Segregation In Alloys Research Articles

Investigating the Effects of Boron Nitride on Mechanical Properties of CoCrFeNiMn Composites

High‐entropy alloys (HEAs) are gaining attention for their exceptional properties but achieving precision and surface quality in powder metallurgy (PM) can be challenging. This study investigates the mechanical properties of CoCrFeNiMn HEAs produced through the flake powder metallurgy (FPM) technique and laser powder bed fusion (LPBF) preservative industrial, which established outstanding antiwear properties. To assess the enhancement of mechanical behavior and wear resistance by incorporating boron nitride (BN) into CoCrFeNiMn composites is fabricated via FPM. By incorporating BN into the composites, the mechanical behavior is significantly enhanced, leading to improved resistance against wear. Furthermore, the study delivers comprehensions into the serration behavior microstructure and shear localization in a high‐strain‐rate‐deformed CoCrFeMnNi entropy‐high alloy produces through metallurgy in powder. The serration behavior of the CoCrFeMnNi HEA is discussed concerning strain rate and temperature. Accordingly, the study employs the LPBF technique to fabricate the CoCrFeMnNi alloy from the prepared flakes. Microstructural characterization using scanning electron microscopy and X‐ray diffraction techniques to explore phase distribution, grain size, and possible segregation in the CoCrFeMnNi alloy produced through FPM‐LPBF is performed. In this particular investigation, a gradient microstructure is achieved through fusion in a laser‐powder bed of CoCrFeMnNi HEA onto highly deformed equal‐atomic HEA sheets. The resulting combination of partially recrystallized regions, exhibiting elevated yield strength, and fully recrystallized regions, demonstrates enhanced strain hardenability and elongation, and yields superior mechanical properties. The results obtained from the FPM‐LPBF CoCrFeMnNi alloy will be compared with those of conventionally processed CoCrFeMnNi to highlight the advantages and improvements achieved through the proposed method. The study findings reveal that the fundamental understanding of HEAs’ behavior under LPBF with FPM affords valuable insights into optimizing the processing parameters and achieving superior mechanical attributes.

Effect of heat treatment on the microstructure and mechanical properties of AZ91D magnesium alloy fabricated via GTA wire arc additive manufacturing

This study aims to address elemental segregation and poor mechanical properties in as-deposited AZ91D magnesium alloy fabricated via wire arc additive manufacturing through heat treatment. Three distinct heat treatments are investigated: solution treatment (T4), artificial aging treatment (T5), and a combination of solution and aging treatment (T6). Microstructural analysis reveals equiaxed grains in all samples. In the T4 state, the eutectic phase along grain boundaries dissolvs into the matrix, reducing elemental segregation but increasing grain size. In the T5 state, elemental segregation is further reduced; however, due to the relatively low aging temperature, the microstructural features of the as-deposited state are largely retained. In the T6 state, solution treatment increases grain sizes, and the uneven distribution of the reprecipitated β phase leads variations in mechanical properties between vertical and horizontal directions. Compared to the as-deposited state, the T6 treatment achieves a synergistic improvement in both strength and ductility. The ultimate tensile strength increases to 262.3 MPa, and the elongation rises to 13.8%, representing improvements of 12.5% and 32.7%, respectively, compared to the as-deposited values of 233.2 MPa and 10.4%.

Grain boundary diffusion in additively manufactured CoCrFeMnNi high-entropy alloys: Impact of non-equilibrium state, temperature and relaxation

Grain boundary diffusion of Ni in the equiatomic CoCrFeMnNi high-entropy alloy, produced by additive manufacturing, is measured using a radiotracer technique in an extended temperature interval of 350 to 703 K. A strongly non-monotonic temperature dependence of the Ni grain boundary diffusion coefficients (with a spectacular intermittent retardation of the diffusion rates with increasing temperature) is seen and explained by relaxation of a non-equilibrium state induced by rapid solidification during fabrication. The grain boundary excess energy of the non-equilibrium state of these grain boundaries, as estimated from the diffusion data, is found to be larger than 0.3 J/m2. This corresponds to an increase of about 30% of the interface energy compared to relaxed general high-angle grain boundaries. The temperature-induced evolution of the grain boundary state is analyzed in terms of the concomitant structure evolution, segregation, phase stability and precipitation in the multi-component alloy.

