Increasing the heating rate during the sintering process of Li6.5La3Zr1.5Ta0.5O12 (LLZTO) solid-state electrolyte is an effective method for suppressing lithium volatilization, tailoring grain ...growth, and achieving rapid densification. However, the ultrafast heating rate can lead to uneven surface temperature distribution of the electrolyte, subsequently affecting its microstructure and performance. Herein, we systematically investigate the microstructural evolution and densification behavior of LLZTO ceramics under rapid heating conditions using the ultrafast high-temperature sintering (UHS) method. An intercalibration technique combining finite element simulations and experimental data from infrared spectral measurements is used for prediction of the temperature distribution of the sample. Adjustment of the size ratio of the graphite felt to the sample diameter resulted in uniform temperature distribution, which aids in improving sintering quality. The densification mechanism of the samples during the UHS process is revealed by thermodynamics. Under sintering conditions with a current of 25 A applied for 30 s, the ultrafast heating rate (up to 104 °C/min) results in fine grains with a diameter of approximately 3.4 μm and a relative density of 93.2 %. The preferred samples finally exhibit good ionic conductivity stability, maintaining 3.09 × 10−4 S cm−1 at dynamic pressures of 5–50 MPa at room temperature.
•Fast sintering of LLZTO was achieved using the UHS method at 25 A held for 30 s.•The uniformity of surface temperature in LLZTO pellets impacts sintering quality.•The decomposition of LiCO3 improved the densification of the LLZTO during UHS.
Microstructures and mechanical properties of Al-Ni-Co-Fe-Cr high-entropy alloys (HEAs) were investigated by systematically varying transition metals instead of Al, within the chemical formula of ...Al2M14 (M represents different mutations of transition metals). The formation of different crystal structures (FCC, BCC, or FCC+BCC mixture) and its effects on the resulting mechanical properties of this series of HEAs, both in tension and compression, were evaluated. It was found that, in the BCC-dominated HEAs, ordered B2 precipitates were always coherently dispersed in the BCC solid-solution matrix. The shape of these B2 precipitates was strongly affected by the lattice misfit between the disordered BCC and ordered B2. A uniform distribution of cuboidal B2 particles could be obtained by properly adjusting M, thus the lattice misfit, in a manner similar to that in Ni-based superalloys. Strengthening effects caused by different BCC/B2 morphologies were also estimated and compared with experimental measurements. The optimal strengthening as a function of the shape and size of the coherent precipitates was discussed in light of the lattice misfit in these HEAs.
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Herein, the phase evolution, densification and grain growth process of the high entropy ceramics during flash sintering were systematically characterized and quantified to understand the ...microstructural evolution for the first time. It was demonstrated that the densification rate of (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 by flash sintering in this work was generally around 60 times that of conventional sintering at 1600 °C, while the grain growth rate by flash sintering was only around 1.5–6 times that of conventional sintering, indicating that grain growth was suppressed during flash sintering. The grain growth mechanisms by flash sintering and conventional sintering could be both attributed to surface diffusion and volume diffusion. In addition, the flash sintered high-entropy ceramics as promising immobilization materials for high-level radioactive waste (HLW) exhibited excellent aqueous durability with normalized leaching rates of Nd, Gd and Zr approximately 10−6∼10−7 g m−2 d−1 after 42 days, which were much lower than most reported pyrochlore materials.
The poor oxidation resistance of tantalum-tungsten alloy at high temperatures limits its development as an ideal candidate for thermal components in aerospace. In this study, a composite coating ...including an internal TaSi2-WSi2 layer and an external ZrB2-MoSi2-ZrSi2 layer was prepared on the surface of Ta10W alloy by combining slurry sintering and halide activated pack cementation (HAPC). The composite coating has a dense structure, and the coating samples remains intact after oxidation at 1500°C for 6 hours, while the unprotected substrate oxidizes to powder within 3 minutes. The coated samples also withstood more than 300 shock heating cycles from room temperature to 1500°C. Its notable resistance to high-temperature oxidation was ascribed to the development of a compact and uniform SiO2 oxide layer containing oxide crystals comprising ZrSiO4, ZrO2, and Ta2O5 during the oxidation process, which effectively mitigated the diffusion of oxygen into the interior of the coating.
•A composite coating was prepared on the surface of Ta10W alloy.•The oxidation characteristics in the microstructure of the composite coating were analyzed.•The coating sample exhibited durability following exposure to oxidation at 1500 ℃.•The coating demonstrated the self-repair capability and strong resistance to thermal shock.
The distribution characteristic of reinforcements is a critical issue for the final performance of magnesium matrix composites (MMCs). Here, the microstructural evolution of graphene nanoplatelets ...(GNPs) reinforced MMCs fabricated by thixomolding process was investigated. The GNPs successfully penetrated through the grain boundaries and embedded in two adjacent grains. Those intragranular GNPs/Mg interfaces not only could provide efficient barriers to pin dislocations, but also contribute to the multiplying dislocations to accumulate rather than dissipate. In addition, the finer MgO particles and elongated Mg17Al12 were anchored around the GNPs, which could effectively improve the interfacial load transfer due to the increased load-bearing capability and the effective interfacial shear strength. These special distribution characteristic of GNPs was beneficial to the significant enhancement of mechanical properties for the composites. The concept of intragranular reinforcements anchored by finer particles in this work may bring positive and directive significance for the structural design of the composites.
•The GNPs successfully penetrated through the grain boundaries.•The finer MgO particles and elongated Mg17Al12 were anchored around the GNPs.•The special GNPs distribution characteristic contributed significantly to the strength improvement.
