Phase Decomposition Research Articles

Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies, coarsening kinetics, elemental partitioning behaviors, grain structures, and tensile properties were explored in detail for L12-strengthened Ni39.9Co20Fe15Cr15Al6Ti4–xNbxB0.1 (x = 0 at.%, 2 at.%, and 4 at.%) high-entropy alloys (HEAs). By substituting Ti with Nb, the spheroidal-to-cuboidal precipitate morphological transition, increase in the coarsening kinetics, and phase decomposition upon aging at 800 °C occurred. The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L12 to lamellar-structured D019 phase upon the long-term aging duration. By partially substituting Ti with Nb, the chemically complex and thermally stable L12 phase with a composition of (Ni58.8Co9.8Fe2.7)(Al12.7Ti5.8Nb7.5Cr2.3) ensures the stable phase structure and clean grain boundaries, which guarantees the superb high-temperature mechanical properties (791 ± 7 MPa for yielding and 1013 ± 11 MPa for failure) at 700 °C. Stacking faults (SFs) were observed to prevail during the plastic deformation, offering a high work-hardening capability at 700 °C. An anomalous rise in the yield strength at 800 °C was found, which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L12 particles.

Characterization of volatiles evolved during vacuum sintering of lunar regolith simulants

This work describes an experimental vacuum setup, designed for sintering of lunar regolith simulant materials, and its application to real-time characterization and identification of volatile species, resulting from in vacuo decomposition of the non-lunar components in the materials. We carried out a thorough experimental system characterization, highlighted critical aspects (temperature, pressure) with respect to regolith sintering, identified the volatile release / sample environment correlations, and postulated the need for high-vacuum capabilities in light of the high gas loads, generated by the simulants. We detailed the identification process of specific released gas species (H2O, NO, H2S, CO2, SO2, SO3, HCl, HF, etc.) using mass spectrometry, and advanced the use of them as markers for thermally activated processes, such as dehydration of hydrates, decomposition of major non-lunar phases (carbonates and sulfates) and lesser-represented components (nitrates, fluorides, chlorites, etc.). Quantification of individual decomposing phases is feasible within reason despite the inherent limitations of mass spectrometry. The methods, detailed in this paper, are intended for use in developing a better understanding of lunar regolith simulants with respect to sintering applications.

Effect of Ag microalloying on the magnetic properties and microstructure of Fe–P–C amorphous alloys

In this work, a series of Fe80−xAgxP13C7 (x = 0, 0.4, 0.6, and 1.0 at. %) amorphous/nanocrystalline alloys with high saturation magnetization (Ms) and excellent bending ductility were produced through melt spinning. The effect of Ag microalloying on the thermostability, microstructure, and soft magnetic properties was then investigated in detail. We observed that the doping of Ag could increase the value of Ms, without significantly deteriorating the coercive force. The increase in Ms was due to the precipitation of α-Fe nanocrystals, caused by Ag, which had a positive mixing enthalpy and immiscibility with iron, resulting in phase decomposition and acting as sites for α-Fe nanocrystalline nucleation. This work offers a promising method for preparing soft magnetic materials with amorphous/nanocrystalline structures and good bending ductility, without annealing.

Two novel metal complexes based on 2,2′-bipyridine and 2,4-dihydroxybenzoic acid: Synthesis, crystal structure and catalytic performance

To further enhance the catalytic performance of copper/lead 2,4-dihydroxybenzoate salt, two novel ternary complexes, Cu(C10H8N2)2(C7H5O4)·3H2O (complex 1) and [Pb(C10H8N2)(C7H5O4)2]·H2O (complex 2) were designed by introducing carbon framework ligand. The molecular structures of complexes 1 and 2 were determined by single crystal X-ray diffraction analysis. Both complex 1 and complex 2 were crystallized in the monoclinic crystal system. The difference is that the central Cu ion in complex 1 is connected to one oxygen atom of the carboxyl group of 2,4-dihydroxybenzoic acid, while the other 2,4-dihydroxybenzoic acid anion only acts as a counterion to balance the charge. But the central Pb ion in complex 2 is connected to four oxygen atoms of the carboxyl groups of two 2,4-dihydroxybenzoic acid and forms a six-coordinated structure. The Hirshfeld surface and associated 2D fingerprint analysis offer valuable insights into the intermolecular interactions within the crystal structure. Importantly, the thermal analysis results show that the thermal decomposition temperature of complex 1 is 204.1 °C, which has better thermal stability than complex 2. Furthermore, complex 1 showed remarkable catalytic activity on RDX compared to complex 2, and changed the thermal decomposition of RDX from melting decomposition to solid phase decomposition. This study will provide a new insight into development of combustion catalysts.

