Two-phase Structure Research Articles

Achieving excellent strength and ductility synergy by δ phase strengthening in low-density high-manganese steel

ABSTRACT In this study, we developed a novel lightweight Fe-Mn-Al dual-phase with high strength, excellent ductility, and considerable toughness. The study involved a thorough exploration of heat treatment processes and mechanical behaviour. The microstructure of the Fe-Mn-Al dual-phase steel consists of austenite and δ-ferrite. In the early stages of plastic deformation, austenite undergoes primary deformation, leading to a faster increase in dislocation density and microhardness. In later stages, strain is transferred from austenite to δ-ferrite through high-density dislocation walls, resulting in coordinated deformation of the two-phase structure. The excellent strength-ductility combination of Fe-Mn-Al dual-phase steel is attributed to multiple stages of continuous work hardening. Initially, dislocation nodes in δ-ferrite contribute to work hardening. Subsequently, high-density dislocation structures and the strengthening effect of hard δ-ferrite enhance work hardening. Finally, deformation twins in austenite, along with the TWIP effect, further increase work hardening, emphasizing the importance of these interactions in improving mechanical performance.

Microstructure regulation and internal friction behavior of Fe45Mn20Cr15Co20 duplex high-entropy damping alloy

Damping alloys not only have the physical property of converting mechanical vibration energy into heat energy and dissipating it but also have good mechanical properties, corrosion resistance, structural stability, and so on. In this paper, a Fe45Mn20Cr15Co20 high-entropy alloy was prepared by vacuum arc melting, and the effects of annealing temperature on the evolution of its structure and internal friction behavior were investigated in depth. The results show that the annealing treatment improves the alloy’s damping performance significantly. The best damping performance was obtained after annealing at 1000 °C: Q−1max= 0.0729, an increase of 22.5% compared with the as-cast state. Combining mechanical and damping properties, the alloy is annealed at 1000 °C to obtain the best combined strength-damping properties. Thus, the design requirement of structural-functional integration is achieved. In addition, the alloy still maintains the two-phase structure of γ austenite and ε martensite after annealing treatment, and the content of ε martensite phase increases gradually with the increase of annealing temperature. The alloys formed a regular arrangement of chevrons and annealed twins when annealed at 1000 °C. The magnetic properties of the alloys gradually increase with the annealing temperature. The tensile strength of the annealed alloy tends to increase and then decrease with the increase of annealing temperature and reaches a peak (Rm = 780.8 MPa) at 1000 °C. This paper provides a reference of experimental data for realizing structural-functional integration of high entropy damping alloys.

Molecular dynamics simulation of the heterostructure of the CoCrFeMnNi high entropy alloy under an impact load

There have been numerous experimental studies conducted on the CoCrFeMnNi high entropy alloys (HEAs) at the macroscopic level. However, it is challenging to quantitatively analyze the shock behavior of the HEAs from a microscopic level through experiments. In this study, we construct single-crystal, twin-crystal, multilayer, hole, and two-phase structures of the CoCrFeMnNi HEAs using the molecular dynamics method. The effects of impact loading on the microscopic level are investigated for CoCrFeMnNi HEAs with different structures. By analyzing the evolution of their microstructure and the changes in physical parameters, the response laws and propagation characteristics of shock waves in various heterogeneous of CoCrFeMnNi HEAs are revealed at the atomic scale.

Mechanism of adding carbonyl iron powder to enhance the magnetic properties of Fe-Si-B-Cu-Nb nanocrystalline soft magnetic composites

In this study, Fe-Si-B-Cu-Nb powders were transformed from amorphous structure to nanocrystalline/amorphous two-phase structure by heat treatment, then mixed with different ratios of carbonyl iron powder (CIP) and pressed into composite cores. The effect of the ratio of CIP on the magnetic properties of Fe-Si-B-Cu-Nb/CIP composite core was studied, and the mechanism of the effect was explained. It was found that as the ratio of CIP increased, the structure of the composites changed from "single-particle" to "two-particle" and then back to "single-particle" structure, and the magnetic properties of the composites also changed with the structural change. When the ratio of CIP was 30 wt%, the composites had the highest relative density of 84.12%, the highest permeability of 35.3, the lowest coercivity of 23.34 A/m and the lowest core loss 281.87 mW/cm3 (Bm=50 mT, frequency=100 kHz). Thus, it was confirmed that adding appropriate amount of CIP as gap-filling particles to Fe-Si-B-Cu-Nb nanocrystalline powders can improve the magnetic properties of the composites.

