Non-equilibrium Solid Solution Research Articles

Underwater wet welding of high-strength low-alloy steel using self-shielded flux-cored wire with highly exothermic Al/CuO mixture

Underwater wet welding processes with minimal energy consumption and high efficiency is the future goal for researchers in the field. Al/CuO exothermic mixtures in consumables for land welding process can improve welding process stability and decrease the electrical dependence of the welding process. To confirm the beneficial effects of Al/CuO thermite and gain a better understanding of metallurgical properties of the thermite welds in underwater environment, this study investigated underwater wet self-shielded flux-cored arc welding (FCAW-S) of high-strength low-alloy steel with highly exothermic Al/CuO mixture. The effects of Al/CuO thermite on arc stability, weld metallurgical behavior and melting characteristics of flux-cored wire were clarified. The ratio of chemical heat in the flux to the heat required to melt welding wire increased from 4.7% to 38.4% with the addition of Al/CuO, potentially accelerating the melting of flux-cored wires. The arc stability with thermite addition was superior to that without the addition of thermite. The presence of copper as a result of metallurgical reactions had a substantial impact on metallurgical behavior of underwater wet welds. The ferrite weld was a non-equilibrium Cu supersaturated solid solution, which increased the microhardness and tensile strength of wet welds. The measured cooling time proved that the incorporation of Al/CuO had the capacity to reduce cooling rate. The incorporation of 50 wt% thermite yielded a 7.8% increase in welding process efficiency compared to the base formulation. Current work provided an important direction for the development of a chemically self-heating flux-cored wire.

Micro-mechanisms of deformation and strengthening during high pressure torsion of CoCuFeMnNi high entropy alloy

The equiatomic FCC CoCuFeMnNi HEA subjected to high pressure torsion (HPT) till 5 turns, showed a fourfold increase in hardness and three orders of magnitude decrease in grain size with an insignificant change in strain rate sensitivity and activation volume. There is a gradual transition from planar slip at low strain to twinning followed by shear banding at high strain accompanied by conversion of low angle to high angle grain boundaries, resulting in average grain size of 55 ± 33 nm at the periphery of 5 turn HPT sample. Atom probe tomography revealed that nano-clusters present in FCC matrix in the homogenized sample underwent partial dissolution during HPT to increase the copper concentration from 8.24 ± 3.29 to 17.02 ± 1.51 atomic%, contributing to significant solid solution strengthening in addition to dislocation and grain size strengthening, leading to tremendous increase in hardness (539 to 1941 MPa). Transient instrumented micro and nano-indentation tests yield activation volume between 12 and 17 b3 for the homogenized and HPT processed samples with three orders of magnitude difference in grain size and dislocation density. This indicates that the dislocation–solute environment interaction is the rate controlling mechanism during the deformation of the investigated HEA. The present work provides a unique pathway to design high entropy alloys that can explore and utilize non-equilibrium solid solution strengthening as a major strengthening mechanism to achieve high strength.

A Molecular Dynamics Simulation to Shed Light on the Mechanical Alloying of an Al-Zr Alloy Induced by Severe Plastic Deformation

In a recent experimental work, as a result of severe plastic deformation, a non-equilibrium solid solution was obtained despite the very limited solubility of zirconium (Zr) in aluminum (Al). This opens up a new path in the development of heat-treatable alloys with improved electrical and mechanical properties, where mechanically dissolved elements can form intermetallic particles that contribute to precipitation strengthening. In the present study, molecular dynamics simulations were performed to better understand the process of mechanical dissolution of Zr within an Al model, with Zr atoms segregated along its grain boundaries. Stress–strain curves, radial distribution functions, and mechanisms of plastic deformation and dissolution of Zr in Al were analyzed. It is revealed that orientation of the grain boundary with segregation normal to the shear direction promotes more efficient mixing of alloy components compared to its parallel arrangement. This happens because in the second case, grain boundary sliding is the main deformation mechanism, and Zr tends to remain within the interfaces. In contrast, the involvement of dislocations in the case of normal orientation of grain boundaries with Zr segregation significantly contributes to deformation and facilitates better dissolution of Zr in the Al matrix. The findings obtained can provide new insights considering the role of texture during mechanical alloying of strongly dissimilar metals.

