Composition Segregation Research Articles

Effect of Laser re-melting on the microstructure of High Entropy Alloys

• A low density equiatomic High Entropy Alloy Zr-Nb-V-Ti-Al was synthesized. • The as-cast alloy had coarse dendritic structure with compositional inhomogeneities. • Re-melting the as-cast alloy with Nd-YAG Laser resulted in formation of fine grains. • Laser Re-melting significantly reduced inhomogeneities in distribution of Zr & Al. • The microhardness of Laser melted region was found to be similar to the BCC phase. A low density equiatomic High Entropy Alloy of composition Nb-Zr-V-Ti-Al was synthesized for nuclear reactor applications. The alloy in as-cast condition showed a coarse dendritic structure with a major BCC/B2 type dendritic phase while segregation of Zr and Al in the form of intermetallics was noted in the inter-dendrtitic region. Re-melting a part of the alloy surface with an Nd-YAG Laser resulted in formation of very fine grains (∼500 nm) with a significant reduction in compositional segregations with respect to Zr and Al. The microhardness of the Laser melted region was found to be similar to the BCC phase in the as – cast microstrucure. This indicates that Laser melting could be an effective method in reducing compositional and microstructural inhomogeneities and push the multiphase structure towards single phase.

Interparticle bonding and interfacial nanocrystallization mechanisms in additively manufactured bulk metallic glass fabricated by cold spray

Amorphous Zr-based bulk metallic glass deposit was produced by cold spray additive manufacturing. The bonding mechanism of metallic glass particles was systematically investigated through studying the deformation behavior of individual particles after deposition. We revealed two collective particle bonding mechanisms that contributed to the formation of metallic glass deposits, i.e., high-velocity impact induced localized metallurgical bonding at the fringe of the interface, and high particle temperature induced viscosity reduction and the resultant annular metallurgical bonding band. Moreover, the dynamic evolution mechanism of the amorphous phase into nanocrystal structures at severely deformed interfacial regions during cold spray was carefully investigated. For the first time, we observed different amorphous/nanocrystal structures in cold sprayed metallic glass deposits, which can represent different evolution stages in nanocrystallization process. Based on the observation, it is inferred that the nanocrystallization process can be divided into following three stages: composition segregation, the formation of ordered 1D and 2D transition structures, and 3D nanocrystals. The current study provides new insights into the bonding mechanisms and the mechanistic nanocrystallization origins in cold sprayed bulk metallic glass deposits.

Characteristics of β-type Ti-41Nb alloy produced by laser powder bed fusion: Microstructure, mechanical properties and in vitro biocompatibility

A β-type Ti-41Nb alloy with high relative density has been successfully fabricated by laser powder bed fusion (l-PBF) using pre-alloyed powders for potential implant application. The homogeneous microstructure can be achieved in l-PBF fabricated (l-PBFed) Ti-41Nb alloy but slight composition segregation was detected along molten pool boundaries. The l-PBFed alloy was dominated by typical epitaxial columnar grains with strong 〈001〉 grain orientation along building direction (BD), and cellular structure was distinguished within the columnar grains. The main reasons for this microstructure can be attributed to effective thermal gradient and epitaxial growth. l-PBFed alloy exhibited higher mechanical strength compared with cold rolling plus annealing (CRA) alloy due to the finer grains, dislocations accumulation and different TRIP behaviors, accompanied by good ductility. It also exhibited much lower thermal conductivity and better hydrophilic feature than those of CP-Ti. Besides, the l-PBFed alloy exhibited slightly better cell spread and cell proliferation rates compared with CP-Ti. Moreover, l-PBFed alloy presented better alkaline phosphatase (ALP) activities and extracellular matrix (ECM) mineralization, which suggests that the l-PBFed alloy can stimulate the osteogenic differentiation of rat bone mesenchymal stem cells. The Ti-41Nb alloy, fabricated by l-PBF, reveals a good combination of mechanical properties, physicochemical properties and biocompatibility, exhibiting the great potential as the dental implant.

In-situ investigation of composition segregation and deformation streamline in bainitic steel on mechanical properties

The influence of segregation bands and deformation streamline on mechanical properties is investigated via in-situ technology. The results reveal that the anisotropy is caused by the segregation bands. The microstructure and grain orientation of the matrix were obviously rotated, and the strain gradient between matrix and segregation promotes crack initiation.