Development of Co–Ni–Al–V–Ta–Cr-based superalloys with high-temperature strength and excellent oxidation resistance

In this study, the effects of V on the microstructure, lattice misfit, solvus temperatures, and oxidation behaviors of the Co–Ni–Al–V–Ta-based superalloys are investigated. The results show that all these alloys exhibit the γ’/γ two-phase structures. More importantly, the γ’ solvus temperature increases, and the oxidation resistance is significantly improved with the reduction of V concentration. Subsequently, the Ti (2 at.%) was added into Co–40Ni–10Al–4Ta–1V–12Cr alloy to further raise the properties of the superalloy. It is found that the Co–40Ni–10Al–4Ta–1V–12Cr–2Ti alloy exhibits a higher solvus temperature reaching 1200 °C of the γ′ phase, a low density of 8.48 g/cm3, a low lattice misfit of only +0.37 %, as well as exceptional oxidation resistance and mechanical properties.

Determining the technological parameters of electron-beam welding of high-strength titanium alloys

It is usually quite difficult to carry out deep penetration of thick-walled products from titanium alloys using conventional welding technologies. In this study, it was proposed to use electron beam welding under high vacuum conditions for the realization of 40 mm thick melting of VT23, VT3-1 alloys. This paper considers the possibility of obtaining high-quality welded joints from high-strength titanium alloys having (a+β) two-phase structures. For the implementation of research works, samples were made from selected materials, samples were welded according to the specified modes, metallographic analysis was performed, and the level of mechanical properties was determined. The research results were verified under laboratory conditions. The technological features of the processes of electron-beam welding of products with a thickness of 40 mm were considered; the parameters affecting the weldability of titanium alloys and their structure were determined. The welded samples were checked by X-ray non-destructive testing, the microstructure of the welds was studied, and the physical and mechanical properties of the welded joints were checked. It was established that a feature of titanium alloys VT3-1, VT23 is the need for heat treatment after welding under the base metal regimes to improve the characteristics of the welded joint. The resulting strength limit of the alloys after heat treatment reached values of 1250 MPa and more, while the impact toughness was at the level of 48–50 J·cm-2. Modeling the welding process has made it possible to ensure the reproducibility of the characteristics of the welded joint at a level close to that of the base metal, to increase the quality indicators of welded joints, and to reduce the time required to test the technology. The studies of simulator samples showed compliance of the quality of welded joints with the predefined parameters.

Influence of eutectoid structures on the mechanical properties and oxidation behavior of Hf-27%Ta and Hf-42%Ta alloys

Hf–Ta alloys have recently received much attention due to their favorable oxidation resistance and mechanical properties at temperatures beyond 1200 °C. However, at lower temperatures the oxidation resistance becomes poor and the alloys experience brittle to ductile transition. Both effects are likely due to their two-phase structure formed during eutectoid transformation. In the present work, we report mechanical properties and oxidation behavior of Hf-27%Ta and Hf-42%Ta (compositions are in atomic percent), which experience eutectoid transformation at ∼1083 °C and contain two phases, Hf-rich HCP and Ta-rich BCC, below this temperature. Alloys of the compositions corresponding to these two eutectoid phases, Hf-90%Ta (BCC) and Hf-3%Ta (HCP), were also produced and studied for comparison. The results show that in the temperature range of 25–1000 °C (i.e., below the eutectoid transformation) the compression strengths of Hf-90%Ta and Hf-3%Ta are considerably smaller than the strength of Hf-27%Ta or Hf-42%Ta, while above the eutectoid transformation, where the Hf-27%Ta and Hf-42%Ta alloys have single-phase BCC structures, the Hf-90%Ta and Hf-3%Ta become stronger than either Hf-27%Ta or Hf-42%Ta. It was concluded that the interfaces in the eutectoid structure of Hf-27%Ta produce considerable strengthening, which can exceed the strengths of the phases which comprise the microstructure. At 800 °C and 1000 °C, Hf-90%Ta oxidized rapidly in comparison to Hf-3%Ta due to the extensive formation of Ta2O5. It is suggested that the observed poor oxidation behavior of Hf-27%Ta below the temperature of the eutectoid transformation is likely controlled by the oxidation kinetics of the Hf-90%Ta BCC phase in the eutectoid structure of this alloy.

