Precipitation Of Secondary Phases Research Articles

Influence of Ta/Zr minor-alloying on the high-temperature microstructural stability of cladding Fe–Cr–Al ferritic stainless steels

The Fe–Cr–Al-based ferritic stainless steels have attracted more attention as accident-tolerant-fuel cladding materials of nuclear reactors due to their prominent comprehensive properties. The present work investigated systematically the influence of Ta and Zr minor-additions on the microstructural stabilities at high temperatures (HTs) of Fe–Cr–Al–Mo–Nb alloys. Alloy compositions were designed in light of a cluster formula approach. Alloy ingots, in turn, were homogenized at 1473 K for 2 h, hot-rolled at 1073 K, aged at 1073 K for 24 h, and then retreated at different temperatures (1273–1473 K) for 1 h. The microstructures at different heat-treatment states were characterized with optical microscopy (OM), scanning electron microanalysis (SEM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM), respectively. The experimental results indicated that fine second-phase precipitates (primarily Laves phase) are distributed homogeneously in the ferritic matrix of aged alloys. These precipitates would be re-dissolved into the matrix when these aged alloys were retreated at a HT above 1273 K. Most of the Laves precipitates are dissolved into the matrix in the Mo/Nb/Ta-containing alloys after retreatment at 1,473 K for 1 h, while the further addition of minor Zr (about 0.1 wt %) can retard this dissolution effectively, resulting in a desirable microstructure with second-phase precipitates uniformly distributed into the refined matrix grains at HTs. Moreover, it was also found that the Zr addition can contribute to the formation of core-shell particles, in which the inner-core is enriched by Zr to form a cubic Zr2Fe phase and the out-shell exhibits a Laves phase structure segregated by Ta and Nb.

Influence of Sintering Temperature on Microhardness and Tribological Properties of Equi-Atomic Ti-Al- Mo-Si-W Multicomponent Alloy

This work aim to investigate the effect of sintering temperature on microhardness and tribological properties of novel equi-atomic TiAlMoSiW HEA fabricated via spark plasma sintering. The influence of Spark plasma sintering temperature on morphological evolution and phase formation was also investigated. The microstructure and the phases formed for the developed HEA were examined using scanning electron microscopy (SEM) and X-ray diffractometry (XRD) respectively. The microhardness and tribological properties were studied using a diamond base microhardness tester Rtec tribometer. It was noticed that sintering temperature has an effect on microhardness and tribological properties. Phase analysis of the samples displayed that the alloy exhibited a bcc matrix with secondary phase precipitate of ordered fcc TiSi2 phase. The developed HEA showed improved mechanical properties as the sintering temperature increases.

Precipitates in Additively Manufactured Inconel 625 Superalloy.

Laser-based additive manufacturing processes are increasingly used for fabricating components made of nickel-based superalloys. The microstructure development, and in particular the precipitation of secondary phases, is of great importance for the properties of additively manufactured nickel-based superalloys. This paper summarizes the literature data on the microstructure of Inconel 625 superalloy manufactured using laser-based powder-bed fusion and directed energy deposition processes, with particular emphasis on the phase identification of precipitates. The microstructure of Inconel 625 manufactured by laser-based directed energy deposition in as-built condition is investigated by means of light microscopy and transmission electron microscopy. Phase analysis of precipitates is performed by the combination of selected area electron diffraction and microanalysis of chemical composition. Precipitates present in the interdendritic areas of as-built Inconel 625 are identified as MC and M23C6 carbides as well as the Laves phase.

Ni-Concentration Dependence of Directed Energy Deposited NiTi Alloy Microstructures

Localized re-melting and re-heating in regions between overlapping passes and layers drive localized microstructure development during laser-based additive manufacturing processes. Ti-rich (Ti52.1Ni47.9 at.%) and Ni-rich (Ni53Ti47 at.%) alloy builds were fabricated using a laser-based directed energy deposition (LDED) process and sectioned in three orthogonal cross-sections to provide a 3-dimensional view of the grain and precipitate morphologies in the interpass and interlayer regions. Fine equiaxed grains, which were an order of magnitude smaller than the columnar grains in the bulk regions, were observed in the interfacial regions. While these columnar grains showed preferential orientation in the highest heat flow direction corresponding to the build height, the fine equiaxed grain structures in the interfacial regions showed no orientation preferences. In addition to these changes in grain structures, precipitation of second phases is also impacted in these regions. The re-melting and re-heating characteristic of the interfacial regions produces precipitates in the Ni-rich alloys with fine oriented morphologies different from those observed in the bulk regions.

