Reactive Hot Pressing Research Articles

Mechanical behaviours of reactive hot-pressed B4C-based composites with ZrB2-ZrC

Three B4C-based composites with 10, 20 and 30 vol% ZrB2-ZrC were prepared by reactive hot-pressing of Zr and B4C powders at 1900°C for 2 h under 20 MPa in Ar. Relative density of ∼96 % was obtained for 10 vol% ZrB2-ZrC, while relative density of >99 % was obtained for 20 and 30 vol% ZrB2-ZrC. The resulting composites were composed of B4C, ZrB2 and ZrC. The elastic moduli and hardness were insensitive to the ZrB2-ZrC content, whereas the fracture toughness increased with increasing ZrB2-ZrC content. In addition, the three compositions were fractured in four-point flexure at temperatures ranging from room temperature to 1600°C. At or below 1500°C, only linear stress-strain response was observed for each composition. Nonlinear response was observed in the stress-strain curves for temperature at 1600°C. The strength displayed by the three compositions was closely associated with the amount and characterization of ZrB2 and ZrC phases formed during the sintering.

Design, preparation, and enhanced mechanical properties of novel plate-like grains toughened (Zr, Ti, W, Ta)C medium-entropy ceramics fabricated by in-situ reaction hot-pressing

Novel plate-like (Ti, Zr)B2 toughened (Zr, Ti, W, Ta)C medium-entropy ceramics were designed based on the atomic scale, nanoscale, and microscale, and prepared via the in-situ reaction hot-pressing adopting (Ti, W, Ta)C and ZrB2 powders. The multiphase ceramics were primarily composed of the (Zr, Ti, W, Ta)C matrix and plate-like (Ti, Zr)B2. When the addition of ZrB2 exceeds 50 mol%, the residual ZrB2 causes the secondary primary phase to change from (Ti, Zr)B2 to (Zr, Ti)B2. The improvement of fracture toughness is primarily influenced by the plate-like (Ti, Zr)B2 on crack propagation mechanisms such as bridging effect and crack deflection. (Ti, W, Ta)C- 50 mol% ZrB2 (raw powder composition) exhibits outstanding mechanical properties, boasting a Vickers hardness of 24.9 GPa, fracture toughness of 5.5 MPa·m1/2, and flexural strength of 552 MPa, which present a novel avenue for achieving a synergistic enhancement in Vickers hardness and fracture toughness within medium-entropy carbide ceramics.

Vacancy ordering in zirconium carbide with different carbon contents

Zirconium carbide (ZrCx) ceramics with different carbon contents were prepared by reactive hot-pressing. The rock-salt structure of ZrCx was the only phase detected by x-ray diffraction of the hot pressed ceramics. The relative densities of ZrCx decreased as carbon content increased, in general. The actual carbon contents were measured by completely oxidizing the ZrCx ceramics to ZrO2. For most compositions, the actual carbon contents were higher than nominal batched compositions, presumably due to carbon uptake from the graphite furnace and hot press dies. Selected area electron diffraction and neutron powder diffraction revealed the presence of carbon vacancy ordering in ZrCx for 0.6 < x < 0.75. Rietveld refinement of the neutron diffraction patterns determined that the crystal structure of the ordered phase was hexagonal, and the carbon site occupancies were higher than nominal batched carbon stoichiometry.

Reactive hot pressing parameters on reaction, densification, and mechanical properties of Ti3AlC2 ceramic

AbstractTi3AlC2 ceramic was produced using a 3Ti–Al–1.8C powder mixture by reactive hot pressing at 2–10 MPa and 1100°C–1300°C for 15–60 min. X‐ray diffraction patterns confirmed the Ti2AlC phase up to 1200°C, and at 1300°C, Ti3AlC2 remained as a primary phase with a tiny peak of TiC. Ti3AlC2 and TiC phases varied from 93 to 96 vol.% and 7 to 4 vol.%, respectively, with different pressure and time at 1300°C. An applied pressure of 10 MPa and 1300°C resulted in a density of 4.18 g/cm3 (98.4% relative density) in the samples. It was observed that lower temperature (1100–1200°C) and low applied pressure (2 MPa) do not attain the required density, whereas soaking time is insignificant to achieve the required density at 1300°C. The microhardness and flexural strength were 6.32 ± .20 GPa and 592 ± 42 MPa, respectively.