Structural and compositional segregation of the gut microbiota in HCV and liver cirrhotic patients: A clinical pilot study

Gut microbial dysbiosis during the development of Hepatitis C virus and liver-related diseases is not well studied. Nowadays, HCV and liver cirrhosis are the major concerns that cause gut bacterial alteration, which leads to dysbiosis. For this purpose, the present study was aimed at correlating the gut bacterial community of the control group in comparison to HCV and liver cirrhotic patients. A total of 23 stool samples were collected, including control (9), liver cirrhotic (8), and HCV (6). The collected samples were subjected to 16 S rRNA Illumina gene sequencing. In comparison with control, a significant gut bacterial alteration was observed in the progression of HCV and liver cirrhosis. Overall, Firmicutes were significantly abundant in the whole study. No significant difference was observed in the alpha diversity of the control and patient studies. Additionally, the beta diversity based on non-metric multidimensional scaling (NMDS) has a significant difference (p = 0.005) (ANOSIM R2 = 0.14) in all groups. The discriminative results based on the LEfSe tool revealed that the HCV-infected patients had higher Enterobacteriaceae and Enterobacterial, as well as Lactobacillus and Bacilli in comparison than the liver-cirrhotic patients. These taxa were significantly different from the control group (p < 0.05). Regarding prospects, a detailed analysis of the function through metagenomics and transcriptomics is needed.

Development of a novel high strength and toughness Al–Cu–Li alloy casting billet with a new process

In this study, a novel Al–5Cu-0.6Li-0.5Mn-0.3Mg-0.15Ti alloy was developed, in which Li is as low as 0.6 wt% and Ti is added, no Ag, Zn, Zr, RE, etc., and high-performance Al–Cu–Li alloy casting billets were prepared by squeeze casting (SC) with ultrasonic treatment (UT). The results show that compared with traditional gravity casting (GC), the porosity and compositional segregation in the SC billets are significantly reduced, and the grain size is also reduced from 80 μm to 38 μm, especially the microstructure of UT + SC billets is further optimized. After UT + SC, the ultimate tensile strength (UTS), yield strength (YS) and elongation of Al–Cu–Li alloy billets are 300 MPa, 190 MPa and 16.5%, which are 27.1%, 35.7% and 230% higher than those of the GC billets, and these as-cast properties are increased by 13.2%, 15.2% and 37.5%, compared with Li-free Al–5Cu-0.5Mn-0.3Mg-0.15Ti alloy billet prepared by UT + SC, respectively. The improvement of the strength and elongation of Al–Cu–Li alloy billet prepared by UT + SC is attributed to the reduction of porosity, grain refinement and uniform distribution of second phases.

Reversible Phase Segregation and Amorphization of Mixed-Halide Perovskite Nanocrystals in Glass Matrices.

Mixed-halide perovskites have attracted great attention in applications of lighting and photovoltaic devices due to their excellent properties. Understanding the phase segregation mechanism of mixed-halide perovskite has significance for suppressing the performance degradation of optoelectronic devices. Herein, we investigate the mixed-halide perovskite nanocrystals (NCs) in isolation from the external factors (oxygen, moisture, and pressure) using glass encapsulation, which shows excellent photostability against phase segregation. By monitoring the structural evolution of the NCs in glass matrices, the coexisting phase segregation and amorphization of mixed-halide perovskites are observed in real-time. The results show that thermal-induced local temperature increase plays a dominant role in the phase segregation of mixed-halide perovskite NCs. The recovery process is driven by the spontaneous crystallization of the amorphous mixed-halide phase. The clarified dynamic equilibrium process between the compositional segregation (mixing) and structural disorder (order) gives us a better insight into the reversible phase segregation mechanism of mixed-halide perovskite.