Effect of Solution Treatment on the Microstructure and Performance of S31254 Super Austenitic Stainless Steel

The second phases in the S31254 super austenitic stainless steel is easily precipitated, and the solution treatment plays an important role on the microstructure and performance. In this work, the solution treatment is performed on S31254 steel at 1180, 1200, and 1220 °C for 1, 2, and 4 h, respectively. Given the grain size and precipitation of second phase, the 1200 °C for 4 h or 1220 °C for 2 h may be suitable as the solid‐solution parameters. The corrosion resistance and impact toughness of the steel after solution treatment at 1200 °C are further investigated by means of microstructural analysis and electrochemical experiments. The sample after solution treatment for 4 h, where no second phase can be observed, exhibits a lower corrosion rate and a better mechanical properties, compared to that after annealed for 1 or 2 h. The electrochemical impedance spectroscopy, Mott–Schottky, and XPS results indicate that the passive films of the samples, with a significantly decreased donor and acceptor densities and increased Cr2O3 and Mo‐riched oxides, may be responsible for the improvement of corrosion resistance. The impact toughness results show that the samples after solution treatment for 4 h has the highest impact energy among the studied samples.

Evaluación ultrasónica de las fases formadas de un acero inoxidable dúplex 2205 tratado térmicamente a 750 °C y su comportamiento frente a la corrosión

El presente estudio evalúa el comportamiento de las variables ultrasónicas en probetas de acero inoxidable dúplex 2205 (UNS31803/ EN1.4462), tratadas térmicamente a 750 °C durante distintos tiempos, con el objetivo de promover la precipitación de fases secundarias, chi (χ) y sigma (σ). Se estudió la evolución microestructural mediante microscopía óptica y electrónica de barrido, y utilizando la técnica ultrasónica pulso eco de contacto. Finalmente, se realizaron ensayos electroquímicos con el objetivo de evaluar la resistencia a la corrosión. Los resultados revelaron que a medida que aumenta el tiempo de tratamiento térmico se produce un incremento de las cantidades relativas de fases χ y σ a lo largo de las interfases ferrita/ferrita y ferrita/austenita, especialmente hacia el interior del grano ferrítico. La velocidad de onda longitudinal y el coeficiente de atenuación presentaron una tendencia que coincide con los cambios microestructurales generados por efecto del tratamiento térmico. La evaluación electroquímica reveló una alta resistencia a la corrosión uniforme. No obstante, se observó una correlación entre el aumento del tiempo de tratamiento térmico con la pérdida de la resistencia a la corrosión localizada.

Duplex and Superduplex Stainless Steels: Microstructure and Property Evolution by Surface Modification Processes

The paper presents an overview of diffusion surface treatments, especially nitriding processes, applied to duplex and superduplex stainless steels in the last five years. Research has been done mainly to investigate different nitriding processes in order to optimize parameters for the most appropriate procedure. The scope has been to improve mechanical and wear resistance without prejudice to the corrosion properties of the duplex and superduplex stainless steels. Our investigation also aimed to understand the effect of the nitriding layer on the precipitation of secondary phases after any heating step.

Effect of Self-Tempering Temperature on Precipitation of Secondary Phase in V–Ti–N High-Strength Seismic Bars

The effect of self-tempering temperature on precipitation of secondary phase is studied by using a V–Ti–N hot-rolled anti-seismic reinforced bar (HRB500E) at 800–900 °C. The results show that the secondary phase precipitated in steel is mainly square titanium nitride and elliptical vanadium carbonitride, and the size of TiN is above 50 nm, while that of V(C, N) is below 20 nm. As the self-tempering temperature decreases, the size of V(C, N) decreases, but the amount of precipitates at 850 °C is close to 900 °C, and the size is slightly less than that at 800 °C. With the decrease in self-tempering temperature, ferrite grains are gradually refined, yield strength increases, and strength ratio and elongation decrease. By controlling self-tempering temperature at 850 °C, the excellent comprehensive performance can be acquired with matched strength and toughness simultaneously.

