Core-rim Structure Research Articles

Fracture mode of Ti(C, N)-based cermets investigated by experiment and simulation calculation

The high hardness and good wear resistance of Ti(C, N)-based cermets confer excellent applicability as cutting tools. However, the lack of in-depth research on the fracture mechanism limits further improvements in toughness. A typical commercial Ti(C, N)-based cermet with a core–rim structure was prepared through low-pressure sintering. The interface characteristics between the complex core, rim, and binder phases were characterised using transmission electron microscopy (TEM), while the actual hardness and modules of these phases were measured through in situ nanoindenter assessments. The crack propagation behaviour of the cermet was analysed using a 70° angle observation method combined with electron backscatter diffraction in scanning electron microscopy (EBSD/SEM). This study identifies a commonly occurring but long-overlooked fracture mode, termed “trans-rim” fracture. The experimental results were verified through simulation calculations. This novel idea offers insights into enhancing the toughness of Ti(C, N)-based cermets.

Effects of sintering temperature on the microstructure and mechanical properties of double-hard-phase TiB2–TiC cermets

The mechanical properties and microstructure of double-hard-phase TiB2–25-wt.%-TiC–20-wt.%-CoNi cermets prepared at different temperatures were investigated by performing scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results showed that the transverse rupture strength, indentation fracture toughness, hardness, and relative density of the cermets reached the maximum values of 1654 MPa, 11.06 MPa m1/2, 90.1 HRA, and 99.42 %, respectively, when the sintering temperature was 1462 °C. Solid- and liquid-phase sintering occurred at temperatures below and above 1354.5 °C, respectively. The cermets were composed of TiB2, TiC, TiB, and CoNi phases when sintered at 1440 °C and below, but the TiB phase disappeared when sintered at 1462 °C and above. Both TiB2 core–(Ti, Co, Ni)B rim and TiC core–(Ti, Co, Ni)C rim structures appeared in cermets sintered at 1440 °C and above. In addition, a few TiC core–(Ti, W)C inner–(Ti, W, Co, Ni)C outer double rim structures were formed. A coherent interface was found between the (Ti, Co, Ni)B rim and TiB2 core. A thin CoNi amorphous metallic layer existed at the interface between the (Ti, Co, Ni)C rim and TiC core. In addition, amorphous CoNi was observed in the interfacial region between the CoNi binder and (Ti, Co, Ni)C rim phase and inside the CoNi binder phase. These multi-interface and core–rim structures resulted in excellent strength and toughness of the cermets.

Clustering and interfacial segregation of radiogenic Pb in a mineral host-inclusion system: Tracing two-stage Pb and trace element mobility in monazite inclusions in rutile

Abstract Accessory minerals like zircon, rutile and monazite are routinely studied to inform about the timing and nature of geological processes. These studies are underpinned by our understanding of the transfer processes of trace elements and the assumption that the isotopic systems remain undisturbed. However, the presence of microstructures or Pb-bearing phases in minerals can lead to the alteration of the Pb isotopic composition. To gain insight into the relationship between Pb isotopic alterations from inclusions and microstructures, this study focused on inclusions from an ultra-high-temperature metamorphic rutile. The studied inclusions are submicrometer monazites, a common mineral rich in Pb but normally not present in rutile. The sample is sourced from Mt. Hardy, Napier Complex, East Antarctica, an ultra-high-temperature (UHT) metamorphic terrane. By applying correlative analytical techniques, including electron backscatter diffraction mapping, transmission electron microscopy (TEM), and atom probe tomography, it is shown that monazite inclusions are often in contact with low-angle boundaries and yield no preferred orientation. TEM analysis shows the monazite core has a mottled texture due to the presence of radiation damage and nanoclusters associated with the radiation damage defects that are rich in U, Pb, and Ca. Some monazites exhibit a core-rim structure. The rim yields clusters composed of Ca- and Li-phosphate that enclose Pb nanoclusters that are only present in small amounts compared to the core, with Pb likely diffused into the rutile-monazite interface. These textures are the result of two stages of Pb mobility. Initial Pb segregation was driven by volume diffusion during UHT metamorphism (2500 Ma). The second stage is a stress-induced recrystallization during exhumation, leading to recrystallization of the monazite rim and trace element transport. The isotopic signature of Pb trapped within the rutile-monazite interface constrains the timing of Pb mobility to ca. 550 Ma.

