantly NiMo alloy’s high lattice distortions ensure a smooth stress response. The results suggest that mechanical properties of 2- to 4-component alloys with optimized composition may be superior to those of equiatomic HEAs without short-range order.

Establishment of Multi-Physics coupling model and analysis on thermal stress and crack risk in directional growth of TiAl alloys under electromagnetic confinement

The electromagnetic confinement directional solidification (EMCDS) technique is an optimal method for preparing large-size and non-contamination directional TiAl alloy crystals. Despite its advantages, the high temperature gradient inherent to this process induces thermal stress within the ingot, which increases the risk of cracking. To address this challenge, an innovative Integrated Multi-Physics Coupling Model was established to map and study the thermal stress field during EMCDS in this study. It synchronized the computation of the electromagnetic field, temperature field, solute field, flow field, and stress field during the crystal growth of TiAl alloy, and its high accuracy was proved by the micro-indentation experiment. Our analysis reveals that transverse temperature differences are crucial in inducing thermal stresses, and identifies that hot cracks and cold cracks are prone to occur respectively at the area of radial 23R along the sample (X = 23R) and on the sample surface, which aligns with experimental observations impressively. This model can more accurately and efficiently optimize the process parameters of the EMCDS process to avoid cracks and promote its industrial application.

Effect of rare earth ions on the optical and electronic properties of defect chalcopyrite: Experimental and theoretical investigation

The present research is a systematic experimental and computational study focused on structural, electronic, and optical properties of ZnGa2S4 doped with neodymium rare earth ions for the first time. High-intensity, narrow-band luminescence peaks are observed in the visible and infrared regions by doping the matrix with Nd ions. These peaks in the background of the broadband spectrum are due to intercenter 4f-4f transitions of Nd3+ ions. The fact that the Raman peaks of the alloy crystal are more intense than that of the pure crystal confirms that the neodymium does not move freely in the lattice and occupies the place of defects. This confirms the results from our X-ray structural analysis. The crystal lattice parameters of the studied materials were determined as follows: a = 5.496 Å, c = 10.99 Å, c/a = 2. To explain the electronic, optical and magnetic properties, using ATK-DFT method, electronic energy band structure, density of states and optical spectrum for pure and doped with ZnGa2S4:Nd compound are computed and discussed. DFT result shows that impurity Nd leads to a decreased bandgap of ZnGa2S4 due to the hybridization of Nd-4d with S-3p orbital in the forbidden gap. The peaks in the optical spectrum are shifted toward the lower energy range for doped supercell.

Study on micro-deformation in micro-grinding of selective laser melting FeCoCrNi high-entropy alloy based on molecular dynamics

Microscopic deformation behavior is an important part of the study of micro-grinding processing. This paper investigates the removal mechanism in the micro-grinding of selective laser melting FeCoCrNiAl0.5 high-entropy alloy. The micro-grinding process is analyzed through simulation using the molecular dynamics simulation method. The study examines how the thickness of the undeformed chip and the grain size affect the microdeformation behaviors of shear strain, micro-grinding force, the evolution of dislocations, and microstructural transformation. The experimental results confirm that the molecular dynamics simulation model accurately predicts the micro-deformation behavior of FeCoCrNiAl0.5 high-entropy alloy during micro-grinding. The micro-deformation mechanism involves dislocation slip and twinning deformation, while the removal mechanism mainly includes the elimination of axial and dendritic crystals. During micro-grinding, the grinding force experiences small to large fluctuations, and the total length of dislocations gradually increases. As the undeformed cutting thickness increases, the micro-grinding force also gradually increases. Simultaneously, the type of dislocations and laminar dislocation nuclei increase, leading to growth in the total length of dislocations. The number of high shear strain atoms, the micro-grinding force, and the degree of fluctuation in the single crystal high-entropy alloy are higher than those in the polycrystalline material, but the total dislocation length is significantly smaller.

Effect of carbon on the shape memory effect of [formula omitted]−Oriented Cr20Fe20Mn20Co35Ni4.9C0.1 high-entropy alloy single crystals under tension

For the first time, the effect of low carbon concentration CC = 0.1at.% on the development of the FCC–HCP martensitic transformation, the value of the shape memory effect (SME), and the degree of shape recovery in “load-unloading” cycles of the [1¯44]−oriented Cr20Fe20Mn20Co35Ni4.9C0.1 (at.%) high-entropy alloy crystals under tension was studied. It was shown that, at CC = 0.1at.%, the maximum SME was achieved due to cycling: 9.4 % with increasing stresses from 50 to 150 MPa in the “cooling-heating” cycle and 12.4 % with increasing transformation strain in the “load-unloading” cycle and subsequent heating. The degree of shape recovery with increase in the number of “load-unloading” cycles (n = 1–15) was 98–100 % with a transformation strain of 10 % in cycle.

