Reactive Hot Pressing Research Articles

Mechanical, thermal physical properties and thermal shock resistance of in situ (TiB2+SiC)/Ti3SiC2 composite

Dense (TiB2+SiC)/Ti3SiC2 composite with 20 vol% TiB2 and 10 vol% SiC was synthesized by in situ reaction hot pressing. The room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composite were investigated and compared with SiC/Ti3SiC2 composite as prepared by the same process. The synergistic action of TiB2 and SiC produced a significant strengthening and toughening effect and (TiB2+SiC)/Ti3SiC2 composites exhibit superior room- and high-temperature mechanical properties compared with SiC/Ti3SiC2 composite. Especially, (TiB2+SiC)/Ti3SiC2 composite shows higher Young's modulus from 25 °C to high temperature and can retain a high modulus of 313 GPa (about 81% of room temperature value) up to 1200 °C. From 25 °C to 800 °C, (TiB2+SiC)/Ti3SiC2 composite has higher thermal conductivity and slightly lower coefficient of thermal expansion. Both water quenching test results and calculation of thermal shock resistance parameters, R, Rst and RIV indicate an improved thermal shock resistance of (TiB2+SiC)/Ti3SiC2 composite, with an increase of the critical thermal shock temperature difference ΔTc from 325 °C for SiC/Ti3SiC2 to 440 °C for (TiB2+SiC)/Ti3SiC2. Superior properties render (TiB2+SiC)/Ti3SiC2 composite a promising prospect as high temperature structural materials.

Carbon‐content‐dependent phase composition, microstructural evolution, and mechanical properties of SiBCN monoliths

AbstractThree SiBCN monoliths: carbon‐lean, ‐moderate, and ‐rich monoliths with the same Si/B/N mole ratio have been prepared by mechanical alloying followed by reactive hot pressing. The role of carbon on phase composition, microstructural evolution, and mechanical properties were investigated. XRD and TEM investigations showed that the as‐sintered monoliths contain several components: Si metal, SiC, t‐BN, and BN(C), strongly correlated with the carbon content. TEM results also revealed that BN(C) enwraps SiC grain forming capsule‐like structure. Raman, NMR, and HRTEM studies suggested that the structural evolution within the BN(C) areas includes the graphitization of amorphous carbon and the lateral growth of BN(C), accompanied by an increase in the defect concentration. The defects form within BN (0002) basal planes resulting from the disordered carbon imbedded, which in turn determines the morphological evolution of BN(C). The ill‐developed microstructures for carbon‐lean monoliths and the excessive carbon in carbon‐rich ones both deteriorate their mechanical properties and densities.

Nanolaminated ternary carbide (MAX phase) materials for high temperature applications

There is a clear need for high-strength (≥300MPa), thermally stable, conductive materials that are also thermal shock resistant. Some MAX phases – ternary nano-laminated carbides and nitrides – are reported to fulfil all these requirements and can be considered as potential structural materials for high-temperature applications. In this work, a set of quaternary (M,M′)AX phase materials based on the Nb-Al-C system were synthesized by reactive hot pressing, starting from M-hydride powders. The possibility to substitute Nb with at least 10at% of other M elements (M′=Ti, Zr, Hf and Ta) in the crystal lattice was investigated. The crystal structure of the produced solid solutions was studied by X-ray diffraction and the lattice parameters were calculated by Rietveld refinement. The material behaviour in an inert atmosphere was tested by measuring the elastic properties – Young's Modulus and internal friction – as a function of temperature up to 1500°C, and the effect of the substitution on the room temperature flexural strength was assessed.

Dramatically enhanced impact toughness of two-scale laminate-network structured composites

TMCs are well known materials for their potential use in many fields. However, there are few research on the toughness especially impact toughness of TMCs. To improve impact property of TMCs, one novel kind of titanium matrix composites with two-scale laminate-network structure was fabricated via powder metallurgy and reaction hot pressing. From macro-scale, the novel composites were constituted by Ti6Al4V alloy layers and TiBw/Ti6Al4V composite layers. Moreover, the composite layer exhibited network structure and refined matrix microstructure from micro-scale. Layer thicknesses were well controlled by the masses of Ti6Al4V powder and TiB2-Ti6Al4V mixture powders, and the volume fraction of TiBw reinforcement in the composite layer was adjusted by controlling TiB2 raw material. Bending test results exhibited that the ultimate strain of two-scale composite doubled while bending strength was similar compared with TiBw/Ti6Al4V composites. The impact toughness of two-scale composites was nearly fivefold enhanced compared with monolithic TiBw/Ti6Al4V composites. This phenomenon can be attributed to the introduction of laminate structure, microstructure refinement and interface crack deflection. The analysis of impact curves and fractographs suggests that introducing Ti alloy layers can enhance plastic stage. Increasing TiBw reinforcement volume fractions can enhance the energy absorbed during crack propagation stage.

