Search

Refractory High entropy alloy (RHEA) is a potential material for coating gas turbine blades and pipeline due to its high-temperature mechanical and chemical properties. In this paper, a series of new NbTiCrFeMoX (x = 0.2, 0.4, 0.6, 0.8, 1) RHEAs were coated on GTD-111 nickel base superalloy by the laser cladding method. The effects of the Mo amount on the microstructure and tensile, creep, corrosion, and wear properties were investigated. XRD results showed that the microstructure of all five coatings included the B2 regular phase, the BCC irregular phase, C14-Laves (FeTi2), and C15-Laves (Cr2Nb). However, with the increase of Mo from 0.2 to 1, the amount of the BCC phase increased from 24.1 to 29.5%, the C14 phase increased from 55.1 to 61.4%, and the amount of the C15 phase decreased from 11.2 to 1.8%. The yield strength increased by increasing the volume fraction of BCC and C14-Laves phases from 328 MPa for the Mo0.2 sample to 685 MPa for the Mo1 specimen. The same factor increased the creep life of RHEA from 43 to 54 h under a force of 450 N and temperature of 800 °C by increasing the places of dislocation locking. The simultaneous presence of the BCC solid solution and Laves phase was one of the factors that reduced the coefficient of friction during the wear test from 0.63 to 0.44 with increasing Mo. Electrochemical tests in 3.5 wt% NaCl solution showed that the RHEAs showed significant corrosion resistance. Specimen Mo1 with the smallest Icorr (1.6103 × 10–6 A cm2) and the highest Ecorr (− 1.2025 V) showed the best corrosion resistance. [ABSTRACT FROM AUTHOR]

Nickel-based superalloys exhibit exceptional suitability for operating in environments characterized by corrosive agents and elevated temperatures. Strategic allocation of this expensive material solely to the functional surface areas yields significant economic advantages. The poor tribological property profile of Inconel 718 can be significantly improved through a boriding process. In this study, the possibility of combining a coating process with boriding technology and in situ heat treatment was investigated. Layers of Inconel 718 were deposited to an austenitic stainless steel using wire-based electron beam cladding (EBC) and subsequently subjected to boriding. Based on results from annealing experiments, boriding treatments were performed at various temperature/time regimens with the aim of inducing in situ age hardening during boriding. The focus was on the investigation of the influence of the temperature/time regime during boriding on the microstructure and hardness, as well as examining the wear and corrosion behavior of the resulting borided layers. The results showed that the desired target hardness range was achieved after in situ aging with all boriding variants. Furthermore, it was demonstrated that boriding significantly improved the wear resistance but decreased corrosion resistance. [ABSTRACT FROM AUTHOR]

The pronounced brittleness of hard Laves phase intermetallics is detrimental to their tribological properties at room temperature. In this study, we utilized a heterogeneous structure to engineer an ultrastrong dual-phase (Laves + B2) AlCoFeNiNb high-entropy alloy that exhibits a low wear rate (3.82×10−6 mm3/(N·m)) at room temperature. This wear resistance in the ball-on-disc sliding friction test with the counterpart of Al2O3 balls stems from the activated deformation ability in the ultrafine Laves lamellae under heterogeneous interface constraints. Furthermore, as tribological stress intensifies, the surface deformation mechanism transitions from dislocation slip on the basal and pyramidal planes to a unique combination of local shear and grain rotation within the Laves phase. Our study illuminates fresh perspectives for mitigating the embrittling effect of Laves phase intermetallics under tribological loading and for the development of wear-resistant materials. [ABSTRACT FROM AUTHOR]

This study was conducted on the quality control of Inconel625 alloy thin-walled structural components by CMT-WAAM (Cold Metal Transfer Based Wire and Arc Additive Manufacture) process. The solid solution + two-stage aging treatment process was proposed for the heat treatment of thin-walled parts. The effect of solid solution treatment temperature on the microstructure and mechanical properties of thin-walled parts was studied under the same aging treatment conditions. The experimental results show that the deposited microstructure of Inconel625 thin-walled structural parts is mainly columnar dendrites, which grow epitaxially along the deposition direction. The basic microstructure of specimen is γ-Ni solid solution, and many blocky or chain-like Laves phases are distributed in the grains and grain boundaries. After solid solution and two-stage aging treatment, the Laves phase of the specimen dissolves back, δ phase precipitates and significant changes in mechanical properties. With the increase of solution treatment temperature, the Laves phases dissolve gradually, accompanied by δ phases precipitation. As the solid solution treatment temperature increases, the Laves phase gradually dissolves back, accompanied by δ Phase precipitation. When the solid solution treatment temperature rises to 1200 °C, the Laves phase is completely dissolved, δ The phase disappears and the grain size significantly increases. As the solution treatment temperature increases, the microhardness and tensile strength show a trend of first increasing and then decreasing, while the yield strength shows no significant change. [ABSTRACT FROM AUTHOR]

