Supersaturated Solid Solution Research Articles

Crystallization of FeC Iron Monocarbide During Peritectoidal Transformation of the Lamellar Eutectoid of Ledeburite White Eutectic Cast Iron

The previously unknown process of homogeneous and heterogeneous crystallization of FeC iron monocarbide and its co-crystallizations with ε-carbide Fe2C from a supersaturated solid solution based on ε-carbide Fe2C or polycarbide quasi-eutectic formed in the process of peritectoid decomposition during prolonged heating (isothermal annealing) of the lamellar eutectoid ledeburite in cast eutectic white iron has been investigated. Crystallization of 2D monolayers of FeC monocarbide allotropes in the form of translucent extended and elastic crystalline nanofilms has been experimentally proved. The carbide phases in white cast iron can be characterized as a single isomorphic and isostructural quasi-carbide solid solution, which structurally crystallizes as a mixture of carbide phases as a quasi-eutectic, in which the carbon content is free to vary widely without indentification of the carbide phases proper. The decomposition product of the lamellar eutectoid as a result of peritectoid transformation during isothermal annealing is polycarbide with a gradient crystal lattice of solid solutions corresponding in carbon concentration to this or that carbide.

Formation of metastable phases during spark plasma sintering of Fe@Au core–shell particles

ABSTRACT The present work reports the formation of Fe-Au metastable alloys by galvanic replacement, pressureless spark plasma sintering (SPS) and acid treatment. The reaction of partial galvanic replacement between micrometer-sized iron particles and HAuCl4 solution allowed synthesising core–shell Fe@Au particles. When the Fe@Au particles were treated in HCl solution, iron was selectively dissolved and hollow Au particles were formed. Pressureless SPS of the Fe@Au core–shell particles led to the formation of a metastable intermetallic phase, FeAu3, and a supersaturated solid solution, Au(Fe). The latter was resistant to HCl solution; at the same time, the FeAu3 phase decomposed during HCl treatment to form a porous gold structure. This work shows that a combination of galvanic replacement, SPS and acid treatment can be used for the preparation of metastable Fe-Au alloys with different phase compositions and particle morphologies.

Rapid-thermal-process pre-treatment promoted precipitation towards strengthening hard magnetism of Sm2Co17-type magnets

Dislocations can act as heterogeneous nucleation sites but their annihilations upon heating sacrifice point defects nearby, weakening the driving force for precipitate nucleation. In order to keep a high density of dislocations and point defects together, here rapid-thermal-process (RTP) pre-treatment was carried out to promote the precipitate nucleation in Sm2Co17-type permanent magnets that evolve gradual formation and dissociation of dislocations during concurrent precipitation and recrystallization. As exhibited in a model magnet Sm25Co49.3Fe17.1Cu5.6Zr3.0 (wt.%) with high-density dislocations at solution-treated state, the RTP pre-treatment can induce excess point defects owing to the migration of solute atoms towards equilibrium state and the quenching of point defects, and suppress the early-stage diffusion-controlled dissociation of dislocations. After whole-process isothermal aging and slow cooling, the RTP pre-treated magnet possesses much higher fraction of hexagonal SmCo5 (1:5H) nanoprecipitates than the non-pre-treated one, giving rise to effective enhancements in coercivity Hcj from 26.20 to 30.34 kOe and in knee-point field Hk from 14.43 to 19.20 kOe. These findings add insight into controlling precipitation in dislocation-bearing supersaturated solid solutions, which are feasible for strengthening the Sm-Co-based magnets.

