Segregation Phase Research Articles

Experimental studies on dilution, microstructural, mechanical and wear characteristics of Inconel 718 deposited over stainless steel 304 employing cold metal transfer process

In this research, the sinewave weaving technique is used by the Cold Metal Transfer process for hard-facing the Inconel 718 over the Stainless Steel 304. A microstructural study exhibited columnar grains that were transformed into equiaxed grains. Scanning Electron Microscopy (SEM) – Energy Dispersive X-ray analysis (EDX) results confirm the element’s segregation and secondary phase’s formation. Outcomes of hardness, tensile, impact measurements, and wear analysis show that a hard-faced layer offers better hardness and wear resistance.

Effect of ultrasonic vibrations on mass efficiency and microstructure of laser direct deposition Inconel 718 superalloy

Laser direct deposition (LDD) is widely used to repair and manufacture high-value industrial components. However, it faces various defects, such as porosity, cracks, non-uniform microstructure, lack of fusion, keyhole phenomenon, element segregation, and undesirable secondary phases. A method to manage these defects is to concurrently apply ultrasonic vibrations (USV) during the LDD process. This study investigates the effect of USV on the mass efficiency and microstructure of LDD Inconel 718 superalloy to understand how incorporating USV can change the performance and structural integrity of single passes produced using the LDD process. For this purpose, USV is applied to a substrate during the LDD process. The resulting samples are characterized and analyzed using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicate that applying USV below a threshold power value increases mass deposition by over 25%, while exceeding this threshold reduces it. Attention to this threshold power value is crucial for determining the process parameters, including laser power and speed. Additionally, USV transforms the microstructure from columnar to equiaxed and increases subgrain formation. This implementation also enhances the cooling rate, significantly decreasing the Laves phase by over 30% in all process parameters.

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.

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.

Brazing Zircaloy-4 to Ti3AlC2 using Ti–Zr–Cu–Ni amorphous alloys: Microstructural evolution and mechanical properties

Brazing of Zircaloy-4 (Zr-4) to Ti3AlC2 ceramic was investigated using a Ti–13Zr–21Cu–9Ni (wt.%) amorphous braze alloy (ABA) at temperatures ranging from 840 °C to 990 °C for 5 min. The interfacial reactions of the brazed joint were comprehensively analyzed, revealing a distinct multilayer structure consisting of Ti3AlC2/ZrC/α-Zr(Ti) + [Zr(Ti)]2Cu + [Zr(Ti)]2[Cu(Ni)]/α-Zr(Ti)/α-Zr. The formation of the ZrC carbide particle layer primarily results from the reaction between the molten ABA and decomposed Ti3AlC2 caused by the de-intercalation of Al. Additionally, the dissolution of Zr-4 into the ABA induces an alteration in the composition of the Zr-4/ABA interface, leading to the formation of α-Zr(Ti) isothermal solidification dendrites. The effect of brazing temperature on the mechanical properties of the joint was investigated and detailed fracture analysis was conducted to determine the failure mechanism. The fracture path result indicated that the joint primarily fractured within the ZrC layer due to the mismatch of hardness, rather than within the reticular [Zr(Ti)]2[Cu(Ni)] segregation phase. The maximum shear strength of the brazed Ti3AlC2/Zr-4 joint with an average strength of 187 ± 34 MPa was achieved by brazing at 930 °C for just 5 min. With the increase in brazing temperature, the thickness of the ZrC ceramic layer became thicker in an undulating manner, and the content of Zr-based alloy particles in the ZrC layer increased gradually, resulting in a slight decrease in the bearing capacity of the joint. The presented results provide valuable insights into the microstructural evolution, interfacial reactions, and mechanical properties of the Ti3AlC2/Zr-4 brazed joints using amorphous alloys.