Composition regulation of composite materials in laser powder bed fusion additive manufacturing

Understanding and controlling the composition segregation during powder spreading is of key importance in the additive manufacturing (AM) of composite materials. Under this circumstance, the segregation behavior of WC/316 L composite powders during spreading in laser powder bed fusion (LPBF) AM was numerically investigated by the discrete element method. The effects of process conditions (i.e., spreader velocity and geometry) and powder properties (i.e., size and shape of the WC powder) on the powder bed composition segregation and related characteristics were systematically analyzed. Corresponding mechanisms were identified from microscopic scale in terms of particle velocity, motion trajectory, mechanical behavior, and energy information. Finally, proper solutions in designing and constructing WC/316 L composite materials with desired gradient structures were proposed. The results show that the small blade velocity ( V ) will enhance the negative segregation, increase the average packing density ρ ¯ , and decrease uniformity ρ vc in the WC/316 L composite powder bed. Compared with the blade, the roller can increase the negative segregation ( Se roller = −0.027 < Se blade = −0.019) and the average packing density ( ρ ¯ roller = 0.31> ρ ¯ blade = 0.20). When the WC/316 L size ratio increases from 25 μm/45 μm to 45 μm/45 μm, the negative segregation becomes weaker, and its value increases from −0.084 to −0.007. When the size ratio increases to 65 μm/45 μm, the powder behaves positive segregation with Se max = 0.017; in this case, the packing density is the lowest (0.14), and the uniformity is the worst (0.17). In comparison with spherical shape, polyhedral WC powder can reduce the negative segregation of the powder bed ( Se sphere = −0.019 < Se polyhedron = −0.008), while the WC shape has less effect on the packing density and uniformity. The density difference of the WC and 316 L powders leads to the difference in energy and force, resulting in different motion and segregation behaviors in the composite powder bed. For WC/316 L composite powder with a fixed composition, the condition of V = 0.025 m/s, WC/316 L size ratio = 25 μm/45 μm, roller spreader, and spherical WC can realize the proper composition gradient along the spreading direction in the composite powder bed. • The spreading of binary WC/316L composite powders was numerically investigated. • The parametric studies on segregation behavior were systematically conducted. • The underlying mechanisms were identified from microscopic scale. • Composition control of composite materials was realized by regulating segregation.

The effect of composition segregation of mold powder produced by spray granulation on the sintering performance

The composition uniformity of mold fluxes particles produced by spray granulation has an essential influence on its properties. Therefore, in the paper, the distribution of composition in mold fluxes during the heating process and the corresponding effect on the sintering properties of mold flux was comprehensively considered through TG-DSC, XPS, XRF, XRD, ICP, Infrared absorption, SEM-EDS, and sintering measurement. The results showed that: The difference in outward diffusion ability of different raw materials in the granulation process led to the enrichment of Na and C on the surface of mold flux particles. The partial aggregation of Na induced the centralized generation of sintering phase cuspidine (Ca4Si2O7F2) to inhibit the sintering of mold flux, and the sintering of mold flux can be hindered by increasing the particle size of mold flux to adjust the uniformity of mold flux composition.

Influence of the Synthesis and Crystallization Processes on the Cation Distribution in a Series of Multivariate Rare-Earth Metal-Organic Frameworks and Their Magnetic Characterization.

The incorporation of multiple metal atoms in multivariate metal–organic frameworks is typically carried out through a one-pot synthesis procedure that involves the simultaneous reaction of the selected elements with the organic linkers. In order to attain control over the distribution of the elements and to be able to produce materials with controllable metal combinations, it is required to understand the synthetic and crystallization processes. In this work, we have completed a study with the RPF-4 MOF family, which is made of various rare-earth elements, to investigate and determine how the different initial combinations of metal cations result in different atomic distributions in the obtained materials. Thus, we have found that for equimolar combinations involving lanthanum and another rare-earth element, such as ytterbium, gadolinium, or dysprosium, a compositional segregation takes place in the products, resulting in crystals with different compositions. On the contrary, binary combinations of ytterbium, gadolinium, erbium, and dysprosium result in homogeneous distributions. This dissimilar behavior is ascribed to differences in the crystallization pathways through which the MOF is formed. Along with the synthetic and crystallization study and considering the structural features of this MOF family, we also disclose here a comprehensive characterization of the magnetic properties of the compounds and the heat capacity behavior under different external magnetic fields.

