Outstanding specific yield strength of a refractory high-entropy composite at an ultrahigh temperature of 2273 K

We reported on the mechanical properties and microstructural evolution of a W_(20)Ta_(30)Mo_(20)C_(30)(at.%)re-fractory high-entropy composite(RHEC).The RHEC exhibited outstanding yield strength at both 2273 and 1873 K.Microstructural investigations revealed that the as-cast RHEC had a triple-phase structure con-sisting of FCC dendrites,HCP matrix,HCP-BCC eutectic structure,and FCC-BCC eutectoid structure,and exhibited high-density defects owing to the complex phase transformations during solidification.After annealing at 2273 K,the precipitation of the BCC phase from the FCC dendrites and the decomposition of the HCP phase into the FCC-BCC eutectoid structure was observed to significantly refine the grain sizes of all triple phases.After compression at 2273 K,the ceramic phases and solid solution precipitated out from each other,which helps to avoid persistent softening after the yielding of RHEC.Further analyses sug-gested that the dominant deformation mechanisms of the BCC phase and HCP phase are dislocation glide and transformation-induced plasticity;whereas those of the FCC phase are twinning-and transformation-induced plasticity.The outstanding yield strength of this RHEC at ultrahigh temperatures may originate from the high-content ceramic phases and the structural metastability of the multi-principal composition.These findings provide a novel strategy to design RHECs by alloying high-content nonmetallic elements,which contributes to further breaking through their performance limits at ultrahigh temperatures.

Low thermal conductivity and high porosity ZrC and HfC ceramics prepared by in-situ reduction reaction/partial sintering method for ultrahigh temperature applications

Porous ultra-high temperature ceramics(UHTCs) are potential candidates as high-temperature thermal insulation materials. However, high thermal conductivity is the main obstacle to the application of porous UHTCs. In order to address this problem, herein, a new method combining in-situ reaction and partial sintering has been developed for preparing porous Zr C and Hf C with low conductivity. In this process, porous Zr C and Hf C are directly obtained from ZrO2/C and HfO2/C green bodies without adding any pore-forming agents. The release of reaction gas can not only increase the porosity but also block the shrinkage. The asprepared porous Zr C and Hf C exhibit homogeneous porous microstructure with grain sizes in the range of 300–600 nm and 200–500 nm, high porosity of 68.74% and 77.82%, low room temperature thermal conductivity of 1.12 and 1.01 W·m-1 K-1, and compressive strength of 8.28 and 5.51 MPa, respectively.These features render porous Zr C and Hf C promising as light-weight thermal insulation materials for ultrahigh temperature applications. Furthermore, the feasibility of this method has been demonstrated and porous Nb C, Ta C as well as Ti C have been prepared by this method.

General Trends in Electronic Structure, Stability, Chemical Bonding and Mechanical Properties of Ultrahigh Temperature Ceramics TMB_2(TM=transition metal)

The electronic structure, stability, chemical bonding and mechanical properties of 3d, 4d and 5d transition metal diboride TMB2 were investigated using first-principles calculations based on density functional theory. All the primary chemical bonds, i.e., metallic, ionic and covalent have contributions to the bonding of TMB2. The number of valence electrons of transition metals or the valence electron concentration(VEC) of TMB2 has strong effects on the lattice parameters, stability and mechanical properties of TMB2. Both lattice constants a and c decrease with VEC, but c decreases faster than a, which is attributed to the enhanced TM de B p(sp2) bonding. Bulk modulus B of TMB2 increases continuously with VEC due to the enhanced TM de B p(sp2) and TM dd bonding. Shear modulus G increases with VEC,reaching a maximum at VEC=3.33, and then decreases with further increase of VEC. YB2 and Mn B2 have low Young’s modulus and are predicted to have good thermal shock resistance. According to Pugh’s criterion(G/B < 0.571), Mn B2, Mo B2 and WB2are predicted as ductile or damage tolerant ultrahigh temperature ceramics(UHTCs).

Fine structure characterization of an explosively-welded GH3535/316H bimetallic plate interface

An explosion-welded technology was induced to manufacture the GH3535/316H bimetallic plates to provide a more cost-effective structural material for ultrahigh temperature,molten salt thermal storage systems.The microstructure of the bonding interfaces were extensively investigated by scanning electron microscopy,energy dispersive spectrometry,and an electron probe microanalyzer.The bonding interface possessed a periodic,wavy morphology and was adorned by peninsula-or island-like transition zones.At higher magnification,a matrix recrystallization region,fine grain region,columnar grain region,equiaxed grain region,and shrinkage porosity were observed in the transition zones and surrounding area.Electron backscattered diffraction demonstrated that the strain in the recrystallization region of the GH3535 matrix and transition zone was less than the substrate.Strain concentration occurred at the interface and the solidification defects in the transition zone.The dislocation substructure in 316H near the interface was characterized by electron channeling contrast imaging.A dislocation network was formed in the grains of 316H.The microhardness decreased as the distance from the welding interface increased and the lowest hardness was inside the transition zone.