Outstanding specific yield strength of a refractory high-entropy composite at an ultrahigh temperature of 2273 K

We reported on the mechanical properties and microstructural evolution of a W20Ta30Mo20C30 (at.%) refractory high-entropy composite (RHEC). The RHEC exhibited outstanding yield strength at both 2273 and 1873 K. Microstructural investigations revealed that the as-cast RHEC had a triple-phase structure consisting of FCC dendrites, HCP matrix, HCP-BCC eutectic structure, and FCC-BCC eutectoid structure, and exhibited high-density defects owing to the complex phase transformations during solidification. After annealing at 2273 K, the precipitation of the BCC phase from the FCC dendrites and the decomposition of the HCP phase into the FCC-BCC eutectoid structure was observed to significantly refine the grain sizes of all triple phases. After compression at 2273 K, the ceramic phases and solid solution precipitated out from each other, which helps to avoid persistent softening after the yielding of RHEC. Further analyses suggested that the dominant deformation mechanisms of the BCC phase and HCP phase are dislocation glide and transformation-induced plasticity; whereas those of the FCC phase are twinning- and transformation-induced plasticity. The outstanding yield strength of this RHEC at ultrahigh temperatures may originate from the high-content ceramic phases and the structural metastability of the multi-principal composition. These findings provide a novel strategy to design RHECs by alloying high-content nonmetallic elements, which contributes to further breaking through their performance limits at ultrahigh temperatures.

The cracking behavior of the new Ni-based superalloy GH4151 in the triple melting process

In this paper, the mechanism of crack formation in VIM, ESR, and VAR of GH4151 alloy ingots is discussed. The cracks in VIM ingots are hot tearing, mainly caused by severe segregation of elements such as Nb, Ti, and Al in VIM electrodes, which promotes the precipitation of (γ+γ′) eutectic and Laves phases and thus leads to the formation of hot tearing. On the other hand, cracks in ESR and VAR ingots are cold cracks caused by the uneven distribution of γ′ phases. The temperature range for the precipitation of (γ+γ′) eutectic phases is 1160°C–1148 °C, while that for γ′ phases is 1137°C–1072 °C. At temperatures below 884 °C, the uneven precipitation of γ′ phases results in a maximum internal stress of 1096 MPa at room temperature. Based on computational fluid dynamics (CFD) simulations, to avoid large internal stresses caused by the precipitation of γ′ phase, the ingot should be removed from the water-cooled crystallizer within 57 min. Annealing is an effective measure to improve the quality of electrodes. After annealing treatment, the γ′ phase in the alloy transforms from short rod-shaped to square-shaped, and the decomposition of Laves phase promotes the precipitation of μ phase and σ phase from the matrix, improving the mechanical properties of the alloy.

Clustering evolution during low temperature aging and thermal stability during two-step aging in Al-Mg-Si alloys

The age-hardening response during two-step aging in Al-Mg-Si alloys depends on the nanoclusters formed at the early stage of phase decomposition. However, understanding of the evolution and thermal stability of the nanoclusters is still lacking. This paper discusses clustering behavior during natural aging (NA) and pre-aging (PA) at 50 ℃ through various experiments [hardness, differential scanning calorimetry (DSC), electrical resistivity and three-dimensional atom probe (3DAP)]. A rapid increase in hardness is confirmed due to the acceleration of cluster formation during PA, and a larger area of an endothermic peak is confirmed during DSC thermal analysis compared to NA specimens. Hardness and electrical resistivity were clearly decreased in the PA specimens at the initial stage of two-step aging since the formation of thermally unstable clusters, the Mg-enriched clusters identified by 3DAP, was accelerated during PA. A long aging time at 50 ℃ produces Mg-enriched clusters rather than facilitating the transition from Cluster (1) to Cluster (2). To investigate the atomic arrangement of a cluster, we conducted further analysis for chemical composition of clusters through the normalization for the different sizes of cluster. We found that the Mg-rich clusters with the core-shell structure where Mg atoms are segregated around the cluster-matrix interface, cause thermally unstable at the initial stage of two-step aging. The direct experimental evidence primarily proves that atomic arrangement inside a cluster is a more critical factor than its size for the thermal stability of a cluster in Al-Mg-Si alloys.