Multivalent manganese-based composite materials for sodium energy storage in ether electrolyte

The structural collapse of manganese-based materials during cycling greatly restricts its development in Sodium-ion batteries (SIBs). Hence, two-phase structures with different manganese valence states are designed based on an alternating voltage approach to mitigate volume expansion via two-phase coordination. Here distinct manganese valence states of precursor are driven via atmosphere. And in the following heat-induced phase transition, the elemental valence state is the decisive key for the structural and compositional evolution. In a batch of synthetic materials, MnO/MnS with bivalent states of manganese performs best as expected. Furthermore, NaPF6 in DEGDME is taken to replace the traditional carbonate electrolyte which is prone to produce unstable SEI film and leading to polarization. A series of characterization methods confirm that the energy barrier of charge transfer at the interface between electrolyte and electrode is the main factor determining the electrochemical characteristics of the interface and thus the energy storage performance. Ultimately, carbon dots (CDS) with rich functional groups are introduced in an effort to further promote the conductivity of MnO/MnS, and the in-situ generated C-S-Mn bond enhances electrochemical kinetic behaviors. This work provides an approach for designing manganese-based materials and the selection of electrolytes in sodium energy storage.

In-situ phase separation constructing robust hydrophobic ionogels with multifunction

Ionogels, as the promising alternative to hydrogels, have garnered great attention due to their excellent ionic conductivity, thermal stability, and non-volatility. However, the design and construction of robust hydrophobic ionogel still is a significant challenge at present, especially with a facile method. Herein, inspired by the two-phase structure of the dermis in human skin (stiff collagen fiber scaffold embedded in soft elastin matrix), a one-pot in-situ phase separation design strategy is proposed to construct the robust hydrophobic ionogel through the random copolymerization of hydrophobic monomers with the solubility difference in hydrophobic ionic liquid (IL). The resultant ionogel forms a homogeneously dual phase network containing solvent-rich soft domains and solvent-poor hard domains. The mechanical robustness can be synergistically achieved by the hard domain strong skeleton and the soft domain elastic matrix. Benefiting from multiple reversible interactions including hydrogen bonding, ion–dipole and ion-ion interactions, and physical entanglement, the ionogel shows strong IL retention, excellent self-healing, and regenerating abilities, while can adhere to different substrates. The unique phase separation structure brings temperature-dependent transparency and high-damping features. The ionogel also presents high ionic conductivity due to the abundance of movable ions. As application demonstrations, the ionogel is successfully used for dynamic pattern encryption, shock attenuation, and strain sensing. What’s more, this strategy can be also applied to improve the mechanical properties of hydrophobic ionogels based on other monomers. Hence, this work establishes an effective and general design strategy for hydrophobic ionogels demanding versatility.

A new approach to predict microhardness of two-phase in cutting S32760 duplex stainless steel

The uneven distribution of microhardness in the two-phase structure of S32760 duplex stainless steel after cutting is attributed to variations in the crystal structure, which significantly impact the material's performance. This paper presents a new approach to predict the microhardness of two-phase based on the flow stresses in the austenitic and ferrite. The effect of strain, strain rate, and temperature on the flow stress in the shear plane of orthogonal cutting S32760 was analyzed, and the prediction model for microhardness of two-phase considering the two-phase flow stress was established to obtain a mapping relationship between the two-phase flow stress and the two-phase microhardness of S32760. The impact of cutting dosages on shear strain, strain rate, and temperature in the shear plane was investigated. A function relationship between cutting dosages and microhardness of austenite and ferrite in the shear plane was established, two-phase microhardness experiments were conducted, and the model's accuracy was validated with a prediction error of less than 6%. This study provided insights into the impact of strain, strain rate, and temperature in the shear plane on the microhardness of the two-phase, thus contributing to the theoretical foundation of processing techniques in duplex stainless steel.