Effect of High-Energy Ball Milling Time on the Density and Mechanical Properties of W-7%Ni-3%Fe Alloy

The present work was aimed at the investigation of the effect of high-energy ball milling (HEBM) time on the sintering kinetics, structure, and properties of the heavy tungsten alloy (HTA) W-7%Ni-3%Fe. The HTA samples were obtained from nanopowders (20–80 nm) using conventional liquid-phase sintering (LPS) in hydrogen and using spark plasma sintering (SPS) in vacuum. The HTA density was shown to depend non-monotonously on the HEBM time that originates from the formation of nonequilibrium solid solutions in the W-Ni-Fe systems during HEBM. The SPS kinetics of the HTA nanopowders was shown to have a two-stage character, the intensity of which depends on the Coble diffusion creep rate and on the intensity of diffusion of the tungsten atoms in the crystal lattice of the γ-phase. The kinetics of sintering of the initial submicron powders has a single-stage character originating from the intensity of the grain boundary diffusion in the γ-phase. The dependencies of the hardness and of the yield strength on the grain sizes were found to obey the Hall–Petch relation. The hardness, strength, and dynamic strength in the compression tests of the fine-grained tungsten alloys obtained using SPS and LPS were studied.

Combustion synthesis of ultra-high-temperature solid solutions (ZrxNb1-x)B2. Part 1: The mechanisms of combustion and structure formation

The self-propagating high-temperature synthesis (SHS) in Zr–Nb–B system has been thoroughly investigated with a focus on its macrokinetics and phase formation mechanisms. The activation energies were determined by analyzing the relationship between the combustion rate and temperature. For compositions NbB2–40%ZrB2 and NbB2–50%ZrB2, the increase in the initial temperature has resulted in a significant change in combustion rate's dependence on temperature, signifying a possible shift in the reaction mechanisms. The quenched combustion front technique (QCF) method in conjunction with thermodynamic and ab initio calculations were used to analyze the sequence of phase formation events, including the formation of niobium and zirconium borides through gas transport reactions involving volatile boron suboxide BO in the heating zone, followed by the emergence of a zirconium-boron melt in the combustion zone and formation of main fraction of niobium and zirconium borides. Interactions between the primary niobium and zirconium borides and the melt result in the formation of solid solutions; however, at sub-optimal combustion conditions multiple non-equilibrium solid solutions are retained in the products. To address this issue, a machine learning model was developed, attaining coefficients of determination (R2) of 89% for combustion temperature and rate predictions, thus enabling the fine-tuning of macrokinetic parameters of the SHS process in the system.

Non-equilibrium BaxSr1-xSO4 solid solution compositions at elevated Sr2+ concentration, ionic strength, and temperature

