Segregation Of Elements Research Articles

Enhancement of Mechanical Properties of NiCo Alloy Induced by Inhomogeneous Gradient of Solute Element Segregation

Inhomogeneous gradient nanocrystals have better mechanical properties than uniform gradient nanocrystals. The segregation of solute elements in inhomogeneous gradient nanocrystals is expected to induce the re‐enhancement of mechanical properties of alloys. Herein, solute element segregation structures of inhomogeneous gradient structure nanocrystalline NiCo alloy are established, simulating tensile deformation via molecular dynamics. The results show that the segregation of solute elements in the inhomogeneous gradient structure of nanocrystals can improve the mechanical properties of the alloy, especially the structure of the intragranular segregation. The intragranular segregation of solute elements induces the decrease of grain boundary energy and greatly enhances the stability of grain boundaries. In addition, the segregation of solute elements within grains can hinder the dislocation movement to a certain extent, and the hindering effect on dislocation movement of stable grain boundaries induced by intragranular segregation of solute elements further enhances the mechanical properties of nanocrystalline alloys. This strategy of combining heterogeneous gradient structure and solute element segregation structure provides a positive and interesting perspective for the design of advanced alloys with excellent properties.

Crack suppression in directed energy deposition of molybdenum

Additive manufacturing (AM) is a valuable tool for the fabrication and repair of refractory parts such as molybdenum alloys. Cracking is a common defect encountered during AM processing of refractory parts, that is generally associated with the segregation of light elements to grain boundaries which affect grain boundary cohesion and, ultimately, affect the final performance of the part. Similarly, because of the high melting points of refractory metals, lack-of-fusion defects are also common. The effect of build substrate and small alloying additions on suppression of defects during multilayer builds was investigated using directed energy deposition (DED) printing. Identical sample matrices were printed on three different build substrates: molybdenum (Mo), commercially pure titanium (Cp-Ti), and 316 stainless steel (316). Twenty-six usable samples were produced. Samples were cross sectioned, polished, and were characterized for total cross-section defect area. Additionally, samples from each substrate material were analyzed for grain boundary oxygen content. The strongest defect suppression, producing crack free material, was observed in samples printed on a Cp-Ti build substrate with a ten atomic percent addition of titanium in the molybdenum powder feed. The part quality was enhanced due to three factors: 1) the moderation of thermal diffusivity through a change in build plate material, 2) the suppression of light element segregation via increased solubility through titanium addition, and 3) a lack of brittle phase formation due to metallurgical compatibility of the build material with the build substrate. Analysis of defect area versus dimensionless number, π1, shows that increasing π1 reduced defects throughout the part.

From Powders to Alloys: A Study of the Mechanical Alloying Process and Sintering

This study explores the intricacies of mechanical alloying, aiming to unlock its potential in modern engineering. It investigates the impact of milling duration, ball-to-powder ratio, and sintering temperature on the microstructure and mechanical properties of Al-7Zn-2.5Mg-2.5Cu alloy and Al7075 alloy. Mechanical alloying can produce alloys with exceptional hardness, strength, and ductility, while grain size can be controlled by adjusting milling parameters. Experimental techniques like X-Ray diffraction and transmission electron microscopy reveal microstructural changes during mechanical alloying, aiding in understanding metastable phases and element segregation, which influence the alloy’s properties. Sintering, the subsequent consolidation step, determines final properties, with a trade-off between grain size and mechanical qualities observed at different sintering temperatures. This trade-off presents an intriguing avenue for developing materials with optimal properties. The study also explores potential applications of mechanical alloying across industries, including aerospace, biomedical, and energy. These unique mechanical alloys are attractive for various uses, from structural to catalytic and magnetic materials. They have the potential to revolutionize industries and drive technological advancements.