Kinetics of Transformation, Border of Metastable Miscibility Gap in Fe–Cr Alloy and Limit of Cr Solubility in Iron at 858 K

The study was aimed at determination of the position of the Fe-rich border of the metastable miscibility gap (MMG) and of the solubility limit of Cr in iron at 858 K. Towards this end, a Fe73.7Cr26.3 alloy was isothermally annealed at 858 K in vacuum up to 8144 hours and Mössbauer spectra were recorded at room temperature after every step of the annealing. Three spectral parameters viz. the average hyperfine field, 〈B〉, the average isomer shift, 〈IS〉, and the probability of the atomic configuration with no Cr atoms in the two-shell vicinity of the probe Fe atoms, P(0,0), gave evidence that the transformation process took place in two stages. All three parameters could have been well described in terms of the Johnson–Mehl–Avrami–Kolmogorov equation, yielding kinetics parameters. The first stage, associated with the phase decomposition, proceeded much faster than the second stage, associated with the alpha-to-sigma phase transformation. The most reliable estimation of the position of the MMG and that of the value of the Cr solubility limit was obtained from the annealing time dependence of 〈B〉, namely 24.5 at. pct Cr for the former and 20.3 at. pct Cr for the latter. A comparison of these figures with the recent phase diagrams pertinent to Fe–Cr system was done.

Enabling metal substrates for garnet-based composite cathodes by laser sintering

In this study, a laser irradiation method developed by Fraunhofer ILT was used to sinter screen-printed cathode layers of LiCoO2 (LCO) and Li7La3Zr2O12 (LLZO) directly on a stainless steel current collector. The laser sintering method was proved to be a promising method to sinter the composite cathode onto steel with greatly reduced side reactions and material degradation. In comparison, conventional sintering and another light-absorption-based sintering method, namely rapid thermal processing (RTP) in a lamp furnace, led to almost complete decomposition of the LLZO phase accompanying with the detrimental formation of LaCoO3 and CoO. Phase and morphology analysis of the laser-sintered cathodes using Raman spectroscopy, X-ray diffraction, and scanning electron microscopy confirmed sintering of LCO and LLZO through the layer with small amounts of secondary phases (LaCoO3, Li0.5La2Co0.5O4 and CoO). The resulting porous matrix of the laser-sintered cathode was infiltrated with a polyethylene oxide (PEO) electrolyte to connect the cathode to an LLZO separator and a Li metal anode without an additional sintering step. This model cell was used to evaluate the electrochemical activity of the laser-sintered composite cathodes, which exhibited a specific discharge capacity of 102 mAh g−1 at 4.0 V in the first electrochemical cycle.

Efficient pre-reduction of chromite ore with biochar under microwave irradiation

Pre-reduction of chromite ore is crucial for producing ferrochromium alloy by submerged arc furnace (SAF) smelting. In this study, an efficient method for low-temperature pre-reduction of chromite ore using biochar under microwave irradiation was explored to produce good burden for subsequent SAF smelting. The thermodynamic analysis proved the feasibility of the easy decomposition of the main phase in the ore, FeCr2O4 to Cr2O3, and finally to ferrochromium, mainly (Fe,Cr)7C3, in the commonly examined temperature range for pre-reduction. The characterizations of electromagnetic properties, microwave absorption capabilities, and temperature rising behaviors of the main raw materials and their mixtures showed that microwave heating could greatly improve the carbothermic reduction of chromite ore. Under the condition of C/O molar ratio of 1.2, addition of 2 wt% Na2B4O7, pre-reduction temperature of 1100 °C, and dwell time of 3 h, the chromite ore-biochar composite briquettes had efficient pre-reduction, producing pre-reduced briquettes with (Fe,Cr)7C3 as the main alloy phase. The iron and chromium metallization degrees (ηFe and ηCr) reached 98.1% and 86.8%, respectively. Compared with the conventional pre-reduction, they were increased by 15.9% and 51.7%, respectively. The results proved the feasibility of using microwave for efficent pre-reduction of chromite ore.