Deformation-induced grain boundary segregation in a powder-metallurgy ultrafine-grained MoNbTaTiV refractory high-entropy alloy

A powder-metallurgy MoNbTaTiV refractory high-entropy alloy synthesized by mechanical alloying (MA) and spark plasma sintering was subjected to hot deformations at different temperatures and strain rates. The microstructural morphologies were characterized, and component element segregation was elucidated. With grain refinement and lattice strain increase, the large inhomogeneous milled powder became refined and homogeneous after the MA. Component element segregation was observed at relatively low deformation temperatures and high strain rates. As the deformation temperature increased and the strain rate decreased, the segregation gradually disappeared, which was attributed to dislocation movement.

Study of Effects of Post-Weld Heat Treatment Time on Corrosion Behavior and Manufacturing Processes of Super Duplex Stainless SAF 2507 for Advanced Li-Ion Battery Cases.

Lithium-ion batteries are superior energy storage devices that are widely utilized in various fields, from electric cars to small portable electric devices. However, their susceptibility to thermal runaway necessitates improvements in battery case materials to improve their safety. This study used electrochemical analyses, including open-circuit potential (OCP), potentiodynamic polarization, and critical pitting temperature (CPT) analyses, to investigate the corrosion resistance of super duplex stainless steel (SAF 2507) applied to battery cases in relation to post-weld heat treatment (PWHT) time. The microstructure during the manufacture, laser welding, and PWHT was analyzed using field-emission scanning electron microscopy, X-ray diffraction, and electron backscatter diffraction, and the chemical composition was analyzed using dispersive X-ray spectroscopy and electron probe micro-analysis. The PWHT increased the volume fraction of austenite from 5% to 50% over 3 min at 1200 °C; this increased the OCP from -0.21 V to +0.03 V, and increased the CPT from 56 °C to 73 °C. The PWHT effectively improved the corrosion resistance, laying the groundwork for utilizing SAF 2507 in battery case materials. But the alloy segregation and heterogeneous grain morphology after PWHT needs improvement.

Radiation-induced Fe segregation in the dual-phase FeCrNiMnAl high-entropy alloy under high-dose helium ion irradiation

This study investigates the radiation-induced Fe segregation in the dual-phase FeCrNiMnAl high-entropy alloy (HEA) under the helium (He) ion irradiation with a dose of 5 × 1017 ions/cm2. The superior bubble swelling resistance of the chemical-ordered NiAlMn phase with B2 structure is observed compared to the FCC FeCrMn matrix, which can be attributed to their differences in atomic packing factors. Fe segregation at phase boundaries (PBs) is detected after irradiation, as evidenced by a higher concentration of Fe at PBs than in the matrix by about 10 at.%. This phenomenon can be ascribed to the migration of Fe interstitials induced by irradiation cascades and the punching mechanism during bubble growth. In essence, the Fe segregation in this study is driven by the radiation-enhanced diffusion and metastable state of FCC-structured FeCrMn with supersaturated Fe atoms. These insights provide theoretical foundations and innovative perspectives for the radiation-induced segregation (RIS) in HEAs.

"Phase segregation and surface properties in liquid Ga-Tl alloys predicted by modified self-association model "

"Phase segregation and surface properties in liquid Ga-Tl alloys predicted by modified self-association model "

Triple Junction Segregation Dominates the Stability of Nanocrystalline Alloys.

We present large-scale atomistic simulations that reveal triple junction (TJ) segregation in Pt-Au nanocrystalline alloys in agreement with experimental observations. While existing studies suggest grain boundary solute segregation as a route to thermally stabilize nanocrystalline materials with respect to grain coarsening, here we quantitatively show that it is specifically the segregation to TJs that dominates the observed stability of these alloys. Our results reveal that doping the TJs renders them immobile, thereby locking the grain boundary network and hindering its evolution. In dilute alloys, it is shown that grain boundary and TJ segregation are not as effective in mitigating grain coarsening, as the solute content is not sufficient to dope and pin all grain boundaries and TJs. Our work highlights the need to account for TJ segregation effects in order to understand and predict the evolution of nanocrystalline alloys under extreme environments.

Coupling Photogeneration with Thermodynamic Modeling of Light-Induced Alloy Segregation Enables the Identification of Stabilizing Dopants

Coupling Photogeneration with Thermodynamic Modeling of Light-Induced Alloy Segregation Enables the Identification of Stabilizing Dopants

Pb Segregation in SnPb eutectic solder alloy induced by gamma irradiation

Pb segregated and formed a Pb segregation area in SnPb eutectic solder joint after 100 h of gamma irradiation. With the irradiation time increased to 150 h, Pb segregating stopped. The Pb segregation was attributed to the oxidation of β-Sn to SnO2 induced by the gamma irradiation. The Pb element content in the segregation area predicted by the segregation heat formulas is 53.5 %, which is consistent with the experimental result of 49.11 %. Density functional theory (DFT) calculation and Molecular dynamics (MD) simulation results were used to prove that the Pb segregation is caused by the oxidation of β-Sn.