Evolution of microstructure and mechanical properties of TiB2-7.5 wt.% WC-based cermets sintered at different temperatures

The evolution of microstructure and mechanical properties of TiB2–7.5 wt% WC-based cermets sintered at different temperatures were investigated. The results showed that eutectic liquid phase appeared at around 1335 °C, indicating that solid-phase sintering occurred below that temperature, while liquid-phase sintering occurred above that temperature. TiB2, W2CoB2, WB and CoNi phases were detected in the XRD patterns of cermets sintered at different temperatures, and TEM analysis showed the presence of TiC phase. So the following chemical reaction should occur: TiB2 + WC + Co → W2CoB2 + WB + TiC. The TiB2 core - (Ti, W, Co, Ni)(B, C) rim phase were observed in the cermet sintered at 1397 °C. As the increase of sintering temperature, the solubility of WC and TiB2 in CoNi binder phase, and the number of core-rim structure hard phase particles increased. The formation of core-rim structure enhanced the interfacial bonding strength between TiB2 phase and CoNi binder, which was beneficial for improving the strength and toughness of cermets. The cermets sintered at 1465 °C had good comprehensive mechanical properties, with a relative density of 99.62%, a hardness of 17.34 ± 0.43 GPa, a transversal rupture strength of 2038 ± 53 MPa, and an indentation fracture toughness of 12.15 ± 0.32 MPa·m1/2.

Effect of Ta on the tribological behavior of in-situ TiC/Ni composites

The tribological behavior of TiC/Ni composites is optimized by adding Ta, due to the improved oxidation resistance and hardness. The (Ti,Ta)C particles with core-rim structure are formed, and particle size decreases from 1.03 µm to 0.75 µm by adding 8 wt% Ta, and the hardness increases from 630.43 HV to 669.13 HV. At 20 N load, the primary wear mechanism is oxidation wear and the tribological property is influenced by oxidation resistance. At 50 N load, the main wear mechanism includes abrasive, adhesive, delamination and oxidation wear, and the tribological behavior is mainly influenced by the hardness and oxidation resistance.

Post–peak petrochronology and C–O–H fluid influx in the lower crust: A case study of garnet–orthopyroxene granulites from the Rauer Islands, East Antarctica

Garnet–orthopyroxene granulites from the Rauer Islands (East Antarctica) provide a spectacular example to investigate the late fluid evolution, metamorphic duration, and behavior of monazite and zircon in response to metamorphic reactions and fluid–rock interaction. Here, we characterize the secondary fluid inclusions in peritectic garnet and orthopyroxene, which occur as multiphase inclusions along micro–fractures. Inclusions are composed of siderite, pyrophyllite, calcite, quartz and residual CO2, representing stepdaughter phases resulting from the interactions between C–O–H fluid and its hosts at variable temperatures during retrogression. Zircon grains show clear core–rim structure, which yield 206Pb/238U ages of 540–507 Ma and 527–490 Ma, respectively. Index inclusions and internal structures suggest that the cores document the timing of peak and post–peak decompression while the growth of rims corresponds to melt crystallization during the final cooling. The UPb systems in zircons are considered to have not been obviously affected by fluid or melt–mediated modification. The unusual formation of monazites in garnet–orthopyroxene granulites may be linked with the elevated phosphorus budget as a result of apatite dissolution during the prograde melting of the rocks. Detailed investigations suggest that the crystallization of monazites occurred both at peak and post–decompression stages, whose isotopic systematics has been completely reset due to melt–mediated dissolution–precipitation. The spurious dates for monazites (522–495 Ma) are highly coinstantaneous with the dating results for zircon rims, further supporting this view. Therefore, we conclude that the late carbonic fluid influx cannot result in marked U(Th)–Pb resetting in zircon and monazite. Instead, anatectic melt may have played an important role in the disturbance of isotopic systematics in monazites, especially for long–lived high–grade metamorphic terranes. Combined with previously published data, we propose that the Pan–African metamorphic event in the Rauer Islands may have reached the peak at around ∼540 Ma, followed by a protracted post–peak evolution that lasted for at least ∼50 Myr. This study highlights the importance of an integrated investigation of fluid and index mineral inclusions, as well as the chemical signatures of zircon and monazite, to interpret chronological data correctly.