Design and development of a novel non-equiatomic Ti-Nb-Mo-Ta-W refractory high entropy alloy with a single-phase body-centered cubic structure

Element segregation and consequent phase separation are challenging problems in refractory high entropy alloys (RHEAs). In this study, thermodynamic parameters and calculation of phase diagrams (CALPHAD) were implemented for RHEA design to suppress segregation and phase separation. A novel non-equiatomic RHEA, Ti1(NbMoTa)2W0.5 alloy, was designed with a minimized difference between liquidus and solidus temperatures (ΔTl−s) and a wide temperature range between solidus temperature and transformation temperature (ΔTsingle) and compared with equiatomic RHEA. As-cast Ti1(NbMoTa)2W0.5 alloy maintained a single-phase body-centered cubic (BCC) structure, corresponding to the Scheil–Gulliver model, and segregation of constituent elements was suppressed owing to the minimized ΔTl−s. Ti1(NbMoTa)2W0.5 alloy showed excellent strength (approximately 1100 MPa) and ductility (1.6 times higher than TiNbMoTaW alloy). This study demonstrated a novel approach for obtaining single-phase BCC-structured RHEAs with suppressed elemental segregation and phase separation by utilizing combined use of ΔTl−s and ΔTsingle as alloy design indexes.

Quantum fluctuation in the segregation phase of two-component weakly interacting Bose–Einstein condensates

Our investigation focuses on the condensate density, excited and condensed fractions of bosons in a mixture of two-component segregated weakly interacting Bose–Einstein condensates (BECs) at zero temperature based on the Gross–Pitaevskii (GP) and Cornwall-Jackiw-Tomboulis (CJT) theories. We find that the repulsive interaction among interspecies of bosons is the origin of the local behavior and the decline of quantum fluctuation. These features are disparate to the single BEC or miscible BECs. The numerical evaluation of the condensed fraction is provided to support the experimental observation.

Microstructure evolution of extruded Mg-RE-Ag alloy during short-time heat treatment

Microstructure regulation via short-time heat treatment is conducive to the optimization in the microstructure and properties of precipitable magnesium (Mg) alloys, but there is currently a lack of relevant studies. In this work, the microstructure evolution of a Mg-RE-Ag alloy during different short-time heat treatments was characterized and discussed. The results show that extreme short-time heat treatment (ESHT, e.g., 2 min) at 450–480 °C can greatly increase solute concentration in Mg matrix through the rapid re-dissolution of the second-phase and simultaneously maintain fine grains, while the ESHT at a too high temperature (e.g., 510 °C) is not suitable due to excessive grain growth and coarse second phase regenerated at grain boundaries. It is found that 480 °C is the approximate critical temperature for appropriate ESHT, and further prolongation of the time will lead to excessive grain growth. It is suggested that in addition to grain boundary migration, grain rotation is activated, resulting in the annihilation of high-angle grain boundaries with relatively low misorientation, as well as the reduction in the ability of the residual second phase to pin grain boundaries. In addition, the reasons for the abnormal grain boundary segregation and grain boundary continuous phase were analyzed from the perspective of interfacial energy. This study provides a basis for effective microstructure regulation of Mg-RE alloys.

X-ray computed tomography analysis of calcium chloride hexahydrate solidification

Latent Thermal Energy Storages (LTESs) are receiving increasing interest, since they offer many advantages to the field of Thermal Energy Storages (TESs), such as a nearly isothermal solid–liquid phase transition and a high energy storage density. To boost the transition towards a climate-neutral future, a lot of work is surely needed on Phase Change Materials (PCMs) which are used inside LTESs. Salt hydrates, a particular class of PCMs, are very attractive in terms of cost and volumetric latent enthalpy but present drawbacks that still limit their effective deployment in real applications, such as supercooling and phase separation (or segregation). The current work assesses the suitability of X-ray Computed Tomography (XCT) as a novel technique in the field of LTESs to study the solidification of phase-change materials. In this study, calcium chloride hexahydrate (CaCl2·6H2O) is chosen as a representative PCM and the results derived are shown to be repeatable. For the first time, the direct measurement of the volumetric liquid fraction is correlated to temperature measurements allowing for the visualisation of supercooling and the validation of an enthalpy-porosity numerical model, which could be useful in the design of LTES. XCT methodology offers unmatchable features as compared to the other method to measure the volumetric liquid fraction. Given its accuracy and the rigorous protocol, it can be considered as a benchmark for new liquid fraction measurement technologies but it also permits to study how the different PCMs melt and solidify, giving new insights on the mechanisms at the basis of the most crucial challenges, among those: subcooling, segregation, and metastable phase change.

