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The fracture behaviour of grains and grain boundaries plays an important role in the design of novel ceramics with improved macromechanical deformability. This was investigated first time in a dual-phase high-entropy carbide/boride ceramic, using microcantilever bending, linear beam theory and micro/nanofractography. Microcantilevers were focused ion beam milled from the carefully polished sample surface with random locations containing both carbide and boride grains and grain boundaries in the beam volume. Scanning electron microscopy-based fractography analysis revealed that the bending strength of grain/phase boundaries varies in the range of 0.9–6.7 GPa, depending on the orientation and location of the boundaries in the beam. The fracture of carbide grains was found to be initiated mainly on submicron size volume defects with strength values of 4.5–9.5 GPa, while the fracture origin in most boride grains was identified as surface defects introduced by specimen preparation, which resulted in higher fracture strengths of 9.0–12.3 GPa. [ABSTRACT FROM AUTHOR]

The phase and grain boundary characteristics of recently developed fine-grained dual-phase high-entropy (Ti-Zr-Nb-Hf-Ta)C/(Ti-Zr-Nb-Hf-Ta)B 2 was investigated throughout all the accessible length scales using scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The system exhibits relatively homogeneous grain size distribution where the average size is approximately 0.97 µm, with chemical composition (Ti 0.14 Zr 0.2 Nb 0.2 Hf 0.2 Ta 0.26)C + (Ti 0.38 Zr 0.18 Nb 0.22 Hf 0. 115 Ta 0.105)B 2. SEM analyses revealed no micro-crack formation and second – phase segregation at the boundaries or micro-pores at the triple – points. Investigation down to the sub-nanometer scale revealed that the phase and grain boundaries were typically clean and sharp with an indistinct 1 – 1.5 nm thin gradient of metallic elements at boride/boride and carbide/carbide interfaces. The sharp phase and grain boundaries do exhibit elemental enrichment from a trace amount of Fe being incorporated in interstitial positions of carbide and boride grains locally at boride/carbide boundaries or are present in boride and carbide grains in the form of continuous thin layer at boride/boride and carbide/carbide interfaces with the probably origin from starting powders. [ABSTRACT FROM AUTHOR]

Fine-grained (Ti-Zr-Nb-Ta-Hf)C/(Ti-Zr-Nb-Ta-Hf)B 2 dual-phase high-entropy boride/carbide ceramic (HEC/HEB) was prepared from powders synthesized via a boro-carbothermal reduction and sintered by spark plasma sintering. The developed ceramic has a very high density and relatively homogeneous chemical composition of HEC and HEB grains with a size of HEC grains from 0.1 μm to 1.5 μm and HEB grains from 0.1 μm to 5 μm. The nanohardness of the HEC and HEB grains are very high with values of 37.4 ± 2.3 GPa and 43.3 ± 2.9 GPa, respectively. The nanohardness in the vicinity of grain boundaries is 30.0 ± 5.2 GPa. The Young's modulus of HEC with a mean value of 536.5 ± 34.2 GPa is significantly lower in comparison to Young's modulus for HEB grain with a mean value of 766 ± 45.7 GPa. Vickers hardness HV1 of the developed HEC/HEB ceramic is very high with a value of 29.4 ± 2.0 GPa which is the highest between the up-to-now reported dual-phase high-entropy ceramics. The developed system shows good indentation fracture resistance with a value of 3.9 ± 0.62 MPa m1/2. The most significant toughening mechanism is crack branching in larger HEC grains with sizes from 1.0 μm to 1.5 μm. [ABSTRACT FROM AUTHOR]

A novel dual-phase (Ti-Zr-Nb-Hf-Ta)B 2 + (Ti-Zr-Nb-Hf-Ta)C high-entropy, compositionally complex composite was synthesized by reactive spark plasma sintering using commercial ZrB 2 , NbB 2 , HfB 2 , TaB 2 and TiC powders as starting materials. The composite with high density, consists predominantly of boride (∼39.7 vol%) and carbide (∼56.4 vol%) phases and with residual amount of (Hf-Zr)O 2. The microstructure is homogenous and relatively fine due to the reaction and solid solution coupling effect with average grain sizes of 3.4 μm and 2.2 μm of the boride and carbide phases, respectively. Large-area SEM analyses confirmed no microcracks or micropore formation and no presence of large grains or clusters. Investigation from micro to nano and atomic level, focused on local chemical composition and spatial distribution of constituent elements, revealed that Ti preferentially dissolves into the boride and all other elements to the carbide phase creating a (Ti 0.82 Zr 0.04 Nb 0.08 Hf 0.03 Ta 0.03)B 2 + (Ti 0.49 Zr 0.12 Nb 0.13 Hf 0.11 Ta 0.15)C system. The analysis of grain and phase boundary interfaces revealed a continuous, <1.5 nm wide, segregation of impurity atoms of Fe and Co. Continuous (>20 nm) and atomically flat basal interfacial termination planes (001) decorated with monoatomic Zr enriched layer in the boride phase were observed. Triple points at grain and phase boundaries exhibit metal-rich composition due to impurities from the starting raw powders. [ABSTRACT FROM AUTHOR]

Abstract: Silicon nitride+1wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si3N4 grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9MPam0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3MPam0.5. The highest value of K IC was 9.9MPam0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5MPam0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging. [Copyright &y& Elsevier]

Abstract: Silicon nitride+1wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as filler. The influence of the addition of GPLs on the microstructure development and on the fracture toughness of Si3N4 +GPLs composites was investigated. The GPLs with thickness from 5nm to 50nm are relatively homogeneously distributed in the matrix of all composites, however overlapping/bundle formation of GPLs was found, containing 2–4 platelets as well. The single GPLs and overlapped GPLs are located at the boundaries of Si3N4, and hinder the grain growth and change the shape of the grains. The fracture toughness was significantly higher for all composites in comparison to the monolithic Si3N4 with the highest value of 9.9MPam0.5 for the composite containing the GPLs with smallest dimension. The main toughening mechanisms originated from the presence of graphene platelets, and responsible for the increase in the fracture toughness values are crack deflection, crack branching and crack bridging. [Copyright &y& Elsevier]