Effect of binder content and sintering time on the microstructure and mechanical properties of gradient cemented carbides with Ni binder

Replacement of Co by alternative Ni binder in gradient cemented carbide is an effective way to reduce the health concerns associated with Co powders. In the present work, gradient cemented carbides with different Ni contents and gradient sintering times were prepared by traditional powder metallurgy with a two-step sintering method. The microstructure of cemented carbide including the gradient layer formation was analyzed by XRD, SEM, EDS, TEM, and EBSD. The thickness of the gradient layer is almost proportional to the square root of sintering time, indicating that the gradient layer formation is a diffusion-controlled process. The atomic migration can be accelerated to form a thicker gradient layer with the increase of the binder content. The coherent interface of the Ti(C, N) core-rim structure for the gradient cemented carbides with Ni binder was revealed. With the increase in binder content, the average grain size of the WC phase is increased. The hardness and fracture toughness of the cemented carbides were also studied and analyzed according to the crack propagation behavior and the distribution of dislocation and stress indicated by KAM and GND. The variation of the hardness and the fracture toughness with the sintering time is mainly related to the density and the grain size of hard phases, while the change of the mechanical properties along with the Ni content probably results from the local dislocation density and the stress in the cemented carbides. The cemented carbide can obtain a good combination of hardness and fracture toughness with 10 wt% Ni gradient sintered for 2h.

The microstructure and mechanical properties of steel-bonded TiB2 modified by adding Mo2C

In this study, the microstructure and interface of steel-bonded TiB2 were adjusted by adding Mo2C, so as to improve its comprehensive mechanical properties. The addition of Mo2C led to the formation of two kinds of core-rim structures, including TiB2 core-(Ti, Fe, Ni, Cr, Mo)B rim, and TiC core-(Ti, Fe, Ni, Cr, Mo)C rim. Micro - nano scale of TiB2, TiC and (Mo, Cr)C particles were formed in the steel-bonded TiB2. The introduction of Mo element caused the redistribution of elements in Fe-based binder phase, and two types of binder phases were generated, one was Ni-rich binder phase, and the other was Cr-rich binder phase. A full coherent interface was formed between the core and rim phase, and the amorphous metal thin layer was formed between the rim and binder phase. A coherent interface was formed between TiB2 and (Mo, Cr)C phase, (Mo, Cr)C and Ni-rich binder phase, as well as TiB2 and Ni-rich binder phase. Both the coherent interface and amorphous metal thin layer greatly enhanced the interface bonding strength, and improved the strength and toughness of the steel-bonded TiB2. The steel-bonded TiB2 with 6.0 wt% Mo2C exhibited excellent comprehensive mechanical properties, and its relative density, hardness, transverse rupture strength and fracture toughness were 99.94 ± 0.06%, 92.3 ± 0.5 HRA, 1278 ± 37 MPa and 10.90 ± 0.28 MPa·m1/2, respectively.

Broadening of the carbon window and the appearance of core-rim carbides in WC-Fe/Ni cemented carbides

Among several separate challenges, the major one for replacing cobalt in cemented carbides is the difficulty to obtain alternative binder materials with a C-window broad enough to be robustly processed under conventional industrial control on the C content. The C-window is defined as the C content range for which phases that are detrimental to the mechanical properties are avoided. The present paper has two main objectives: first, to show that the processing C-window of Fe-Ni based systems is in fact wider than what thermodynamic equilibrium calculations predict, and that its width can be controlled moderately by tweaking the initial WC grain size and the cooling rate used in the material’s processing. Secondly, in case those detrimental phases are not avoided, this work gives insight on how to make their appearance less detrimental for the mechanical properties. The morphology, volume fraction and particle size distribution of the detrimental phases, specifically η6-carbides at low C contents, are investigated to explore desirable combination of hardness and toughness of alternative binder cemented carbides. During this study it was also discovered that in samples with carbon contents below the low-C limit of the C window a carbide with hexagonal lattice known as κ, not commonly seen in cemented carbides, appeared and formed the core of a core-rim structure together with the more common η6-phase. It is believed that the κ-carbide form due to local high concentrations of tungsten during solid state sintering and that it has an impact on the precipitation characteristics of the η6-phase.