The solid solution design strategy bolsters the comprehensive mechanical properties of high/medium entropy alloys (H/MEAs), yet its utilisation within hexagonal close-packed (HCP) H/MEAs remains notably limited. Here, exploring the impact of reduced Cr content (x = 0.3, 0.1, and 0) from the HfZrTiCr0.5 prototype alloy, our study successfully obtained a novel HCP single phase MEA and investigated the relationship between microstructure and mechanical properties. Decreasing Cr content led to the diminishing presence of the Laves phase, culminating in a single HCP phase solid solution at x = 0.1. The Laves phase, arising from negative enthalpy between Cr and other elements and lower atomic radius of Cr, acted as crack initiation sites, resulting in brittle fracture (x = 0.5) alloy and limited plasticity strain of 3.5% (x = 0.3). Conversely, the HfZrTiCr0.1 alloy exhibited impressive combined yield strength (1360 MPa) and plasticity strain (14.5%), attributed to prominent solid solution strengthening and enhanced dislocation movement within the solid solution structure. These findings offer insights into manipulating Cr content to form HCP solid solution structures, potentially broadening solid solution alloy design strategies for superior mechanical properties in HCP H/MEAs. [ABSTRACT FROM AUTHOR]

The alloys of the Ti–Zr–Mn system based on the C14-type Laves phase are considered as ones of the most promising materials for safe storage and transportation of hydrogen. These alloys have appropriate parameters for activating the processes of absorption and release of hydrogen, a low cost, and a fairly high cyclic stability. In this work, the microstructure and phase composition of the starting alloys and the crystal structure of the hydrides synthesized from them are studied. Possible ways to reduce the cost of the final products are shown. The fact that changing the method of the alloy fabrication does not significantly affect its hydrogen absorption properties is shown. On the example of the considered alloys, it is shown that, as expected, alloying with an element with a larger atomic radius that forms a stable chemical compound with hydrogen results in an increase in the hydrogen capacity. This is explained by both the increased radius of the tetrahedral interstitial sites, where hydrogen atoms are located after dissolution, and the higher total amount of the element interacting with hydrogen.

The evolution of tensile properties and microstructure in three 9% Cr martensitic rotor steel with varying contents of W and Mo elements is investigated after aging at 630 °C for different durations, with a primary focus on the characteristics of martensite laths and blocks, grain boundaries, dislocations, and precipitates. The results indicate that the presence of W and Mo elements not only influences the precipitation and coarsening behavior of Laves phases but also that of M23C6 carbides, which differs from conventional beliefs. The elevated content of Mo and reduced content of W in the steel leads to a reduction in the equilibrium Cr concentration, promoting enhanced precipitation of Cr atoms and increases nucleation sites of M23C6 carbides. During the aging treatment, the M23C6 carbides in the 2.0W0.5Mo steel exhibits the highest content and the lowest coarsening rate. The strength of 9% Cr martensitic rotor steel is primarily enhanced through dislocation strengthening and grain boundary strengthening. The presence of fine M23C6 carbides effectively immobilizes dislocations and interfaces, resulting in the exceptional stability of the microstructure in the 2.0W0.5Mo steel and a minimal reduction in yield strength.

The electrochemical corrosion mechanisms of Mg alloys were extensively studied in previous investigations of different chemical compositions, modified surface states and various electrolyte conditions. However, recent research focused on the active state of Mg dissolution, leading to unresolved effects of secondary phases adjacent to a stable α-solid solution passive layer. The present study investigates the fundamental electrochemical corrosion mechanisms of three different Laves phases with varying phase morphologies and phase fractions in the passive state of Mg-Al-Ca alloys. The microstructure was characterized by (transmission-) electron microscopy and synchrotron-based transmission X-ray microscopy. The electrochemical corrosion resistance was determined with a standard three-electrode setup and advanced in-situ flow cell measurements. A new electrochemical activity sequence (C15>C36>α-Mg>C14) was obtained, as a result of a stable passive layer formation on the α-solid solution. Furthermore, nm-scale Mg-rich precipitates were identified within the Laves phases, which tend to inhibit the corrosion kinetics.

For developing the low-cost, high-performance eutectic medium-entropy alloy, Cofree eutectic medium entropy alloys Cr2 FeNiNbx (X=0.4,0.59,0.8) and CrFe1.5 Ni1.5 Nby (Y =0.4,0.5,0.61) were designed based on the elemental radius, mixing enthalpies and simple eutectic mixture method. Two alloy systems were prepared by vacuum arc melting. The microstructures of alloys were analyzed by XRD, EDS and SEM, and the mechanical properties of alloys were valued by Vickers hardness and universal electro-mechanical tests. The microstructure transformations and strengthening-toughening mechanisms were eventually discussed. The results shown that, with the increasing Nb content, the microstructures of both alloy systems underwent hypoeutectic eutectic hypereutectic transformations. The primary phase of Cr2 FeNiNbx (X=0.4,0.59,0.8) evolved from BCC solid solution to Laves phase, whereas that of CrFe1.5 Ni1.5 Nby (Y=0.4,0.5,0.61) changed from FCC solid solution to Laves phase. The Vickers hardness, fracture strength and fracture strain of Cr2 FeNiNbx (X= 0.4,0.59,0.8) were 726~822 HV,2638~2835 MPa and 7.98%~9.22%, respectively, and thoese of CrFe1.5 Ni1.5 Nby (Y=0.4,0.5,0.61) were 359~460 HV,1880~2 115 MPa and 20.48%~25.65%, respectively. Attractively, the alloy CrFe1.5 Ni1.5 Nb0.4 was manifested with excellent combination of strength and toughness, with high fracture strength of 2 110 MPa and fracture strain of 24. 25%. The outstanding comprehensive mechanical properties were attributed to synergistic effects of Laves phase strengthening, interface strengthening, solid solution strengthening, second phase toughening and alloying toughening mechanisms. [ABSTRACT FROM AUTHOR]

