Reactive Hot-pressing Sintering Research Articles

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.

High temperature and room temperature tribological behaviors of in-situ carbides reinforced Ni-based composites by reactive sintering Ni and Ti2AlC precursor

The in-situ carbides reinforced Ni-based composites were fabricated by reactive hot-pressing sintering Ni and Ti2AlC powders. Tribological performance and mechanisms at room temperature and 800 °C were studied. Sintered composites contain Ni-based solid solution, Ni2TiAl, Ni3Al, TiCx, Ti3NiAl2C and Al2O3. At room temperature, the elastic modulus, hardness and H/E of composites enhanced due to the increase of carbides, resulting in wear rate declined with the rise of Ti2AlC. At 800 °C, because of the formation of reducing-friction and anti-wear oxide layers consisting of TiO2, NiO, Al2O3, NiTiO3, and NiAl2O4, wear rate was further decreased to 4.2 × 10−6 mm3/Nm. Composition differences on the surface between worn and static oxidized at 800 °C show that tribo-chemical reaction promotes the generation of NiO and NiTiO3.

Comparative study on NiAl and FeAl intermetallic-bonded diamond tools and grinding performance for Si3N4 ceramic

Metal-bonded diamond tools are widely used for machining hard and brittle materials such as ceramic and stone. The metal bonds are normally based on alloys composed of Fe, Co, Ni, Cu, Sn, Zn, or Cr. However, high heavy metal concentrations are hazardous to the environment and tool operators. Furthermore, the high density of these metal bonds results in excessively heavy tools, requiring high energy and intensive labor during operation. Therefore, we propose two environmentally friendly and lightweight metal bonds based on NiAl and FeAl intermetallics for diamond tools by reactive hot-press sintering. The effect of sintering temperature on the microstructure and mechanical properties of NiAl and FeAl was examined and the tribological behavior of NiAl and FeAl on Si3N4 ceramic balls was investigated. Grinding performance of the sintered diamond tools on Si3N4 ceramic was also compared. Our results reveal that B2-ordered NiAl and FeAl are obtained at sintering temperatures in the range of 700–1100 °C. The relative density of sintered NiAl is higher than that of FeAl, while the Vickers hardness and flexural strength of NiAl are lower than that of FeAl. Compared with FeAl, NiAl demonstrates higher wear resistance in a friction test against Si3N4 ceramic, while FeAl-bonded diamond tools demonstrate a higher grinding ratio than NiAl-bonded diamond tools due to enhanced self-sharpening ability. Nevertheless, diamond tools based on both NiAl and FeAl can be used to grind Si3N4 ceramics.

Microstructural evolution and mechanical properties of in situ nano Ta4HfC5 reinforced SiBCN composite ceramics

The in situ nano Ta4HfC5 reinforced SiBCN-Ta4HfC5 composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering. The microstructural evolution and mechanical properties of the resulting SiBCN-Ta4HfC5 were studied. After the first-step milling of 30 h, the raw materials of TaC and HfC underwent crushing, cold sintering, and short-range interdiffusion to finally obtain the high pure nano Ta4HfC5. A hybrid structure of amorphous SiBCN and nano Ta4HfC5 was obtained by adopting a second-step ball-milling. After reactive hot-pressing sintering, amorphous SiBCN has crystallized to 3C-SiC, 6H-SiC, and turbostratic BN(C) phases and Ta4HfC5 retained the form of the nanostructure. With the in situ generations of 2.5 wt% Ta4HfC5, Ta4HfC5 is preferentially distributed within the turbostratic BN(C); however, as Ta4HfC5 content further raised to 10 wt%, it mainly distributed in the grain-boundary of BN(C) and SiC. The introduction of Ta4HfC5 nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics, reaching to 344.1 MPa and 4.52 MPa·m1/2, respectively. This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix, which is beneficial to the real applications of SiBCN ceramics.