Core-rim structured MXene@SiO2 composites as oil-based additives for enhanced tribological properties

Herein, we have prepared SiO2 particles uploaded MXene nanosheets via in-situ hydrolysis of tetraetholothosilicate. Due to the large number of groups at the edges of MXene, SiO2 grows at the edges first, forming MXene@SiO2 composites with a unique core-rim structure. The tribological properties of MXene@SiO2 as lubricating additive in 500 SN are evaluated by SRV-5. The results show that MXene@SiO2 can reduce the friction coefficient of 500 SN from 0.572 to 0.108, the wear volume is reduced by 73.7%, and the load capacity is increased to 800 N. The superior lubricity of MXene@SiO2 is attributed to the synergistic effect of MXene and SiO2. The rolling friction caused by SiO2 not only improves the bearing capacity but also increases the interlayer distance of MXene, avoiding accumulation and making it more prone to interlayer slip. MXene@SiO2 is adsorbed on the friction interface to form a physical adsorption film and isolate the friction pair. In addition, the high temperature and high load induce the tribochemical reaction and form a chemical protection film during in the friction process. Ultimately, the presence of these protective films results in MXene@SiO2 having good lubricating properties.

Mechanisms of microstructural evolution in high-volume TiC reinforced steel composites: Insights into particle coarsening, morphology, and thermochemical reactions

Titanium carbides reinforced steel matrix (TiC-steel) composites exhibit a range of excellent properties, including increased strength-to-weight ratio, high-temperature strength, and outstanding wear resistance. Prior research has demonstrated that the mechanical properties of TiC-steel composites are significantly affected by particle coarsening and the associated core-rim structure. Nevertheless, the fundamental mechanisms of thermochemical reactions that govern the formation of the final microstructure in these composites, including the core-rim structure, remain incompletely explored. This study elucidates the microstructure evolution mechanism of high-volume TiC-steel composites, particularly particle coarsening, through thermodynamic calculations and chemical analysis. The investigation unveils the coarsening process, shedding light on the formation of more stable phases and associated compositional changes, including the incorporation of substitutional elements and carbon depletion (resulting in low stoichiometry, x < 1) within the newly developed (Ti,M)Cx. Additionally, the increased carbon content contributes to pearlite emergence within the steel matrix. Notably, key factors influencing the distinct morphologies observed in growing TiC particles are identified, attributed to variations in Mo, V, and W concentrations, as elucidated by rigorous thermodynamic and first-principles calculations. This study represents significant progress in understanding the intricate microstructure development of high-volume TiC-steel composites.

Preparation of Ultrafine Co- and Ni-Coated (Ti,W,Mo,Ta)(C,N) Powders and Their Influence on the Microstructure of Ti(C,N)-Based Cermets.

The use of metal-coated ceramic powders not only effectively enhances the wettability of the metal-ceramic interface but also promotes a more uniform microstructure in Ti(C,N)-based cermets, which is advantageous for improving their mechanical properties. In this study, ultrafine Co- and Ni-coated (Ti,W,Mo,Ta)(C,N) powders were synthesized via the spray-drying-in-situ carbothermal reduction method. Subsequently, Ti(C,N)-based cermets were effectively fabricated using the as-prepared ultrafine Co- and Ni-coated (Ti,W,Mo,Ta)(C,N) powders. The impact of reaction temperature, heating rate, and isothermal time on the phase and microstructure of prepared powders was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Additionally, the microstructure of the as-sintered cermets was experimentally investigated. The findings reveal that the complete reduction of Co and Ni metal salts, pre-coated on the surface of (Ti,W,Mo,Ta)(C,N) particles, can be achieved through rapid heating (10 °C/min) in a specific temperature range (600-1000 °C) with an isothermal time of 3 h at a lower reduction temperature (1000 °C). The synthesized powders have only two phases: the (Ti,W,Mo,Ta)(C,N) phase and Co/Ni phase, and no other heterogeneous phases were observed with an oxygen content of 0.261 wt.%. Notably, the conventional core-rim structure was not dominant in the cermets obtained from the prepared Co- and Ni-coated (Ti,W,Mo,Ta)(C,N) powders. Moreover, the heterogeneous segregation effect of the Co/Ni coating on the ultrafine powder particles resulted in a finer microstructure than the traditional cermets with the same composition. However, the grain size is mainly in the range of 0.5-0.8 μm. The weaker residual stresses at the core and rim interfaces and the finer particle distributions could theoretically enhance the toughness of Ti(C,N)-based cermets, simultaneously.