CALPHAD-Based Modeling and Experimental Validation of Silicon Effect on the Solidification Behavior of Novel SiNb Cast Irons

The low oxidation resistance of SiMo ductile cast irons used as exhaust manifold material at high temperatures necessitates the development of new generation ductile cast iron compositions. New alloy designs can be made using CALPHAD methodology, and solidification sequence, segregation and critical phase transformation temperatures can be determined, especially for the solidified bulk materials. Thus, in commercial practicality, castable compositions with a raised A1 temperature can be obtained. In this study, novel SiNb cast irons with varying silicon contents were developed as candidate materials for exhaust manifolds. Solidification sequence, microsegregation, phase transformations, equilibrium phases of hypereutectic compositions containing 4 to 7 wt pct Si were calculated by CALPHAD-based modeling. The bulk materials of the studied compositions were cast as Y blocks and metallurgical analyzes were carried out. Studies revealed that; (i) in the ferritic matrix of the cast irons, graphite, Nb-rich carbides and some pearlite existed, (ii) pearlite formation was due to the negative segregation of silicon and positive segregation of manganese during solidification, (iii) as silicon content increased the amount of silicon dissolved in ferrite phase increased in the solidified structure and as a result pearlite formation decreased at the cell boundaries, and amount of vermicular graphite increased, (iv) depending on the silicon content the critical A1 temperature varied between 860 °C to 1013 °C and these values were higher than that of SiMo cast iron. All these findings revealed that SiNb cast irons had phase stability at higher temperatures compared to SiMo cast iron.

Effect of Hf addition on structural transformation and aging stability of Cu–15Ni–8Sn alloy

The Cu–15Ni–8Sn-xHf (wt.%, x = 0–0.6) alloys were prepared by vacuum melting, and the effect of Hf on the micro-structure of as-cast, as-solution-treated and as-aged alloys was systematically investigated, as well as the alloy aging stability. The Hf addition provided a significant role on refining the solidification structure and reducing the micro-segregation degree, and it also made γ-D03 phases more likely to form in needlelike-shape rather than lamellar-shape. With the presence of more Hf atoms, their existence form was accordingly changed from solid solute atoms to nano-scale precipitation phases and then to micron-scale segregation phase. A small content of Hf (0.1 wt%) could effectively improve the aging stability by inhibiting the growth of γ-D03. Once its content reached or exceeded 0.3 wt%, the harmful discontinuous precipitation was completely inhibited, and the common lamellar-shape γ-D03 locating around the grain boundary was no longer formed. Even the alloy failed at higher temperatures, it could only be carried out through the continuous precipitation method, namely the generation of needlelike-shape γ-D03 uniformly in the matrix. The appropriate Hf addition was capable of extending the alloy aging window, and the hardness of Cu–15Ni–8Sn-0.3Hf alloy could still exceed 360 HV even after 48 h aging.

Development of a novel autogenous double pulse tungsten inert gas welding technique for joining IN625 in concentrated solar power plants

The research focuses on the challenges of joining the IN625 alloy for high-temperature applications. Tungsten inert gas (TIG) welding has drawbacks such as higher heat input (HI), which causes elemental segregation and secondary phases, ultimately affecting the quality of weldments. Previous literature indicates that single-pulsed TIG (SP-TIG) welding has shown improvements over TIG welding. In this study, a new technique called double-pulsed TIG (DP-TIG) welding was developed to address the HI and elemental segregation issues during the welding process. The study also compares the improvements in properties of DP-TIG with SP-TIG welding. The results demonstrate that DP-TIG significantly reduces HI by 53% compared to SP-TIG. It effectively decreased residual stress by 26% and suppressed secondary phase formation. Moreover, DP-TIG exhibits superior mechanical properties, with an 11% increase in hardness and a 10% increase in tensile strength compared to SP-TIG. These findings establish DP-TIG as the preferred method for joining the IN625 alloy.