Low‐temperature reactive hot‐pressing of Ta0.2Hf0.8C–SiC ceramics at 1700°C

AbstractTemperature above 2000°C and additional pressure is generally required to achieve the full densification of TaxHf1−xC‐based ceramics. This work proposed a novel method to fabricate dense Ta0.2Hf0.8C ceramics at relatively low temperature. Using a small amount of Si as a sintering aid, Ta0.2Hf0.8C was densified at 1700°C by reactive hot‐pressing (RHP), with SiC formed in situ. Microstructure evolution mechanisms of the ceramics during RHP were investigated. The effect of silicon content on the densification and mechanical properties of the ceramics was revealed. It is indicated that the apparent porosity of the Ta0.2Hf0.8C–SiC ceramics was as low as 0.5%, whereas bending strength and fracture toughness of the ceramics were as high as ∼637 MPa and 6.7 MPa m1/2, respectively, when the silicon content was 8 wt.%. This work provides a new idea for the low‐temperature densification of TaxHf1−xC and other ultrahigh temperature ceramics with high performance.

Preparation, mechanical properties and long-term ablation resistance of Cf/SiBCN–ZrB2 composites

The Cf/SiBCN–ZrB2 composites were prepared by dipping and winding combined with reactive hot pressing. The flexural strength and fracture toughness reached 261 MPa and 11.96 MPa • m1/2, respectively, through continuous carbon fibers debonding, pulling and bridging mechanisms. Excellent mechanical properties ensured that the Cf/SiBCN–ZrB2 composites remained intact after exposure to a plasma flame with a heat flux of 9.37 MW/m2 for 300 s, with the mass and linear ablation rates of 1.78 mg/s, 1.01 μm/s, respectively. The excellent ablation resistance was due to the formation of dense oxide layers separating the matrix from the plasma flame. The SiO2 formed in the low-temperature areas away from the center was the main ablation-resistant barrier, while the ZrO2/SiO2 double oxide layer formed in the high-temperature region at the center was the major ablation-resistant barrier.

Microstructures and Enhanced Mechanical Properties of (Zr, Ti)(C, N)-Based Nanocomposites Fabricated by Reactive Hot-Pressing at Low Temperature

Dense and enhanced mechanical properties (Zr, Ti)(C, N)-based composites were fabricated using ZrC, TiC0.5N0.5, and Si powders as the raw powders by reactive hot-pressing at 1500-1700 °C. At the low sintering temperature, both (Zr, Ti)(C, N) and (Ti, Zr)(C, N) solid solutions were formed in the composites by adjusting the ratio of ZrC to TiC0.5N0.5. During the sintering process, the Si added at a rate of 5 mol% reacted with ZrC and TiC0.5N0.5 to generate SiC. With the increase in Si addition, it was found that the residual β-ZrSi was formed, which greatly reduced the flexural strength of composites but improved their toughness. The reaction and solid-solution-driven inter-diffusion processes enhanced mass transfer and promote densification. The solid solution strengthening and grain refinement improved the mechanical properties. The ZrC-47.5 mol% TiC0.5N0.5-5 mol% Si (raw powder) composite possessed excellent comprehensive performance. Its flexural strength, Vickers hardness, and fracture toughness were 508 ± 33 MPa, 24.5 ± 0.7 GPa, and 3.8 ± 0.1 MPa·m1/2, respectively. These reached or exceeded the performance of most (Zr, Ti)(C, N) ceramics reported in previous studies. The lattice distortion, abundant grain boundaries, and fine-grained microstructure may make it possible for the material to be resistant to radiation.

Microstructures and tensile properties of low-cost TiBw/Ti–6Al–4V composites by vacuum reactive hot pressing

Coarse Ti–6Al–4V powder with particle size of 150–250 μm are the by-conduct of the processing of spherical fine powder, and usually considered as waste powder. In order to reduce carbon emission during recycle process and obtain titanium-based material with low cost and good mechanical properties, the TiBw/Ti–6Al–4V composites using coarse Ti–6Al–4V powder with a particle size of 150–250 μm were designed and successfully fabricated by vacuum reactive hot pressing (VRHP). The microstructures and tensile properties of low-cost TiBw/Ti–6Al–4V composites at room temperature and elevated temperatures were studied to optimize the content of TiBw. Besides, the strengthening mechanisms were analyzed based on the analysis of plastic deformation behavior and the fracture morphologies. The results showed that the composites reinforced by 1.0 vol% TiBw exhibited superior strength and ductility synergy, with a high tensile strength of 952 MPa and an elongation of 18.4% at room temperature, which are 24.3% and 145.3% higher than that of monolithic Ti–6Al–4V alloy, respectively. In addition, the 1.0 vol% TiBw/Ti–6Al–4V composites also showed excellent mechanical properties under high temperatures. This work shows that combining the network microstructure strategy with coarse spherical powder offers a pathway to design a low-cost titanium matrix composite with promising mechanical performance.