Fe microalloying during the in-situ synthesis of homogeneous CuCrFe alloys by aluminothermic self-propagating

Microalloying is an effective approach to improve the performance of CuCr alloys. This paper presents a new idea for the preparation of CuCrFe alloys by self-propagation reduction in-situ microalloying. It is found that the addition of Fe alloying components reduces the Gm and ΔGm of the CuCr melt, which inhibits the occurrence of the liquid phase decomposition process. When the Fe content of CuCrFe alloy is 1.5%, compared with the Cr-rich phase of CuCr alloy, the second phase undergoes significant refinement, the average particle size refined from 21.7 μm to 19.3 μm and the standard deviation reduced from 10.59 to 8.73, and the addition of Fe is beneficial to the spheroidization and refinement of the Cr-rich phase. Fe is mainly combined with Cr, and CuCrFe alloy still exists in the form of two phases. After solution + aging treatment, the average particle size of the second phase in CuCr alloy decreases from 21.8 μm to 17.9 μm, and that in CuCrFe alloy decreases from 19.6 μm to 18.6 μm. The conductivity of CuCrFe alloy increases from 10.5 MS m−1 to 15.9 MS m−1, and the hardness increases from 95.0 HB to 117.0 HB. After heat treatment, the nano-spherical are present in the Cr-rich phase of the CuCrFe alloy, while Fe is completely solid-soluble in the Cr-rich phase; the Cu-rich matrix shows a diffuse distribution of fine second phases, which play a role in strengthening the second phase in the matrix.

Thermal and combustion characteristics of honge, jatropha, and honge‐jatropha mixed biodiesels

AbstractThermal characteristics of biodiesels are useful in system design, modeling, and operation. Such investigations are extensively being carried out in combustors, engine, and process industries. This article examines the thermal characteristics of jatropha (Jatropha curcas), honge (Pongamia pinnata), and their equal mixing from thermogravitometry and differential scanning calorimetry (TG‐DSC) curves for the specific 10°C/min heating rate under atmospheric air. Fuel properties are measured following ASTM standards to compare with diesel properties. Each experiment was repeated three times, and the properties showed insignificant scatter. The average properties of the repeated tests are presented. Two phases of decomposition were observed for diesel, whereas three (viz., devolatilization of aqueous fractions, combustion of methyl esters, and combustion of carbonaceous residues) in biodiesels. Jatropha oil methyl ester (JOME) is thermally stable compared to honge oil methyl ester (HOME). Mixed biodiesel (JOME+HOME) is prone to oxidation due to the high content of oleic and linoleic acids. Recorded onset and offset temperatures of mixed biodiesel are low compared to pure biodiesel. Mixed biodiesel exhibited high volatility resembling diesel characteristics. It exhibited an enthalpy of 240 J/g, whereas the enthalpy of diesel, jatropha, and honge exhibited enthalpies of 130, 321, and 570 J/g, respectively. The combustion index (S) of diesel, jatropha, honge, and mixed biodiesel was 41.6, 82.8, 77.74, and 64.6, respectively. Mixed biodiesel reduces the intensity of combustion (Hf), promising better combustion characteristics. Thus, mixed biodiesel shows the potential of an efficient alternative energy source.

Phase Stability of SrTi1-XFexO3- δ Under Solid Oxide Cell Fuel-Electrode Conditions: Implications for Related Exsolution Electrode Materials

SrTi1-xFexO3-δ (STF) has shown promise as a fuel electrode, and as a basis for fuel electrodes with catalytically active nanoparticle exsolution. This study aims to determine trends in performance and stability of STF in highly reducing environments as Fe-content is varied from x = 0.5 to 0.8. Here we report that, while polarization resistance of STF fuel electrodes decreases with higher Fe-content, this is accompanied with lower stability in highly reducing conditions. For too-reducing conditions and too-high x, partial decomposition of the original perovskite phase is observed with the formation of metallic Fe and a Ruddlesden-Popper STF phase.