Superior strength-ductility synergy in additively manufactured CoCrFeNi high-entropy alloys with multi-scale hierarchical microstructure

Additive manufacturing offers a feasible route for fabricating net-shape metallic materials with advanced physicochemical and mechanical properties, which is unattainable through conventional routes. This study focuses on the processing parameters, microstructural, and mechanical properties of the CoCrFeNi high-entropy alloys (HEAs) fabricated via laser powder bed fusion (LPBF). A near-dense LPBF CoCrFeNi HEA was manufactured by fine-tuning the parameters. Compared with conventionally processed CoCrFeNi HEAs, the as-built samples exhibit excellent strength-ductility synergy with 200 % higher yield strength and unsacrificed ductility. Microstructural analyses of the LPBF samples demonstrate a hierarchical structure including melt pools, columnar grains, dislocation cells, and elemental segregations. This excellent strength is attributed to the high-density dislocation cellular structure and Cr2O3 nanoparticles that impede the dislocation motion. The excellent ductility correlates to the steady work-hardening regulated by the interactions between cellular structure, dislocations, deformation twins, and nanoprecipitation. This work provides promising outlook towards fabricating high-performance alloys with tailored hierarchical microstructure.

New insights of the microcrack initiation induced by Mo addition in Ni-based single crystal superalloys during creep at 900 °C

Mo has received much attention in Ni-based single crystal superalloys as a main strengthening element at elevated temperatures above 1000 °C. However, the effects of Mo addition in new generation single crystal superalloys at medium temperature conditions are still not clear. This paper investigated the creep property at 900 °C and 392 MPa in a 4th generation Ni-based single crystal superalloy with different Mo contents, indicating that Mo addition decreased the creep life and induced large amounts of microcracks. Higher Mo content could weaken the interface strength by inducing more stacking faults, causing crack initiation along the interface. The elemental segregation along the stacking faults triggered the nucleation of other precipitates, which could further lead to cracks. The precipitation of topologically close-packed (TCP) phases introduced by more Mo addition caused crack initiation along the interfaces between the TCP phases and γ′ phase. By investigating the microcrack formation induced by Mo during creep at 900 °C, we provided a new perspective for the alloy design.

Unexpected thermal aging effect on brittle fracture and elemental segregation in modern dissimilar metal weld

A full-scale dissimilar metal weld safe-end mock-up, precisely replicating a critical component of a modern nuclear power plant, was investigated. The brittle fracture behavior, carbide evolution and nanoscale elemental segregation in the heat-affected zone (HAZ) of low alloy steel (LAS) were analyzed under both post-weld heat-treated and thermally-aged conditions (400 °C for 15,000 h, equivalent to 90 years of operation) using analytical electron microscopy and atom probe tomography. The observed increase in grain boundary (GB) decohesion and intergranular cracking on the fracture surface and the decrease of fracture toughness are primarily attributed to P and Mn segregation to GBs and the coarsening of carbides upon long-term thermal aging. The direct observations of significant elemental segregation to GBs and the consequent reduction in fracture toughness in the HAZ are unexpected for modern low-phosphorus LASs, highlighting potential concerns for evaluating the structural integrity of modern nuclear power plants.

Laves phase control and tensile properties optimization of DED-arc repaired 718Plus components through the addition of TiC and Cr2C3

Directed energy deposition-arc (DED-arc) additive manufacturing technology was used to repair the damaged 718Plus components. This work shows that TiC/Cr2C3 addition to 718Plus alloy is an effective way to suppress the formation of the unfavorable Laves phase. TiC additions to 718Plus alloy can alleviate the elemental segregation, refine the dendritic structure and promote the formation of blocky TiC-NbC core-shell carbides and NbC carbides, while Cr2C3 additions enable the precipitation of rod-like NbC carbides. During the deposition process, the TiC/Cr2C3 additions were dissolved into the molten pool and decomposed into Ti, Cr, and C. The introduction of additional carbon in the melt drastically consumed the Nb available for Laves phase. The tensile tests show that TiC addition to 718Plus alloy contributed to an increased tensile strength of about 120 MPa due to the reduced amount of Laves phase and the reinforced effect of carbides. The fracture behaviour of carbides was explained in detail. The critical shear stress for blocky carbides to crack is higher than that required for rod-like ones, suggesting that TiC additions were desirable for better ductility compared with Cr2C3 additions.