Preparation and Study of Carbide-Sialon Composite in SiC-SiAlON-Al2O3 System

Goal: Synthesis of SiAlON by reaction coating method using aluminosilicate natural raw material geopolymer (kaolin), corundum and silicon carbide and on its basis obtaining a composite with high physical and technical properties by hot pressing for use in armor and rocket technology. For the intensification of sialon formation and sintering processes, the influence of various additives was studied, such as: aluminum powder, elemental silicon, yttrium and magnesium oxides. Method: A SiAlON-containing composite with an open porosity of 15% - 16% was obtained by the metallothermic process and the method of reactive annealing in nitrogen. The resulting material was milled to a dispersion of 1 - 3 μm and hot pressed at 1620°C to obtain a product with high density and performance properties. We studied the process of sialon formation and the microstructure of the composite by X-ray phase, optical and electronmicroscopy analysis methods. Result: In the selected composition the β-SiAlON was formed at 1400°C instead of 1800°C, which was due to the mutual influence of the initial raw materials:geopolymer kaolin, perlite, corundum, aluminum, silicon, SiC, the development of the process is facilitated by the vitreous dopant perlite (96 glass phase). The use of perlite, which is eutectic with geopolymer at low temperatures, creates a good prerequisite for intensive diffusion processes with other components. Conclusion: A SiAlON-containing composite with high physical and technical properties was obtained in the SiC-SiAlON-Al2O3 system by the method of reactive sintering and hot pressing, with the following properties: the strength limit in compression is 1940 MPa, and in bending it is 490 MPa. The process of making sialon has been studied using X-ray phase and electron microscopy analysis methods. The physical and technical properties of the obtained composite are studied by modern research methods.

Fabrication and characterisation of ZrSi2 ceramics via reactive hot-pressing

ABSTRACT ZrSi2 was fabricated via reactive hot-pressing technique for 45 min at 1250°C and 1350°C, respectively. The hot-pressed sample exhibited Vickers hardness of 10.93 ± 0.32 GPa and indentation fracture toughness of 3.16 ± 0.54 MPa.m1/2. Thermal conductivity (RT-2.99 W (m.K)−1) and diffusivity (RT–18.2 mm2 s−1) of the sample decreased with increasing temperature due to phonon scattering. Oxidation studies were carried out for the hot-pressed samples from 1000°C to 1200°C for 5, 48 and 100 h in air. The oxidation test results showed that ZrSi2 followed selective oxidation with Si diffusing into the oxide layers at high temperatures. At 1200°C, the surface of ZrSi2 oxidised completely to form a high-temperature stable phase, zircon (ZrSiO4). The autoclave test at 250°C and 250 bar shown negligible mass gain (∼0.022 g) which suggests the usage of ZrSi2 for nuclear fuel cladding applications.

Synthesis, microstructure, and formation mechanism of a potential neutron shielding material: WAlB

• Dense, predominately single-phase samples of WAlB were synthesized using a reactive hot press sintering method for the first time. • DFT calculations and Rietveld analysis of WAlB indicate that WAlB most likely forms via a topotatic transformation reaction, by the intercalation of Al layers into WB crystal structure. • The calculated linear attenuation coefficients for different energy gamma-rays imply WAlB has a high gamma-ray shielding capacity.

Microstructure characterization and enhanced mechanical properties of quasi-continuous network structured Ti3AlC2-Al3Ti/2024Al composite

Al matrix composites (AMCs) with high specific strength are highly desirable for a wide range of critical applications due to their unique mechanical and electrical characteristics. On the target of the effective production of high-performance AMCs, a novel quasi-continuous network structure based on Ti3AlC2 was devised rationally and developed a heterogeneous Ti3AlC2-Al3Ti synergistically reinforced 2024 Al composites by the technique of reactive hot press. Herein, the produced Ti3AlC2-Al3Ti/2024Al composite exhibited superior mechanical strength, importantly, it exhibits a minimal ductility reduction in comparison with pure 2024Al. Compressive yield strength, tensile ultimate and yield strength along with bending strength increased with the increasing Ti3AlC2 content; the optimal values for 5vol%Ti3AlC2/2024Al composite were increased by 16.9% in compressive yield strength, 11.5% in UTS and 30.0% in YTS compared with 2024Al. The mechanical properties increment was attributed to the synergistic effect of the quasi- network structure, dispersion strengthening, load transfer strengthening and thermal mismatch strengthening mechanism. The approach reported in this study is expected to open new avenues for advanced AMCs synthesis with tunable morphology and mechanical strength for their widespread applications.

