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Abstract This paper is aimed to evaluate the high temperature tribological behaviors of CoCrFeNiNb x eutectic high entropy alloys which would be highly-promising candidates for high-temperature applications. The effects of testing temperature and material structure on the friction and wear properties are investigated. The CoCrFeNiNb x alloys display a low harness decrement rate about 35% from room temperature to 1000 °C, indicating good softening resistance. The main wear mechanisms are abrasion wear at room temperature, change to adhesion wear at 400 °C, and translate into oxidation wear and mechanical wear over 600 °C. The wear rates increase from room temperature to 400 °C and decrease afterwards. The effects of material structure are dominated by the hardness of alloy, the content of chemically active Nb element and the structure of oxidized tribo-layer. The compact oxidized tribo-layers with excellent anti-wear properties are formed on the worn surface of CoCrFeNiNb 0.65 and CoCrFeNiNb 0.8 alloys at 800 °C, leading to extremely low wear rate. Highlights • Tribological behaviors of CoCrFeNiNb x alloys at elevated temperatures are studied. • CoCrFeNiNb x eutectic high entropy alloys possess excellent softening resistance. • Wear rate increases from room temperature to 400 °C and decreases afterwards. • Oxidized tribo-layers with excellent anti-wear properties are formed at 800 °C. • Effects of material structure on wear behaviors and mechanisms are investigated. [ABSTRACT FROM AUTHOR]

Abstract The face centered cubic (FCC) high entropy alloys (HEAs) showed highly attractive properties because of their simple crystal structures and low stacking fault energies. However, deep understandings of micro-alloying in FCC HEAs, including solid solubility, precipitates formation and stacking fault energy variation, remain unsatisfactory. In present study, we systematically investigated the solubility of Ti, Al, and Mo in the CoCrFeNi HEA and evaluated the effect of every single element on the precipitation behavior. The results showed that at 750 °C the CoCrFeNi matrix can separately dissolve up to 0.6 at.% Ti, 4 at.% Al, and 3 at.% Mo. The roles of both additional and base elements in the precipitation behaviors were identified. Specifically, Co can stabilize the γ′ phase in the Ti-doped HEAs, while Al leads to the formation of B2 phase in the CoCrFeNiAl x system. The annealing stacking faults and twins in the matrix changed with the addition of different elements, which was mainly due to stacking fault energy variations. These insights will greatly benefit the design of precipitate-hardening HEAs and provide a deeper understanding of metallurgical behaviors of HEAs. Highlights • The CoCrFeNi HEA dissolve 0.6 at.%, 4 at.% Al and 3 at.% Mo at 750 °C, respectively. • In the CoCrFeNi system, Co stabilizes the γ′ phase through decreasing the VEC. • The Al leads to the formation of B 2 phase at 750 °C, rather than the γ′ phase. • The Al and Ti decrease the stacking fault energy of CoCrFeNi while Mo increases it. [ABSTRACT FROM AUTHOR]

Based on binary phase diagrams and thermodynamic calculation, CoCrFeNiTa x (x = 0.1, 0.3, 0.43, 0.5 and 0.7) eutectic high entropy alloys were designed. With increasing Ta content, as-cast microstructure changed from hypoeutectic (primary γ phase) to eutectic and to hypereutectic (primary Laves phase), and CoCrFeNiTa 0.43 eutectic alloy was composed of γ phase with face-center cubic (FCC) structure and C14 type Laves phase with hexagonal close-packed (HCP) structure. Volume fraction of Laves phase increased with increasing Ta content, and thus microhardness was basically positive correlation with Ta content. Meanwhile, microhardness and elastic modulus of primary phase increased from primary γ phase to primary Laves phase, while hardness and elastic modulus of eutectic phases firstly decreased and then increased with increasing Ta content. In addition, based on nanoindentation load-displacement curves, the reverse analysis algorithms were used to obtain initial yield stress σ y and strain hardening exponent n of primary γ phase and eutectic phases. Moreover, with increasing Ta content, the plasticity of CoCrFeNiTa x alloys decreased, while the compression strength firstly increased and then decreased, and CoCrFeNiTa 0.43 EHEA showed relatively high strength (2377 MPa) and relatively high plastic strain (17.5%). [ABSTRACT FROM AUTHOR]