Intrinsic heterogeneity of grain boundary phase transitions in the Cu–Bi system: insights from grain boundary diffusion measurements

Abstract Diffusion of Bi and Ag in a series of polycrystalline Cu–Bi alloys is investigated using a radiotracer technique and applying the 207Bi and 110m Ag isotopes, respectively. Together with the previous measurements (Divinski S., Lohmann M., Herzig C., Straumal B., Baretzky B., Gust W. Grain-boundary Melting Phase Transition in the Cu−Bi System. Phys. Rev. B 2005, 71, 104104), a temperature–concentration interval of strong, by orders of magnitude, enhancements of Bi grain boundary diffusion rates is distinguished and the results are interpreted in terms of a grain boundary pre-wetting/wetting phase transition. Grain boundary diffusivity of Ag exhibits as well a step-wise increase with rising Bi content, mirroring the behaviour observed for the Bi tracer. However, contrary to the Bi tracer atoms for which grain boundary enhancement is observed at about 60 ppm of Bi in Cu–Bi alloys, this transition is revealed by the Ag tracer atoms at a significantly higher concentration, specifically between 90 and 100 ppm of Bi at 1080 K. The Ag diffusion rates in alloys with a moderate Bi content turn out to be not affected by the Bi-induced grain boundary phase transition and the measured grain boundary diffusion coefficients of Ag are nearly the same as those determined for pure polycrystalline Cu. This spectacular result suggests a strong heterogeneity of Bi segregation and Bi-induced phase transition for general high-angle grain boundaries in a given alloy. The behaviour is discussed in terms of the extrinsic grain boundary defects and their impact on mechano-chemical coupling which is accompanying the grain boundary phase transitions.

Solidification Segregation Behavior and Homogenization Process of a Difficult-to-Deform Superalloy Used at 850 °C

Solidification segregation behavior and homogenization heat treatment processes of a difficult-to-deform superalloy for use at 850 °C were studied. Additionally, the effect of carbon content on homogenization, the thermal treatment process, and pore growth behavior within the alloy were discussed. Our results revealed that Al, Ti, and Nb elements are distributed in the interdendritic space, while W and Mo elements are distributed in the dendrite. There is a significant quantity of γ-γ′ eutectic and MC carbide precipitates in the interdendritic space. Notably, for the alloy containing 0.1 wt% C, a homogenization heat treatment at 1200 °C for 48 h can effectively eliminate the segregation and undesirable phases. As carbon content increased, the γ-γ′ eutectic phases diminished, and the homogenization time decreased accordingly. In this context, pores are smaller and more dispersed, which may enhance alloy forging properties.

Microzone characterization and corrosion resistance of water-based epoxysilane/Ti/Zr composite chemical conversion coating on multi-metals

A water-based epoxy silane and titanium/zirconium composite chemical conversion coating was prepared on the surfaces of 6061 aluminum alloy, 7075 aluminum alloy, and galvanized steel simultaneously. Our findings revealed that the composite chemical conversion coating exhibited the strongest anti-corrosion effect on multi-metals surfaces as the concentration of silane was 0.4 vol% and the conversion time was 90 s. The analysis showed that a notable deposition of TiO2 and ZrO2 in close proximity to intermetallic compounds, segregation phases enriched with copper, and inclusions. Notably, the formation of the composite chemical conversion coatings on the multi-metals surface primarily follows a synergistic mechanism involving adsorption, oxidation, and readsorption. Consequently, a crosslinked siloxane network structure is formed, which offers additional filling sites for surplus particles of Al2O3, TiO2, ZrO2, and Fe2O3.

Effect of ultrasonic power on degassing and microstructure of large-scale 7085 aluminum alloy ingots

In this work, five sets of large-scale 7085 Al alloy ingots (diameter 830 mm × length 3600 mm) were manufactured by introducing different ultrasonic powers (0–750 W) in the Direct-Chill (DC) casting process. The difference among the ultrasonic degassing, the α-Al grains, elemental segregation and eutectic phase with ultrasonic treatment were investigated. The results indicated that with the increase of ultrasonic power, the efficiency of ultrasonic degassing was gradually improved, which was decreased from 0.085 cm3/100 g to 0.056 cm3/100 g in ingot center part. The grain size was also dramatically refined, in which the grain refinement rate reached 38.2%, 42.8% with 550 W, 750 W power, respectively. The segregation of alloying elements (Zn, Mg, Cu) in the ingot was significantly suppressed, which the maximum positive deviations ∆C of Zn, Mg, and Cu were reduced by 59.5%, 55.9%, and 60.8%, respectively. In addition, the new Al14Cu2Fe3 (α−Fe) phase was the first to be discovered in the low power state and the coarse eutectic phase was more refined in the high power. The mechanisms of degassing, grain refinement and suppression of macroscopic segregation during ultrasonic casting were further revealed by calculating the acoustic energy density (I).