An inspection into the unexpected gamma precipitation in supersaturated U-13 at.% Nb subjected to aging

Phase separation of the U-Nb (13 at.%) alloy at an aging temperature of 723 K was investigated through transmission electron microscopy. Chemical redistribution initiated from a supersaturated state was identified after aging for half an hour before the discontinuous precipitation (DP) process. Such redistribution was accompanied by a fine two-phase structure and non-lamellar morphology. Among the decomposition products at this early stage, a lath-shaped Nb-rich phase was confirmed to be the bcc (γ) phase, with Nb content in the range of 35∼45 at.%. Meanwhile, discrete regions of much smaller size were detected to bare an Nb content approaching the equilibrium of the α phase. It is envisaged that the γ precipitation was a preceding event of the α. Thermodynamic estimation was carried out to reveal the decomposition mechanism responsible for this unexpected behavior. The results were in favor of a nucleation and growth mechanism over the spinodal decomposition. These results provide a deeper understanding and a new sight into the aging phenomenon of U-13Nb alloys, which are also worth consideration in other binary systems slightly outside the coherent spinodal regions such as Fe-20Cr.

Two-phase flow structures in a helically coiled microchannel: An experimental investigation

At the microfluidic scale, the utilization of helically coiled channels (HCCs), also known as a spiral channel, for two-phase flow offers numerous advantages in various applications. Existing articles mainly focus on the macro-scale transport, examining secondary flows induced in curved channels. The increasing demand, however, for innovative miniature equipment for thermal energy management emphasizes the importance of comprehending gas–liquid micro-scale flow in curved channels. Unfortunately, despite a vast body of literature on this paradigm, there is still a lack of systematic investigations into the underlying facets of two-phase micro-scale transport in HCCs. To address this gap, our study conducted experiments on adiabatic two-phase air–water flow inside an up-flow helical micro-scale tube. The tube had a hydraulic diameter of 0.87 mm, a coil diameter of 50 mm, and a helical pitch of 20 mm. The primary aim was to explore the impact of centrifugal force on flow pattern, void fraction, and frictional pressure drop characteristics. Additionally, we carefully examined the phase separation phenomenon influenced by the secondary flows induced by the curved channel. In particular, we compared the gas-core flow pattern (either throat-annular flow or annular flow), void fraction, and frictional pressure drop obtained from our experiments on the helical tube with corresponding results based on straight micro-scale channel configurations for an Eötvös number of approximately 0.01. In summary, this study delves deep into the crucial aspects of two-phase micro-scale transport in HCCs, contributing to a better understanding of these systems for future advancements in micro-channel applications.

Extraordinary role of Nd3+ doping on high temperature stable multiferroic and magneto electric properties of Bi1-xNdxFeO3-Bi2Fe4O9/Sr1-xCaxTiO3 dual phase ceramic composite

The Bi1-xNdxFeO3-Bi2Fe4O9/Sr1-xCaxTiO3 (BNFO-SCT) ceramic material displays multiferroic properties after being synthesized via using both the traditional solid-state method and the mechanical alloying technique. Nd3+ was added to the BNFO-SCT as a dopant, and the effects on the microstructure, dielectric characteristics, ferroelectric properties, and piezoelectric properties were examined using a variety of analytical techniques. With addition of Nd3+ dopants in BNFO-SCT, a two-phase structure that changed from rhombohedral perovskite (R3c) to triclinic (P1) and a significant increase in dielectric properties were observed. The remnant polarization, coercive electric field, and magnetocapacitance coefficient showed notable enhancement with increasing Nd3+ doping (x) up to 0.6, thereafter, all these properties began to decrease. Moreover, the leakage current of the BNFO-SCT material decreased with higher Nd3+ content. Interestingly, the dielectric properties at room temperature exhibited a substantial increase when subjected to an applied magnetic field.