Slow precipitate coarsening kinetics in extremely fine nanograined Ni-Cr alloys

Grain boundaries significantly influence the nucleation and growth of precipitates in supersaturated solid solutions. However, due to the inherent instability of grain boundaries in nanograined metals, the precipitation process usually occurs concurrently with grain growth during thermal annealing, making it difficult to reveal the grain size effect on the precipitate coarsening behavior. In this study, nanograined Ni-48 Cr (at.%) supersaturated alloys with grain sizes ranging from about 85 nm to 10 nm were fabricated by plastic deformation, in which the relaxed grain boundaries led to the extremely fine nanograined structure having good thermal stability. During the high-temperature annealing process, nanoscale Cr particles were precipitated from the nanograined matrix, forming a nanograined dual-phase structure. The growth kinetics of the Cr precipitates in the extremely fine nanograined structures are different from the larger grain size samples, showing a trend that the smaller the as-deformed grain size, the slower the precipitate coarsening rate. Detailed microstructural characterization and kinetic analysis indicate that the extremely fine nanograined structure may change the rate-limiting process for precipitate coarsening from lattice diffusion to grain boundary diffusion as grain size is refined to about a few tens nanometers, which leads to a slow coarsening rate. The nanograined dual-phase Ni-Cr alloy of the annealed 10 nm sample shows the highest strength among these samples as well as excellent thermal stability, which provides guidance for the design of high-performance alloys.

THERMOPHYSICAL PROPERTIES OF HYPOEUTECTIC, EUTECTIC AND HYPEREUTECTIC Al-Si AUTOMOTIVE ALLOYS UNDER AGEING TREATMENT

Influence of different levels of silicon under a variety of thermal treatment is investigated on the thermophysical behaviour of Al-Si-Cu-Mg automotive alloys. Conventional cast alloys are subjected to T6 heat treatment for age hardening. Hardness values, thermal conductivity along with microstructural observation under different heat-treated conditions are well thought-out to comprehend the ageing performance and the precipitation behaviour of the alloys. From these investigational results it is indicated that two successive hardening peaks take place in the agedalloys. Ageing sequence consists of different phases like a supersaturated solid solution, GP zones, intermediate βʹʹ, intermetallic βʹ, equilibrium β and the Q phase but GP zones and the metastable phases are most likely responsible for these ageing peaks. Thermal conductivity decreases for these precipitates formation during ageing and increases for relieving the internal stress, dissolution of metastable phase and coarsening the precipitation of the alloys. Addingup of Si to the alloy showed earlier ageing peaks with higher intensities intended for its increasing properties of heterogeneous nucleation and diffusion kinetics. After ageing around at 200°C for four hours the alloys offer the height hardness. Microstructural study reveals that eutectic silicon makes the grain boundary coarsen and beyond the eutectic composition the star-shaped blocky primary Si is formed. Subsequent to ageing at 350oC for one hour the alloys reach completely recrystallized state.

High-strength and high-conductivity additively manufactured Cu-O alloy enabled by cellular microstructure

Printing Cu alloys with simultaneously high strength and high electrical conductivity by laser powder bed fusion remains a challenge. In conventionally manufactured Cu alloys, nanoprecipitates form through alloying elements to impart significant strengthening effects while having a lesser impact on electrical conductivity. However, the ultrahigh cooling rate of laser powder bed fusion promotes the supersaturated solid solution of alloying elements, resulting in severe lattice distortion in the Cu matrix and a significant compromise in electrical conductivity. Therefore, it is vital to select Cu alloying elements that substantially prevent the formation of a supersaturated solid solution in laser powder bed fusion to achieve outstanding strength while maintaining sufficient electrical conductivity. In this study, we fabricated a Cu-O alloy with high strength (491.6 MPa) and high electrical conductivity (68.0 % IACS) using laser powder bed fusion. The alloy features a cellular microstructure in which Cu2O nanoprecipitates are configured orderly as the cellular boundary. This cellular microstructure can promote strength by impeding dislocation motion and maintain a high electrical conductivity by preserving a longer free path for conductive electrons. This study explores the potential of combining the alloy composition and extreme process conditions of laser powder bed fusion to provoke unique microstructures and overcome the dilemma between strength and electrical conductivity in Cu alloys.