The BaxSr1-xSO4 solid solution is ubiquitously present in both geological and industrial processes, where they mostly form under non-equilibrium conditions. Compared with those formed under equilibrium conditions, the BaxSr1-xSO4 solid solution formed at non-equilibrium condition has significantly higher Sr incorporation at the same aqueous phase compositions. The solid composition of BaxSr1-xSO4 formed at non-equilibrium condition is critical for the study of chemical palaeoceanography as well as the solid solution nucleation and growth kinetics. However, few studies have been conducted to investigate the composition of the BaxSr1-xSO4 solid solution when it precipitates at non-equilibrium conditions. In this study, the distribution coefficient of Ba2+ and Sr2+ between the BaxSr1-xSO4 solid solution and the aqueous phases (KD,Sr-Barite) at non-equilibrium conditions was studied with barite saturation index (SIbarite) from 0.9 to 1.5, [Sr2+]/[Ba2+] molality ratio from 0.33 to 30, temperature (T) from 50 to 90 °C and ionic strength (IS) from 0.01 M to 3 M as NaCl, with celestite being undersaturated. The composition of the BaxSr1-xSO4 solid solution formed at non-equilibrium conditions can then be calculated from the KD,Sr-Barite values. The results show that the KD,Sr-Barite value decreases with the increase of aqueous Sr2+ concentration at fixed SIbarite and T conditions. The IS effect on the KD,Sr-Barite value is small. Based on the experimental results, a new empirical model is developed to accurately predict the measured compositions of BaxSr1-xSO4 solid solution at non-equilibrium conditions under a wide T and IS conditions as follows (the plot of the predicted log10KD,Sr-Barite versus the measured log10KD,Sr-Barite with R2=0.9995):KD,Sr-Barite=-1.21-1.84SIbarite+1928.1SIbarite×T+aSr2+,aqaBa2+,aq-1Several theoretical models have also been compared against the experimental data. The birth and spread crystal growth model (B + S model) could accurately predict the solid composition of BaxSr1-xSO4 at higher barite SI and/or higher T conditions (barite SI = 1.5 at 70 °C and barite SI = 1.2–1.5 at 90 °C with [Sr2+]/[Ba2+] = 0.33–10). However, the B + S model predictions show larger deviations at lower SI and/or lower T conditions (barite SI = 0.9 and 1.2 at 50 °C and barite SI = 0.9 at 70 °C with [Sr2+]/[Ba2+] = 0.33–10 in this study). For other theoretical models, such as the CNT model and the BCF model, the predicted solid compositions of BaxSr1-xSO4 are significantly higher than the measured results. This quantitative study of the BaxSr1-xSO4 solid solution compositions could help reconstruct oceanic physical conditions and chemistry. It also establishes a solid foundation to further investigate the kinetics of the BaxSr1-xSO4 solid solution formation during non-equilibrium geological and industrial processes.

Microstructure and resistivity of Au-B supersaturated solid-solution films prepared by sputter deposition

• A series of Au–B films was deposited by magnetron sputtering. • Au-B supersaturated solid solutions without AuB 2 compounds confirmed in the films. • The microstructure of the Au–B supersaturated solid-solution films was studied. • The relationship between the resistivity and b content in the films was revealed. • The Au–B supersaturated solid-solution films were stable at room temperature. Au-B films are of interest for superconductor and hohlraums materials. A series of gold-boron (Au-B) films with different B contents has been deposited by magnetron sputtering of Au-B pseudo-alloy targets in an Ar atmosphere. The B atomic concentration of the Au-B films was varied by adjusting the B contents of the Au-B targets and sputtering pressure. The composition, microstructure and resistivity of the Au-B films were investigated by X-ray photoelectron spectrometry, X-ray diffraction and the four-probe method. Due to the mutually immiscibility of Au and B, non-equilibrium supersaturated solid solutions of B in Au with nanocrystalline microstructures instead of AuB 2 metal diboride were observed in the as-deposited films. With increasing B content, the average grain size of the films decreased and the d -spacing of Au (111) increased. The resistivity of the Au-B supersaturated solid-solution films was linearly related to the B content and no superconductivity was observed. The structure of the Au-B films was stable at room temperature for a long time, while annealing caused phase transition.

Electronic and Lattice Thermal Conductivity Switching by 3D−2D Crystal Structure Transition in Nonequilibrium (Pb1−xSnx)Se

AbstractDynamic control of thermal transport in solid materials is highly desired for thermal management technology. However, the development of a material exhibiting large modulation of thermal conductivity (κ) by external stimuli remains a major challenge. Here, the large κ modulation is reported by the reversible 3D to 2D crystal structure transition in a nonequilibrium solid solution of (Pb1−xSnx)Se, where Pb2+ stabilizes a 3D cubic structure while Sn2+ does a 2D layered structure. The phase boundary of these phases is induced in (Pb0.5Sn0.5)Se bulk polycrystals by thermally quenching the high‐temperature solid solution phase. Through the 3D–2D phase transition, the 1/2.9‐times decrease of lattice κ (κlat.) is achieved by strong phonon scattering in the 2D layered structure, and the electronic κ (κele.) is also decreased by 5 orders of magnitude due to the electronic phase transition from a 3D high conductivity state to a 2D semiconducting state. The total κ (=κlat. + κele.) modulation ratio κ3D phase/κ2D phase = 3.6 is attained at 373 K. The present strategy will lead to a novel concept for designing thermal management materials through crystal‐structure dimensionality switch using nonequilibrium phase boundaries.