Densification, mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC1−x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH2 powders at 30MPa and 1500–2100°C. The two-step reactive hot pressed ZrC1−x ceramic has a higher relative density (>95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100°C. A cubic Zr2C-type ordered phase forms in the ZrC1−x sample obtained at a ZrC/ZrH2 molar ratio of 0.6 at a relatively low temperature of 1100°C. The decrease in C/Zr ratio is beneficial to densification of ZrC1−x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.

Li1.2Zr1.9Sr0.1(PO4)3 as a Solid-State Electrolyte

In recent years there has been a resurgence in the use of Li metal as an anode, because such use can to lead to rechargeable batteries with higher energy than currently exist. Using a Li anode with a liquid electrolyte can lead to dendrite shorting and flammability issues. To overcome these problems Li-ion conducting solid electrolytes are attracting considerable attention. The major requirements for the solid Li-ion conducting electrolyte are: high Li-ion conductivity, low electronic conductivity, high relative density, chemical/electrochemical stability with the Li anode and cathodes, adequate mechanical properties. One possible electrolyte under consideration based on the NaSCION structure is Li1.2Zr1.9Sr0.1(PO4)3. It is the purpose of this presentation to discuss the following: 1] ionic/electronic conductivity 2] elastic modulus/hardness/fracture toughness and 3] Li /water stability of dense (~97%) fine-grain (~10 mm) Li1.2Zr1.9Sr0.1(PO4)3 consolidated by reactive hot-pressing. The results will compared to other Li-ion solid-state electrolyte conductors based on the NaSICON, perovskite and garnet structures.

Impact-resistant silicon-carbide-based ceramic materials

The bullet resistance is determined by an indirect method, by evaluation of time of delay of penetration by bullet of the silicon-carbide-based ceramics obtained by reactive sintering, liquid-phase sintering, and hot pressing.

Formation of metastable tungsten tetraboride by reactive hot-pressing

Abstract The synthesis of tungsten tetraboride (WB 4 ) powders prepared by reactive hot-pressing in a vacuum from elemental tungsten (W) and amorphous boron (B) powders is reported in this work. This study systematically investigates the effects of the synthesizing temperature, B/W molar ratio, pressure and time on phase formation. The XRD pattern for the as-synthesized powders was analysed. Metastable tungsten tetraboride was synthesized in a vacuum at 1350 °C with a B/W molar ratio of 8.0 under a low uniaxial pressure (30 MPa) for 1 h. The scanning electron microscopy and transmission electron microscopy results showed that the particles had a tendency to form agglomerates. Amorphous boron was found to exist in the final product, and formation of the crystal phase WB 4 was confirmed.

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.

Effects of first-scale TiBw on secondary-scale Ti5Si3 characteristics and mechanical properties of in-situ (Ti5Si3+TiBw)/Ti6Al4V composites

Abstract In order to further improve mechanical properties, Ti6Al4V matrix composites reinforced with secondary-scale Ti 5 Si 3 particles and first-scale TiB whiskers were designed and prepared by reaction hot pressing. The effects of first-scale TiBw fractions on secondary-scale Ti 5 Si 3 microstructure characteristics and mechanical properties of two-scale network structured composites were investigated. With increasing TiBw fractions, the size of Ti 5 Si 3 particles in β phase increased, the quantity of Ti 5 Si 3 particles near TiBw reinforcement increased and the morphology changed from isolated particles to interconnected particles. These changes resulted in that the room and high temperature tensile strength of the composites increased, moreover, the creep resistance of the composites improved. Compared with the Ti6Al4V alloy, the room and high temperature tensile yield strength of the composites with 3.4 vol%TiBw increased by 36.5% and 70%, and the elongation remained at 5.0% and 11.5%, respectively. The composites possess approximately one order of magnitude lower steady-state creep rate and about eight times higher time-to-rupture at 873 K/200 MPa. The superior tensile and creep properties can be attributed to adjustment of secondary-scale Ti 5 Si 3 characteristics and two-scale network structure.