Mineralizations of Nb-Ta-Rb-Zr and rare-earth elements in Boziguoer, South Tianshan, NW China: Geochronology and geochemistry of monazite and bastnäsite

The Boziguoer alkaline intrusion is located in the South Tianshan Orogenic Belt on the northern margin of the Tarim Craton. The intrusion is entirely mineralized and forms a super-large rare metal (RM) and rare-earth element (REE) deposit dominated by Nb-Ta-Rb, accompanied by Zr-REE. The primary RM minerals include pyrochlore, astrophyllite, and zircon, while the main REE minerals consist of fluocerite, monazite, xenotime, and bastnäsite, which are commonly present as granular aggregates or singularly filling the interstices between gangue minerals. Through a combination of geochronological and geochemical analyses of different types of monazite and bastnäsite in the mineralized alkaline rocks, this study elucidates the role of magmatic-hydrothermal evolution on the Boziguoer RM-REE mineralization and reconstructs the geochronological framework of alkalic magmatic-hydrothermal evolution and mineralization processes, as well as establishing mechanisms responsible for RM-REE enrichment. Petrographic observations and back-scattered electron (BSE) imaging revealed several types of monazite and bastnäsite with different characteristics as follows: (1) type Ia monazite (Mnz-Ia) experienced intense hydrothermal alteration, forming residual cores of monazite mantled by apatite and allanite coronas; (2) type Ib monazite (Mnz-Ib) was partially eroded, forming concentric zoning patterns with a core of monazite, a mantle of apatite, and an outer rim of allanite coronas; (3) type II monazite (Mnz-II) is slightly modified, and commonly associated with fluorite; (4) bastnäsite and fluocerite exhibit a core-rim structure, with the core of fluocerite being brighter than the rim of bastnäsite in BSE images. According to the paragenetic relationships and compositional variations, all two types of monazite are of primary magmatic origin, while bastnäsite is of hydrothermal origin. In addition, principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) results indicate that Mnz-Ia and Mnz-Ib belong to the same group, different from Mnz-II. The latter has higher (Ce/Gd)N and (Th/U)N ratios, indicating relatively low-temperature crystallization of Mnz-II at late stages. In-situ LA-ICP-MS U-Th-Pb dating of monazite and bastnäsite show that Mnz-Ia and Mnz-Ib have ages of ca. 290 Ma, consistent with the zircon age of the intrusion, whereas the Mnz-II and bastnäsite have younger ages of ca. 280 Ma, representing a post-magmatic hydrothermal mineralization event at Boziguoer. These new ages indicate that both magmatic and subsequent hydrothermal processes played critical roles in the RM-REE mineralization in this deposit. The latest findings also support the model that the formation of the alkaline belt where the Boziguoer ore-bearing intrusion was temporally and spatially linked to the Tarim Large Igneous Province that is genetically related to a mantle plume.

Synthesis of (Ti, W, Mo) CN based cermets with different carbides configurations for demanding applications: Study of the crystal structure, microstructure, and mechanical properties

In this study, based on different element configurations within constant atomic ratio of elements, (Ti0.93W0.07Mo0.07)C–20%Ni and (Ti0.93W0.07Mo0.07)CN0.3-20%Ni derived cermets have been synthesized. The basis for the difference in the production route was whether the carbides were formed by carbothermic reaction from the metal oxide together or separately, or in the case of Mo2C, the carbide is added to the mixture together with the binder after reduction and just before consolidation. Another basis for the difference was whether the cermet was a carbide or a carbonitride. To investigate the influence of the different production routes, the crystal structure, microstructure, and mechanical properties of the cermets produced were examined using XRD, FESEM, STEM, and Vickers indentation. The XRD spectra of all the cermets were found to be very similar to those of TiC-based cermets, indicating that the additive carbides in the TiC or Ti(CN) phases of the cermets dissolve perfectly during the high vacuum sintering process at 1510 °C. The highest toughness (14.65 MPa m1/2) was obtained in (Ti0.93W0.07) C–8%Mo2C–20Ni cermets with a core-rim structure. In addition, the use of nitrogen leads to a dramatic reduction in particle size. The use of molybdenum and tungsten in the form of separate carbides had little effect on limiting the expansion of crystal size and grain size compared to the scenario where the dissolution of these elements took place within the primary core-rim structure. However, in terms of hardness and toughness, it was found that, in addition to grain size, the route taken in the addition of molybdenum and tungsten was also important.