Defect-free-induced Na+ disordering in electrode materials

The guest-ion disordered and quasi-zero-strain nonequilibrium solid–solution reaction mechanism provides an effective guarantee for realizing long-cycle life and high-rate capability electrode materials.

Fabrication and Characterization of Immiscible Fe-Cu Alloys using Electrical Explosion of Wire in Liquid

Iron and copper are practically immiscible in the equilibrium state, even though their atomic radii are similar. As non-equilibrium solid solutions, the metastable Fe-Cu alloys can be synthesized using special methods, such as rapid quenching, vapor deposition, sputtering, ion-beam mixing, and mechanical alloying. The complexity of these methods (multiple steps, low productivity, high cost, and non-eco-friendliness) is a hinderance for their industrial applications. Electrical explosion of wire (EEW) is a well-known and effective method for the synthesis of metallic and alloy nanoparticles, and fabrication using the EEW is a simple and economic process. Therefore, it can be potentially employed to circumvent this problem. In this work, we propose the synthesis of Fe-Cu nanoparticles using EEW in a suitable solution. The powder shape, size distribution, and alloying state are analyzed and discussed according to the conditions of the EEW.

Formation of non-equilibrium ductile solid solutions and textures in NbCr2 bulks produced by mechanical milling and spark plasma sintering

Bulk NbCr2 samples with ultra-fine grains have been fabricated by mechanical milling and subsequent spark plasma sintering. The room-temperature fracture toughness of these bulk samples achieves as high as 11.3 ± 0.5 MPa m1/2, which is remarkably improved compared to the as-cast monolithic NbCr2 Laves phase (1.2 MPa m1/2). The microstructural analysis indicates the presence of abundant non-equilibrium Nb and Cr solid solutions among the grain boundaries. The formation of Nb and Cr solid solutions is attributed to the applied pulsed electric current during spark plasma sintering, which can decrease the critical nucleation undercooling, accelerate the atomic diffusion and enhance the nucleation rate. Based on the microstructural characterization, the enhancement in fracture toughness is ascribed to formation of non-equilibrium Nb and Cr solid solutions toughening and grain boundary toughening mechanisms. Furthermore, strong texture components have been detected in these bulk samples. For the sample sintered with the powder milled for 30 h, the <100> direction of NbCr2 Laves phase is perpendicular to the compressive axis while the Nb and Cr solid solutions show random orientation. When sintered with the powder milled for 60 h and 90 h, the texture component in NbCr2 Laves phase transforms into <110> fiber texture, and the Cr solid solution also show strong <100> and <110> texture components perpendicular to the compressive axis. The evolution of texture component is discussed by considering the crystallization behaviour of the milled powders.

Synthesis of a binary alloy nanoparticle catalyst with an immiscible combination of Rh and Cu assisted by hydrogen spillover on a TiO2 support.

This work demonstrated the use of TiO2 as a promising platform for the synthesis of non-equilibrium RhCu binary alloy nanoparticles (NPs). These metals are regarded as immiscible based on their phase diagram but form NPs with the aid of the significant hydrogen spillover on TiO2 with concurrent proton–electron transfer. The resulting RhCu/TiO2 exhibited 2.6 times higher catalytic activity than Rh/TiO2 during hydrogen production from the hydrolysis of ammonia borane (AB), due to a synergistic effect. Theoretical simulations showed a higher energy value for the adsorption of AB on the RhCu alloy and a lower activation energy for the rate determining N–B bond dissociation by the attack of H2O during AB hydrolysis compared to monometallic Rh. High-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the formation of RhCu alloy NPs with a mean diameter of 2.0 nm on the TiO2. H2-temperature programmed reduction and in situ X-ray absorption fine structure analyses at elevated temperature under H2 demonstrated that Rh3+ and Cu2+ precursors were simultaneously reduced only on the TiO2 support. This effect resulted from the improved and limited reducibility of Cu2+ and Rh3+, respectively. The rate of hydrogen spillover of TiO2 is faster as compared to γ-Al2O3 and MgO as evidenced by sequential H2/D2 exchanges during in situ Fourier transform infrared analyses. Density functional theory calculations also showed that the migration of H atoms on TiO2 proceeds with a lower energy barrier than that on Al2O3, and the reduction of Cu2+ species is facilitated by H spillover on the support rather than by direct reduction by H2. These results confirm the vital role of TiO2 in the formation of the alloy and may represent a new strategy for the synthesis of different non-equilibrium solid solution alloys.