High entropy alloys (HEAs) are facing difficulties in balancing strength and ductility, and till now there is no satisfactory solution. In this paper, the CoCrFeNiNb 0.25 HEA with good ductility was strengthened by a face centered cubic (FCC) precipitate after annealing heat treatment. When the CoCrFeNiNb 0.25 HEA was annealed at 750 °C, a lath-shaped FCC precipitates with nano basket-weave microstructures appeared and distributed randomly in the proeutectic FCC phase. Due to the precipitation strengthening, the yield stress of the CoCrFeNiNb 0.25 HEA almost doubled but its ductility was just slightly damaged. [ABSTRACT FROM AUTHOR]

Based on surveying the existing binary phase diagrams with eutectic points, a strategy of designing eutectic high entropy alloys (EHEAs) with desired high strength and ductility is proposed. Based on the computer-aided thermodynamic calculations, the pseudo eutectic binary alloy system of CoCrFeNiNb x (x = 0.1, 0.25, 0.5 and 0.8) was designed. The experimental results show that the eutectic alloys are composed of a ductile face centered cubic (FCC) phase and a hard Laves phase with fine laminar structures. The designed alloys show excellent integrated mechanical properties of ductility and strength. For the CoCrFeNiNb 0.5 alloy, the compressive fracture strength and strain can reach above 2300 MPa and 23.6%, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Glasses with different Bi2O3 contents (37–42mol%) have been prepared by conventional melt quench technique. The IR and Raman studies indicate that these glasses are made up of [BiO6], [BiO3], [BO3] and [BO4] basic structural units. The vibrations of [BiO3] and [BO3] become stronger as the content of Bi2O3 increases, which makes glass structure loosened. Viscosity of the glasses was measured by using a Rheotronic III paralleled plate rheometry, which shows that the viscosity of glass samples decreased when the content of Bi2O3 increased at the same temperature (400–460°C). The temperature range which suits for glasses sealing was calculated by using the approximation of Arrhenian behaviour. The wetting performance of Bi2O3–ZnO–B2O3 glasses was described by using high-temperature microscope, which also proves that the structure of investigated Bi2O3–ZnO–B2O3 glasses become loosened due to the increasing of the content of Bi2O3. [Copyright &y& Elsevier]

• The L1 2 phase exhibits a high volume fraction in our explored HEAs. • Both Re and Ru do not change the phase constitution of Ni 29 Co 27 Fe 27 Cr 3 Al 7 Ti 7. • Only minor Re and Ru (1 at%) can significantly enhance the coarsening resistance of L1 2 phase in high entropy alloy. • The addition of Ru increases the microhardness, because it enhances the volume fraction and decreases the size of L2 1 phase. The thermal stability of precipitates is a major concern for L1 2 -type strengthening high entropy alloy at elevated temperatures. In this study, we systematically investigated the effect of Re and Ru on the phase stability an d coarsening kinetics of L1 2 precipitates in the Ni 29 Co 27 Fe 27 Cr 3 Al 7 Ti 7 HEA. The L1 2 phase exhibited a high volume fraction of 40.29% in the explored HEA and remained a stable morphology during the long-term exposure. Re addition did not change the volume fraction of L1 2 phase, while Ru addition decreased it to 34.42%. Both Re and Ru facilitated the formation of L2 1 precipitates slightly. Minor Re significantly decreased the coarsening rate of L1 2 phase and minor Ru also retarded the coarsening of L1 2 phase to some degree, which were mainly ascribed to the sluggish diffusion of Re and Ru atoms. These findings provide a feasible strategy to improve the coarsening resistance of L1 2 precipitates in HEA system through sluggish atomic diffusion. [ABSTRACT FROM AUTHOR]