Research on tribological properties of new Ni-based single crystal alloy containing Re

Abstract The particles in high-temperature and high-speed airflow in the battlefield environment will form sliding friction and wear on the aeroengine turbine blades, thus reducing the service performance of the blades. However, few studies has been reported on the tribological properties of Ni-based single crystal alloy. Accordingly, the tribological properties of Ni-based single crystal alloys with different contents of Re (0 wt%, 1.5 wt%, 2.5 wt%, 3.5 wt%, 4.5 wt% and 5.5 wt%) are investigated by tribological experiments and molecular dynamics simulations in this paper. The results of tribological experiments show that Ni-based single crystal alloy without Re exhibits the characteristics of abrasive wear and adhesive wear, while the wear state is significantly improved after adding Re element. In particular, the worn surface of Ni-based single crystal alloy containing 5.5% Re (NSCA5) is the smoothest and only a few minor defects are observed. In addition, the micro-tribological characteristics of Ni-based single crystal alloy are analyzed by molecular dynamics simulations, the results show that Re atoms can inhibit the dislocation movement and reduce the system potential energy, which enhance the stability and hardness of Ni-based single crystal alloy, thereby the wear resistance of the material are improved.

Influence of enhanced Laves phase shape and distribution on atomic-scale frictional wear mechanisms in nickel-based single crystal alloys

This study aims to simulate the influence of different shapes and distribution states of Laves phases on the friction-wear behavior of nickel-based alloys using molecular dynamics (MD). The investigation systematically examined the mechanical properties, friction coefficient, number of worn atoms, dislocations, temperature, and other micro-deformation behaviors of materials incorporating horizontally and vertically distributed short rod-shaped, spherical, and short strip-shaped Laves phases. The presence of the Laves phase significantly impedes temperature transfer, defect motion, and atomic displacement in the workpiece, resulting in reduced dislocation glide rate and shorter average dislocation lengths. High dislocation densities accumulate at the Laves/γ phase interface, enhancing surface wear resistance. The short rod-shaped Laves phase, due to its large surface area at the Laves/γ interface, impedes defect motion more effectively than spherical and short strip-shaped phases. dislocation tangle, higher friction force, fewer worn atoms, a higher friction coefficient, and improved wear resistance. However, vertically distributed short strip-shaped and short rod-shaped Laves phases exhibit less effective defect interaction, resulting in increased wear and significant deformation. The spherical Laves phase, with its geometric symmetry, shows consistent wear resistance regardless of distribution state. Short rod-shaped Laves phase provides the best reinforcement due to its effective defect motion impedance, while the spherical Laves phase offers stable performance across different distribution states, making it the most suitable shape for Laves phase reinforcement.

Morphology of dissipative structures formed during the high-temperature synthesis of the MgB2 compound

The interaction of components during high-temperature annealing of compressed magnesium and boron powders in the technologies for obtaining the superconducting compound MgB2 in works using standard synthesis technology and hot gas-static pressing technology is considered. The effect of the kinetics of crystal growth and diffusion of components released at the interface on the morphology of dissipative structures formed during phase transformation is studied in a computer model of crystallization of a binary alloy. The conditions and mechanism of formation of “dendrite-like” and “layered” structures arising during crystallization of MgB2 from magnesium melt (Mg –%B) are analyzed. It is shown that the application of pressure during the synthesis of the compound makes it possible to control the morphology of the resulting dissipative structures that determine the degree of development of porosity and liquation heterogeneity of the composition in a massive superconductor.

Structural heredity in the casting of eutectic cast iron

It has been shown that the effect of structural inheritance in casting eutectic cast irons can be explained from the standpoint of nanostructured crystallization of foundry alloys. Proposed is a mechanism of structural heredity when casting eutectic cast irons. This mechanism is determined by the stability of the crystallization centers of microcrystals of austenite, graphite, cementite. This stability has been shown to depend on the concentration of adsorbed oxygen atoms. The higher this concentration, the less stable the centers of crystallization of microcrystals of austenite, graphite, cementite in melts of eutectic cast irons, and vice versa. With an increase in overheating and (or) the holding time of melts, the concentration of adsorbed oxygen atoms in them increases. As a result, structural stability during remelting of eutectic cast irons is reduced and the effect of structural heredity is disturbed.

Data-driven modeling of dislocation mobility from atomistics using physics-informed machine learning

Dislocation mobility, which dictates the response of dislocations to an applied stress, is a fundamental property of crystalline materials that governs the evolution of plastic deformation. Traditional approaches for deriving mobility laws rely on phenomenological models of the underlying physics, whose free parameters are in turn fitted to a small number of intuition-driven atomic scale simulations under varying conditions of temperature and stress. This tedious and time-consuming approach becomes particularly cumbersome for materials with complex dependencies on stress, temperature, and local environment, such as body-centered cubic crystals (BCC) metals and alloys. In this paper, we present a novel, uncertainty quantification-driven active learning paradigm for learning dislocation mobility laws from automated high-throughput large-scale molecular dynamics simulations, using Graph Neural Networks (GNN) with a physics-informed architecture. We demonstrate that this Physics-informed Graph Neural Network (PI-GNN) framework captures the underlying physics more accurately compared to existing phenomenological mobility laws in BCC metals.