Особенности массопереноса при ионной имплантации

Исследованы продукты реакции, полученные при ионной имплантации азота, кислорода и фосфора в вольфрамовую мишень. Выяснены условия создания новых соединений при высокодозной имплантации, приводящей к процессам быстрого массопереноса ионов в материал мишени. Установлено, что ионная имплантация азота в вольфрам приводит к возникновению фазы внедрения W2N с ГЦК-структурой; имплантация кислорода — к образованию фазы с плотноупакованной ГЦК-структурой, близкой по структуре и параметрам к W20; фосфора — к образованию фосфида вольфрама W2P с орторомбической структурой после дополнительного отжига при температурах, превышающих 800-900 0С. Сделана оценка локального давления потока ионов на поверхность мишени, которое приводит к локальному искривлению кристаллической решетки, благодаря чему возможен быстрый массоперенос имплантированных ионов в материале мишени.&#x0D; Образование фаз внедрения при ионной имплантации является результатом локализации сдвиговой деформации в условиях ускоренного массопереноса. В условиях высокодозной ионной имплантации формируются высоконеравновесные пересыщенные твердые растворы, а также высокодефектные структурно-неравновесные состояния с высокой кривизной кристаллической решетки.

High power CW YAG laser weldability of dissimilar Ti to Ni metal plates through beam shift

Incompatibilities of thermo-physical-mechanical as well as metallurgic properties between dissimilar Ti and Ni metals present technical challenges to their fusion joining. In this manuscript, using a high power continuous wave (CW) YAG laser, a direct butt welding of Ti and Ni plates was performed with a relatively large beam offset from the contact interface between two base metals (BMs) toward either side of the BMs. As a result, a large amount of rapidly solidified non-equilibrium solid solution occurred in the main fusion zone (FZ); while the total amount of prior intermetallic compounds (IMCs) such as Ti2Ni, Ni3Ti and NiTi significantly decreased, most of which were primarily located as a band (less than 200 μm wide) along one of the fusion interfaces with the BMs. When the laser beam was shifted 0.5 mm toward the Ni side, the toughness of the FZ was sufficiently improved, leading to a crack-free joint. In this study, the microstructures, chemical compositions, and microhardness in the laser-welds thus obtained were examined to learn the influences of beam offset on laser weldability of Ti to Ni metals.

The Ni-rich nanostructured solid solutions of Ni1−xMx (M = Cd, Co and Cu)

The nanostructured solid solutions (NSSes) of Ni–Cd (Cu, Co) with the FCC lattice were obtained by a simple chemical method. The NSSes consist of nanocrystallites (10–25 nm) which form the primary aggregates (40–80 nm) composing the secondary aggregates (100–500 nm). The Ni–Cd and Ni–Cu nanocrystallites were shown to be anisometric, which is unusual for the FCC lattice. The Ni–Cd aggregates contain the structural units consisted of the stratification of the nano-thick crystallites by the (111) and (200) faces with a wide surface in common and spatial orientation. The spatial heterogeneity of compounds for the aggregates of Ni–Cd and Ni–Cu can be explained because of the differences in the nanocrystal chemical compositions. The main reason of it is in the differences in the redox potentials of the components of the bimetallic NSSes obtained by chemical reactions. The surface of the primary aggregates contains H2O, O2, CO2 and, probably, hydroxide and oxide as nano-islets in a layer whose thickness is approximately 1–3 nm. These inclusions can be desorbed or decomposed at 100 °С–140 °С, 120 °С–220 °С and 300 °С–400 °С accordingly. The obtained non-equilibrium solid solutions (up to 5% of Cd or Cu) have saturation magnetization in low magnetic fields also decrease in this magnetization and coercivity (80–260 Oe at 300 K).