Multi-component alloys with good mechanical properties and casting liquidity have become a hotspot in the materials field. Typically, the addition of minor metal elements plays a vital role in strengthening the mechanical properties. However, doping metalloid elements such as Si into multi-component alloys is also an effective method to enhance the mechanical properties. In the present study, the effect of Si (0 ∼ 4 at%) on the microstructure and mechanical properties was investigated within both as-cast and thermal mechanical treatment states. In as-cast state, all 4 alloys exhibited dendrites and interdendrites structure. The fine structure enhanced tensile strength for Si1 and improved ductility for Si2. Once the Si content reached 4 at%, both the strength and ductility dropped dramatically. Phase selective recrystallization and aging greatly increased the tensile strength and ductility simultaneously. The alloys with addition of Si had similar mechanical properties after thermal mechanical treatment process. Si2 after recrystallization and aging was considered a promising candidate for further application with a yield strength of 960 MPa, tensile strength of 1463 MPa and fracture strain of 20.5%. This work provided a new prospect to design multi-component alloys with nonmetallic elements, which may be applied to wear, corrosion and oxidation resistance fields. • The effects of metalloid Si on the microstructure and mechanical properties of Fe 36 Ni 36 Cr 10 Mo 1 Al 17−X Si X was investigated. • The addition of Si enhanced tensile strength to 1305 MPa for Si1 and improved ductility to 24.8% elongation for Si2. • Si2-R-A showed mechanical properties with yield strength of 960 MPa, tensile strength of 1463 MPa and ductility of 20.5%. [ABSTRACT FROM AUTHOR]

The W and Mo are in the same groups of periodic table and are usually used as solid solution elements. In this investigation, the minor alloying behaviors of W and Mo in Fe 37 Ni 36 Al 17 Cr 10 alloy were explored. The addition does not change the casting microstructure greatly, with dendrite region of FCC + B2 and inter-dendrite region of FCC phase. For tensile properties at ambient temperature, as-cast Fe 36 Ni 36 Al 17 Cr 10 Mo 1 alloy exhibited higher yield strength and fracture strength compared with Fe 37 Ni 36 Al 17 Cr 10 alloy. Then, dual-phase alloys with Mo content of 2 and 3 at% were further designed and carefully studied. It is found that a nano-scale structure occurred in as-cast Fe 35 Ni 36 Al 17 Cr 10 Mo 2 alloy, where a large number of spherical BCC precipitates are distributed in the dendrite region of B2 phase. Meanwhile a mixture of irregular B2 phase and FCC phase is contributed to the inter-dendrite area. The Fe 35 Ni 36 Al 17 Cr 10 Mo 2 alloy exhibited the most excellent and comprehensive mechanical properties due to the fine structure and its high work hardening ability. The yield strength, ultimate tensile strength and fracture elongation of as-cast Fe 35 Ni 36 Al 17 Cr 10 Mo 2 alloy are 863 ± 20 MPa, 1285 ± 17 MPa and 16.0 ± 1.0 %, respectively, which is higher than the most of reported dual-phase as-cast FeNiCrAl multi-component alloys. • The alloying behaviors of W and Mo in Fe 37 Ni 36 Al 17 Cr 10 alloy were explored. • Effect of Mo amount on the microstructure and mechanical properties were studied. • A nano-scale structure consisted of FCC, B2 and BCC occurred in as-cast alloy. • The Fe 35 Ni 36 Al 17 Cr 10 Mo 2 alloy exhibited the most excellent mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract The poor rate performance and inferior cycling life of Ni-rich layered oxide cathodes, which are related to low lithium conductivity and structure degradation during cycling, are the obstacles to commercial utilization. Herein, we apply a solid grinding approach based high temperature calcination to synthesize Na-substituted, F-substituted and Na-F-substituted LiNi 0.6 Co 0.2 Mn 0.2 O 2. The synergistic coupling effect of both sodium and fluorine substitution on particle morphology and structure is systematically investigated. The galvanostatic charge/discharge results show that Na and F co-substituted sample displays improved rate performance as well as superior capacity retention. The cooperative substitution of Na and F has synergistic effect on enlarging the interlayer spacing, which is favorable for increasing lithium diffusivity and enhancing the rate capability. Meanwhile, the Na incorporated into the Li layer and the partial substitution of metal-O bond by stronger metal-F bond can help to stabilize the crystal system, thus enhancing the structural stability. This work presents some new thought into synthesis of high-performance cathode materials through strategic cationic-anionic co-substitution. Highlights • Effect of Na and F substitution on morphology and structure is investigated. • Na and F co-substituted sample displays improved electrochemical performance. • The co-substitution enlarges interlayer spacing and enhances structure stability. [ABSTRACT FROM AUTHOR]