Effects of WC/TiC addition on the thermodynamic behavior of TiB2-based cermet during sintering process

The heat absorption, weight change and exhaust behavior of TiB2-based cermet powder mixtures with additions of WC, TiC and WC-TiC were analyzed by a simultaneous thermal analyzer coupled with a mass spectrometer (TG/DSC-QMS). Thermodilatometry was used to investigate the densification behavior due to sintering process of cermets. During the sintering process, the order of degassing is: water vapor, CO2 released by the reduction of adsorbed oxygen in the raw material powder, CO and CO2 released by the reduction of surface oxides of Co and Ni powder, WC powder, TiB2 and TiC powder. The addition of WC, TiC and WC-TiC accelerated the reduction of oxides on the surface of Co and Ni powders, and decreased the temperature point of liquid phase formation in TiB2-based cermet. WC and TiC dissolved in liquid CoNi binder and then precipitated on the undissolved TiB2 particles to form a typical core-rim structure during the sintering. This study is expected to lay a solid foundation for the development of high-performance TiB2-based cermets based on TiB2-CoNi, TiB2-WC-CoNi, TiB2-TiC-CoNi and TiB2-WC-TiC-CoNi cermets in the future.

On the effects of B4C addition on the microstructure, mechanical properties, wear and thermal stability of TiCN- Co -Cr3C2-Si3N4 cermets developed by vacuum hot pressing

This paper reports the development of TiCN - Co - Cr3C2 - Si3N4 cermets modified with the addition of B4C. Cermets of three different compositions with varying B4C in TiCN - Co - Cr3C2 - Si3N4 were fabricated. The TiCN, Co, Cr3C2, Si3N4 and B4C powders in the combinations 80%TiCN-5%Co-5%Cr3C2–5%Si3N4–5%B4C; 75%TiCN-5%Co-5%Cr3C2–5%Si3N4–10%B4C; 70%TiCN-5%Co-5%Cr3C2–5%Si3N4–15%B4C were ground in a ball milling unit for six hours at a speed of 300 rpm at a ball-to-powder ratio of 10:1. The powders were then sintered in a vacuum hot press at a temperature of 1600 °C and a pressure of 50 MPa with soaking time of 90 min. For comparison purpose, 90%TiCN-5%Co-5%Cr3C2 cermet was prepared. The microstructure of the sintered cermets revealed the core-rim structure. The Vickers hardness test was done with a load of 10 Kg and for duration of 10 s. Fracture toughness was measured using the crack length data obtained from the hardness test using scanning electron microscopy. The cermet with a composition 75% TiCN-5%Co-5% Cr3C2–5% Si3N4–10% B4C produced higher hardness of 26.53 ± 0.56 GPa and fracture toughness of 10.29 ± 0.18 MPam and 70% TiCN-5%Co-5% Cr3C2–5% Si3N4–15% B4C showed high fracture toughness value of 11.23 ± 0.09 MPam with hardness value of 21.05 ± 0.10 GPa. The cermet composition 75% TiCN-5%Co-5% Cr3C2–5% Si3N4–10% B4C retained the hardness of 16.85 GPa even after annealing at 600 °C. The sintered cermets tend to form TiO2 which determines the hardness of the annealed cermets at a range of 600 °C – 1000 °C. The presence of these oxides tends to decrease the hardness. The wear test was performed taking the cermet as the pin material. The abrasive and adhesive wear mechanisms were dominant in the cermets. The harder precipitates formed tend to increase wear resistance by reducing the wear rate. The harder precipitates (TiN and TiB2) formed mainly due to the addition of Si3N4 and B4C, this addition promotes adhesive wear by a tribolayer formation.

Phase-field simulation of core-rim structure at the early sintering stage in TiC-WC-Ni cermet

Understanding the formation mechanism of core-rim structure in TiC-based cermets is essential for optimizing their mechanical properties, such as strength and toughness. In this work, the formation process of core-rim structure in TiC-WC-Ni cermet at the early sintering stage was investigated by means of the multiphase and phase-concentration modeling framework coupled with CALPHAD (CALculation of PHAse Diagrams) databases. The effect of W content in the Ni binder phase on the rim thickness was examined. The results reveal that W in the Ni binder phase diffuses into the undissolved TiC particles at the early sintering stage, leading to the formation of (Ti,W)C solid solution phase on the particle surface. This solid solution phase serves as the rim which constitutes the core-rim structure together with TiC particles. Furthermore, there is a positive correlation between the rim thickness and the W content in the Ni binder phase. Phase-field simulation exhibits a good agreement with the scanning electron microscopy (SEM) observations of the annealed WC-Ni/TiC–Ni cermet diffusion couple. This study offers a novel approach for exploring the formation mechanism of core-rim structure in TiC-based cermets.