Reactive-sintering B4C matrix composite for armor applications

The B4C matrix composite intended for armor applications still presents restrictions, such as low sintering density, production of large parts and inherited brittleness. Herein, research on this topic is discussed in detail. First, the material outlook of armor applications is organized from the development of composite armor and ceramic materials for armor to the B4C matrix composite and reactive-sintering method. In the second section, the technologies are reviewed for reactive pressureless sintering, reactive hot-pressing sintering, reactive discharge plasma sintering and self-propagating high-temperature sintering. Thereafter, our previous works on the TiB2/SiC/B4C composite and laminated Ti/B4C composite are employed to illustrate their microstructural evolution, phase transformation and fracture model. These studies provide a potential method for producing tough and high-strength ceramic composites for armor application. In the final section, the mechanism, evaluation method and influencing factors of anti-penetration for ceramic armor and B4C matrix composite are reviewed.

Ti2AlC triggered in-situ ultrafine TiC/Inconel 718 composites: Microstructure and enhanced properties

In situ ultrafine TiC dispersion reinforced Inconel 718 alloy with enhanced mechanical properties was fabricated by the technique of reactive hot-press sintering Ti2AlC and In718 powders. The effect of Ti2AlC precursor additions (5 vol.%, 10 vol.%, 15 vol.%) on microstructure and mechanical properties of TiC/In718 composites were investigated. A relationship of microstructural characteristics, room and elevated temperature mechanical performance, and underlying strengthening mechanisms were analyzed. The results show that initial Ti2AlC precursor transformed completely into ultrafine TiC particulate (∼230 nm) and distributed uniformly in the matrix after sintering 5 and 10 vol.% Ti2AlC/In718. However, TiC particulates tended to aggregate to stripes with the addition of Ti2AlC up to 15 vol.%, which, in adverse, weaken the properties of In718. The 5 vol.% Ti2AlC/In718 sample showed a higher tensile strength of 1404 ± 13 MPa with a noticeable elongation of 9.8% at room temperature compared to the pure In718 (ultimate tensile strength (UTS) = 1310 MPa, elongation = 21.5%). At 600 °C, 700 °C, 800 °C and 900 °C, tensile strength of the as-sintered 5 vol.% Ti2AlC/In718 composite was determined to be 1333 ± 13 MPa, 1010 ± 10 MPa, 685 ± 25 MPa and 276 ± 3 MPa, increased by 9.2%, 14.6%,14.2% and 55%, respectively, compared with that of monolithic In718 alloy. The excellent tensile properties of TiC/In718 composite can be ascribed to the combined mechanisms in term of increased dislocation density, dispersive Orowan and load transfer mechanisms.

Fine and high-performance B6.5C-TiB2-SiC-BN composite fabricated by reactive hot pressing via TiCN–B–Si mixture

In this study, B6.5C-TiB2-SiC-BN composite was fabricated via reactive hot pressing from a powder mixture of TiCN, B, and Si. The reaction process of the powder mixture was investigated by heat treatment at elevated temperatures. The as-reacted powder products possessed a core-shell structure of TiB2 surrounded by h-BN. During densification, the h-BN in this structure acts as a diffusion channel and lubricant, which promoted the densification process. The extremely small size, fresh surface of in-situ formed phases, and uniform dispersion in synthesized powders also contributed to sintering. The designed reactive hot-pressing sintering effectively resolves the difficulty of obtaining uniformly-dispersed BN. The obtained bulk B6.5C-TiB2-SiC-BN composite ceramic shows high relative density (98.8%) and good comprehensive mechanical properties including hardness, bending strength, and fracture toughness of 19.6 GPa, 801 MPa, and 5.31 MPa m1/2, respectively.

Effects of TiC particle size on microstructures and mechanical properties of B4C–TiB2 composites prepared by reactive hot-press sintering of TiC–B mixtures

Effects of the starting TiC particle size on the phase compositions, microstructures and mechanical properties of B4C–TiB2 composites prepared by reactive hot-press sintering of TiC–B mixtures were investigated. B4C–TiB2 composites were prepared from a single grade amorphous B powder (0.9 μm) and three different TiC powders (50 nm, 0.8 μm and 3.0 μm), respectively. The composites prepared from 50 nm and 3.0 μm TiC powders exhibited finer microstructures and higher hardness (29–30 GPa). The composite prepared from 0.8 μm TiC powder showed the coarsest microstructure but possessed the highest strength of 659 MPa due to its homogenous phase distribution without C impurity and abnormal grain growth that presented in the other two composites. The results indicated that the mechanical properties of B4C–TiB2 composite could be controlled by the particle size matching between TiC and B powders.