A comparative study on cryogenic formability and paint baking property of AA7075 alloy sheets tempered at fast retrogressed and pre-aged conditions

Forming at cryogenic temperatures provides a new approach to the formability improvement of 7000-series high-strength aluminum alloys. However, the cryogenic forming process based on solution-treated (W) or peak artificial aged (T6) alloys cannot meet the demands of autobody manufacturing for good formability, high post-formed strength and high efficiency simultaneously. The fast retrogression (FR) treatment of AA7075-T6 and pre-aging (PA) treatment of AA7075-W are elaborately designed to tailor a pre-hardened AA7075 alloy, which has good cryogenic formability and relatively high initial strength, and can be further strengthened by paint baking in high efficiency. The results show the optimum fast retrogression temperature for AA7075-FR is 400 °C, at which the sample can acquire a cryogenic elongation of 29.2% and obtain 90% of initial T6 sheet tensile strength by the combined effect of cryogenic pre-deformation and subsequent paint baking treatment. The optimum pre-aging temperature for AA7075-PA is 120 °C, at which the sample can acquire a cryogenic elongation of 31.0% and exceed 96% of tensile strength of AA7075-T6 after paint baking treatment. The comparative study demonstrates that the supersaturated solid solution matrix with an appropriate proportion of GP zones and η′ phases, but few Cr-containing dispersoids and precipitate free zones, is an ideal pre-hardened microstructure to get a good combination of cryogenic formability and final strength. The precipitation of GP zones and η′ phases is prior to the formation of precipitate free zones during pre-aging of AA7075-W, making the pre-aged hardening cryogenic forming process more suitable for automobile industry.

Precipitation behavior of supersaturated solid-solubility CuCrZr alloy by additive manufacturing

As a typical precipitation-strengthened alloy, the improvement of CuCrZr alloy performance can be achieved by promoting the precipitation of alloying elements. However, the low solubility of Cr limits further enhancement of the performance of conventionally processed CuCrZr alloys. In this study, a supersaturated solid solution CuCrZr alloy was prepared using laser powder bed fusion (LPBF), and the precipitation of solute atoms was promoted through heat treatment, resulting in an alloy with excellent comprehensive performance. The influence of different heat treatment processes on the precipitation behavior and performance of the alloy was investigated. Microstructural analysis revealed the presence of micrometer-scale Cr particles in the specimens after solution treatment (ST) and solution-aging treatment (SAT), while the direct-age-hardening (DAH) specimen exhibited dispersed nanoscale Cr precipitations. Notably, a Nishiyama-Wassermann (N-W) relationship was observed between the Cr precipitations and the Cu matrix, significantly enhancing the mechanical properties of the alloy. The DAH specimen exhibited an ultimate tensile strength (UTS) of 661.54 MPa, a hardness of 231.03 HV and an electrical conductivity of 65.05 % IACS, demonstrating the best match between mechanical properties and electrical conductivity. The outstanding properties can be credited to the uniformly distributed Cr nano-precipitates and slim lattice distortion. Additionally, the main strengthening mechanism is precipitation strengthening for CuCrZr alloy.

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.

Microstructural evolution and mechanical properties of the rapidly-solidified and hot-extruded 2196 Al-Li alloy

The microstructural evolution and mechanical properties of the 2196 Al-Li alloy prepared by the vacuum gas atomization, hot-isostatic-pressing (HIP) and hot-extrusion were investigated. The results show that prior particle boundaries (PPBs) exist in the microstructure of the HIPed alloy, and hot-extrusion effectively breaks PPBs to restrict the element micro-segregations. The as-extruded alloy exhibits fine grains, and bears higher solution treatment temperature due to the absence of low-melting-point micro-segregations. In the ageing process, the alloy hardness continues to rise, and reaches the peak value at 48 h. The precipitation sequence of the alloy is determined to be: supersaturated solid solution (SSS) → GP zone + fine T1 + δ′ → T1 + θ′ + δ′, where T1 phase dominates the whole ageing process. The diameter of the T1 platelets increases gradually with the increase of ageing time, meanwhile the T1 number density slightly drops in the over-aged alloy. The 3% pre-stretching promotes the nucleation of T1 phase, and the peak-ageing time is shortened to 28 h. Besides, no over-ageing phenomenon is observed in the pre-stretched samples. The Mg-Cu and non-Cu clusters formed on the dislocations accelerate the precipitation of T1 phase and prevent the nucleation of θ′ phase. The yield strength (YS) and ultimate tensile strength (UTS) of the T8 sample are 490 MPa and 515 MPa, respectively, which is notably higher than the T6 sample. The tensile strength of the present alloy is relatively higher than that of the Al-Li alloys with a comparable Li content.