Joining of Al-Alloy Tubes by Carbon Steel Tubes using MPW Technique

Background/ Objectives: Joining aluminum alloys with low carbon steel (DC01) through fusion welding is a challenging task due to the occurrence of solidification defects like porosity, alloy segregation, and hot cracking. However, these issues can be addressed by employing solid-state welding techniques like explosive welding and MPW (magnetic pulse welding). MPW is a cost-effective and high-speed solid state welding method that involves creating joints in an overlap configuration. Methods: The MPW technique was employed to join dissimilar aluminum alloy AL 7075T6 hollow tube and DC01 steel rod in this study. A central composite design model was created to analyze the relationship between key MPW process parameters, including stand-off distance (SoD), overlap length, and discharge energy, and the bonding strength of the joints. Experimental setups were established and correlate with tensile-shear fracture load and interface hardness of the joints with the MPW process parameters. Findings: It is understood that the maximum TSFL of 2.404 kN was obtained under the welding condition of 23kJ discharge energy, 2.25 mm Stand-off distance, 8.5 mm overlap length, which is the optimum MP welding state for AA 7075 T6 with DC01 and confirmed by RSM. In addition, the cultivated connection can be aptly leveraged to anticipate the TSFL of MPW joints with a 95% confidence level. Novelty: The parametric mathematic samples were developed for forecasting TSFL of joints. The MPW parameters were optimized to enhance TSFL of joints. Aluminium alloys with low carbon steel joints were developed using MPW for cars and ships applications without fusion welding defects. Keywords: Magnetic Pulse Welding, Dissimilar Joint, (RSM) Response Surface Methodology, Tensile Shear-Fracture Load

Understanding grain boundary segregation and solute drag using computational and machine learning studies

Grain boundary (GB) solute segregation has been used as a strategy to tailor the properties and processing pathways of a wide range of metallic alloys. GB solute drag results when segregated alloying elements exert a resistive force on migrating GBs hindering their motion. While GB segregation has been the subject of active research, a detailed understanding of solute drag and the migration kinetics of doped boundaries is still lacking, especially in technologically-relevant alloys. Through theoretical analysis, mesoscale modeling, and machine learning studies, we investigate GB segregation and solute drag and establish design maps relating drag effects to relevant alloy and GB properties, i.e., the complete alloy design space. We find that solute drag is dominant in immiscible alloys with far-from-dilute compositions in agreement with experimental observations of GB segregation in metallic alloys. Our analysis reveals that solute–solute interactions within the GB and the degree of segregation asymmetry greatly influence solute drag values. In broad terms, our work provides future avenues to employ GB segregation to control boundary dynamics during materials processing or under service conditions.

Effect of solutes segregation in a binary TiAl alloy: A first-principles calculation method

Comprehending the interfacial structures and energetics of interfaces between γ-TiAl and α2-Ti3Al phase is critical to understanding the stabilities of the binary-phase TiAl-Ti3Al alloy. Here a comprehensive first-principles investigation is reported on the structures and energetic properties of γ-TiAl(111)/α2-Ti3Al(0001) interfaces with 19 segregated transition metals considered. The results indicate that all considered atoms have a tendency to segregate at the interface. The interfacial strength enhancement occurs after V, Cr, Mn, Fe, Nb, Mo, Tc, Ru segregated in TiAl, Ag segregated in Ti3Al, and almost all solute atoms can improve interface stability. Compared with the pure interface, the bonding between the segregated atoms and the surrounding atoms is stronger at the interface after V or Nb segregation, while the bonding between Cd and the surrounding atoms is weaker. Our calculations provide the necessary interfacial structures and energetics for predicting the stabilities of binary TiAl alloy.