Effects of TaC addition on microstructure and mechanical properties of SiBCN composite ceramics

In this work, SiBCN–TaC composite ceramics were prepared by a combination of mechanical alloying and reactive hot-pressing sintering techniques. The effects of TaC addition on microstructure, mechanical properties and oxidation resistance of the resulting composite ceramics were studied in detail. Results suggested that the as-sintered SiBCN–TaC composite ceramics composed of three phases: SiC, BN (C) and TaC. Some TaC~10–20 nm dispersed randomly within the ceramic matrix while others ~3–5 nm were scattered in turbostratic BN(C). The introduction of nanocrystalline TaC can effectively improve the flexural strength and fracture toughness of SiBCN ceramics. The incorporation of 10 wt% TaC showed the optimized mechanical properties with flexural strength and fracture toughness reaching to 399.5 MPa and 4.26 MPa m1/2 respectively. The TG-DTA analysis confirmed the oxidation damage of BN (C) phase can be effectively alleviated by the introduction of TaC nanocrystals.

Synthesis and Characterization of Double Solid Solution (Zr,Ti)2(Al,Sn)C MAX Phase Ceramics.

Quasi phase-pure (>98 wt %) MAX phase solid solution ceramics with the (Zr,Ti)2(Al0.5,Sn0.5)C stoichiometry and variable Zr/Ti ratios were synthesized by both reactive hot pressing and pressureless sintering of ZrH2, TiH2, Al, Sn, and C powder mixtures. The influence of the different processing parameters, such as applied pressure and sintering atmosphere, on phase purity and microstructure of the produced ceramics was investigated. The addition of Sn to the (Zr,Ti)2AlC system was the key to achieve phase purity. Its effect on the crystal structure of a 211-type MAX phase was assessed by calculating the distortions of the octahedral M6C and trigonal M6A prisms due to steric effects. The M6A prismatic distortion values were found to be smaller in Sn-containing double solid solutions than in the (Zr,Ti)2AlC MAX phases. The coefficients of thermal expansion along the ⟨ a⟩ and ⟨ c⟩ directions were measured by means of Rietveld refinement of high-temperature synchrotron X-ray diffraction data of (Zr1- x,Ti x)2(Al0.5,Sn0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti2(Al0.5,Sn0.5)C solid solution were compared with those of the Ti2AlC and Ti2SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)2(Al0.5,Sn0.5)C double solid solution MAX phases as compared to the Zr2(Al0.5,Sn0.5)C and Ti2(Al0.5,Sn0.5)C end-members.

Exploring the interfacial state and tensile behaviors in nickel matrix composite with in-situ TiC and γ′-Ni3(Al,Ti) reinforcements

This paper presents the study on the formation of a novel nickel matrix composite reinforced with in-situ formed TiC and coherent γ′-Ni3(Al,Ti) precipitates (TiC-γ′/Ni) processed by reactive hot-press sintering Ti2AlC and Ni precursors. Detailed interfacial analysis was carried out using high-resolution transmission electron microscopy (HRTEM). Semi-coherent carbide/nickel interface was determined and the orientation relationship between TiC and Ni phases appears to be [001] TiC//[011] FCC Ni and (020) TiC//(1¯11¯) Ni. Coherent γ′ -Ni3(Al,Ti) with ultrafine grain size (less than 100 nm) was separated out from γ-Ni matrix. The fabricated composites exhibits substantially higher yield strength (YS) of 654 ± 38 MPa and 769 ± 44 MPa than monolithic Ni. The great strength enhancement in these composites can be attributed to a refinement grain size led by growth pinning of TiC-γ′ particulates in the nickel matrix coupled with the formation of well-bonded and high-strength TiC/Ni and γ/γ′ interfaces.