Microstructure characterization of a novel nanostructured zirconia-alumina coating prepared by plasma spraying YSZ-Al reactive system

A YSZ-Al reactive system was used as raw materials for the fabrication of a novel YSZ-Al2O3 composite coating by plasma spraying. Thermite reaction was triggered in the plasma jet and phases like alumina and Zr–Al intermetallics were in-situ formed in the reaction zone. Solid solution reaction occurred in the rapid solidification of the homogeneously mixed Y–Al–Zr–O superheated melt and high solubility of Al3+ was also observed in the supersaturated solid solution. Phase transitions from the reactive precursor to the coating composition and the microstructure formation of the coating lamellae were revealed by extensive characterizations including XRD, XPS, Raman, SEM, EDS, and EPMA. Good compatibility within the coating compositions and compact interlamellar bonding were obtained in the as-prepared YSZ-Al2O3 composite coating. The coating's surface and cross-section Vickers hardness values were 7.48 GPa and 10.40 GPa, respectively. Additionally, the micro-zone hardness value of the eutectic area could reach 19.43 GPa.

Realizing High Thermoelectric Performance in GeTe‐Based Supersaturated Solid Solutions

AbstractAs a highly promising thermoelectric material in the mid‐temperature region, pristine GeTe shows deteriorated thermoelectric properties due to the low Seebeck coefficient and the intrinsic high thermal conductivity. Herein, it is discovered that the introduction of strongly correlated d, f electrons by alloying rare‐earth atoms at Ge sites will increase the band effective mass and promote band convergency, which can largely improve the electric transport property. Moreover, the heavy atoms (Pb, Bi) doping induces optical phonon softening and the avoided crossing between acoustic and optical phonon branches, decelerating both optical and acoustic phonon velocities. The modulated composition wave from the fluctuated multi‐component distribution introduces a complicated hierarchical structure including point defects and nanoprecipitates, which provides all‐scale scattering sources for heat‐carrying phonons and results in low lattice thermal conductivity. Consequently, a high zT of 2.4 at 800 K can be obtained in the Ge0.86Pb0.09Bi0.03Ce0.005Te sample due to the combination of strong correlation for d, f electrons and the full‐spectrum scattering for phonons. This work provides an effective strategy to increase the thermoelectric performance of IV–VI compounds by combining the modulated band structure and the softening phonon dispersion.

Direct artificial aging of the PBF-LB AlSi10Mg alloy designed to enhance the trade-off between strength and residual stress relief

The AlSi10Mg alloy is one of the most studied alloys processed by the Powder Bed Fusion-Laser Beam (PBF-LB). Many already published works focus on post-process heat treatments to reduce residual stress or improve mechanical strength. Instead, the present study aims to identify direct artificial aging (AA) heat treatment able to optimize both aspects, thus enhancing the trade-off between strengthening and residual stress relief for the PBF-LB AlSi10Mg alloy produced using a no-heated platform. Higher temperatures (190–240 °C) than those typically used in AA heat treatment were selected based on thermal analysis to benefit both residual stress relief and precipitation of reinforcing phases from the supersaturated solid solution of the metastable as-built alloy. The effects of AA heat treatment on mechanical properties (i.e. hardness) and residual stress were evaluated by plotting aging curves and by XRD and Raman analyses and demonstrated that different trade-offs between strengthening and stress relief can be achieved by tuning heat treatment parameters (temperature and time). In particular, the exposure at the lowest temperature (190 °C) induced a partial decrease in residual stress and a slight increase in hardness. By increasing heat treatment temperature and soaking time, the relief was more significant, whilst the decrease in hardness was rather limited. The results are supported by the microstructural changes observed on the samples due to the different heat treatment conditions applied and show the feasibility of designing an AA heat treatment for the PBF-LB AlSi10Mg alloy capable of satisfying the mechanical response required by the final application.