SOLUTION TREATMENT EFFECTS IN COBALT-NICKEL SECONDARY HARDENING STEEL

Effect of the austenitizing temperature on the microstructure,strength, and toughness of 16NiCo and 23NiCo alloys was studied. With increasing austenitizing temperature, the reductions in strength and hardness are probably caused by the coarsening of M_2C carbides. The use of higher austenitizing temperature resulted in improved impact toughness due to the dissolution of undissolved (Cr, Fe, Mo)_(23)C_6 carbides.As the austenitizing temperature is increased to 1050℃ for 16NiCo and 23NiCo alloys impact toughness levels are reduced on average by 31 and 53 J/cm￣2 respectively.The use of 1050℃ austenitizing temperatures resulted in intergranular fracture for 23NiCo alloy in 482℃ aged condition. It was suggested that this grain boundary embrittlement was the result of the formation of cementite particles during ageing.

High strength and high porosity YB2C2 ceramics prepared by a new high temperature reaction/partial sintering process

Porous ultrahigh temperature ceramics(UHTCs) are potential candidates as reusable thermal protection materials of transpiration cooling system in scramjet engine. However, low strength and low porosity are the main limitations of porous UHTCs. To overcome these problems, herein, a new and simple in-situ reaction/partial sintering process has been developed for preparing high strength and high porosity porous YB2C2. In this process, a simple gas-releasing in-situ reaction has been designed, and the formation and escape of gases can block the shrinkage during sintering process, which is favorable to increase the porosity of porous YB2C2. In order to demonstrate the advantages of the new method, porous YB2C2 ceramics have been fabricated from Y2O3, BN and graphite powders for the first time. The as-prepared porous YB2C2 ceramics possess high porosity of 57.17%–75.26% and high compressive strength of 9.32–34.78 MPa.The porosity, sintered density, radical shrinkage and compressive strength of porous YB2C2 ceramics can be controlled simply by changing the green density. Due to utilization of graphite as the carbon source, the porous YB2C2 ceramics show anisotropy in microstructure and mechanical behavior. These features render the porous YB2C2 ceramics promising as a thermal-insulating light-weight component for transpiration cooling system.

Theoretical predictions and experimental verification on the phase stability of enthalpy-stabilized HE TMREB_(2)s

Transition metal diborides(TMB_(2)s)are the materials of choice in extreme environments due to their excellent thermal and chemical stabilities.However,the degradation of oxidation resistance of TMB_(2)s at elevated temperature still hinders their applications.To cope with this challenge,it is effective to incorporate rare earth elements to form high-entropy transition and rare-earth metal diborides(HE TMREBs).To obtain thermodynamically stable single-phase structures for HE TMREB_(2)s,a“16×16 mixed enthalpy matrix”is constructed using first-principles calculations to predict the single-phase formation ability of120 two-component diborides(TCBs).Through the use of the“16×16 mixed enthalpy matrix”of TCBs,specific combinations of TMB_(2)s and REB_(2)s that are most likely to form single-phase HE TMREB_(2)s are confirmed.Subsequently,based on the energy distribution of the local mixing enthalpies of all possible configurations,the enthalpy and entropy descriptors of HE TMREB_(2)s(RE=Sc,Lu,Tm,Er,Ho and Dy)are investigated.It is found that the mixing enthalpy plays a critical role in the stability of the single-phase HE TMREB_(2)s,i.e.,HE TMREB_(2)s are enthalpy-stabilized materials.The experimental results further confirm that enthalpy dominates the thermodynamic domain and drives the stability of REB_(2)s in HE TMREB_(2)s.This study validates that enthalpy-stabilized HE TMREB_(2)s can further expand the compositional space of ultrahigh temperature ceramics(UHTCs)and is expected to further improve the oxidation resistance and high temperature properties of UHTCs.

A novel Hf-Si-O wire drawing phenomenon after ablation of SiC_(nws)/ HfC-SiC coating on C/C composites

A SiC_(nws)/HfC-SiC coating was fabricated by a two-step chemical vapor deposition.After introducing SiC nanowires,the fracture toughness of HfC-SiC coating increases by 228%.After ablation under oxyacetylene torch for 60s,a bone-like HfO_(2) reticular structure was formed in the central ablation area.Interestingly,a novel Hf-Si-O wire-drawing phenomenon was happened in the transition area of ablated SiC_(nws)/HfC-SiC coating,providing a new toughness mechanism under ablation environment.Moreover,HfO_(2) precipitated from Hf-Si-O glass wires to generate nanocrystalline grains during ablation,which explains the formation of bone-like HfO_(2) reticular structure observed from central ablation area.