The alloys of the Ti-Zr-Mn system based on the C14-type Laves phase are considered as ones of the most promising materials for safe storage and transportation of hydrogen. These alloys have appropriate parameters for activating the processes of absorption and release of hydrogen, a low cost, and a fairly high cyclic stability. In this work, the microstructure and phase composition of the starting alloys and the crystal structure of the hydrides synthesized from them are studied. Possible ways to reduce the cost of the final products are shown. The fact that changing the method of the alloy fabrication does not significantly affect its hydrogen absorption properties is shown. On the example of the considered alloys, it is shown that, as expected, alloying with an element with a larger atomic radius that forms a stable chemical compound with hydrogen results in an increase in the hydrogen capacity. This is explained by both the increased radius of the tetrahedral interstitial sites, where hydrogen atoms are located after dissolution, and the higher total amount of the element interacting with hydrogen. [ABSTRACT FROM AUTHOR]

• Mg-rich nm-scale precipitates exist in the C36 and the C14 laves phases. • The electrochemical activity (C15>C36>α-SS>C14) accounts in the passive state. • Interconnected C14 laves phases increases the corrosion current density. • Small and dispersed C15 laves phases decrease the corrosion current density. • Interconnected C36 laves phases cause minimal corrosion current densities. The electrochemical corrosion mechanisms of Mg alloys were extensively studied in previous investigations of different chemical compositions, modified surface states and various electrolyte conditions. However, recent research focused on the active state of Mg dissolution, leading to unresolved effects of secondary phases adjacent to a stable α-solid solution passive layer. The present study investigates the fundamental electrochemical corrosion mechanisms of three different Laves phases with varying phase morphologies and phase fractions in the passive state of Mg-Al-Ca alloys. The microstructure was characterized by (transmission-) electron microscopy and synchrotron-based transmission X-ray microscopy. The electrochemical corrosion resistance was determined with a standard three-electrode setup and advanced in-situ flow cell measurements. A new electrochemical activity sequence (C15>C36>α-Mg>C14) was obtained, as a result of a stable passive layer formation on the α-solid solution. Furthermore, nm-scale Mg-rich precipitates were identified within the Laves phases, which tend to inhibit the corrosion kinetics. [Display omitted] [ABSTRACT FROM AUTHOR]

Laser cladding is a promising surface modification and repair technology for fabricating Inconel 718 parts. However, laser cladding may produce coarse grains and brittle Laves phase, leading to a significant decrease in performance. The application of high-intensity ultrasonic vibration in laser cladding is able to refine grain and reduce Laves phase. This study investigates the distribution characteristics and the evolution mechanism of the grains and Laves phase in laser cladding with high-intensity ultrasonic vibration. The effects of high-intensity ultrasonic vibration on alteration of grain characteristics and the reduction of Laves phase were analyzed and discussed through ultrasonic vibration-assisted laser cladding experiments. Results indicate that high-intensity ultrasonic vibration promoted a columnar-to-equiaxed transition, and reduced the grain size from 75.6 μm to 48.8 μm. Furthermore, the Laves phase had a decrease of 47.1% in the volume fraction, and transformed from a long-striped shape into a fine granular shape, leading to a more homogeneous distribution of elements in Laves phase. The change in the grains and the Laves phase is attributed to dendrite fragmentation and inhibition of element segregation by nonlinear effects of ultrasound including cavitation and acoustic streaming. The findings of this study confirm the significant benefits of applying high-intensity ultrasonic vibration in laser cladding and provide insight into the underlying evolution mechanisms of high-intensity ultrasonic vibration for grains and Laves phase.

The effect of yttrium on the 475 °C brittleness and microstructure stability of Fe–13Cr–6Al–2Mo-0.5Nb-0.15Zr alloy after long-term aging at 475 °C has been investigated. At the early stage of aging, the nanoscale metastable transition phase precipitates in large quantities. After aging for 10,000 h, the nanophase in the Y-free alloy has completely disappeared, and only a great deal of granular Laves phases are left, while a large number of nanoscale phases still exist in the grain of Y-containing alloys. This is because the addition of yttrium combines with vacancies, hinders the diffusion of Laves phase-forming elements, and has the result that the transformation of nanoscale phases Fe2(Mo0.3Nb0.7)Al to Laves phase is inhibited during the aging process of the Y-containing alloys. During aging, precipitation-free zones (PFZ) are formed in the surrounding area where the Laves phase and grain boundaries exist. The elongation of Y-containing alloys decreases with yttrium content, but it is still higher than that of Y-free alloy. The addition of yttrium improves the aging plasticity and weakens the degree of 475 °C brittleness of the alloy, and the effectiveness is diminished with the yttrium. This plays a key role in the further study of Fe–Cr–Al alloys for fuel cladding materials.