Improved Tribocorrosion Behavior Obtained by In-Situ Precipitation of Ti2C in Ti-Nb Alloy

Novel in-situ Ti-based matrix composites (TMCs) were developed through the reactive hot pressing of Ti + NbC powder blends. Due to the chemical reaction that occurred in the solid-state during processing, the produced samples were composed of an Nb-rich β-Ti phase that formed a metallic matrix along with Ti2C as a reinforcing phase. By employing different proportions of Ti:NbC, the phase composition of the alloys was designed to contain different ratios of α-Ti and β-Ti. The present work investigated the corrosion and tribocorrosion behavior of the composites, compared to unreinforced Ti, in a phosphate-buffered solution (PBS) at body temperature. Corrosion tests included potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Tribocorrosion tests were carried out using a ball-on-plate tribometer with sliding performed at open circuit potential (OCP) and under anodic potentiostatic conditions. Results showed that the stabilization of the β phase in the matrix led to a decrease in the hardness. However, the formation of the in-situ reinforcing phase significantly improved the tribocorrosion behavior of the composites due to a load-carrying effect, lowering the corrosion tendency and kinetics under sliding. Furthermore, localized corrosion was not observed at the interface between the reinforcing phase and the matrix.

Overcoming the strength-ductility trade-off dilemma in TiBw/TC18 composites via network architecture with trace reinforcement

In order to improve the ductility of TC18 (Ti–5Al–5Mo–5V–1Cr–1Fe) titanium alloy without sacrificing its strength, TiBw/TC18 composites with low reinforcement content (≤2.0 vol%) were designed and successfully fabricated. The large spherical TC18 powers and fine TiB2 particles were selected for low energy ball milling (LEM) followed by reaction hot pressing (RHP) process. In situ synthesized TiB whiskers by Ti and TiB2 reaction are distributed only on the surface of spherical TC18 powders, generating a three-dimensional network microstructure. The results showed that both the tensile strength and elongation of the as-sintered 0.5 vol% TiBw/TC18 composite (1171 MPa, 15.6%) were remarkably enhanced compared with those of TC18 alloy (1151 MPa, 0.6%). This phenomenon can be mainly attributed to the network microstructure, β grain refinement, and elevation of the proportion of α-colony brought about by the introduction of TiBw. However, with the further increase of TiBw volume fraction (above 1.0 vol%), the strength and the ductility of the composites were decreased. Consequently, the network microstructure strategy with minor reinforcement addition offers a pathway to design engineering composites overcoming the strength-ductility trade-off dilemma.

Reaction, densification and mechanical properties of Ti 2 AlC x ceramics at low applied pressure and temperature

Ti2AlCx ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with the molar ratio of 2:1:1-0.5 at 10–20 MPa, 1200–1300°C for 60 min. XRD analysis confirmed the Ti2AlC with TiC, Ti3Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHP at 10 MPa, 1300°C exhibited ≥95 vol.% Ti2AlC with TiC as a minor phase. The density of samples increased from 3.69 - 4.04g/cm3 at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 - 4.07g/cm3 (2:1:1 to 2:1:0.5). The samples made at 10 MPa, 1300°C, exhibited a density from 3.95 - 4.07g/cm3. Reaction and densification were studied for 2Ti-Al-0.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti-Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti2AlC phase was ≥1000°C; further, as the temperature increased, Ti2AlC peak intensity was raised, and other phases intensities were reduced. The sample made at 700°C showed a density of 2.87g/cm3, while at 1300°C exhibited 3.98g/cm3; further, soaking for 60 min resulted in a density of 4.07g/cm3. Microhardness and flexural strength of Ti2AlC0.8 sample were 5.81±0.21 GPa and 445±35 MPa. This article is protected by copyright. All rights reserved

Arc erosion behaviour of in-situ TiB2/Cu composites with a three-dimensional network structure

Copper matrix composites (CMCs) with tailored heterogeneous structures at the mesoscopic scale are promising candidates for electrical contact materials. In this work, CMCs reinforced by an in situ formed three-dimensional network of TiB2 particles were synthesized from Cu, Ti and B powder mixtures by reactive hot-pressing. The arc erosion behaviour of the fabricated CMCs was investigated by an electrical contact test. The distribution state of in situ TiB2 depends on the particle size of the Cu powder. The critical size for forming a continuous network in 3 wt%TiB2/Cu composites is estimated to be 24 μm. Once the continuous network is formed in CMCs, the arc energy and duration suddenly change to ultrasmall and stable values, and the erosion area and total mass loss after 5000 cycles of the contact test remarkably decrease. The results indicate that the CMCs reinforced by in situ networks of TiB2 particles exhibit excellent arc erosion resistance.