Effect of Heat Treatment on Microstructure and Properties of GH3536 Fabricated by Selective Laser Melting

Selective laser melting (SLM) forming technology to prepare nickel-based superalloy parts can significantly save costs and solve bottleneck problems. The extremely high-temperature gradient and large residual stress during SLM lead to structural defects and compositional segregation. The parts formed by SLM urgently need heat treatment to control the microstructure composition and improve mechanical properties. Results showed that the heat treatment did not significantly change the microcracks and pores in the SLM sample, but the carbides in the grain boundary gradually changed from a granular distribution to a continuous strip distribution. After heat treatment, the elongation increased significantly, but the yield strength decreased. The tensile fracture of the SLM samples changed from a transgranular fracture to a ductile fracture, and obvious plastic deformation occurred, confirming that heat treatment can improve the benefits of the SLM sample.

Formation Mechanism and Control of Solidification Cracking in Laser-Welded Joints of Steel/Copper Dissimilar Metals

The solidification cracking behavior in laser welds of steel/copper dissimilar metals was systematically investigated. T2 copper and SUS304 stainless steel were used in the study. The results showed that the occurrence of solidification cracking in welds was the synergistic effect of ε phase liquation, inclusions and composition segregation. During the welding process, the liquation of grain boundaries substantially reduced the cohesion between adjacent grains, as well as the resistance for intergranular crack propagation. The composition segregation inside the grains could induce lattice distortion, thus reducing the plastic deformation capacity of the material itself and concurrently increasing the susceptibility to cracks. In addition, an effective solution for inhibiting solidification cracking was proposed by using an oscillating laser, and the inhibition mechanism was further discussed. Laser oscillating welding significantly promoted grain refinement, solute diffusion and the formation of uniformly distributed ε-Cu precipitated phases in welds. It can improve the intergranular bonding, reduce the susceptibility to solidification cracking and increase the resistance to plastic deformation. The tensile strength of joints using laser oscillating welding is 251 MPa, 35.7% more than 185 MPa using laser welding. Meanwhile, the strain of joints using laser oscillating welding is 3.69, a 96% increase compared to 1.88 using laser welding.

Homogenizing the composition of in-situ fabricated Ti2AlNb-based alloy via manipulating the droplet transfer mode of twin-wire arc additive manufacturing

The twin-wire arc additive manufacturing (TWAAM) technology is an effective method for the in-situ fabrication of Ti2AlNb-based alloys relative to conventional cast and wrought routes. However, composition segregation is still challenging in the field of in-situ TWAAM. To homogenize the chemical composition, here we proposed a novel single droplet pre-alloying transfer mode of TWAAM based on the original droplet transfer mode of double wires directly fed into the molten pool. The designed nominal composition of Ti-25Al-23Nb was set as the target alloy. The as-printed parts were investigated systematically. The results showed that this unique droplet transfer mode could not only improve the forming accuracy but also effectively mitigate the composition segregation. In addition, the inconsistency of microstructure caused by the composition segregation was successfully solved by this mode and appears a fully lamellar microstructure. The improvement of composition uniformity played a positive role in enhancing the mechanical properties of the alloy, in which the tensile strength of room and high temperature were improved by 76.8% and 74.1% respectively compared with our previous research. This work is of great significance to alleviate the composition segregation of intermetallics and further promote the in-situ TWAAM fabrication of Ti2AlNb-based alloys.

The Microstructure Prediction during the Welding Process of Fe–C–Si Steel Using Physical and Numerical Simulation: A Comparative Study

Overall, medium‐carbon, multiphase steels subjected to welding processes can be described as difficult and require dedicated process conditions. In these investigations, an attempt is made to characterize the welding process of commercial 55Si7 steel with a high carbon equivalent (CE = 0.73). An experimental tungsten inert gas (TIG) welding process is performed by the use of preheating. Then, to understand and explain the processes during welding, a numerical (finite element method [FEM]) and physical (dilatometry) simulation is carried out. Measurement points located in the heat‐affected zone (HAZ) are selected based on the obtained data of thermal cycles. Significant differences in the results obtained with the three approaches are assessed and indicated. The cause of the discrepancies in the test results is also identified. The essence of the implementation of material data, which precisely defines the kinetics of phase transformation, is confirmed. A detailed analysis of the contribution of the phases is performed, together with their morphological assessment, as well as the hardness distributions are conscientiously compared. Moreover, it seems that the segregation of the chemical composition of austenite and the initial microstructure plays a crucial role in terms of the quality of the prediction.