Machine-Learning-Assisted Development and Theoretical Consideration for the Al2Fe3Si3 Thermoelectric Material.

Chemical composition alteration is a general strategy to optimize the thermoelectric properties of a thermoelectric material to achieve high-efficiency conversion of waste heat into electricity. Recent studies show that the Al2Fe3Si3 intermetallic compound with a relatively high power factor of ∼700 μW m-1 K-2 at 400 K is promising for applications in low-cost and nontoxic thermoelectric devices. To accelerate the exploration of the thermoelectric properties of this material in a mid-temperature range and to enhance its power factor, a machine-learning method was employed herein to assist the synthesis of off-stoichiometric samples (namely, Al23.5+ xFe36.5Si40- x) of the Al2Fe3Si3 compound by tuning the Al/Si ratio. The optimal Al/Si ratio for a high power factor in the mid-temperature range was found rapidly and efficiently, and the optimal ratio of the sample at x = 0.9 was found to increase the power factor at ∼510 K by about 40% with respect to that of the initial sample at x = 0.0. The possible mechanism for the enhanced power factor is discussed in terms of the precipitations of the metallic secondary phases in the Al23.5+ xFe36.5Si40- x samples. Furthermore, the maximum achievable thermal conductivity of Al2Fe3Si3 estimated by the Slack model is ∼10 W m-1 K-1 at the Debye temperature. An avoided-crossing behavior of the acoustic and the low-lying optical modes along several crystallographic directions is found in the phonon dispersion of Al2Fe3Si3 calculated by ab initio density functional theory method. These preliminary results suggest that Al2Fe3Si3 can have a low thermal conductivity. The calculated formation energies of point defects suggest that the antisite defects between Al and Si are likely to cause the Al and Si off-stoichiometries in Al2Fe3Si3. The theoretically obtained insight provides additional information for the further understanding of Al2Fe3Si3.

Effects of the precipitation of secondary phases on the erosion-corrosion of 25% Cr duplex stainless steel

This study examined the effect of aging at 850 °C on the precipitation of secondary phases and the resultant erosion-corrosion behavior of 25% Cr duplex stainless steels (DSS). An aging time of 30 min, which led to a ratio of reactivation current to activation current (ir/ia × 100) of ˜1% in a DL-EPR test and a current density of ˜0.8 mA cm−2 at 9 m s−1 in a jet impingement test, is regarded as a boundary aging condition for erosion-corrosion. Further, the current density response of ˜0.8 mA cm−2 can be considered a criterion for the surface depassivation resulting from erosion-corrosion.

The Influence of Segregation Bands and Hot Rolling on the Precipitation of Secondary Phases during Aging at 750 °C for Nickel Alloy 625

For Inconel 625, where the γ” and δ phases precipitate, the influence of prior hot rolling on the process is not well covered. The influence of segregation bands and prior hot rolling on the precipitation of secondary phases during aging at 750 °C for different times was investigated. Prior hot-rolling was conducted on a hot rolling mill at 1050 °C and 1150 °C with three different deformation levels. The hot rolled samples were aged at 750 °C for 1, 5, 25 and 125 h. The γ″ precipitated in both the deformed and recrystallized grains in the segregation bands containing a high concentration of niobium and molybdenum and a lower concentration of nickel, chromium and iron. The opposite was observed between the segregation bands where no γ″ precipitate was found. There was a smooth transition in the density and the size of the γ″ particles in the deformed grains at the border of the segregation bands, while a more complex transition occurred in the recrystallized grains. This occurred in the area where the average niobium concentration decreased from 4.5 to 2.7 wt. %, which influenced the mechanical properties.