Additive manufacturing of a new titanium alloy with tunable microstructure and isotropic properties

Conventional titanium alloys have a high propensity in developing columnar grains with strong textures during additive manufacturing (AM), which causes pronounced anisotropy in mechanical properties and hampers their practical applications. In this study, a new metastable β titanium alloy with high Fe addition (Ti-3Al-6Fe-6V-2Zr, wt.%) was designed for AM, and the strategies and underlying mechanisms to eliminating the structural heterogeneity including grain morphology, Fe element segregation and phase constituent distribution were elaborately investigated. It was demonstrated that the alloy has an outstanding intrinsic capability of forming equiaxed grains during direct energy deposition (DED) due to the positive effect of Fe on constitutional supercooling. The final microstructure of bulk sample was determined by the directly deposited microstructure and subsequent microstructure coarsening caused by cyclic heating, and homogeneously equiaxed microstructure without texture could be achieved when the heat-affected zones can fully cover the formerly deposited layer. Despite of very high level of Fe addition, both microscale and macroscale Fe segregation were completely suppressed during DED, by taking the advantages of fast solidification rate and tailoring the heating effect between the adjacent layers. Moreover, the problem related to the heterogeneity in α phase distribution along the building direction was solved by mitigating the heat accumulation during DED. On the basis of these understandings, homogeneously equiaxed Ti3662 alloy with isotropic mechanical properties of high strength (~1200MPa) and decent ductility (~10%) was finally fabricated by DED, which stands a good chance for practical application. This study demonstrates that the special metallurgical process during AM largely expands the design space of titanium alloys on the aspects of composition and microstructure, which can be utilized to fabricate titanium alloys with desirable microstructure and excellent properties.

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.

Effect of ultrasonic micro-forging on the microstructure and properties of GH4169 superalloy deposited by laser direct energy deposition

Nickel-based superalloys prepared by laser direct energy deposition (DED) still suffer from microstructure, elemental segregation, and unstable strength, which cannot meet the service requirements and severely limit the application and development of DED-prepared superalloy parts. In this paper, GH4169 specimens with high density and free of defects were prepared by ultrasonic micro-forging assisted laser direct energy deposition (UMT-DED) method. The effects of UMT on the dendritic structure, elemental segregation, and precipitation phases of the alloys prepared by UMT-DED, as well as the densities, hardness, and tensile properties at room and high temperatures were investigated. The contribution of the strengthening mechanism to the yield strength was estimated. The grain refinement mechanism and the deformation mechanism of the Laves phase in GH4169 were discussed. The results show that UMT achieves grain refinement by forming fine crystal strips between the layers. The proportion of fine grain areas increased from 5.9 % to 33.3 %. The plastic deformation introduced by UMT provides external work for dislocation movement to break and fracture part of the Laves phase during plastic deformation, with dislocation cutting of the Laves phase being the main deformation mechanism. The residual stresses was transformed into residual compressive stress. The tensile strength, yield strength, and elongation of the specimens at 25 °C were increased by 4.23 %, 6.42 %, and 23.67 % respectively.

Microstructure forming mechanism of inconel 625 alloy fabricated by laser/ultra-high (UHF) induction hybrid deposition method

To reveal the microstructure forming mechanism of laser/ultra-high frequency (UHF) induction deposition, this paper developed a microscopic phase-field (PF) model to numerically investigate dendrite growth during solidification. The macroscopic model of molten pool evolution is adopted to provide the solidification conditions for the microscopic PF model. The dendrite growth during laser deposition is simulated to evaluate the effect of UHF induction heat on the dendrite growth. Results show that because of the high temperature gradient and cooling rate, the PDAS of laser-UHF induction hybrid deposited layer is less than that of the laser deposited layer. The UHF induction heat also leads to a high flow velocity of the molten metal during laser-UHF induction hybrid deposition. The high flow velocity contributes to the decrease in PDAS by inhibiting the interdendritic enrichment of solute. During laser-UHF induction hybrid deposition, a higher solute gradient is present in the tip region of dendrite arm, leading to a faster dendrite growth rate. The UHF induction heat also increases the solute distribution coefficient during deposition, which further inhibits the element segregation. Under the action of UHF induction heat, a low interdendritic solute gradient and an evenly distributed solute can be obtained, thus helping increase interdendritic undercooling degrees and decreasing the PDAS. The simulated PDAS and solute distribution have good consistency with the experimental results. The spectral analysis of EDS line detection indicates that the laser-UHF induction hybrid deposited layer has a more refined microstructure and weaker element segregation than the laser deposited layer does.