Laser direct deposition (LDD) is widely used to repair and manufacture high-value industrial components. However, it faces various defects, such as porosity, cracks, non-uniform microstructure, lack of fusion, keyhole phenomenon, element segregation, and undesirable secondary phases. A method to manage these defects is to concurrently apply ultrasonic vibrations (USV) during the LDD process. This study investigates the effect of USV on the mass efficiency and microstructure of LDD Inconel 718 superalloy to understand how incorporating USV can change the performance and structural integrity of single passes produced using the LDD process. For this purpose, USV is applied to a substrate during the LDD process. The resulting samples are characterized and analyzed using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicate that applying USV below a threshold power value increases mass deposition by over 25%, while exceeding this threshold reduces it. Attention to this threshold power value is crucial for determining the process parameters, including laser power and speed. Additionally, USV transforms the microstructure from columnar to equiaxed and increases subgrain formation. This implementation also enhances the cooling rate, significantly decreasing the Laves phase by over 30% in all process parameters.

Hydrogen is now being used as a renewable clean energy carrier. One of the main issues with the application of hydrogen energy is a shortage of security and efficient hydrogen storage technology. TiMn-based alloys are considered promising hydrogen storage materials, but their comprehensive hydrogen storage properties and cyclic stable performance limit their further practical application. The hydrogen storage properties of alloys can be enhanced by substituting transition metal elements. Therefore, the comprehensive hydrogen storage performance of the Ti 0·9 Zr 0·1 Mn 0·95 Cr 0·7 V 0.2 M 0.15 (M ​= ​Fe, Co, Ni, Cu, Mo) alloys was systematically investigated according to the Mn element on the B side is partially replaced by variety of transition metal elements. The M ​= ​Ni alloy, which showed the highest hydrogen storage capacity among the group of alloys, was used to explore cycle stability. The plateau pressures of the series alloys decreased in order, Fe ​> ​Co ​> ​Ni ​> ​Cu ​> ​Mo. Aspects of hydrogen absorption kinetics, all of the alloys can reach full hydrogen absorption saturation within 400 ​s at 303 ​K. The Ti 0·9 Zr 0·1 Mn 0·95 Cr 0·7 V 0·2 M o 0.15 alloy possessed the fastest hydrogen absorption kinetic rate (t 0.9 ​= ​65 ​s) and the smallest hysteresis factor. This suggests that the substitution of Mo elements is effective in improving the hysteresis of the Laves phase alloys. Among the series of alloys, the M ​= ​Ni alloy exhibited the best overall hydrogen storage performance, which hydrogen storage capacity can reach 1.81 ​wt% and 97% of its capacity is kept after 100 cycles. [Display omitted] • Hydrogen storage properties of Ti 0·9 Zr 0·1 Mn 0·95 Cr 0·7 V 0.2 M 0.15 (M ​= ​Fe, Co, Ni, Cu, Mo) alloys is systematically studied. • The hydrogen storage capacity of M ​= ​Ni can reach 1.81 ​wt% and 97% of its capacity is kept after 100 cycles. • All of the alloys can reach full hydrogen absorption saturation within 400 ​s at room temperature. [ABSTRACT FROM AUTHOR]

Abstract—Multicomponent polycrystalline TbInxCo2 – x (with х = 0–0.2) solid solutions are prepared for the first time, and their crystal structure and magnetic, magnetocaloric, and magnetostrictive properties are studied. X-ray diffraction patterns taken at room temperature demonstrate mainly the presence of the cubic C15 Laves phase in all samples. As the indium content increases to x = 0.1, the lattice parameter is found to increase; the further increase in the indium content to х = 0.2 leads to a decrease in the lattice parameter. In this case, the Curie temperature TC monotonically increases to 245 K. The isotheral magnetic entropy change ΔSmag is calculated in accordance with magnetic measurements using the thermodynamic Maxwell's relation. At a magnetic field change from 0 to 1.8 T, the maximum entropy change monotonically decreases and, for composition with x = 0.2, is 1.8 J/(kg К). As the indium content increases to x = 0.05, the volume magnetostriction increases. The further increase in the indium concentration leads to the decrease in the peak values and their shift to high temperatures. [ABSTRACT FROM AUTHOR]

P91 and P92 steels are widely used in power plants and petrochemical industry for long-term service components. Due to high resistance to creep, fatigue and corrosion, the use of grade 91 and grade 92 steels allows ultrasupercritical conditions (600 °C, 300 bar). To achieve this goal the alloy design was based on the following metallurgical concepts: stable tempered martensitic microstructure, precipitation strengthening through M23C6 (Cr, Fe, Mo or W carbides) and fine MX (V, Nb carbides), solution hardening through elements as Mo or Mo/W and high Cr content. This study is focused on the microstructure evolution of grade 91 and grade 92 steels under aging and creep conditions. Three sets of laboratory-aged specimens heated in oven at 550°C, 600°C and 650°C were examined. Furthermore, the influence of stress on the microstructure was evaluated. The microstructures were characterized by several means of investigations and the results were compared to literature. [ABSTRACT FROM AUTHOR]

An ODS steel with nominal composition Fe–14Cr–2W–0.4Ti-0.3Y2O3 (wt.%) was produced by mechanical alloying and compacted by hot isostatic pressing (HIP) followed by hot cross rolling (HCR). To check the effects of thermal aging at relevant temperatures of operation in fusion power plants, the alloy was thermally aged at 873 K for 2000 h. In this work, small-angle neutron scattering (SANS) and X-ray absorption spectroscopy (XAS) techniques are used for the advanced characterization of secondary phases and the oxide nanoparticle dispersion. SANS results show that the oxide nanoparticles remain stable after the thermal aging treatment. Composition of the oxide nanoparticles was identified as Y2TiO5 or Y2Ti2O7 by SANS, while non-stoichiometry was found by XAS analysis. Laves phase precipitation after the thermal aging treatment is further confirmed by SANS, from the magnetic anisotropic contribution to the scattering intensity associated to this metallic phase, and by XANES.