In-situ TiC and γ′-Ni3(Al,Ti) triggered microstructural modification and strengthening of Ni matrix composite by reactive hot-press sintering pure Ni and Ti2AlC precursor

In the present study, a novel type of Ni-based composites was successfully fabricated from pure Ni and Ti2AlC powder mixtures by hot-press sintering route at 1400 °C and 30 MPa. The microstructural analysis indicates that the initial reinforcement Ti2AlC particles transformed into TiC phase, while the additional AlTi atoms decomposed from Ti2AlC diffused into the Ni matrix. The de-intercalated AlTi atoms would subsequently react with Ni to give rise to the formation of γ′-Ni3(Al,Ti) precipitates according to element compositions. SEM observations on 30 vol%Ti2AlC/Ni composite after etched showed that L12-centered γ′ phase is ultrafine (about 100 nm) and uniformly distributed in the Ni matrix. The mechanical behavior in relationship with the different fractions of Ti2AlC (10 vol%, 20 vol%, 30 vol% and 40 vol%) were carefully investigated. Yield strength, ultimate compressive strength, flexural strength and Vickers hardness except for fracture strain and toughness increased with the increasing Ti2AlC content; the optimal values for 30Ti2AlC/Ni composite were approximately 1056 ± 29 MPa, 1776 ± 33 MPa, 1284 ± 59 MPa, and 4.06 ± 0.07 GPa respectively, with 19.37 ± 0.75% fracture strain and 26.31 ± 2 MPa m1/2 fracture toughness. The enhancement of mechanical properties was attributed to the synergistic effect of fine-grain strengthening, austenitic solid-solution strengthening and dispersion strengthening mechanism.

High toughness and electrical discharge machinable B4C-TiB2-SiC composites fabricated at low sintering temperature

Dense and electrical discharge machinable B4C-TiB2-SiC triple-phase ceramic composites with high fracture toughness and high hardness were successfully fabricated via reactive hot-pressing sintering using B4C and nano-layered Ti3SiC2 with different ratios as raw materials at a relatively low temperature of 1850°C. The results showed that the mechanical properties of B4C-TiB2-SiC composite were remarkably enhanced than that of monolithic B4C ceramic. The micro-hardness, flexural strength, fracture toughness and compressive strength of B4C-TiB2-SiC composite prepared with B4C-30vol% Ti3SiC2 starting powder were 31.6GPa, 492.3MPa, 8.0MPa·m1/2 and 2727MPa, respectively. High hardness was attributed to the low open-porosity and grain refinement, while the high toughness was mainly ascribed to the crack deflection due to the dispersed TiB2 and SiC particles in the B4C matrix. Moreover, the composites exhibited significantly improved machinability than monolithic B4C, as evidenced by electrical discharge machining.

Synthesis of MAX Phases in the Zr-Ti-Al-C System.

This study reports on the synthesis and characterization of MAX phases in the (Zr,Ti)n+1AlCn system. The MAX phases were synthesized by reactive hot pressing and pressureless sintering in the 1350-1700 °C temperature range. The produced ceramics contained large fractions of 211 and 312 (n = 1, 2) MAX phases, while strong evidence of a 413 (n = 3) stacking was found. Moreover, (Zr,Ti)C, ZrAl2, ZrAl3, and Zr2Al3 were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr0.33,Ti0.67)3Al1.2C1.6 stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed M6X octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase. The decomposition of the secondary (Zr,Ti)C phase was attributed to the miscibility gap in the ZrC-TiC system.

Fabrication of transparent LaYZr2O7 ceramic by reactive hot-pressing sintering from commercial raw powders

A transparent LaYZr2O7 ceramic sample was successfully prepared by reactive hot-pressing sintering using commercially available La2O3, Y2O3, and ZrO2 powders as raw materials. By applying mechanical pressure, the sample was successfully sintered at 1700°C, which is lower by 100–200°C compared to the temperature of used in pressureless sintering, leading to a fine average grain size (i.e., 1.5µm). The in-line transmittance reached 50.8% at 500nm and 70.4% at 1100nm (1mm thick).