Deformation strengthening mechanism of laser remelted high manganese steel

In order to further improve the deformation hardening ability of high manganese steel (HMnS) under low stress or small deformation, a low speed laser remelting technology was proposed to process the HMnS surface. Through parameters optimization, microstructure characterization, wear testing, and deformation strengthening analysis, the deformation strengthening mechanism of laser remelted HMnS (LR-HMnS) was systematically studied, and excellent wear resistance properties was also achieved. Firstly, by optimizing the parameters of laser remelting, high-quality microstructures with large remelting depth and fewer porosities were obtained. Then, the sliding wear tests showed that although the surface hardness of LR-HMnS (252.83HV0.3) was only 2.56 % higher than that of continuous casting HMnS (CC-HMnS) (246.53HV0.3), its volume wear rate (1.87×10−5 mm3/N·m) was only 10 % of CC-HMnS (18.71×10−5 mm3/N·m), which exhibited excellent wear resistance properties. Finally, for quantitative evaluating its work hardening at high strain rates, Hopkinson compression test of LR-HMnS was conduced according to the operating conditions of HMnS parts. The microstructure after small compression deformation was analyzed using EBSD and TEM. It was found that the larger grain size of LR-HMnS was helpful of reducing the nucleation stress of twins, forming high-density nano-twins, high-density stacking faults, and high-density dislocations. The synergistic effect of stacking faults, dislocations, and twins which segmented the matrix and refined the grains, and achieved the dynamic Hall-Petch effect. The superior deformation strengthening and wear resistance exhibited by larger grain HMnS were called the inverse grain size effect. Additionally, the large number of C-Mn strong bond networks formed by element segregation could enhance the pinning effect of dislocations, promote deformation hardening rate and wear resistance properties under low stress or small deformation. The research results provided a new method for pre-hardening of HMnS, and will also expand the application potential of HMnS in combined conditions of high, medium, and low impact.

Hydrothermal Synthesis of a Valence State Constant High-Entropy Perovskite Sr(TiZrHfVNb)O3 with Improved Photoresponsiveness.

A vanadium ion valence state constant high-entropy perovskite system was synthesized using the hydrothermal method with a trivalent vanadium ion as the vanadium source. The B-site of the perovskite crystal lattice was loaded with five atoms in equal proportions. We tried to synthesize the Sr(TiZrHfVNb)O3 high-entropy system using different methods. However, the valence state of the vanadium ion could only be kept constant using the hydrothermal process in the valence balanced high-entropy composition system. There was significant vanadium element segregation and second phase in the Sr(TiZrHfVNb)O3 system prepared using the solid-state reaction process. Also, obvious vanadium ion valence state ascending from V3+ to V5+ appeared in this high-entropy system with an increase in calcination temperature. Inconspicuous vanadium element segregation appeared at 900 °C, the significant segregation phenomenon and second phase appeared at 1200 °C, and the particle size increased with the temperature. This meant that the high-entropy value could not only stabilize the crystal phase, but also stabilize the ionic valence state. Moreover, the constant trivalent vanadium ion valence state could provide coordinated performance with a wide optical response range and a low band gap for the high-entropy system. This suggests that the system might grow a potential ceramic material for optical applications.