CoCrFeNi-(Nb, Ta) high-entropy alloys (HEAs) were prepared by a vacuum arc melting process, and the synergistic effect of Nb and Ta concentrations on the microstructure evolution and mechanical properties was investigated in detail. Nb and Ta concentrations in the HEAs affect the microstructure compositions, eutectic phase lamella spacings, Laves phase size topographies, phase volume fractions and compositions. The alloy with an equal atomic ratio of Nb/Ta show an eutectic microstructure of the face centered cubic (FCC) crystal matrix and Laves phase, while those with non-equal atomic ratio of Nb/Ta exhibit the microstructure of the eutectic (FCC+Laves) phase and primary Laves phase. The volume fraction and grain size of the primary Laves phase monotonically increase with the increase of Nb/Ta atomic ratio. The compressive yield strength of HEAs is monotonously improved with the volume fraction of Laves phase, whereas the ultimate strength is hardly affected by the microstructure composition. Compression plasticity is negatively correlated with the volume fraction, type, and size distribution of Laves phase. The strengthening mechanism of CoCrFeNi-(Nb, Ta) HEAs was analyzed with theoretical calculation, and the relationship between the microstructure composition and the alloy strengths was established. The results indicate that the fine-grain strengthening and second phase strengthening of Laves phase are the main factors for the improvement of yield strength.

Inconel 718 alloy (IN718) is a popular choice for aerospace hot-end components due to its exceptional mechanical properties. This study investigates the impact of processing parameters (i.e., laser power, powder feeding rate, and scanning speed) on the microhardness, bond strength, and tensile strength of layer-cladded IN718. The results show that IN718 coatings have high microhardness (277.15 HV0.1), strong metallurgical bonding strength (33.97 kN), substantial yield strength (794.09 MPa), impressive ultimate tensile strength (1171.81 MPa), and notable elongation at failure (8.24 %) under laser power is 1.2 kW, powder feeding rate is 250 mg/s, and scanning speed is 4.5 mm/s. In particular, decreased laser energy input enhances microhardness, yield strength, and ultimate tensile strength but reduces bonding strength and elongation. This is attributed to improved Nb element segregation, the reduction of the Laves phase, and grain refinement. However, inadequate energy input leads to cracks and unmelted powder, negatively affecting metallurgical bonding strength. The shear and tensile fracture mechanism of the IN718 coatings is a typical ductile fracture with a microvoid accumulation fracture. The study can facilitate the fabrication of the IN718 coatings with the excellent mechanical properties and the applications in the engineering field.

This study investigated the first hydrogenation behavior of TiFe-based hydrogen storage alloy with the laves phase. A TiFe 0.9 Cr 0.1 alloy was prepared through vacuum arc melting, and it had a dual-phase microstructure with B2 and laves phases. The first hydrogenation kinetics was measured by applying a hydrogen pressure of 40 bar at 30 °C, where the alloy absorbed hydrogen without thermal activation. The first hydrogenation kinetics was divided into three sections to understand the first hydrogenation behavior, and each section was analyzed using a solid–gas reaction model. The model's goodness of fit was evaluated by fitting each section. In the early hydrogen absorption stage, a surface-controlled mechanism (Chemisorption model) was analyzed to be the most dominant. In the second stage, where hydrogen was quickly absorbed, an interface-controlled mechanism (CV3D model) was most suitable. In the last hydrogen absorption stage, where hydrogen absorption was saturated, a diffusion-controlled mechanism (GB3D model) was most suitable. • The TiFe 0.9 Cr 0.1 alloy had a dual-phase microstructure with B2 and laves phases. • The alloy absorbed hydrogen without any thermal activation process. • The first hydrogenation kinetics was divided into three sections for analysis. • Hydrogen absorption behavior was analyzed through the solid-gas reaction model. [ABSTRACT FROM AUTHOR]

Copyright of Rare Metals is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Rare Metals is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The thermal stability of the grain structure and mechanical properties of the high-entropy two-phase TiCoCrFeMn alloy produced by powder metallurgy, assessed based on microhardness measurements, was analyzed in this work. For this purpose, material obtained via sintering using the U-FAST method was subjected to long-term heating at a temperature of 1000 °C for up to 1000 h in an argon atmosphere. For homogenization times of 1, 10, 20, 50, 100, and 1000 h, grain size changes in the identified phase components of the matrix were assessed, and microhardness measurements were conducted using the Vickers method. It has been shown that the changes in the analyzed parameters are closely correlated with non-monotonic modifications in the chemical composition. It was found that the tested alloy achieved structural stability after 100 h of annealing. A stable grain size was obtained in the BCC solid solution of approximately 2 µm and the two-phase BCC+C14 mixture of roughly 0.4 µm. Long-term heating for up to 1000 h caused the grain structure to grow to 2.7 µm and 0.7 µm, respectively, with a simultaneous decrease in hardness from 1065 HV to 1000 HV. The chromium and titanium diffusion coefficient values responsible for forming the BCC solid solution and the Laves C14 phase, including the material matrix, were also determined at this level to be DCr = 1.28 × 10−19 (m2·s−1) and DTi = 1.04 × 10−19 (m2·s−1), demonstrating the sluggish diffusion effect typical of high-entropy alloys. [ABSTRACT FROM AUTHOR]