Tailorable fragile-to-strong kinetics features of metal oxides nanocomposite phase-change antimony films

A fragile-to-strong (F-S) kinetics feature is desirable in phase-change supercooled liquids, for decoupling the contradictory relation between good thermal stability nearby glass transition temperature and fast crystallization speed around melting temperature. We designed three metal oxides (MOs), i.e., the fragile TiO2, the moderate ZnO, and the strong GeO2, doping into a very fragile Sb matrix, which is an ideal elementary phase-change material (PCM) for overcoming the composition segregation in fatigued device, to tailor the F-S transition behavior in their supercooled liquids. Depending on the crystallization kinetics analyses of three representative films, which all exhibit the similar thermal stability with crystallization temperature of 200 ± 6 ºC, crystallization activation energy of 2.83 ± 0.09 eV, and the temperature for ten years data retention of 101 ± 7 ºC, we found no F-S behavior in the Sb62.1(TiO2)37.9 film, but weak and strong F-S behavior in Sb82.7(ZnO)17.3 and Sb73.6(GeO2)26.4 film, respectively. It shows that, the Sb based nanocomposite films with different MOs that have various fragilities can bring tailorable F-S kinetics features. Nevertheless, although the metal elementaries of Ti, Zn, Ge, have been confirmed could not bond with the Sb atoms, the Sb2O3 phase are detected more or less in Sb62.1(TiO2)37.9 and Sb82.7(ZnO)17.3 but not found in Sb73.6(GeO2)26.4 film. It is thus believed the formation of Sb2O3 would impede the F-S behavior in the supercooled liquids. The Sb73.6(GeO2)26.4 film that has large F-S transition magnitude of 2.3 and high F-S transition temperature of 536 K, is suggested to be a candidate for potential phase-change memory application. Such distinct F-S behavior in nano-size confined Sb73.6(GeO2)26.4 matrix, which consists of phase-changeable Sb and non-phase-changeable GeO2, stems from the competition between short-range ordering cluster that with abundant Peierls-like distortions in fragile Sb phase and medium-range ordering cluster that with energetical favorable networks in strong GeO2 phase.

Segregation Neutralised Steels: Microstructural Banding Elimination from Dual-Phase Steel Through Alloy Design Accounting for Inherent Segregation

Banding in commercial dual-phase steels, such as banded ferrite and pearlite or ferrite and martensite microstructures, is inherited from segregation during solidification in continuously cast material, predominantly from Mn segregation, and subsequent rolling. The banded microstructures lead to anisotropic mechanical properties which is generally undesirable. This paper presents an alloy design approach (termed “segregation neutralised” steels) to remove banding of the second phase by utilising co-segregation of both austenite and ferrite stabilisers to reduce local variability in second phase stability. The new composition proposed also considers achieving the same strength levels through maintaining the same second phase fraction, grain size and solid solution strengthening increments. Phase field modelling has been used to predict the segregation and phase transformation behaviours for a commercial composition dual-phase steel and the new composition segregation neutralised steel. A 5 kg laboratory alloy production route (casting, hot rolling and coiling simulation, cold rolling and annealing) has shown that the banded structure seen in commercial dual-phase steels is accurately reproduced and that banding has been reduced dramatically in both the hot rolled condition as well as after cold rolling and annealing in the new segregation neutralised steel. Chemical analysis has shown that in the segregation neutralised alloy the second phase distribution shows no correlation to the segregation bands, due to the achieved balance in austenite and ferrite stabilisers.

Failure analysis on weld cracking of a centrifugal fan blade in a nuclear power plant

Abstract Abnormal sound occurred during a centrifugal fan startup in a nuclear power plant and the root welds of two blades of the fan were cracked during the maintenance. In this paper, the failure blade and weld were comprehensively analyzed by means of morphology, composition, hardness, metallography and microstructure. It was found that the T-shaped weld at the blade root was convex and poor forming. The overall welding quality of the weld was poor, and there were the defects of arc pits, slag inclusion holes, composition segregation in the weld. The blade was subjected to typical fatigue stress due to fan vibration during operation. Due to poor forming and poor quality of the weld, the stress concentration factor at the weld toe of the fillet weld was large and the fatigue resistance was poor. Finally, blade fatigue cracking occurred during long-term operation.