The new sight to Fe–C system

ABSTRACT The crystalline Fe–C alloy had been the focal point of research in the traditional metallurgical steel industry. However, its corresponding amorphous alloy system had received little attention, with a lack of consensus on the microstructural evolution. Three types of Fe–C unbalanced composite films with different carbon content were fabricated using the magnetron sputtering method. The results indicated that as the carbon content increased, carbon existed in the form of supersaturated solid solution atoms in crystalline Fe, an amorphous Fe–C structure and Fe3C, respectively. The resistivity of the films in these three carbon types gradually increased from 1.37 × 10−4 ohm/cm to 4.53 × 10−2 ohm/cm and then to 5.43 × 10−2 ohm/cm. Simultaneously, the transmittance gradually increased from 0% to 28% and then to 51%. Upon annealing at 400°C, with the crystallisation of Fe in the low-carbon film, unbalanced carbon atoms were pushed out of the lattice and precipitated as Fe3C. After annealing at 650°C, the Fe3C in the high-carbon film decomposed into Fe and C. The as-prepared medium-carbon film displayed commendable corrosion resistance. These findings provided valuable insights for the design and performance development of new materials.

Evading the strength and ductility trade-off dilemma in titanium matrix composites through supersaturated solid solution and phase transformation

This short communication found thermal-deformation is effective in facilitating the dissolve of TiC, Ti5Si3 reinforcement and the transformation of β-to-αs, refining primary α through the dislocation introduced, which results in a significant improvement of mechanical properties with ultimate tensile strength of ∼1300 MPa, and 9.6% elongation, representing a nearly 18% and 3.4 times increment, respectively.

Wear and Corrosion Resistant Eutectic High-Entropy Alloy Al0.3CoCrFeNiMo0.75 Produced by Laser Metal Deposition and Spark-Plasma Sintering

Eutectic high-entropy alloys (EHEAs) with a fine-lamellar structure and homogenous property profile are of particular interest for wear and corrosion protection coatings. High cooling rates in the laser metal deposition (LMD) process can induce microstructure refinement and allow the formation of a supersaturated solid solution in EHEAs. A subsequent solution annealing can create the equilibrium state. In the present study, LMD coatings with an ultrafine-grained Widmanstätten structure were produced from the EHEA Al0.3CoCrFeNiMo0.75 gas atomized powder. High cooling rates during deposition led to a supersaturated solid solution with face-centered cubic (FCC) structure. The LMD coating exhibits the highest average hardness of 734 HV0.5, which drops to approx. 200 HV0.5 due to an increased microstructural domain size after heat treatment. Under oscillating wear, the formation of oxidized wear debris promotes material removal in the heat-treated condition. Corrosion tests reveal a deterioration of the passivation behavior. LMD processes exhibit great potential to create supersaturated solid solutions with refined structure in EHEAs to enhance the property profile.

Thermodynamic modelling application for prediction of diffusion formation of supersaturation solution

Abstract In this paper the effect of diffusion formation of supersaturated solid solution in the Al–Ge system is discussed. It was shown that at relatively low temperatures of diffusion annealing, Ge concentration in Al-based solid solution became higher than the solubility limit at given temperatures. The maximal concentration of Ge was determined by extrapolation of the diffusion profile on the Ge/Al interface. Analysis of diffusivity of Al in Ge allowed to suggest that slow diffusion in Ge restricts the formation of Ge-based solid solution. Qualitative approach confirmed this suggestion.