In this paper, the microstructural evolution and failure behavior of GH4169 superalloy repaired using wire-fed electron beam directed energy deposition (EB-DED) are investigated in detail. Results show that in the deposited zone (DZ), the Laves phase is formed in the interdendritic zone, and the γ'' phase and γ′ phase precipitate around the Laves phase and are distributed gradiently along the building direction. In contrast, the substrate zone (SZ) is composed of fine equiaxed grains and uniform distributed γ'' and γ′ phases. Direct aging heat treatment cannot effectively dissolve the Laves phase, but can further precipitate the γ'' and γ′ phases, resulting in an increase in yield strength and ultimate tensile strength and a decrease in elongation. During the plastic deformation process, dislocation activity preferentially occurs in the γ matrix and forms accumulation at the Laves/γ interface, leading to the formation of cracks in the DZ, so that all tensile samples fail in the DZ. In addition, this study further elucidates the fracture mechanism of the Laves phase within grains and at grain boundaries.

In this study, 1.0 wt%Al element was added into the traditional 26.8Cr-3.7Mo–2Ni super-ferritic stainless steel to tailor the brittle phase precipitation, recrystallization microstructure and mechanical property via solution annealing at temperatures of 950–1150 °C. The experimental results demonstrated that hot-rolled microstructure for 1.0Al sample consisted of surface fine grain region and core elongated grains together with TiN, NbC and Laves phase, while none of these fine grains appeared in 0Al samples. After annealing below 1050 °C, both σ-phase and Laves phase were formed in Al-free samples, while only abundant Laves phase precipitated in 1.0Al sample indicating the inhibition effect of Al on the σ-phase and the promotion effect on Laves phase. When annealing above 1050 °C, recrystallization was completed and neither σ-phase nor Laves phase were observed. Relatively stable mean grain size was produced due to the Laves phase pinning effect during annealing at 1050 °C, while the Nb/Al solute drag effect bore responsibility for the situation at 1050–1150 °C. Sudden grain coarsening phenomenon appeared after annealing above 1150 °C. Al additions gave a solution strengthening and intensive Laves phase precipitation strengthening for the designed alloys, and good comprehensive mechanical properties were obtained demonstrating the probability of developing of the Al-modified ultra-super stainless steels.

A series of CoCrFeNi(Mo, Nb) high entropy alloys (HEAs) were prepared using vacuum arc-melting. The effect of Nb and Mo on the microstructure and high temperature anti-oxidation properties of HEAs was investigated. The addition of only Mo was found to not cause formation of new phase, but did increase the lattice distortion. After the addition of Nb, as-prepared CoCrFeNiMo0.2Nb0.1 and CoCrFeNiMo0.2Nb0.2 alloys are characteristic of duplex-phase structures of FCC and Laves, in which the Laves phase is semi-coherent with FCC matrix. Our studies also revealed that the oxidation kinetics of the alloys followed parabolic rate law and the corresponding oxidation rate constants were calculated. Due to the synergistic effects of Mo and Nb, the anti-oxidation properties of HEAs improved significantly. It was also discovered that the new Laves phase containing Mo and Nb at the inner oxidation layer hindered the outward diffusion of active metal cations, ergo improving the oxidation resistance of HEAs. With increase of Nb content, it was further recognized that the layered structure of Laves phase was thinner and denser. Therefore, the oxygen ion diffusion process in the CoCrFeNiMo0.2Nb0.2 alloy can be inhibited effectively by the new formation of duplex-phase structures with undulated Laves phase and FCC matrix.

An important aspect of energy conservation and emission reduction in automotive industry is to improve the combustion efficiency of gasoline. The full combustion of gasoline will inevitably cause an increase in exhaust temperature. The coarsening and dissolution behavior of precipitates in steel will lead to a decrease in its high-temperature mechanical properties when the engine is used for a long time in such an alternating heating and cooling environment. In order to obtain ferritic stainless steel with good high-temperature resistance, this paper is based on 444 ferritic stainless steel and attempts to partially replace Mo element with W element to obtain Laves phase containing W, Mo, and Nb with more stable high-temperature performance. A simulated cyclic high-temperature annealing process up to 1050 °C was designed to investigate the precipitation behavior in this modified ferritic stainless steel. The results show that using W to replace Mo in 444 ferritic stainless steel can refine the grains of ferritic stainless steel. It can not only provide better solid solution strengthening at elevated temperature, but also stabilize the Laves phase and provide precipitation strengthening. The reason is that the W elements can change the composition of the Laves phase in ferritic stainless steels and precipitate Fe2(Nb, W) with high thermal stability to provide stable precipitation strengthening during long-term alternating annealing. The W-rich Laves phase is prone to precipitation at grain boundaries, effectively pinning grain boundaries and preventing microcracks from propagating along grain boundaries during service, thereby improving the service life of the steel.