Direct Observations of the Coupling between Quartz Dissolution and Mg-Silicate Formation

Although quartz is a stable mineral at Earth surface conditions, field samples have shown its rapid dissolution in combination with the precipitation of Mg-silicate phases. Atomic force microscopy (AFM) experiments were performed to investigate the dissolution of quartz and the precipitation of secondary phases in high-pH, Mg-rich solutions both in situ and ex situ. Experiments were conducted at room temperature with varying MgCl 2 or MgSO 4 concentrations (0.1-100 mM), pH (8.9-12) and ionic strength (<1-530 mM). The results suggest that quartz dissolves by the removal of nanoparticles on the time scale of minutes, and that a nanometer-scale gel-like layer of amorphous silica forms on the quartz surface and is thicker at higher pH. During the in situ experiments, soft and poorly attached precipitates form on the surface when the Mg concentration is high (100 mM). After 20 h in a high-pH, Mg-rich solution, solid Mg-rich precipitates can be observed at places on the surface where the gel-like silica layer is present, predominantly near surface edges where dissolution is enhanced. This suggests a coupling between the dissolution of quartz, that resulted in the gel-like layer, and the formation of secondary phases, indicating an interface-coupled dissolution-precipitation mechanism. The precipitates could not be precisely identified but evidence suggests they are likely to be amorphous Mg-silicate phases. Such a coupled reaction may provide a pathway for Mg-Si phase formation suitable as a new environmentally friendly cement.

The Comparison in the Microstructure and Mechanical Properties between AZ91 Alloy and Nano-SiCp/AZ91 Composite Processed by Multi-Pass Forging Under Varying Passes and Temperatures.

In this study, both AZ91 alloy and nano-SiCp/AZ91 composite were subjected to multi-pass forging under varying passes and temperatures. The microstructure and mechanical properties of the alloy were compared with its composite. After six passes of multi-pass forging at a constant temperature of 400 ℃, complete recrystallization occurred in both the AZ91 alloy and composite. The decrease of temperature and the increase of passes for the multi-pass forging led to further refinement of dynamic recrystallized grains and dynamic precipitation of second phases. The grain size of the nano-SiCp/AZ91 composite was smaller than that of the AZ91 alloy under the same multi-pass forging condition, which indicated that the addition of SiC nanoparticles were beneficial to grain refinement by pinning the grain boundaries. The texture intensity for the 12 passes of multi-pass forging with varying temperatures was increased compared with that after nine passes. The ultimate tensile strength is slightly decreased while the yield strength was increased unobviously for the AZ91 alloy with the decrease of temperature and the increase of the passes for the multi-pass forging. Under the same condition of multi-pass forging, the yield strength of the composite was higher than that of the AZ91 alloy due to the Orowan strengthening effect and grain refinement strengthening resulting from externally applied SiC nanoparticles and internally precipitated second phases. By comparing the microstructure and mechanical properties between the AZ91 alloy and nano-SiCp/AZ91 composite, the strength-toughness properties of the composites at room temperature were affected by the matrix grain size, texture evolution, SiC nanoparticles distribution and the precipitated second phases.

Preparation and mechanical properties of selective laser melted H13 steel

Abstract

Magnetoelectric coupling in multiferroic BiFeO3 by co-doping with strontium and nickel

We report the effects of the Sr2+ and Ni2+ co-doping of BiFeO3 on the crystal structure and multiferroic properties of Bi1−xSrxFe1-yNiyO3 (x = 0.05, 0.0 ≤ y ≤ 0.10, and Δy = 0.05) that is synthesized using assisted high-energy ball milling. The mixtures of Bi2O3, Fe2O3, SrO and NiO were milled for 5 h, pressed at 900 MPa, and sintered at 800 °C in order to obtain cylindrical test pieces. X-ray diffraction and Rietveld refinement elucidated the effects of Sr2+ and Ni2+ on the crystal structure. Co-doping with SrNi in suitable proportions stabilizes rhombohedral BiFeO3. High contents of Ni2+ promote the precipitation of secondary phases in the forms of NiFe2O4 and Bi25FeO40. The magnetic behavior was examined by means of vibrating sample magnetometry. The results showed a change in the magnetic order from antiferromagnetic for the undoped sample to the ferromagnetic order for the co-doped samples. This change is attributed to the modulations in the magnetic moment due to crystal structure distortions. All samples show high relative permittivity values, which were enhanced by doping with Sr2+. Ni2+ cations increase the dielectric dissipation factor; this enhancement is related to their interactions with cations of a different oxidation state, such as Fe3+, Fe2+, Ni2+, Bi3+ and Sr2+ in the crystal structure of BiFeO3. The magnetoelectric coupling that was evaluated using magnetodielectric measurements was above 4% at 1 kHz for the higher applied magnetic field of 18 kOe.