Effect of heat treatment on microstructural characteristics and mechanical properties of hypereutectic B390 aluminium alloys modified by phosphorus and bismuth

This study investigated the effects of solution treatment and aging processes on microstructural characteristics and mechanical properties of hypereutectic B390 Al-Si Alloy modified by phosphorus (P) and bismuth (Bi). The morphology evolution of primary Si, eutectic Si, intermetallic compounds, and secondary dendrite arm spacing (SDAS) of both initial and modified alloys were investigated in as-cast and heat-treated conditions. It was found that the combination of 0.1 wt%P and 0.5 wt%Bi in B390 alloy resulted in a remarkable modification and refinement of primary Si, eutectic Si, and SDAS compared to initial specimens in the as-cast condition. After heat treating, the fine lamellar eutectic Si and intermetallic compounds underwent a higher degree of fragmentation and coarsening, appearing as small round particles, leading to a more pronounced formation of a supersaturated solid solution. Additionally, the peak aging time of the B390-P-Bi alloy was only 7 h, 2 h less than the initial specimen. After T6 treatment and addition of both 0.1 wt%P and 0.5 wt%Bi, the B390-P-Bi alloy exhibited a smaller rounded Mg3Bi2 phase at peak aging with a 62.07% size reduction compared to the B390-Bi alloy. The results show that adding P with Bi enhanced hardness, ultimate tensile strength (UTS) and elongation because of refinement of the SDAS, primary Si and eutectic Si. These refinements played a crucial role in increasing tensile strength by more effective strengthening through precipitation and grain size reduction. The B390-P-Bi alloy is a promising choice for applications requiring a balance between strength and ductility.

Oxidation Behavior of FeNiCoCrMo0.5Al1.3 High-Entropy Alloy Powder.

One of the most promising applications of FeNiCoCrMoAl-based high-entropy alloy is the fabrication of protective coatings. In this work, gas-atomized powder of FeNiCoCrMo0.5Al1.3 composition was deposited via high-velocity oxygen fuel spraying. It was shown that in-flight oxidation of the powder influences the coating's phase composition and properties. Powder oxidation and phase transformations were studied under HVOF deposition, and during continuous heating and prolonged isothermal annealing at 800 °C. Optical and scanning electron microscopy observation, energy dispersive X-ray analysis, X-ray diffraction analysis, thermogravimetric analysis, differential thermal analysis, and microhardness tests were used for study. In a gas-atomized state, the powder consisted of BCC supersaturated solid solution. The high rate of heating and cooling and high oxygen concentration during spraying led to oxidation development prior to decomposition of the supersaturated solid solution. Depleted Al layers of BCC transferred to the FCC phase. An increase in the spraying distance resulted in an increase in α-Al2O3 content; however, higher oxide content does not result in a higher microhardness. In contrast, under annealing, the supersaturated BCC solid solution decomposition occurs earlier than pronounced oxidation, which leads to considerable strengthening to 910 HV.

Analysis of rolling contact fatigue and wear characteristics of aging-treated laminar plasma strip quenching rails

The application of laminar plasma discrete quenching (LPQ) technology to enhance the wear resistance of railway tracks has been a notable advancement. Nonetheless, it has been observed that LPQ-treated rails experience a reduction in rolling contact fatigue (RCF) resistance. Our previous study found that aging treatment can improve the toughness of laminar plasma spot quenched rails. However, the microstructure as well as the mechanical properties of strip quenching are significantly different from those of spot quenching. To tackle this concern, the study delves into the wear and RCF damage mechanisms of aging-treated laminar plasma strip quenching (ALPQ) rails. The findings reveal that the aging process induces the precipitation of supersaturated solid solutions within the hardened layer, consequently elevating the RCF resistance of ALPQ rails. Following appropriate aging treatment, nearly complete elimination of fatigue cracks on the rail surface is achieved.