Novel (Zr, Ti)B2-(Zr, Ti)C-SiC ceramics via reactive hot pressing

Fully dense (Zr, Ti)B2-(Zr, Ti)C-SiC ceramics were prepared by reactive hot-pressing using ZrB2, TiC, and SiC as the initial materials for the first time. Effects of SiC addition on the microstructure evolution and mechanical properties were reported. The in-situ reaction between ZrB2 and TiC as well as the SiC addition leads to the grain refinement. Besides, elongated (Zr, Ti)B2 plate-like grains are obtained due to the occurrence of a transient liquid phase, which leads to the crack deflection in the matrix effectively. Mechanical properties are improved significantly due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects. The ZrB2-10 mol%TiC composite with 10 mol% SiC additional exhibits good comprehensive mechanical properties of the hardness of 20.2 GPa, the flexural strength of 803 MPa, and the fracture toughness of 5.7 MPa m1/2.

Solid‐state formation mechanisms of core–shell microstructures in (Zr,Ta)B2 ceramics

AbstractTransition metal diborides with core–shell microstructures have demonstrated excellent mechanical properties at elevated temperatures. Previous studies concluded that core–shell microstructures were formed by liquid‐assisted mass transport mechanisms, but in this study, we propose a solid‐state formation mechanism for core‐shell microstructures in (Zr,Ta)B2 ceramics produced by reaction hot pressing and in ZrB2‐TaB2 diffusion couples. Diffusion couple experiments demonstrated that core–shell microstructures developed as a result of Ta diffusion along ZrB2 grain boundaries, which occurred concurrently with lattice diffusion of Ta into ZrB2. These findings suggest that with optimization of batching and processing parameters, core–shell diboride materials may be formed through solid‐state processes rather than liquid‐assisted processes, which could assist in raising the upper temperature limits of use for these materials.

Microstructure and mechanical properties of b4c-tib2 composites reactive sintered from B4C + TiO2 precursors

Ceramic composites consisting of a boron carbide (B4C) matrix and titanium diboride (TiB2) secondary phase were obtained by reactive sintering from boron carbide powder with 40 and 50wt.% of titanium dioxide (TiO2) additive. The same sintering temperature of 1850?C and pressure of 35MPa, but different sintering times from 15 to 60min, were applied during reactive hot pressing of the composites in vacuum. The effects of TiO2 content and sintering time on phase compositions, microstructures and mechanical properties of the composites were studied. The TiO2 additive enhanced densification of the B4C-TiB2 ceramic composites. Both Vickers hardness and the fracture toughness of the composites increased with prolongation of sintering time. The highest hardness of 29.8GPa was achieved for the composite with 29.6 vol.% of TiB2 obtained by sintering of the precursor with 40wt.% of TiO2 additive for 60min. The fracture toughness reached a maximum value of 7.5MPa?m1/2 for the composite containing 40.2 vol.% of TiB2, which was fabricated by reactive sintering of the precursor with 50wt.% of TiO2 additive for 60min.

Anisotropy of the bending strength of LaB6–W2B5 reactive hot-pressed ceramics

Refractory composite ceramic material in the LaB6–W2B5 system with a component ratio of 50 : 50 vol.% was obtained by reactive hot pressing in a graphite mold. A heterophase powder containing lanthanum hexaboride, metallic tungsten, and amorphous boron preliminarily ball-milled for 20 h with tungsten balls was used as the initial reaction mixture. The average particle size of the milled mixture was 2.9 μm. A relative density of 92 % was achieved at a temperature of 1800 °C with isothermal holding for 15 min at 30 MPa in an argon atmosphere. The structure and composition of the LaB6–W2B5 material were studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The composition of the ceramics contained two phases – cubic LaB6 lanthanum hexaboride and hexagonal W2B5 tungsten pentaboride. The ceramic structure featured by ordered lamellar W2B5 particles in a LaB6 polycrystalline matrix. During the reactive hot pressing of the LaB6–W–B mixture, the predominant growth of W2B5 crystals along (101) atomic planes was observed. Resulting lamellar W2B5 particles were oriented in the LaB6 matrix perpendicular to the pressing load. Images obtained with electron microscopy were used for the three-dimensional visualization of the LaB6–W2B5 structure. Three-point bending tests were conducted on 3×3×30 mm samples. The dependence of bending strength on the direction of applied breaking load was established. When a breaking load was applied perpendicular to the surface of the lamellar W2B5 particles, the ultimate strength was 420 MPa, while when loaded along the plane of the particles, bending strength increases to 540 MPa. The anisotropy coefficient of ultimate strength was 0.78.