Atomic-Level Description of Chemical, Topological, and Surface Morphology Aspects of Oxide Film Grown on Polycrystalline Aluminum during Thermal Oxidation—Reactive Molecular Dynamics Simulations

Oxidation results in the formation of an oxide film whose properties and structure can be tailored by controlling the oxidation conditions. Reactive molecular dynamics simulations were performed to study thermal oxidation of polycrystalline Al substrates as a function of O2 density and temperature. The structural, chemical, and topological aspects of polycrystalline Al (poly-Al) substrates and oxide films formed upon oxidation were studied. The studies were supported by surface topography and morphology analyses before and after oxidation. An analysis of Al–O atomic pair distribution showed the development of long-range order in the oxide films grown upon exposure to low-density (0.005 g/cm3) and high-density (0.05 g/cm3) O2 gas. The long-range order was more apparent for the high-density environment, as the oxide films formed in low-density O2 gas did not fully cover the poly-Al surface. The dominance of over-coordinated polyhedral units in a tightly packed structure was indicative of medium- and long-range atomic order in the oxide films. The two-phase structure of the oxide was found in the films, with a crystalline phase at the metal/oxide interface and an amorphous phase at the oxide/O2 interface. The combination with topological analyses supported the conclusions of the chemical analysis and enabled us to capture an amorphous-to-crystalline phase transformation in the oxide films with increasing oxygen density and temperature. An important effect of Al surface roughness before oxidation on the behavior of the metal/oxide interface and on the oxide film structure was observed.

Visualization of local two-phase flow structure in a packed bed of spheres

A two-phase flow structure in porous media is essential to clarify the debris cooling characteristics during a severe accident in nuclear power plants. In previous experimental studies, a packed bed of spheres has been used instead of the porous debris bed. Many models to predict the flow and heat transfer characteristics in the packed bed have been proposed based on the experimental data in the sphere-packed bed. However, those models have not considered the local flow structure, which should be studied to enhance the prediction accuracy. In this study, the local flow structure of the air-water two-phase flow in the unit model of the sphere-packed bed was measured by neutron computed tomography. The distribution of the gas phase was reconstructed by acquired images. From these results, the breakup and coalescence of the bubbles in the packed bed were evaluated.

Tunable anti-plane wave bandgaps in 2D periodic hard-magnetic soft composites

In this paper, we present a theoretical model for the analysis of large magneto-deformation and the anti-plane shear wave bandgaps in 2D periodic two-phase hard magnetic soft composite structures subjected to magnetic stimuli. The constitutive behavior of the phases in the hard-magnetic soft composite is described using the incompressible Gent model. To solve the incremental anti-plane wave equations, the finite element method and the Floquet–Bloch theorem for periodic media are utilized. Using the developed framework, we numerically study the dependency of the bandgap width and their location on the direction and magnitude of the applied magnetic flux density vector, material parameter contrasts, and geometry and volume fraction of the inclusion phase. The numerical results reveal that significant tunability of the bandgap is achieved when the applied magnetic flux density direction is along the residual magnetic flux density direction. Also, it is seen that the geometry of the inclusion has significant effect on the bandgap width. The crucial inferences from the present investigation can find their potential use in the design and manufacturing of futuristic smart soft wave devices with tunable band structures.

Phase-Field Study of Exchange Coupling in Co-Pt Nonstandard Nanochessboards.

The Co-Pt binary system can form a two-phase nanochessboard structure comprising regularly aligned nanorods of magnetically hard tetragonal L10 phase and magnetically soft cubic L12 phase. This Co-Pt nanochessboard, being an exchange-coupled magnetic nanocomposite, exhibits a strong effect on magnetic domains and coercivity. While the ideal nanochessboard structure has tiles with equal edge lengths (a = b), the non-ideal or nonstandard nanochessboard structure has tiles with unequal edge lengths (a ≠ b). In this study, we employed phase-field modeling and computer simulation to systematically investigate the exchange coupling effect on magnetic properties in nonstandard nanochessboards. The simulations reveal that coercivity is dependent on the length scale, with magnetic hardening occurring below the critical exchange length, followed by magnetic softening above the critical exchange length, similar to the standard nanochessboards. Moreover, the presence of unequal edge lengths induces an anisotropic exchange coupling and shifts the coercivity peak with the length scale.