Exploring Stable and Selective Anode Materials for the Electrochemical Oxidative Coupling of Methane (EOCM): A Case Study of Doped Titanates

The electrochemical oxidative coupling of methane (EOCM) is an emerging process through which methane can be converted into value-added products such as ethylene. However, this application is hampered by the fact that a stable and ethylene-selective anode material does not currently exist. In this study, we analyse a selection of titanate-based anode materials: SrTi0.3Fe0.63Co0.07O3, SrTi0.65Fe0.35O3 and La0.3Sr0.7TiO3. We analyse the stability of these materials under EOCM operating conditions using post-mortem characterisations involving XRD, XPS and EDS analyses. We also characterise the ethylene production of these anode materials at similar current densities. We find that Fe/Co-doped titanates are unstable under EOCM conditions and undergo significant phase decomposition while La0.3Sr0.7TiO3 remains stable over several weeks of operation.

Investigation of the Reaction Mechanisms in La1-XSrxCo1-YFeyO3 Electrode by DFT Calculations

La1-xSrxCoy-1FeyO3-δ (LSCF) is a widely used oxygen electrode in Solid Oxide Cells (SOCs). Despite its high electrochemical activity combined with a good mixed ionic and electronic conductivity, it is prone to a phase decomposition upon operation decreasing the global cell performance. However, the underlying mechanisms for the reaction mechanism and the degradation are still not precisely understood and require investigations at atomic scale. Therefore, Density Functional Theory (DFT) calculations are used to better unravel the mechanisms occurring in the electrode. In particular, the interaction of the dioxygen gas with the AO-terminated (100) surface is studied. In this context, the oxygen reduction reaction is decomposed in a series of elementary steps including oxygen vacancy formation, adsorption of the dioxygen molecule on the bare surface and into a surface vacancy accompanied with the formation of intermediate oxygen species, followed by the dissociation and incorporation into the bulk. The calculations have been carried out at two different Cobalt contents (y=0.125 and y=0.25).

Insights into Phase Evolution and Mechanical Behavior of the Eutectoid Ti–6.4(wt%)Ni Alloy Modified with Fe and Cr

Titanium alloys gain increasing importance in industry due to the expansion of advanced manufacturing technologies such as additive manufacturing. Conventional titanium alloys processed by such technologies suffer from formation of large primary grains and anisotropy of mechanical properties. Therefore, novel alloys are required. Herein, the effect of ternary alloying elements Fe and Cr on the Ti–6.4(wt%)Ni eutectoid system is investigated. Both elements act as eutectoid formers. Fe and Cr show sluggish transformation behavior, whereas Ni is an active eutectoid‐forming element. Thereby, sluggish refers to slow and active to fast transformation kinetics. The focus of this work is on the combined addition of such elements studied under different heat‐treatment conditions. It is shown in the results that largely varying microstructures can be generated resulting in hardness values ranging from 239 to 556 HV0.1. Moreover, the formation of a substructure within the α phase of direct aged alloys is observed. The formation mechanism of this substructure is investigated in detail. The mechanical properties are discussed based on the microstructural characteristics. The presence of intermetallic Ti2Ni phase increases the Young's modulus, whereas the presence of ω phase results in embrittlement. The results shed light upon the complex phase formation and decomposition behavior of titanium alloys based on Ti–6.4Ni.

Influence of 1:7 H phase stability on the evolution of cellular microstructure in Fe-rich Sm-Co-Fe-Cu-Zr magnets

Fe-rich Sm2Co17-type magnets have many essential applications prospects in high-temperature operation environments. However, the formation of complete cellular microstructure becomes increasingly difficult when the Fe content of Sm2Co17-type magnets exceeds 20 wt%. Herein, the phase stability of 1:7 H phase and its effect on the precipitation of the 1:5 H precipitates in Fe-rich Sm2Co17-type magnets are investigated. The results confirm that the stability of the 1:7 H phase decreases with the increasing of Fe content for the solution-treated magnets. Not only 2:17 R phase but also a lot of 2:17 R′ phase is found in solution-treated magnets when the Fe content exceeds 22 wt%. As the Fe content increases, the density of initial defects decreases for the growth of 2:17 R nanotwins in the solution-treated magnets. Furthermore, the segregation of Cu in the defects-aggregated cell boundaries weakened gradually. Consequently, the nucleation and precipitation of cell boundary precipitates is difficult, and the cellular microstructure becomes incomplete and coarse when the Fe content is above 22 wt%. The present work may provide a further insight for understanding the evolution of cellular microstructure depending on the phase decomposition behavior in Fe-rich Sm2Co17-type magnets.