Characteristics of hydrogen storage of as-milled TiFe-based alloys

The phase structure, activation and kinetic performances of the as-cast and milled Ti1.04Fe0.6Ni0.1Zr0.1Mn0.2Sm0.06 alloys were investigated. The alloys contained TiFe, Ti2Fe and Sm segregation phases. Ball milling resulted in the occurrence of the particle pulverization and aggregation. It also introduced new crystal defects, disordered or amorphous structures in the alloys. The short-time ball milling can increase the activation property, shorten the incubation time and increase the hydrogenation capacity, whereas the long-time ball milling showed negative effects. The alloy milled for 0.5 h showed the optimum activation property and hydrogenation capacity. The activation began at 82 s and reached 1.713 wt% of hydrogenation capacity (423 K). The particle pulverization not only increased the contact surface between hydrogen molecules and the alloys but also increased the content of bare Sm segregation phase. Sm segregation phase provided the original power for the alloy pulverization, which also can be used as the specific diffusion channel of hydrogen atoms. The weakened performances of the alloy milled for 5 h were related to the particle aggregation and disordered or amorphous structures.

Key Role of Interfacial Cobalt Segregation in Stable Low-Resistance Composite Oxygen-Reducing Electrodes.

The development of efficient and stable oxygen-reducing electrodes is challenging but vital for the production of efficient electrochemical cells. Composite electrodes composed of mixed ionic-electronic conducting La1-xSrxCo1-yFeyO3-δ and ionic conducting doped CeO2 are considered promising components for solid oxide fuel cells. However, no consensus has been reached regarding the reasons of the good electrode performance, and inconsistent performance has been reported among various research groups. To mitigate the difficulties related to analyzing composite electrodes, this study applied three-terminal cathodic polarization to dense and nanoscale La0.6Sr0.4CoO3-δ-Ce0.8Sm0.2O1.9 (LSC-SDC) model electrodes. The critical factors determining the performance of the composite electrodes are the segregation of catalytic cobalt oxides to the electrolyte interfaces and the oxide-ion conducting paths provided by SDC. The addition of Co3O4 to the LSC-SDC electrode resulted in reduced LSC decomposition; thus, the interfacial and electrode resistances were low and stable. In the Co3O4-added LSC-SDC electrode under cathodic polarization, Co3O4 turned wurtzite-type CoO, which suggested that the Co3O4 addition suppressed the decomposition of LSC and, thus, the cathodic bias was maintained from the electrode surface to electrode-electrolyte interface. This study shows that cobalt oxide segregation behavior must be considered when discussing the performance of composite electrodes. Furthermore, by controlling the segregation process, microstructure, and phase evolution, stable low-resistance composite oxygen-reducing electrodes can be fabricated.

Subgrain microstructural features in directed energy deposited stainless steel 316L: The influence of morphology on mechanical properties

Recent studies have established that the exceptional combination of strength and ductility of directed energy deposited stainless steel 316 L is primarily due to the barrier effect against mobile dislocations introduced by cellular subgrain feature boundaries. Interestingly, the morphology of the feature, and therefore its boundaries, exhibits a significant variation with process parameters. This article presents an experimental study to determine the mechanism responsible for the formation of cellular subgrain feature with different morphologies and the influence of morphology on mechanical properties. Two builds with cellular subgrain features of different morphologies, tubular and vermicular, are manufactured using different process parameters and scanning strategies. The microstructural analysis of the specimens shows very different element segregation and secondary phase distribution. The mechanism responsible for the formation of different morphologies is the differences in the solidification modes. The barrier effect has two contributions; a hard barrier effect introduced by delta ferrite secondary phases and a soft barrier effect introduced by austenite cell wall regions. Irrespective of different morphologies, the two builds exhibited similar macroscopic mechanical behavior. This indicates that the total barrier effect exerted by the cellular subgrain feature determines the mechanical properties of directed energy deposited stainless steel 316 L, not necessarily its morphology.