We report two typical morphologies (elongated- and blocky-shaped) of the Laves phase in tempered martensite ferritic steels during creep process, originating from the two independent formation paths, i.e. along grain boundaries (GBs) and neighboring M23C6. This is attributed to the multi-element segregation (Cr, Mn, Si and W) at GBs and adjacent M23C6 that serves as suppliers of Laves phase formers. Furthermore, the subsequent different growth mechanisms play crucial roles in the two typical morphologies. Our findings improve the understanding of Laves phase heterogeneous precipitation and provide new insights for the innovation of novel heat-resistant steels with superior creep strength.

The rapid development of modern aerospace and energy industries requires increasingly high service performance of metallic materials. Laves phase intermetallic compounds with the chemical formula AB2-type have attracted much attention due to their high melting point, excellent high-temperature strength, and oxidation resistance, and are regarded as one of the promising candidates for a new generation of high-temperature structural applications. Although it can be identified that Laves phase alloy research has been an active research field in the last 20 years in terms of the trend of literature volume growth, there is still a lack of systematic descriptions and comprehensive overviews of the latest frontiers and hotspots in the field. In this article, a quantitative and visual knowledge mapping of the articles published in the field of Laves phase research in the last 20 years has been carried out based on the bibliometric analysis methodology of Cite Space software. Quantitative analysis of high-frequency keywords to understand the basic development of Laves phase and the evolution of the main research hotspots in each period of time identified the hotspots of common concern seven themes: Laves phase, electronic structure, precipitation, Inconel 718 alloy, phase diagram, high-entropy alloys, and TiAl-based alloys. The analysis of the emergent keywords shows that electronic structure, high-entropy alloys, total energy calculations, and laser cladding can represent the research frontiers of Laves phases. This study provides systematic and valuable reference information to gain insight into the current state of Laves phase research and the evolution of the theme, which will be useful for further research.

The high-temperature phase stability of the NbCr2 Laves phase has not yet been established beyond disagreement. The well accepted C14 phase field has been eliminated from the Nb-Cr phase diagram in a recent study that claimed that the NbCr2 alloy shows the C15 structure up to melting. The present study was conducted to address these ambiguities. The experimental investigations show the presence of C14, C15, and C36 structures in the as-cast alloy. Differential scanning calorimetry studies of the as-cast alloy show the transformation of the eutectic mixture of bcc + NbCr2 to single-phase NbCr2 at 1338 K. An in-situ X-ray diffraction (XRD) study of as-cast alloy shows a phase transformation of C36 → C14 below 1273 K. Density functional theory (DFT) calculations confirm two phase transformations: (a) a metastable transformation of C36 → C14 at 783 K and (b) an equilibrium transformation of C15 → C14 at 1971 K, which was validated by high temperature DTA and TEM studies. Further, extended DFT calculations on supercells with four compositions of NbCr1.9, NbCr2, NbCr2.1, and NbCr2.2 also corroborated the presence of the C14 phase, and consequently, the Nb-Cr phase diagram is modified. This finding resolves the controversy regarding the existence of an equilibrium C14 phase field before melting.

The Laves phase formed during the long-term creep of 9% Cr heat-resistant steels plays an important role in their high-temperature mechanical properties. Detecting and quantifying the Laves phase is one of the main problems in predicting the creep residual life. This study detects and quantifies the Laves phase in various 9% Cr heat-resistant steel samples by anodic polarization. Both T92/P92 samples after creep and 9Cr ferritic martensitic samples after thermal ageing precipitate Laves phase particles, and the content of the Laves phase increases with the service time. Comparing the results obtained by the electrochemical method with those obtained by the traditional SEM observation method, it can be found that there is a similar two-stage linear relationship between the two results in all materials, which is related to the diameter change of Laves phase particles during service. According to the correlation between the Laves phase content and creep time, an extrapolation method for the prediction of the residual creep life is proposed. The results show that the creep life of P92 can be predicted by using the dissolved charge density of the Laves phase with high prediction accuracy. This method has the potential to evaluate the service states of materials and predict the long-term creep life in the industrial field. [ABSTRACT FROM AUTHOR]

A comprehensive research program on the creep of a die-cast Mg--5Al--2Ca (mass%) alloy, which has been conducted in the last two decades by our group, is overviewed. The obtained results are summarized into the following categories: microstructure and creep strength, creep parameters, dislocation analysis, and life assessment. The creep strength of the alloy is predominantly ascribed to the interconnected skeleton of C36--(Mg,Al)2Ca phase, while the fine C15--Al2Ca precipitates have limited effects on creep strength. The change in creep parameters, n and Qc, results from the decreased creep strength driven by the divorce of the interconnected skeleton of C36 phase during creep. At stress levels below the yield stress, the dislocation climb in the primary α-Mg grains is inferred as the rate-controlling process for the alloy. Mostly type dislocations are introduced within the primary α-Mg grains during die-casting, and the dislocation segments are mainly located on the basal plane. The basal segments of dislocations bow out and glide on the basal planes under stress, and the jogs follow the basal segments with the help of climb during creep. The minimum creep rate and creep rupture life follow the phenomenological Monkman--Grant relationship for the alloy. When the Larson--Miller constant is set at 20, the value of Larson--Miller parameter is uniquely described by the logarithm of the applied stress. [ABSTRACT FROM AUTHOR]