Bismuth Stabilizes the α-Phase of Formamidinium Lead Iodide Perovskite Single Crystals

α-FAPbI3 (FA = formamidinium) perovskite offers an optimal bandgap for single-junction solar cells but converts into a more thermodynamically stable photoinactive δ-polymorph at room temperature. FA- or I-site substitutional alloying stabilizes α-FAPbI3; however, it leads to compositional segregation in operational devices. Here, we stabilize α-FAPbI3 single crystals through Pb-site doping with a heterovalent metal–bismuth (Bi). We show that undoped α-FAPbI3 has an α- to δ-phase half-life transition of <0.15 h, while the optimum concentration of Bi extends it by 4 orders of magnitude. Differential scanning calorimetry (DSC) reveals that Bi has effectively decreased the δ- to α-phase onset transition temperature. Density functional theory (DFT) calculations suggest a relatively clean gap, supporting previous findings on the improved photovoltaic performance of Bi-doped α-FAPbI3-based solar cells.

Elimination of Composition Segregation in 33Al-45Cu-22Fe (at.%) Powder by Two-Stage High-Energy Mechanical Alloying.

In the present work, complex powder alloys containing spinel as a minor phase were produced by mechanical alloying in a high-energy planetary ball mill from a 33Al–45Cu–22Fe (at.%) powder blend. These alloys show characteristics suitable for the synthesis of promising catalysts. The alloying was conducted in two stages: at the first stage, a Cu+Fe powder mixture was ball-milled for 90 min; at the second stage, Al was added, and the milling process was continued for another 24 min. The main products of mechanical alloying formed at each stage were studied using X-ray diffraction phase analysis, Mössbauer spectroscopy, transmission electron microscopy, and energy-dispersive spectroscopy. At the end of the first stage, crystalline iron was not found. The main product of the first stage was a metastable Cu(Fe) solid solution with a face-centered cubic structure. At the second stage, the Cu(Fe) solid solution transformed to Cu(Al), several Fe-containing amorphous phases, and a spinel phase. The products of the two-stage process were different from those of the single-stage mechanical alloying of the ternary elemental powder mixture; the formation of undesirable intermediate phases was avoided, which ensured excellent composition uniformity. A sequence of solid-state reactions occurring during mechanical alloying was proposed. Mesopores and a spinel phase were the features of the two-stage milled material (both are desirable for the target catalyst).

Microstructure characterization and mechanical behaviour of laser additive manufactured ultrahigh-strength M54 steel

Ultra high-strength M54 steel blocks were fabricated by laser metal deposition. The microstructure and mechanical behavior of the material were investigated systematically. The microstructure of the as-deposited M54 steel is anisotropic; the cross-section (XOY plane) has a cellular structure, whereas the longitudinal section (XOZ and YOZ planes) shows a mixture of alternating cellular and columnar forms. Compositional segregation is present at the cell walls (interdendritic regions) in the as-deposited state, resulting in retained austenite at the cell walls. The cross-sectional XOY plane contains 10.08% austenite, whereas the XOZ and YOZ planes contain 24.59% and 22.4% austenite, respectively. The retained austenite at the cell wall (interdendritic region) has low thermal and mechanical stability and disappears after the cryogenic treatment or is transformed into martensite during a tensile test. The as-deposited samples show anisotropic mechanical properties. The transverse samples exhibit stronger transformation-induced plasticity (TRIP) and work hardenability with a lower yield strength of 662 MPa and higher ultimate strength of 1982 MPa, corresponding to a higher amount of retained austenite in this direction. The longitudinal ultimate strength and yield strength are 1832 MPa and 997 MPa, respectively. The ductility and toughness are also largely anisotropic, and their reduction in the transverse direction is only 1/3 of that in the longitudinal direction. The Vickers hardness of the microstructure increases slightly from the bottom to the middle and upper part of the sample due to less thermal cycling in the upper part.