Revealing the critical role of vanadium in radiation damage of tungsten-based alloys

Refractory tungsten-based medium- and high-entropy alloys form a promising class of strong, heat- and radiation-resistant materials for nuclear applications. Here, we systematically explore the radiation tolerance of Mo–Nb–Ta–V–W alloys using molecular dynamics simulations and a machine-learned interatomic potential. Going from pure W to W-based equiatomic binaries, ternaries, quaternary, and the quinary high-entropy alloy, we simulate the radiation damage accumulation up to 0.4–0.8 dpa through cumulative collision cascades. We find that the radiation damage and evolution are controlled by a combination of cascade-induced clustering, the balance in migration of vacancies and interstitials, and defect cluster binding energies. These mechanisms are strongly influenced by atom size, among which vanadium as the smallest atom is decisive. In pure W and alloys without V, the microstructure is characterised by large and growing interstitial dislocation loops. In stark contrast, in alloys containing V, vacancy dislocation loops are formed directly in collision cascades and the balanced migration rate of vacancies and interstitials promotes recombination and prevents growth of interstitial loops. The atom size differences also lead to clear segregation, with small atoms (V) in interstitial clusters and larger atoms (Ta, Nb) segregating around vacancy clusters. Furthermore, the small cluster sizes, formation of vacancy loops, segregation, and balanced migration in V-containing alloys lead to low swelling and an exceptionally efficient defect annealing compared to pure W and V-free alloys. Our results highlight WTaV as a promising low-activation material, where almost 90% of defects are annealed at 2000 K during 5 ns compared to <20% in pure W.

Atomistic Simulation Study of Grain Boundary Segregation and Grain Boundary Migration in Ni-Cr Alloys

Using Molecular Dynamics (MD) and Monte Carlo (MC) simulations, we studied the grain boundary (GB) segregation under different temperatures and Cr concentrations in Ni-Cr alloys with two distinct grain-boundary structures, i.e., Σ5(310)[010] and Σ101(200)[100]. Temperature plays a minor influence on Cr segregation for Σ5(310)[010] GB, but Cr segregation rapidly diminishes with elevating temperatures for Σ101(200)[100] GB. We also used the synthetic driving force and corresponding identification methods to investigate the effect of Cr solute segregation on grain boundary stability. All Σ5(310)[010] models have multi-stage grain boundary migration at 800 K. In the first stage, the grain boundary’s slow acceleration time is related to solute concentration. The migration temperature can influence this phenomenon. As temperatures rise, the duration of this slow acceleration phase diminishes. No similar phenomenon was observed in the process of the grain boundary movement of Σ101(200)[100]. The influence of solute concentration on grain boundary migration is complicated. The segregation concentration at the grain boundary cannot be regarded as the only factor affecting the migration of the grain boundary because the Cr atom on the grain boundary does not move with the grain boundary. This work will also discuss the grain boundary migration‘s relationship with lattice distortion and grain boundary atom diffusion. The results and findings of this study provide further insights into the segregation-increase GB stabilization of NC Ni-Cr alloys.

Enhanced grain boundary cohesion mediated by solute segregation in a dilute Mg alloy with improved crack tolerance and strength

Effects of solute segregation at grain boundaries (GBs) on the deformation mechanism and fracture behavior remain obscure for magnesium (Mg) alloys. Here, by introducing Zn segregation at GBs, we obtained an Mg-0.5Al-0.4Mn-0.2Ce-0.4Zn (wt.%) alloy achieving fracture elongation (FEL) of ∼33.6 %, with a remarkable FEL improvement by 100 % in comparison to Zn-free counterpart. Meanwhile, the tensile yield strength (TYS, 195.5 MPa) is increased by ∼27.5 MPa after trace Zn addition. Although trace Zn addition improves the fraction of GBs with high misorientation, it reconciles crack tolerance with enhanced strength. The introduction of Zn not only promotes pyramidal 〈c + a〉 slips and inhibits twinning nucleation, but also enhances the GB cohesion by Zn segregation. Based on in-situ microstructure observation, we found that the enhanced GB cohesion enables the segregation-inspired hierarchical crack buffering, as well as deflecting and branching. Enhanced GBs can also facilitate the continuous emission of 〈c + a〉 dislocations in neighboring grains irrespective of the onset of microcracks, forming a plastic zone to retard local strain concentration, thus avoiding microcrack percolation and attaining a crack-mediated elongation reserve of above 15 %. Besides, the higher TYS in the Zn-containing alloy mainly stems from the enhanced solid-solution strengthening of Zn solutes, thus achieving strength and crack tolerance synergy.