Ultrastrong, high plasticity, and softening-resistant refractory high-entropy alloy via stable isostructural coherent interfaces

Traditional approaches for improving the mechanical performance of alloys entail modifying interfaces, particularly grain boundaries, with elemental segregation or secondary phases. However, these methods face challenges in concurrently improving the strength, plasticity, and high-temperature softening resistance of alloys. Here, we uncovered that stable isostructural coherent interfaces effectively address these challenges. In the model body-centered cubic (BCC) MoTaVW refractory high-entropy alloy (RHEA) fabricated by mechanical alloying and spark plasma sintering, controlling the sintering temperature enhances the preferential segregation of W at interfaces. This results in a distinct BCC W-enriched nanolayer between micrometer-scale grains. This nanolayer facilitates dislocation slip and prevents grain growth, thereby improving both plasticity and resistance to high-temperature softening. Consequently, the MoTaVW RHEA featuring stable isostructural coherent interfaces achieves an ultrahigh yield strength of 1410 MPa and a plasticity of 22 % at ambient temperature. Even at 1200 °C, it maintains a yield strength of 575 MPa under hot compression.

Physical field of dual-frequency ultrasonic vibration and its effect on solidification behavior of MgZnY alloy

A three-dimensional model was established for coupled simulation of the physical field in a liquid subjected to dual-frequency ultrasonic vibration. The effect of sonotrode position, tip shape, and vessel on the distributions of acoustic pressure and acoustic streaming were clarified. The influence of physical fields on the solidification behavior of Mg alloy was investigated. The simulation results show that the interaction between the two ultrasounds was weakened as the sonotrode spacing increased, and the radiating angle had little effect on the acoustic pressure distribution. The volume mean pressure in the liquid radiated by the plane tip was the smallest, but with the greatest acoustic streaming intensity. In contrast, the spherical tip radiated the highest volume mean pressure in the liquid medium, but the acoustic streaming was severely suppressed. Attributed to the smooth spherical surface that facilitated fluid flow, high flow velocity could be achieved using a round-bottomed crucible without severe acoustic pressure attenuation. The experimental results show that cavitation and acoustic flow caused the coarse dendrites to turn into fine isometric crystals. The acoustic flow accelerated the melt flow and alleviated the segregation and precipitation of Zn elements, leading to a decrease in the content and size of the second phase in the billet.

Degradation analysis of plasma sprayed metal-supported solid oxide fuel cell: The evolution of electrode processes under polarization

Metal-supported solid oxide fuel cells (MS-SOFCs) prepared by plasma spraying exhibit promising potential for scale-up applications, where the key lies in breaking through the challenges of limited lifetime and durability. Although electrochemical impedance spectroscopy (EIS) has been widely used to unfold the complex electrode processes in electrochemical reactions, there are still some concerns concerning the in-depth understanding and scientific evaluation of the degradation mechanism of cell long-term performance. In this study, the contribution of each resistance to the overall performance degradation is separately determined by coupled equivalent circuit model (ECM) based on the deconvolution of recorded EIS data at different DC biases. It is found that the deterioration of cathode surface oxygen exchange and O2− conduction causes more than 64 % of the total resistance increase, followed by the anode charge transfer reaction and gas-phase transport (24.91 %), and then the ohmic resistance (9.73 %). However, the correlation between the output voltage drop of MS-SOFC under load and the gas-phase diffusion resistance gradually increases, indicating a shift in the cell degradation mechanism. Combined with the post-test characterization of the aged cell, it can be inferred that the degradation is mainly attributed to the densification of the anodic microstructure and the cathodic elemental segregation. Additionally, attention should be paid to the effects of the thickening of the metal substrate oxide layer and thermal mismatch on the structural stability of MS-SOFC.

Enhancing mechanical properties of medium Mn steel by warm rolling based on laminated elemental segregation

The hot-rolled microstructure of medium Mn steel has coarse grains and severe elemental segregation, resulting in low strength and plasticity. Constructing a multiphase structure, refining the microstructure, and regulating elemental segregation enhance the mechanical properties. In this study, liquid nitrogen treatment created a layered distribution of austenite and martensite. Warm rolling was then used to reduce layer thickness and refine grain structure. After liquid nitrogen and warm rolling treatments, the strength and plasticity of medium Mn steel increased to 1270 MPa and 23.3%, respectively, far exceeding the hot-rolled state (724 MPa, 12.8%). Warm rolling also triggers austenite reverted transformation (ART) and introduces high-density dislocations, further improving austenite stability. This strengthening effect is higher than that from intercritical annealing alone. Improved austenite stability delays the transformation induced plasticity (TRIP) effect, preventing brittle fracture and enhancing deformation coordination between layers, significantly increasing the plastic deformation capacity of medium Mn steel.