Face-centered-cubic (FCC) Multi-principal element alloys (MPEAs) have attracted intensive scientific research interest due to their excellent fracture toughness and ductility. Still, the strengths of single-phase MPEAs are unsatisfactory. Although great progress has been made by utilizing multiple strengthening mechanisms, it remains challenging to take advantage of large-atom solutes and nano-precipitates simultaneously because solutes always partition into the precipitates. In the present work, we successfully achieved an effective combination of solid solution strengthening and precipitation hardening in the NiCrFeWTiAl multi-principal element alloys. Our experimental results showed that W is partitioned into the FCC matrix, while Ti and Al tend to exist in the L1 2 phase. This controlled elemental partitioning leads to a good combination of solid solution strengthening and precipitation hardening effects and results in good tensile properties at temperatures from 25 ℃ to 760 ℃. The strengthening mechanisms were revealed based on the quantified microstructure analysis. The solid solution hardening of all the solute atoms contributes ∼140 MPa in total, but W mainly contributes to solid solution hardening, and L1 2 precipitates enhance the yield strength by ∼330 MPa through ordering strengthening. • The NiCrFeWTiAl MPEAs efficiently combine the solid solution strengthening and precipitation hardening. • The W partitioned into the FCC matrix mainly leads to solid solution hardening. • The L1 2 phase which enriched in Ti and Al enhances the yield strength through the ordering strengthening mechanism. • The effect of Cr on the microstructure and mechanical properties was demonstrated. [ABSTRACT FROM AUTHOR]

Coarsening of the basketweave α + β microstructure in Ti-6Al-4V is a common phenomenon that occurs during thermal cycling in both additive manufacturing and cyclic heat treatment. Such a coarsening process is driven by multiple dissolution and precipitation transformations rather than by conventional Ostwald ripening. It is clear that during the first cycle, the complete dissolution of the thinnest α plates at the heating stage provides large spaces for surviving plates to regrow at the cooling stage, which results in coarsening after the first cycle. However, it is still unclear how the α plates continue to vanish and the remaining α plates sustainably coarsen in the repeated thermal cycles. In this study, cyclic heat-treatment experiments and phase-field simulations were conducted to illustrate the mechanism of sustained coarsening during thermal cycling. It was found that in addition to the complete dissolution of the thinnest α plates in the first thermal cycle, α-plate vanishing continued during repeated thermal cycling in two ways, which has not been reported before. The origin of such sustained vanishing of α plates is related to the Gibbs–Thomson effect. Moreover, the influences of cycling frequency and total duration on the transformation coarsening are detected. Both the experimental and simulation results indicate that long cycling duration results in severe coarsening, while for a given duration time, the final coarsening of the α plates is almost independent of the cycling frequency. [Display omitted] • During the repeated thermal cycling,α–plates vanishing can continue in three ways. • The origin of sustained vanishing of α plates is related to Gibbs–Thomson effect. • The transformation coarsening is more sensitive to cycling time than frequency. [ABSTRACT FROM AUTHOR]