In situ reactive hot press sintering and mechanical properties of Al2O3–TiB2–B4C composites

Dense Al2O3–TiB2–B4C composites are synthesized by in situ reactive hot press sintering of B4C with 20 wt% TiAl2 and TiAl3 at 1700 °C, respectively. The sinterability of B4C is improved by the intermetallic compounds due to the formation of Al2O3 and TiB2. The sizes of Al2O3, TiB2, and B4C grains are less than 2.5 μm. The agglomerates of Al2O3 and TiB2 particles tend to orientationally align in the direction perpendicular to the pressure axis. By incorporation of TiAl2, the flexural strength and fracture toughness reach 482 MPa and 3.6 MPa m1/2, respectively. When TiAl3 is added, the flexural strength is reduced to 408 MPa because of the thermal residual stress, while the fracture toughness is increased to 4.2 MPa·m1/2. The increase of Al2O3 in the composite can reduce the strength and enhance the toughness of the composite due to the thermal residual stress originated from the thermal expansion mismatch between B4C and Al2O3.

Microstructure and mechanical properties of ZrB2–SiC–BN composites fabricated by reactive hot pressing and reactive spark plasma sintering

ZrB2–SiC–BN composites were fabricated from a powder mixture of ZrH2, Si3N4 and B4C via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS). The phase composition, density, microstructure and mechanical properties of the final products were studied and compared. The sample fabricated by the R-SPS process showed higher density, flexural strength and fracture toughness and had a more homogeneous microstructure compared to the one fabricated by the RHP process.

Microstructural Evolution and Mechanical Properties of a Multicomponent Nb-16Si-22Ti-2Al-2Hf-2Cr Alloy Prepared by Reactive Hot Press Sintering

In this paper, the effect of reactive hot press sintering (RHPS) parameters, such as milling time, hot pressing temperature, and heat treatment, on phase constitution, microstructure, and room temperature mechanical properties of a multicomponent Nb-16Si-22Ti-2Hf-2Al-2Cr alloy prepared from ball milled powder mixture is investigated. Evaluation of the microstructure revealed that all as-sintered and heat-treated samples mainly consisted of particles of Nb and Ti solid solutions (NbSS and TiSS), as well as a niobium silicide β-Nb5Si3 matrix, with a small quantity of Hf solid solution particles (HfSS). By sintering at 1773 K and 1873 K (1500 °C and 1600 °C), most of the NbSS phase in the samples prepared from 5 and 10 ball milled powder mixtures showed a narrow strip morphology. Upon increasing the milling time to 20 hours, the morphology changed to a near-equiaxed shape, which became finer with increased milling time. In contrast, the TiSS phase in all as-sintered samples more or less had a near-equiaxed shape. Spheroidizing tendency took place in both NbSS and TiSS phases during an annealing heat treatment at 1773 K (1500 °C) for 50 hours. Interestingly, when the milling time was extended from 5 to 20 hours, the volume fractions of the β-Nb5Si3 and TiSS phases increased, whereas that of the NbSS phase decreased. This resulted in the reduction of the fracture toughness KQ and an enhancement of the Vickers hardness Hv of the bulk as-sintered and heat-treated samples. A fractography analysis was also carried out to elucidate the fracture behavior of phases, with particular emphasis on the interaction between phases and cracks.

Reactive Synthesis of ZrB2-based Ultra High Temperature Ceramics

Reactive processing, such as reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), is effective densification method to prepare ZrB₂-based ultra high temperature ceramics (UHTCs). The present paper reviewed some typical reactive processing of ZrB₂-based UHTCs. All the reactions from the starting materials in the reactive processing are thermodynamically favorable, which generate enough energy and driving force for the densification of the final products under a relatively low temperature. Besides, compared with non-reactive processing, anisotropic ZrB₂ grains, such as ZrB₂ platelets, can only be obtained in the reactive processing, resulting in an improvement of the mechanical properties.

Fabrication, microstructure and mechanical behavior of SiCw-B4C–Si composite

SiC whiskers (SiCw) was prepared and SiCw-B4C–Si composites were fabricated by ball milling together with reactive hot press sintering process at the temperature around 1850K and 30MPa. The compositions, morphologies and mechanical properties of the fabricated composites were investigated. The hardness was decreased with the addition of SiCw and Si. The Si additive improved the relative density of B4C, whereas the addition of SiCw did not change the density to much. The addition of SiCw and Si reinforced the composite in fracture toughness and flexural strength due to the formation of laminated flakes and fiber network structures. At the contents of 5wt% SiCw and 10wt% Si, the relative density, hardness, flexural strength and fracture toughness of the composites could reach 96.5%, HRA93.5, 487MPa, 5.78MPam1/2, respectively.