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.
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.
High-performance grade 300 maraging steels were fabricated by selective laser melting (SLM) and different heat treatments were applied for improving their mechanical properties. The microstructural ...evolutions, nanoprecipitation behaviors and mechanical properties of the as-fabricated and heat-treated SLM parts were carefully characterized and analysed. The evolutions of the massive submicron sized cellular and elongated acicular microstructures are illustrated and theoretically explained. Nanoprecipitates triggered by intrinsic heat treatment and amorphous phases in as-fabricated specimens are observed by TEM. High-resolution TEM (HRTEM) images of the age hardened specimens clearly exhibit massive nanosized needle-shaped nanoprecipitates Ni3X (X=Ti, Al, Mo) and 50–60nm sized spherical core-shell structural nanoparticles embedded in amorphous matrix. XRD analyses reveal austenite reversion and probable phase transformations during heat treatments. The hardness and tensile strength of the as-fabricated and age-treated SLM specimens absolutely meet the standard wrought requirements. Furthermore, the lost ductility after aging can be compensated by preposed solution treatments. Relationships between massive nanoprecipitates and dramatically improved mechanical performances of age hardened specimens are elaborately analysed and perfectly explained by Orowan mechanism. This study demonstrates that high-performance grade 300 maraging steels, which is comparable to the standard wrought levels, can be produced by SLM additive manufacturing.
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•Evolutions of the typical SLMed microstructures are illustrated and theoretically explained.•Precipitation behavior and phase transformation of SLMed maraging steel are characterized by TEM and XRD.•Significant improvement of strength after solution and aging treatment was evaluated and explained.•Relationships between massive nanoprecipitates and improved mechanical performances are elucidated.
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 grain orientation control via twinning activity on deformation features is of great significance to offer a key insight into understanding the deformation mechanism of Mg alloy sheets. The ...{10–12} twinning were performed by pre-strain paths, i.e., tension (6%) and compression (5%) perpendicular to the c-axis along extrusion direction (ED), to investigate the microstructural evolution and mechanical properties of AZ31 Mg alloy sheets. The distinction in the texture evolution and strain hardening behavior was illustrated in connection with the pre-strain paths for the activities of twinning and slip. The result shows that the activation of the deformation mode was closely bound up with the grain orientation and the additional applied load direction. The {10–12} twin-texture components with c-axis//ED were generated by pre-compression, which can provide an appropriate alternative to accommodate the thin sheet thickness strain and enhance the room temperature formability of Mg alloy sheet.
Ni(Nickel)-based superalloys are used as indispensable materials for turbine blades in aero-engines because of their excellent creep, fatigue and corrosion resistance. Co(Cobalt), as one of the new ...key elements in Ni-based superalloys, has attracted extensive attention in Ni-based superalloys field. However, the effect mechanism of Co on Ni-based superalloys is still controversial. To solve this problem, tensile properties and microstructure of Ni-based superalloys doped with or without Co (5 %) have been investigated through molecular dynamics simulation, and the joint impact of temperature and Co on the superalloy mechanical properties has also been analyzed. The findings indicate that 5 % Co doping reduces the yield stress, yield strain, and elastic modulus of the structure, while increasing the flow stress at 300 K–600 K; however, the advantage decreases at 900 K and disappears completely at 1200 K. This can be explained that Co doping leads to more slippage of stacking faults in the elastic stage and hinders the loop formation of dislocations at 300 K–600 K, while more Stair-rod dislocations occur in the plastic stage. These effects disappear at 900 K–1200 K.
•The effects of Co and temperature on mechanical properties of nickel-based superalloys were studied by molecular dynamics.•The combined effect of Co and low temperature results in more stationary Stair-rod dislocations, resulting in increased flow stress.•Co leads to more HCP and stacking faults in γ' phase, hinders the formation of dislocation loops and decreases tensile strength at 300 K-600 K.
•Dislocations preferentially form within the PFZ and accumulate at the interface between the bulk and PFZ is revealed.•The presence of PFZ leads to a softening effect on strength because the PFZ is ...prone to plastic deformation during tensile process.•The level of strain concentration and the degree of dislocation accumulation are not sensitive to the PFZ width.•A PFZ-dependent strength model is developed by considering dynamic strengthening of PFZ to quantitatively calculate PFZ softening on tensile strength effect.
Precipitate free zone (PFZ) consistently forms near the grain boundaries (GBs) in precipitate-strengthened alloys, significantly weakening the materials because of their intrinsic softness compared to the bulk. However, the influence of PFZ near GBs on deformation mechanism remains largely unrevealed. Here, we systematically investigate the effects of PFZ on the macroscopic mechanical behavior and the microstructure deformation mechanism of the modelled precipitate-strengthened Al-Zn-Mg-Cu alloy, using a combination approach of experiments, molecular dynamics (MD) simulations, and theoretical modeling. Four Al-Zn-Mg-Cu alloys with highly different PFZ widths are prepared by tailoring the quenching media and applying the new deformation heat- treatment process proposed by us. Experimental characterizations demonstrate that severe dislocation accumulation occurs at the interface between PFZ and bulk. Meanwhile, MD simulations further reveal that PFZ is prone to plastic deformation during tensile process, contributing to the softening of materials. The PFZ exhibits significant strain concentration, leading to the preferential formation of dislocations within PFZ rather than at GBs. It is found that the level of strain concentration and the degree of dislocation accumulation are not sensitive to the PFZ width. Based on these mechanisms, a PFZ-dependent strength model is developed to quantitatively evaluate the influence of PFZ on tensile strength by considering dynamic strengthening of PFZ. It is predicted that an increase in PFZ width greatly reduces the tensile strength, with a 21 % reduction observed when PFZ width reaches 268 nm, emphasizing the important impact of PFZ width on materials strength.
•The work aims to provide theoretical bases for engineering damage level evaluation.•CF test is interrupted to study micro-evolution and mechanical property of GH4169.•Strength degradation is related ...to the coarsening and shearing of γ″ precipitate.•Formation of voids and slip band leads to decrease in elongation after 50 % lifetime.•A mapping model from micro-evolution to macro-degradation is developed using KAM.
Creep-fatigue interaction is identified as a primary failure mode for components operating under high temperatures. As operational durations extend, this interaction not only alters the material's microstructures but also initiates a gradual degradation in mechanical properties, significantly impacting its deformation and damage behaviors. In this work, the dynamic microstructural evolution of GH4169 superalloy during creep-fatigue was elucidated via qualitative characterization, and damage level evaluation method was subsequently developed by bridging microstructure degradation to mechanical property degradation. Creep-fatigue tests were performed at 650 °C with various tensile holding times and were interrupted at lifetime fractions of 10 %, 50 % and 80 % for further analysis and tensile evaluations. Results revealed that the prolonged exposure to holding times induced the coarsening of γ″ precipitates alongside an increase in low-angle grain boundaries, culminating a reduction in creep-fatigue strength. The development of voids and cracks exacerbated the degradation of elongation, leading to a hybrid fracture mode encompassing both intergranular and transgranular cracking paths. Synthesizing microstructural evolutions to qualitatively categorize diverse degradation levels imparted a robust physical basis for damage evaluation. A mapping model was established to correlate the average kernel average misorientation (micro-degradation indicator) with the tensile plastic strain energy density (macro-degradation indicator). The damage level evaluation method was endowed with quantitative metrics utilizing this model, and its generality was additionally validated in P92 steel. This work offers an insight into the quantitative damage evaluations of creep-fatigue-induced degradations in materials, thereby providing a theoretical basis for the development of operation management and inspection plans of components.
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