• Effects of pre-strain on precipitation kinetics and strengthening mechanisms of γ'-strengthened HEAs were studied. • Pre-strain can accelerate precipitation kinetics but shows a non-monotonous feature with the increase of pre-strain. • The shortened defects existing time by recrystallization can weaken the acceleration effect at high pre-strain. • Strengthening mechanisms can be tuned by different pre-strain to realize targeted properties. Pre-strain is often used as a powerful pre-treatment to tailor the microstructures and mechanical properties of traditional alloys. However, in high-entropy alloys (HEAs), the effect of pre-strain degree on the precipitates is still unclear. In this paper, we investigated the effects of pre-strain degree on the precipitation of Ni 2 CoCrFeTi 0.18 Al 0.12 high-entropy alloys. The HEAs were cold-rolled with different thickness reduction of 30%, 50%, and 70% and annealed at 800 °C for 30 ~ 60 min, followed by high-resolution scanning electron microscope (SEM) analysis. Counter-intuitively, we discovered a non-monotonous effect of pre-strain on the precipitation behavior, that low pre-strain can accelerate the precipitation behavior while such accelerating effect is weakened under high pre-strain. This non-monotonous effect is attributed to the shortened defects existing time by recrystallization under high pre-strain. In addition, pre-strain can controllably tailor the precipitation strengthening, dislocation strengthening, and grain boundary strengthening of HEAs. These results not only uncover the mechanism of the non-monotonous pre-strain effect on the precipitates but also provides a pre-strain mediated scheme for tuning the precipitation-strengthened HEAs to targeted properties. [ABSTRACT FROM AUTHOR]

Abstract: A fluorborate bath being suitable for the continuous electrodeposition system was used to deposit Ni1−x Fe x (0.1< x <0.75) alloys. The obtained alloys were smooth, bright and flexible. The effects of pH, temperature, iron content in electrolytes and current density on alloy composition and plating current efficiency were investigated. The analysis of XRD of Ni1−x Fe x alloys was used to explore the relationship between alloy composition and crystal structures. The surface morphology of alloys was observed using scanning electron microscopy (SEM). [Copyright &y& Elsevier]

A series of NiCoCrSi x (x = 0, 0.1, 0.2, 0.3) medium-entropy alloys were synthesized to investigate the effect of Si addition on the microstructures, mechanical properties and helium irradiation resistance of the NiCoCr-based alloys. The microstructures and the phase evolution of these alloys were examined using electron microscopy as well as the existing models. The results show that the alloys exhibit a single face-centered-cubic structure when the Si content is in the range of 0–6.25 at.%, whereas phase decomposition occurs as the Si content further increases (∼9 at.%). Comparing with NiCoCr, NiCoCrSi 0.2 exhibited improved strength without sacrificing the ductility. The yield strength and ultimate tensile strength increased from 438 to 599 MPa and 921 to 1032 MPa, respectively, whilst the ductility kept as high as ∼50%. Our analysis indicates that the increased strength is mainly ascribed to the grain boundary strengthening. The large ductility is primarily due to the low stacking fault energy and shear modulus of the NiCoCrSi 0.2 alloy, which makes mechanical twinning in small grains accessible and provides a steady strain-hardening rate in a wide strain regime. Besides, the enhanced compositional complexity and local lattice distortion induced by the introduction of Si can improve the helium irradiation resistance of NiCoCr, resulting in reduced average bubble size. • Microstructure,mechanical properties and helium irradiation resistance of NiCoCrSi x MEAs were investigated. • Enhanced strength was obtained in NiCoCrSi 0.2 without the sacrifice of ductility. • Comparing with NiCoCr, NiCoCrSi 0.2 exhibits improved helium irradiation resistance. [ABSTRACT FROM AUTHOR]