Development of spark plasma sintered TiAlSiMoW multicomponent alloy: Microstructural evolution, corrosion and oxidation resistance

Abstract TiAlMoSiW high entropy alloy was fabricated using spark plasma sintering under different sintering temperatures. The microstructural and morphological characteristics of the alloy were investigated using X-ray diffractometry (XRD) and scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The mechanical and corrosion properties of the alloy were tested using diamond base indentor and potentiodynamic polarization. Thermogravimetric analysis and density measurements were used to check the thermal stability and densification of the samples. Microstructural analysis of the samples showed that the samples exhibited a bcc matrix with secondary phase precipitate of TiSi2. The microhardness was found to be subject to sintering temperature and densification with high microhardness values obtained 1000 °C and 98% densification. The presence of elements that form stable protective film were attributed to the high corrosion performance of the samples. These elements also form stable oxide scales in elevated temperatures improving the thermal stability of the alloy. Excessive oxidation was noticed to occur at temperatures above 535 °C showing that the alloy has superior thermal stability than Ti-6V-4Al.

Strain‐Induced Dissolution and Precipitation of Secondary Phases and Synergetic Stengthening Mechanisms of Al–Zn–Mg–Cu Alloy during ECAP

The evolution of secondary phase of 7A60 aluminum alloy before and after equal channel angular pressing (ECAP) deformation and the resultant variation of mechanical properties are studied by means of X‐ray diffractometer (XRD), transmission electron microscopy (TEM), optical microscope (OM), and Instron testing machine. The results show that grains are refined to submicron level after ECAP (from 9.86 µm to about 1 µm). As the strain increases, the main secondary phase (MgZn2) dissolves into the aluminum matrix to form a supersaturated solid solution. Part of the solute atoms diffuses along the vacancies and dislocations under the combined effect of interface energy, strain energy, and grain boundary energy. In addition, grain boundary segregation occurs and further leads to relatively coarse secondary phase. With the increase of temperature, discontinuous precipitation (DP), which occurs from the grain boundary to the grain internal, dominates the formation process of secondary phases. The phase transformation brings about significant precipitation strengthening, resultantly, enhances strength, and ductility are achieved for 7A60 aluminum alloy via ECAP. Moreover, the precipitation strengthening effect, which is closely related with the dissolution and DP, first enhances and then attenuates with the increase of ECAP passes. Dislocation strengthening and Hall‐Petch strengthening aggravate monotonically with the ECAP passes but play less important roles in the overall stress responses.

Effect of Final Annealing Temperature on Corrosion Resistance of SZA-6 Zirconium Alloy Cladding Tubes

The corrosion resistance of SZA-6 zirconium alloy(Zr-0.5Sn-0.5Nb-0.3Fe-0.015Si) cladding tubes finally annealed at 480°C, 510°C and 560°C were studied by static autoclave in 360°C/18.6 MPa pure water and 360°C/18.6 MPa/0.01 mol/L LiOH aqueous solution. The microstructure of the samples before and after corrosion were analyzed by EBSD, TEM and SEM. The results showed that the corrosion weight gains of the three SZA-6 alloy samples were lower than that of Zr-4 alloy after 500 days corrosion in both hydrochemical mediums. After long-term corrosion, the corrosion weight gains of SZA-6 alloy in pure water and LiOH aqueous solution increased obviously with the final annealing temperature, while the corrosion weight gain of unstressed Zr-4 alloy was higher than that of recrystallized under the same condition. With the increase of the final annealing temperature, the high-angle grain boundaries in the alloy larger than 15° became more and recrystallization degree also increased. The Second Phase Precipitates (SPPs) were fine, uniform, and dispersively distributed with an average diameter of about 120 nm. Although the size and distribution of the SPPs were similar, the Nb/Fe ratio in the SPPs increased. The long-term corrosion weight gain of zirconium alloy was related to the number of parallel cracks in the oxide film and the uneven growth degree of the oxide film on the interface of the oxide film/matrix. The corrosion resistance of the alloy in two hydrochemical mediums was related to the degree of recrystallization and the content of Nb in the SPPs. Increasing the final annealing temperature would promote the formation of fine and uniform recrystallized grains, which was benefit to the corrosion resistance, but at the same time it would reduce the content of solid solution Nb in the αZr matrix, which in turn would be detrimental to the corrosion resistance.

