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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]

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: Indentation fatigue of WC–Co systems with different microstructure parameters was investigated using instrumented indentation. A Berkovich indenter was used at maximum loads from 25mN to 200mN, then unloaded to 50% of the maximum load and repeatedly re-loaded to 100 cycles. FIB/TEM observations have been used for characterization of the deformation and damage mechanisms in the zone beneath the indenter to study the influence of microstructure parameters on fatigue and fatigue mechanisms. Deformation induced fcc–hcp phase transformation of Co binder was the main mechanisms in the system with higher volume fraction of binder and slip in WC grains and fracture of WC/WC bridges in the system with lower Co content and higher contiguity. These mechanisms result in different fatigue behavior of the systems. [Copyright &y& Elsevier]

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]

The effect of the addition of carbon nanofibers (CNFs) on the microstructure, fracture/mechanical and electrical properties of the CNF/zirconia composite has been investigated. The microstructure of both ZrO2 and ZrO2–CNF composites consists of a very low grain sized matrix (approximately 160nm) with relatively well dispersed carbon nanofibers in the composite. The mechanical properties slightly decreased after the addition of CNFs to the ZrO2 but the electrical resistivity decreased significantly, exhibiting approximately 0.1Ωcm. [Copyright &y& Elsevier]

Abstract: Sintering and hot pressing have been used for preparation of monolithic ZrO2 and zirconia–carbon nanotube (CNT) composite. The effect of the processing route and the CNT''s addition on the microstructure, fracture/mechanical and electrical properties of the zirconia has been investigated at room temperature. The microstructure of the sintered and hot-pressed monolithic ZrO2 consists of a submicron-sized grains. The matrix of the ZrO2–CNT composite consists of a grains with even smaller size (approximately 140nm) with relatively well dispersed carbon nanotubes. The hardness and the indentation toughness of the sintered monolithic zirconia are 1297kg/mm2 and 8.01MPam0.5 and of the hot-pressed monolithic zirconia 1397kg/mm2 and 6.24 MPam0.5, respectively. The addition of the CNT''s decreased the hardness and indentation toughness to 830kg/mm2 and 5.6MPam0.5 however the electrical resistivity decreased significantly in comparison to the monolithic zirconia to the value of 0.1Ωcm. [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]

Abstract: Tribological behaviur of carbon fibrous phases (nanofibers and nanotubes) containing composites with Si3N4, ZrO2 and Al2O3 matrices was studied by pin-on-disk technique in conditions of dry sliding. Coefficients of friction and wear rates were measured, wear damage mechanisms were observed and identified. The resulting tribological behaviur was related to microstructure and mechanical properties of respective materials. Electrical conductivity was measured in wide range of frequencies by two-point method and effect of volume fraction and distribution of CNTs and CNFs on percolation threshold was evaluated. Both coefficient of friction and electrical resistivity decreased with increasing amount of carbon phases, in both cases the nanofibers were more efficient than the nanotubes. The wear resistance in most cases decreased but for Si3N4–CNT composite a certain optimum (∼5wt.% CNT) was found. [Copyright &y& Elsevier]