A novel machine learning approach for surface roughness quantification and optimization of cast-on-strap lead-antimony alloy via two-point correlation function

Surface roughness has a negative impact on the materials’ lifetime. It accelerates pitting corrosion, increases effective heat transfer, and increases the rate of effective charge loss. However, controlled surface roughness is desirable in many applications. The automotive lead-acid battery is very sensitive to such effects. In our case study, the cast-on-strap machine has the largest effect on the surface roughness of the lead-antimony alloy. In this regard, statistical correlation functions are commonly used as statistical morphological descriptors for heterogeneous correlation functions. Two-point correlation functions are fruitful tools to quantify the microstructure of two-phase material structures. Herein, we demonstrate the use of the two-point correlation function to quantify surface roughness and optimize lead-antimony poles and straps used in the lead-acid battery as a solution to reduce their electrochemical corrosion when used in highly corrosive media. However, we infer that this method can be used in surface roughness mapping in a wide range of applications, such as pipes submerged in seawater as well as laser cutting. The possibility of using information obtained from the two-point correlation function and applying the simulated annealing procedure to optimize the surface micro-irregularities is investigated. The results showed successful surface representation and optimization that agree with the initially proposed hypothesis.

Optimization of Machining Parameters for Enhanced Performance of Glass-Fibre-Reinforced Plastic (GFRP) Composites Using Design of Experiments

A high strength-to-weight ratio, stiffness, fatigue resistance, a low coefficient of thermal expansion, and tailorable properties make glass-fibre-reinforced plastic (GFRP) a popular choice for a wide range of applications, including aircraft structures, automobile chassis, and shipbuilding. However, milling GFRP composites is challenging because of their heterogeneous nature and two-phase structure, which lead to high cutting forces and delamination. A statistical experiment was carried out using the Taguchi design of experiments to investigate the effect of machining settings on GFRP composite performance metrics such as surface delamination, surface roughness, and material removal rate. The L27 orthogonal array was used for the experiment, and it served as the foundation for the choice of material, input variables, levels, and output response variables. The experiment’s outcomes were analysed using MINITAB software® 18 Version and the Analysis of Variance (ANOVA) method. Based on the signal-to-noise (S/N) ratio, the ideal conditions were selected, and confirmation studies were carried out to ensure their applicability. In order to identify the ideal circumstances for the manufacturing and machining parameters, the data were normalised to a range from zero to one. To overcome the difficulties involved in milling GFRP composites, a thorough investigation and optimisation of the manufacturing process factors and machining settings is essential.

Two-Scale Computational Analysis of Deformation and Fracture in an Al-Si Composite Material Fabricated by Electron Beam Wire-Feed Additive Manufacturing

Numerical simulation of deformation and fracture of an AlSi12% alloy additively fabricated by layer-by-layer electron beam melting of a wire is carried out. The microstructure of the alloy is studied by scanning and transmission electron microscopy at different resolutions. The experimental study at a length scale of several dozens of microns reveals a dendritic structure, which can be treated as a composite material consisting of aluminum arms separated by a eutectic network. The volume fraction of dendrites varies with the distance from the base plate in the build direction. The eutectics can also be thought of as a composite with an aluminum matrix reinforced by silicon particles at a scale of a few microns. Particles of different shapes are nearly equally spaced in the matrix. The eutectic and dendritic structures are taken into account explicitly in the calculations. The dynamic boundary-value problems are solved by ABAQUS/Explicit. The isotropic elastic-plastic and elastic models are used to simulate the response of aluminum and silicon. The fracture model includes a maximum distortion energy criterion formulated for the particle and matrix materials in terms of the equivalent stress and plastic strain. A two-scale approach is proposed to investigate deformation and fracture of the AlSi12% alloy. On the eutectic scale, the thermomechanical behavior of the Al matrix-silicon particle two-phase composite is simulated to obtain the homogenized properties of the eutectic composite material, which is then used at a higher scale to investigate the deformation and fracture of a two-phase dendritic structure. Residual stresses formed during cooling of the additively manufactured material were found to decrease the strength of the composite, while the strength increases with the volume fraction of dendrites.