Enhanced mechanical properties and anti-wear performance of Cu45Mn8Ni40Sn7 medium entropy bronze alloy by multi-stage heat treatment

High/ medium entropy brass and bronze alloys exhibit considerable potential for application as structural materials. This study focuses on a Cu45Mn8Ni40Sn7 medium entropy bronze alloy that demonstrates excellent hardness, compressive strength, ductility, and wear resistance through a multi-stage heat treatment technique. The effects of heat treatment on the phase composition, microstructural evolution, mechanical properties, and tribological performance of the alloy were systematically investigated. The alloy, treated by homogenization (820 ℃/4 h)-solution (900 ℃/1 h)-aging (450 ℃/5 h), exhibits a hardness of 362 HV, yield strength of 626 MPa, ultimate strength of 1229 MPa, compressive strain of 47.2 %, fracture toughness of 20.7 MPa·m1/2, and maintains a low wear rate of 1.72×10−5 mm3·N−1·m−1. The processes of homogenization and solution treatment reduce dendrite segregation of elements and eliminate the detrimental lamellar structure at the grain boundaries. During the aging treatment of the alloy, sub-micron scale needle-like precipitates are formed, which play a significant role in enhancing its mechanical properties.

High-temperature creep mechanism of Ti-Ta-Nb-Mo-Zr refractory high-entropy alloys prepared by laser powder bed fusion technology

Creep resistance, which is one of the most important deformation modes, is rarely reported for refractory high entropy alloys (RHEAs). The experiment investigated the high-temperature creep mechanism of Ti-Ta-Nb-Mo-Zr RHEA prepared by laser powder bed fusion (LPBF) technology. The high cooling rate of LPBF suppresses most of the elemental segregation, but there are still over-solidified precipitates and a few continuous precipitates (CP). In the range of 923–1023 K, the stress exponent and activation energy were determined to be 3.2–3.4 and 261.5 ± 19.5 kJ/mol, respectively. Compared with other conventional alloys and HEAs, a large reduction of the minimum creep rate is found in the LPBF-built Ti1.5Ta0.5NbZrMo0.5 RHEA, indicating a significant improvement in high-temperature properties. The dislocation tangles at the interface is formed during the creep process and new Zr-rich CP phases are generated in the dislocation tangles region. The interfacial dislocation tangles is the result of the interaction between dislocations and two-phase mismatch stresses. The dislocation tangles prevents dislocations from further cutting the matrix phase, which is very favorable to the high-temperature creep performance. At the same time, the formation of this dislocation tangles greatly accelerates the nucleation process and growth rate of the new CP phase. The present work provides a pathway to design novel HEAs with improved high-temperature creep resistance.

Formation mechanism of cracks formed during drilling of a cold drawn prestressing 1215 free-cutting steel

1215 steel is a low-carbon free-cutting steel with the largest production, and its quality issues are complex and diverse. The formation mechanism of cracks formed during drilling of a cold drawn prestressing 1215 free-cutting steel was investigated by means of optical microscopy, scanning electron microscopy, high-temperature laser scanning confocal microscopy, and Thermo-Calc software. The failure was caused by abnormally wide pearlite with large amounts of sulfide nearby. We speculate the abnormal microstructure was related to C and S segregation in columnar-to-equiaxed transition zone during continuous casting process. However, element segregation was an inherent property in the continuous casting process. Reheating was an important process for homogenization to eliminate segregation, but the reheating temperature was lowered to reduce production costs. The lower reheating temperature resulted in incomplete elimination of columnar-to-equiaxed transition zone segregation, ultimately leading to the formation of abnormal microstructure. This batch of 1215 steel wire was not suitable for producing component required transverse plasticity during machining, but could be used to produce circular shaft components. In the future, it is suggested to increase the reheating and hot-rolling temperature, or take effective measures to reduce segregation during continuous casting.