The microstructural evolution and the room-temperature tensile properties of fully lamellar Ti-43.5Al–4Nb–1Mo-0.5B alloy exposed at 700 °C for 500 h in air were investigated. In bulk microstructure, ...the precipitation and growth of ellipsoidal ω0 particles were found in equiaxed β0 grains, and the parallel decomposition of coarse α2 lamellae into fine α2 + γ lamellar packets was observed. The α2 lamellae near the surface decomposed first into fine α2 + γ lamellar packets and then blocky γ grains after the full consumption of the α2 phase. A mixed oxide scale with obvious stratification mainly composed of TiO2 and Al2O3 formed on the surface of the specimens. The outward diffusion of Ti atoms on the specimen surface led to the formation of an Al-rich and Ti-lean blocky γ zone between the matrix and the oxide scale. The degradation of room-temperature mechanical properties of the alloy after exposure was primarily caused by the evolution of subsurface microstructure, and the evolution of bulk microstructure had little effect on the mechanical properties at this stage. Under axial stress, microcracks nucleated from the tip of a V-shaped “notch microstructure” at the interface between the blocky γ zone and the matrix, causing premature failure. Removal of the surface layer of the thermally exposed alloy led to room temperature mechanical properties essentially similar to those before thermal exposure.
•The embrittlement of TNM-0.5B alloy during exposure at 700 °C was mainly caused by the oxidation layer and its subsurface.•The outward diffusion of Ti atoms led to the formation of a blocky γ zone, which had high Al content but low Ti content.•Microcracks nucleated from the tip of “notch microstructure” between the blocky γ zone and the matrix.
Microstructural evolution of equiaxed α and lamellar α during hot compression in a bimodal-structure near alpha Ti-6Al-3Nb-2Zr-1Mo alloy were investigated. Focus was concentrated on their own dynamic ...restoration mechanism via electron backscatter diffraction (EBSD) technique. Based on analysis of grain boundary misorientation distribution and misorientation gradient within α grain, it was revealed that limited continuous dynamic recrystallization (CDRX) as well as adequate dynamic recovery (DRV) took place within equiaxed α, whereas CDRX occurred as the main restoration mechanism in lamellar α. Further, the deformation heterogeneous among equiaxed α grains was found to be relevant to the difference in slip systems. Under the same nominal strain of 75% height reduction, process of weak DRV, strong DRV and CDRX coexisted in distinct equiaxed α grains owning to small, medium and high Schmid Factor of prismatic slip. Besides, globularization heterogeneity induced by CDRX in lamellar α was considered to be closely related with its initial orientation to compression axis and the imposed strain. For flat colony α, the microstructure was relatively stable during straining showing a thinning process and a weak dynamic globularization, while the kinked colony α with large strain was globularized sufficiently.
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•It reveals limited CDRX as well as adequate DRV take place within equiaxed α.•Weak DRV, strong DRV and CDRX coexist in equiaxed α due to distinct prismatic slip.•It reveals CDRX occurs as the main restoration mechanism in lamellar α.•Globularization heterogeneity of lamellar α is ascribed to its orientation and strain.
In order to investigate the microstructural evolution and dynamic softening mechanisms of Ti-55531 alloy during hot compression in the α + β phase region, hot compression tests were performed at ...temperatures of 760–840 °C and strain rates of 0.001–1 s−1, and the microstructure of the material under each deformation condition was analyzed by scanning electron microscope and electron backscatter diffraction techniques. The results show that the flow softening phenomenon is influenced by a combination of mechanisms. The deformation heat causes significant flow softening of the material at a strain rate of 1 s−1, while dynamic recrystallization contributes little to flow softening. Due to the shift in αp phase volume fraction, the flow softening fraction f∆Vα reduces as deformation temperature and strain rate increase, from 16.6% at 760 °C and 0.001 s−1 to 1.4% at 840 °C and 1 s−1. The flow softening fraction fΔσSF is 3.95% at 790 °C and 5.34% at 820 °C as a result of the texture evolution.
•The initial stress-strain curve of Ti-55531 alloy was frictionally corrected.•The contributions of deformation heat, α phase evolution, DRX and texture to the flow softening were calculated.•CDRX and DDRX are present in the material, and the contribution of DRX to the flow softening is small.•The effect of deformation heat and α phase content change on softening is significant.
•Cr coating prepared by multi - arc ion plating has good oxidation resistance.•The four-layer structure was observed after oxidation at 1300 ℃: an outer Cr2O3 layer, a residual Cr layer, a new ...Cr-Zr-O layer and a ZrCr2 layer.•The degradation and failure mechanism of Cr coating in steam up to 1400 ℃ was discussed.
This study aimed to investigate the effectiveness of the multi-arc ion plating method in depositing Cr coatings on Zry-4 substrate and to study the oxidation behavior and microstructural evolution of the Cr coatings in steam environments up to 1400 ℃. The results showed that the Cr-coated sample exhibited excellent high-temperature resistance at 1100-1200 ℃, but accelerated oxidation occurred at 1300 ℃, leading to the formation of bubbles and voids in the Cr2O3 layer and the development of a new Cr-Zr-O layer between the residual Cr layer and the ZrCr2 layer. At 1400 ℃, the coating failed completely. These findings provide valuable insights into the oxidation behavior and degradation mechanism of Cr-coated zirconium alloys under accident conditions. The novelty of this study lies in the investigation of the microstructural evolution and failure mechanism of Cr coatings on Zry-4 using multi-arc ion plating method in high-temperature steam environments, which has not been extensively explored in previous research.