Temperature Effects on the Behavior of Lithium Iron Phosphate Electrodes

The systematic study of the effect of temperature (in the range from −45 to +60°C) on the process of lithium extraction from LiFePO4 and its insertion into FePO4 is carried out. At a current of about C/1.5, with decreasing temperature, the capacity decreases, the polarization increases, the range of compositions corresponding to nonequilibrium solid solutions widens, and the slope of the linear section of the galvanostatic curves corresponding to the two-phase system increases. The decrease in the capacity with decreasing temperature is not described by the simple Arrhenius equation. It is assumed that the process on the lithium iron phosphate electrodes has a mixed diffusion-activation nature. The polarization of the anodic and cathodic processes increases with decreasing temperature in a complicated way, and the polarization of the anodic process exceeds that of the cathodic process appreciably.

Evolution of the Structure and Properties of Al/Cu/Mg Ternary Composites during Thermomechanical Treatment

The structure and the mechanical and electrical properties of new ternary composites based on Al‒Mg deformable alloy obtained by fluid extrusion are studied. The evolution of structural and phase transformations in dissimilar fibers (Cu, Mg, and Al–Mg alloy) during thermomechanical treatment are studied using the methods of metallography, scanning electron microscopy, and hardness measurements. It is established that the strengthening of the ternary composite results from solid state reactions at the boundaries of the fiber, which lead to the formation of intermetallide phases (AlCu, Al2Cu, Mg2Cu, and MgCu2) and nonequilibrium supersaturated solid solutions of copper in aluminum and magnesium.

Temperature Effect on the Behavior of a Lithium Titanate Electrode

The effect of temperature (over the–15 to +60°С range) on the insertion of lithium into Li4Ti5O12 is systematically studied. At a current of ~1.2 С, as the temperature decreased the capacity decreased, polarization increased, the range of compositions corresponding to nonequilibrium solid solutions widened, and the slope of the galvanostatic curves’ linear segment corresponding to the Li4 + δTi5O12–Li7–δTi5O12 twophase system increased. The decrease in capacity with decreasing temperature can be explained by the fact that, up to the moment of the abrupt change in the potential, at moderate current values, the diffusion layer’s thickness drops to a value smaller than the material particles’ size. In this case, the capacity’s temperature dependence corresponds to that of lithium’s solid-state diffusion. The latter is described by the Arrhenius equation with the activation energy of ~35 kJ/mol in the temperature range from–15 to +18°С; at higher temperatures the capacity is practically temperature-independent. The polarization of the anodic and cathodic processes is practically identical; it decreases linearly as the temperature increases with a proportionality coefficient of ~2.5 mV/K.

The Secret Life of LiFePO4 Particles

In a recent issue of Nature Materials, Yiyang Li, Saiful Islam, Martin Bazant, William Chueh, and colleagues identify the major role of solvent-assisted lithium migration in LiFePO4 particles along the solid/liquid interface using a combination of X-ray diffraction, microscopy experiments, and ab initio molecular dynamics simulations. This finding suggests that at the particle scale LiFePO4 effectively becomes a three-dimensional Li conductor, and follow-up phase-field simulations suggest that lowering surface diffusivity is a predominant factor in determining the bulk phase transformation behavior during cycling.

High strength nanocrystalline Cu\u2013Co alloys with high tensile ductility

A supersaturated single-phase Cu–26 at.% Co alloy was produced by high-pressure torsion deformation, leading to a nanocrystalline microstructure with a grain size smaller than 100 nm. The nonequilibrium solid solution decomposed during subsequent isothermal annealing. In situ high-energy X-ray diffraction was used to map changes linked to the separating phases, and the development of a nanoscale Cu–Co composite structure was observed. To gain further information about the relationship of the microstructure and the mechanical properties after phase separation, uniaxial tensile tests were conducted on as-deformed and isothermally annealed samples. Based on the in situ diffraction data, different isothermal annealing temperatures were chosen. Miniaturized tensile specimens with a round cross section were tested, and an image-based data evaluation method enabled the evaluation of true stress–strain curves and strain hardening behavior. The main results are as follows: all microstructural states showed high strength and ductility, which was achieved by a combination of strain-hardening and strain-rate hardening.