Optimized DEC: An effective cough detection framework using optimal weighted Features-aided deep Ensemble classifier for COVID-19

Since the year 2019, the entire world has been facing the most hazardous and contagious disease as Corona Virus Disease 2019 (COVID-19). Based on the symptoms, the virus can be identified and diagnosed. Amongst, cough is the primary syndrome to detect COVID-19. Existing method requires a long processing time. Early screening and detection is a complex task. To surmount the research drawbacks, a novel ensemble-based deep learning model is designed on heuristic development. The prime intention of the designed work is to detect COVID-19 disease using cough audio signals. At the initial stage, the source signals are fetched and undergo for signal decomposition phase by Empirical Mean Curve Decomposition (EMCD). Consequently, the decomposed signal is called “Mel Frequency Cepstral Coefficients (MFCC), spectral features, and statistical features”. Further, all three features are fused and provide the optimal weighted features with the optimal weight value with the help of “Modified Cat and Mouse Based Optimizer (MCMBO)”. Lastly, the optimal weighted features are fed as input to the Optimized Deep Ensemble Classifier (ODEC) that is fused together with various classifiers such as “Radial Basis Function (RBF), Long-Short Term Memory (LSTM), and Deep Neural Network (DNN)”. In order to attain the best detection results, the parameters in ODEC are optimized by the MCMBO algorithm. Throughout the validation, the designed method attains 96% and 92% concerning accuracy and precision. Thus, result analysis elucidates that the proposed work achieves the desired detective value that aids practitioners to early diagnose COVID-19 ailments.

Preparation, characterization and activation of Pd catalysts supported on CNx foam for the liquid phase decomposition of formic acid

In this work, we have prepared a series of Pd catalysts on a CNx support for the liquid phase decomposition of formic acid. The structured CNx support was obtained through thermal pyrolysis of melamine foam and the pyrolysis conditions were optimized to achieve high surface area. The resulting support contains high amount of nitrogen with a contribution of pyridinic component. Several Pd catalysts were prepared and under optimized conditions, we were able to obtain small (2.7 ± 0.9) nm Pd particles by using the oxidized support in powdery form. The activity of the optimized catalyst was studied under different conditions in the fresh and the used form. The fresh catalyst did not show significant activity. However, we found that the catalyst activated after use. Activation was understood in terms of the variation of surface Pd oxidation states under the effect of formic acid/sodium formate solutions. We found that the best activity is achieved under an optimal proportion of Pd0/PdII surface states according to previous reports. Under the best conditions, the activity of the best catalyst (8.6Pd/CN0.3) was as high as 9245 h−1, attributable to the small particle size, the Pd0/PdII ratio, the amount of pyridinic nitrogen, and the testing conditions, which included the preadsorption of sodium formate.

Research and Development on Cold-Sprayed MAX Phase Coatings

Cold spraying is an attractive solid-state processing technique in which micron-sized solid particles are accelerated towards a substrate at high velocities and relatively low temperatures to produce a coating through deformation and bonding mechanisms. Metal, ceramic, and polymer powders can be deposited to form functional coatings via cold spraying. MAX phase coatings deposited via cold spraying exhibit several advantages over thermal spraying, avoiding tensile residual stresses, oxidation, undesirable chemical reactions and phase decomposition. This paper presents a review of recent progress on the cold-sprayed MAX phase coatings. Factors influencing the formation of coatings are summarized and discussions on the corresponding bonding mechanisms are provided. Current limitations and future investigations in cold-sprayed MAX coatings are also listed to facilitate the industrial application of MAX phase coatings.