Enhancing core–rim structure control in (K,Na)NbO3-based lead-free piezoceramics via rapid sintering method

Core-rim structure has distinct advantage to improve the performances of KNN-based piezoceramics. Whereas the state of the core-rim structure is difficult to control during sintering. Here, the core-rim structured 0.96(K0.51Na0.47Li0.02)(Nb0.8Ta0.2)O3-0.04CaZrO3 (KNLNT) ceramics were both obtained by conventional sintering (CS) and rapid sintering (RS). KNLNT ceramics prepared by rapid sintering exhibit more outstanding controllability on grain growth and core-rim structure and can hold the core/rim size ratio in a stable and favorable level due to extended effective range of grain boundary diffusion in densification. Benefiting from the controllable microstructure, the RS method prepared samples show excellent performance. The unipolar strain value of RS-1240 (Smax=0.252%) is 2.07 times as much as CS-1110 (Smax=0.122%). Large strain, low hysteresis and low dielectric permittivity features make the core-rim structured KNLNT ceramics a potential material for pulse drive applications and demonstrate that the manual precise control of core-rim structure could create many possibilities on materials design.

Correlation analysis between microstructure and mechanical properties of spark-plasma-sintered Ti(C, N)–W cermets according to changes in titanium, carbon and nitrogen contents

In this study, TiC–50 wt% W, TiN–50 wt% W, TiC0.7N0.3–50 wt% W, Ti0.55C0.13N0.32–50 wt% W, TiC0.5N0.5–50 wt% W and TiC0.5N0.5–70 wt% W cermet specimens with grain sizes smaller than 1 μm were prepared by blending TiC, TiN, TiC0.5N0.5, W and Ti powders followed by the spark plasma sintering of the blended powders. Under Ar gas flow conditions at 1973 K, the TiC0.5N0.5–50 wt% W and TiC-50 wt% W cermet specimens exhibited the first and second highest strengths, respectively, whereas the TiN–50 wt% W and Ti0.55C0.13N0.32–50 wt% W cermet specimens exhibited the lowest strength among all the cermet specimens prepared in this study. In contrast, the Ti0.55C0.13N0.32–50 wt% W and TiC0.5N0.5–50 wt% W cermet specimens exhibited the first and second highest strengths, respectively, at room temperature in laboratory air. Coherent interfaces, whose type differed from those of the TiC0.5N0.5–50 wt% W and TiC0.5N0.5–70 wt% W cermet specimens, were found in (Ti, W)C grains in the TiC–50 wt% W cermet specimen. The coherent interfaces in the TiC–50 wt% W cermet specimen were considered to suppress the grain growth because these interfaces normally suppress the interdiffusion of Ti, W and C atoms. Moreover, the grain growth seemed to be suppressed in the TiC0.7N0.3–50 wt% W, Ti0.55C0.13N0.32–50 wt% W, TiC0.5N0.5–50 wt% W and TiC0.5N0.5–70 wt% W cermet specimens owing to the core–rim structures with coherent interfaces in the cermet specimens. The coherent interfaces and core–rim structures are thus considered to contribute to improving the high-temperature strength of all the cermet specimens except for the TiN–50 wt% W cermet specimen, which contained neither core–rim structures nor coherent interfaces.

Understanding the structural mechanism of electric properties in xBiFeO3-(1-x)BaTiO3 lead-free piezoelectric solid solutions

Chemical distributions, local phase and domain configurations of xBiFeO3-(1-x)BaTiO3 (BF-BT) ceramics with different macroscopic phases were carefully studied to understand the structural mechanisms of electric properties. Universal BF-rich@BT-rich@average-composition triple core-rim structures were found to exist in all the compositions, and the volume fractions of cores decrease with the BF content. The inner cores all show dominant rhombohedral (R) phase, while the rims change from dominant pseudo-cubic (PC) phase with polymorphic polar nanoregions (PNRs) (x = 0.64) to coexistence of R and PC with PNRs and nanodomain (x = 0.7) and then to dominant R phase with PNRs+macrodomain (x = 0.75). The high densities of phase and domain boundaries are responsible for the high and thermal-stable electric-field induced strain in the PC phase. Our results evidence the decisive contribution of the composition-modulated hierarchical microstructures to the electric-field induced strain, and are anticipated to provide the necessary foundation for the effective modulation of the piezoelectricity.