Methane pyrolysis on NiMo/MgO catalysts: The significance of equimolar NiMo alloy resisting nanosize segregation during the reaction

As a perspective catalyst composition for methane non-oxidative decomposition/methane pyrolysis (CH4⇌C+2 H2) yielding clean hydrogen and only solid carbon, the combination of nickel and molybdenum on MgO support was investigated. With deliberately low Ni content and strong metal-support interaction, 7%Ni4%Mo/MgO and 7%Ni12%Mo/MgO catalysts and the monometallic references were prepared. Structural analysis was performed using TPR, XRD, TEM, XPS and Raman spectroscopy in reduced state and after methane decomposition test. Catalytic performance was investigated i) in a highly diluted CH4 flow in a fixed bed reactor under temperature ramp and ii) in 50% CH4/Ar using a horizontal reactor at 800 °C. Synergetic interaction of Mo and Ni was observed under both conditions. The results revealed that the deactivation was coupled with alloy segregation for the low Mo loading, while the more stable, non-segregating Mo/Ni∼1 composition of the individual metal particles in 7%Ni12%Mo/MgO sample resulted in good activity and high carbon nanotube yield.

Surface segregation in high-entropy alloys from alchemical machine learning

High-entropy alloys (HEAs), containing several metallic elements in near-equimolar proportions, have long been of interest for their unique mechanical properties. More recently, they have emerged as a promising platform for the development of novel heterogeneous catalysts, because of the large design space, and the synergistic effects between their components. In this work we use a machine-learning potential that can model simultaneously up to 25 transition metals to study the tendency of different elements to segregate at the surface of a HEA. We use as a starting point a potential that was previously developed using exclusively crystalline bulk phases, and show that, thanks to the physically-inspired functional form of the model, adding a much smaller number of defective configurations makes it capable of describing surface phenomena. We then present several computational studies of surface segregation, including both a simulation of a 25-element alloy, that provides a rough estimate of the relative surface propensity of the various elements, and targeted studies of CoCrFeMnNi and IrFeCoNiCu, which provide further validation of the model, and insights to guide the modeling and design of alloys for heterogeneous catalysis.

CALPHAD-linked analysis of grain boundary segregation and phase-field simulation of solute-drag effect in multicomponent magnesium alloys

The application of Mg alloy sheets is currently limited owing to their poor room-temperature formability. The solute-drag effect is a potential mechanism via which the texture intensity of Mg alloys can be lowered to improve their formability. However, detailed analyses of the effects of solute–solute interactions on the solute drag in multicomponent systems are lacking. Herein, a CALPHAD-linked phase-field model was developed to evaluate the grain boundary segregation and solute-drag effects in a multicomponent system. The model was used to calculate the grain boundary segregation and solute drag in several Mg alloys, and the effect of solute–solute interactions on these phenomena was evaluated. The results showed that Zn and Ca mutually enhanced the grain boundary segregation in Mg alloys, leading to greater solute drag. However, the combination of Al and Zn mutually suppressed the grain boundary segregation and consequently, reduced the solute drag. These results support a previously proposed theory that Mg-Zn-Ca alloys exhibit strong solute drag owing to the co-addition of Zn and Ca, which reduces the texture intensity and improves the room-temperature formability. Furthermore, a new index for describing co-segregation and competitive segregation at grain boundaries, based on Hillert's grain boundary phase model, was introduced. The value of the index was calculated for various combinations of alloying elements in Mg alloys. The results showed that many ternary Mg alloys with a low texture intensity tended to co-segregate alloying elements.

Multi-scale and multi-physics simulation of central segregation in an equiaxed dendritic mushy zone during continuous casting of steel

Centreline segregation during continuous casting of steel is highly detrimental to the end-product mechanical properties. In this study, a 3D multi-scale and multi-physics model is developed and validated that directly predicts alloy segregation occurring in peritectic steel grades having centreline equiaxed grain morphology. The model consists of four models: (1) a 1D macroscale heat transfer and solidification model, (2) a 3D mesoscale dendritic solidification model, (3) a 3D mesoscale dendritic fluid flow model, and (4) a 3D mesoscale solute transport and redistribution model. The use of a mesoscale domain where the solid and liquid phases are separately discretized allows for consideration of physical phenomena affecting segregation that are classically hidden by the averaging procedures of fully continuum approaches. Thus the new model is able to account for the multiple phenomena occurring during continuous casting while also directly considering the stochastic effects resulting from grains having randomly-placed nuclei as well as interactions between the liquid and solid phases. The results demonstrate that solute partitioning combined with intra-dendritic fluid flow leads eventually to liquid channels enriched with solute that result in centreline segregation. The predicted composition in these discrete liquid channels shows excellent agreement with a Mn centreline segregation profile experimentally-measured via microscopic X-ray fluorescence of a commercially-cast steel slab. Finally, the effects of alloy composition, and soft reduction on the degree of centreline segregation are examined.