Weathering in a regolith on the Werenskioldbreen Glacier forefield (SW Spitsbergen). 2. Speciation of Fe, Mn, Pb, Cu and Zn in the chronosequence

The evolution of chemical speciation of Fe, Mn, Pb, Cu, and Zn was investigated in the chronosequence of young sediments, exposed by a currently retreating Arctic glacier on Spitsbergen. Werenskioldbreen is a 27 km 2 subpolar, land-terminated, polythermal glacier in recession, located near the SW coast of West Spitsbergen. Three samples of structureless till were collected at locations exposed for 5, 45 and 70 years. Four grain-size fractions were separated: &gt; 63, 20–63, 2–20, and &lt; 2 μm. Speciation of Fe, Mn, Pb, Cu, and Zn was determined using a 6-step sequential chemical extraction method: 1) 1 M sodium acetate, 2) 1 M hydroxylamine hydrochloride in acetic acid, 3) sodium dithionite in buffer, 4) acid ammonium oxalate, 5) boiling HCl, 6) residuum. The weathering in the proglacial area of the retreating glacier is very fast. The geochemical fates of the metals in question correlate with each other, reflecting a) the geochemical similarities between them, b) the similarities of their primary mineral sources, c) the significant role of incongruent dissolution. The weathering processes dominating the system are redox reactions and incongruent dissolution, followed by precipitation of secondary phases and partial sorption of aqueous species. As a result, the elements released from weathering minerals are only partially transported away from the system. The remaining part transforms by weathering from the coarse-grained fraction (dominated by fragments of primary minerals) into the fine-grained fraction (in the form of secondary, authigenic minerals or as species sorbed onto a mineral skeleton). This is very strongly pronounced within the chronosequence: the content of each of the metals studied correlates identically with the grain size, despite the differences in their chemical character and affinities. The microscope study presented herein indicates that the role of incongruent dissolution previously was underestimated. Also, the formation of coatings of secondary phases on primary mineral surfaces was observed. All these rapid weathering processes affect the mineral speciation of initial soils as well as the composition of mineral suspensions transported away by rivers to the nearby ocean.

Effect of Artificial Aging Temperature on Mechanical Properties of 6061 Aluminum Alloy

Aluminum alloys have been attracted by several engineering sectors due to their excellent strengthweight ratio and corrosion resistant properties. These are categorized into 1, 2, 3, 4, 5, 6, 7and 8xxx on the basis of alloying elements. Among these 6xxx series contains aluminum–magnesium–silicon as alloying elements and are widely used in extruded products and automotive body panels. The major advantages of these alloys are good corrosion resistance, medium strength, low cost, age hardening response no yield point phenomenon and Ludering. 6xxx series alloys generally have lower formability than other aluminum alloys which restrict their utilization for wide applications. Keeping in view of the shortcomings in the set of mechanical properties of 6xxx series the efforts were made to improve the tensile strength and toughness properties through age hardening. In present study heat treatment cycles were studied for 6061 aluminum alloy. Three different age hardening temperatures 160, 200 and 240oC were selected. The obtained results showed that 17.26, 7.69, and 10.51% improvement in tensile strength, toughness and hardness respectively was achieved with solution treatment at 380oC followed by an aging 240oC. Microstructural study revealed that substantial improvements in the mechanical properties of 6061 aluminum alloy under heat treatment were achieved due to precipitation of Mg2Si secondary phase.