FeCrAl alloy is one of potential candidates for accident‐tolerant‐fuel (ATF)‐cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body‐centered‐cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high‐temperature deformation, from only one type of Fe2M (M = Nb, Mo, Ta) Laves phase to Fe2Zr combining Fe2M‐type Laves phase. The Fe2Zr‐type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe2M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future. [ABSTRACT FROM AUTHOR]

The solution treating and cooling experiments were conducted to study the microstructure and mechanical properties of Al modified super ferritic stainless steels with various cooling methods. The results demonstrated that the grain was characterized by the stratified distribution. The grain coarsened gradually with the decrease in cooling rate. And the surface grain coarsened obviously, Nb (C, N) particles were precipitated in the samples in different cooling manners. However, the Laves phase was distributed both at the grain boundaries and in the grains in the furnace-cooled specimen, and the area fraction of Laves phase increased significantly with the increase of Al content. With the cooling rate decreased, the yield strength, ultimate tensile strength and Vickers hardness of the Al-free samples increased, while the elongation decreased, and the furnace-cooled specimens showed brittle fracture during the tensile test. The addition of Al also increased yield strength, ultimate tensile strength and Vickers hardness and deteriorated plasticity of samples with different cooling manners. Meanwhile, the plasticity of furnace-cooled samples was more sensitive to addition of Al than water-quenched and air-cooled specimens. The analysis results showed that the plastic deterioration of furnace-cooled samples was mainly related to the coarsening of Laves phase at grain boundaries. Hence, the precipitation of large size Laves phase at grain boundaries should be inhibited in the process of industrial production.

The Laves phase in the as-built Inconel 718 (IN718) superalloy is known as undesirable phase, which limits the industrial applications of the manufactured parts. Post-processing in the form of the homogenization heat treatment is known as an effective method to eliminate the deleterious Laves phase, where understanding its dissolution kinetics is crucial for optimization of the heat treatment schedule. Accordingly, the IN718 nickel-based superalloy manufactured by the laser powder bed fusion (LPBF), also known as selective laser melting (SLM), was homogenized at temperatures from 950 to 1150 °C for 30 to 120 s, and the Laves phase dissolution was studied by microstructural analysis, its kinetics was modeled by Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis, and the model was validated by the second Fick's law. The activation energy for Laves phase dissolution was determined as ∼160 kJ/mol with an Avrami exponent of ∼1, which was attributed to the high dislocation density and fine microstructure of the LPBF material, and accordingly, the lattice and grain boundary diffusion of Nb in Ni were characterized as the underlying atomistic mechanisms during the dissolution of the Laves phase. These findings are based on the underlying atomistic mechanisms, which can shed light on the principles of dissolution kinetics of Laves phase in IN718 for additive manufactured parts. Moreover, the conducted comparison among LPBF, directed energy deposition (DED), and conventional casting indicated that the fabricating method and the initial segregation play key roles. In other words, due to its lower segregation degree, LPBF has faster Laves phase dissolution kinetics in comparison with other fabrication methods.

We report two typical morphologies (elongated- and blocky-shaped) of the Laves phase in tempered martensite ferritic steels during creep process, originating from the two independent formation paths, i.e. along grain boundaries (GBs) and neighboring M23C6. This is attributed to the multi-element segregation (Cr, Mn, Si and W) at GBs and adjacent M23C6 that serves as suppliers of Laves phase formers. Furthermore, the subsequent different growth mechanisms play crucial roles in the two typical morphologies. Our findings improve the understanding of Laves phase heterogeneous precipitation and provide new insights for the innovation of novel heat-resistant steels with superior creep strength. [ABSTRACT FROM AUTHOR]

In order to solve the problem of poor mechanical properties of laser cladding In718 alloy, different scanning speeds were selected to carry out laser remelting treatment of the cladding layer. Optical microscope, scanning electron microscope and energy dispersive spectrometer were used to observe the microstructure and characteristics and to detect the composition of different phases. The influence of microsegregation on the microstructure was then analyzed. The microhardness and tensile strength of the coating were tested by mechanical testing equipment. The results show that the Laves phase is mainly caused by the segregation of Nb and Mo elements. Compared with the non-remelted coating, the pores of the remelted coating are significantly reduced, and different remelting scanning speeds have different effects on the structure and properties. The Laves phase volume fraction of four coatings respectively decreased from 34. 1% to 24. 6%, 16. 7%, and 19. 6%, the average hardness respectively increased from 250.3 HV to 261.5 HV, 276.9 HV, and 268.0 HV. The tensile strength respectively increased from 678 MPa to 728 MPa, 879 MPa, and 808 MPa. However, the effect of remelted coating on elongation is not obvious. The optimum remelting scanning speed is 15 mm/s, which has the lowest Laves phase content and the highest average microhardness and tensile strength. Laser remelting can effectively improve the morphology of the cladding layer, reduce the porosity, reduce or inhibit the precipitation of Laves phase. Reducing the Laves phase to improve the mechanical properties of In718 alloy. This research establishes a theoretical foundation for the subsequent remanufacturing of centrifugal cast ductile iron pipes molds. [ABSTRACT FROM AUTHOR]