Preparation and mechanical properties of bulk ultrafine (W0.5Al0.5)C0.65 with minor Fe binder by reactive hot-pressing

ABSTRACTBinderless (W0.5Al0.5)C0.65 produced by reactive hot-pressing (RHP) has high hardness but low bending strength and fracture toughness. In this work, bulk ultrafine (W0.5Al0.5)C0.65 with minor addition of Fe as the binder phase has been prepared by RHP. Rapid synthesis and high densification can be achieved for the (W0.5Al0.5)C0.65 with 0.4–1.0 wt-%Fe by RHP at a relatively low temperature. Pure (W0.5Al0.5)C0.65 without sub-carbide phase or brittle η-type phase can be synthesised due to the fact that the carbon vacancy in (W0.5Al0.5)Cy can reach 50%. And thanks to the combined effect of nanoscaled initial powders, low temperature, rapidly reactive sintering and pressure, bulk ultrafine (W0.5Al0.5)C0.65-Fe with a grain size of about 0.2 μm can be obtained by RHP. Thus, with an appropriate amount of Fe (about 0.8 wt-% in this work), the bending strength and fracture toughness of (W0.5Al0.5)C0.65 can be highly improved.

Densification, Microstructure and Mechanical Properties of Reactive Hot-Pressed ZrB<sub>2</sub>-ZrC-Zr Cermets

This study reviews densification behavior, microstructure and mechanical properties of the various composition ZrB2-ZrC0.6-Zr cermets prepared by reactive hot-pressing of Zr + B4C powder mixtures between 1673 K and 2173 K. During reactive sintering, densification consisted of two parts, one resulted from the volume contraction due to the conversion of Zr + B4C mixture into ZrB2 and ZrC0.6, other resulted from the shrinkage due to grain boundary diffusion and grain boundary migration that mostly contributed to the densification. Highly-dense ZrB2-ZrC0.6-Zr cermets were obtained for Zr + B4C mixtures at equal to or greater than 1673 K. The resulting ZrB2-ZrC0.6-Zr cermets consisted of platelet-like ZrB2 grains, equiaxed ZrC0.6 grains and intergranular Zr. The ZrB2 and ZrC0.6 grains were coarsened with increase of the excess Zr amount and/or sintering temperature. On the other hand, the flexural strength and fracture toughness of the cermets strongly depended on the excess Zr amount and/or sintering temperature. In addition, the crack propagation behavior of the cermets was associated with the excess Zr amount. The cracks directly propagated in the samples with less than 16 vol% Zr, however, the multiple cracking behavior was observed for the samples with equal to or greater than 16 vol% Zr.

Chemical process and solid solution formation in reactive hot pressed ZrB2–SiC ceramics doped with W

ABSTRACTZrB2–SiC doped with W was prepared from a mixture of Zr, Si, B4C and W via reactive hot pressing. The fully dense ZrB2–SiC–WB–ZrC ceramic was obtained at 1900°C for 60 min under 30 MPa in an argon atmosphere. Reaction path and solid solution characteristics of the starting powders were studied through a series of pressureless heat treatment at temperatures between 700 and 1500°C. The solid solution phases of (Zr, W)B2, (W, Zr)B and (Zr, W)C were formed directly by reactions between the precursors. Homogeneous distribution of solute atoms in solution and the solid solubilities were also studied.

Electrical, mechanical and chemical behavior of Li1.2Zr1.9Sr0.1(PO4)3

The elastic modulus, fracture toughness, ionic conductivity, Li and water stability of dense (~97%) fine-grain (~10μm) Li1.2Zr1.9Sr0.1(PO4)3 (LZSP) consolidated by reactive hot-pressing was investigated. Low values of elastic modulus (25.2 and 40.9GPa), fracture toughness (0.29 and 0.37MPa-m1/2), total conductivity (2.1×10−5S/cm) at room temperature were exhibited as a result of the presence of microcracks formed during cooling from the hot-pressing temperature. The lattice conductivity of LZSP is 0.85×10−4S/cm with an activation energy for bulk conductivity of 0.29eV. The results of Li/LZSP/Li cell impedance as a function of time and surface examination suggest that LZSP is not stable against metallic lithium. The results of the water stability test suggest that LZSP is a possible membrane for use in aqueous Li-Air cells. The results of this study reveal for the case of lithium zirconium phosphates are as follows: 1] a small grain size is required to prevent microcracking to avoid a reduction in total conductivity and mechanical properties and 2] even though they do not contain a transition metal cation they are not stable against Li at room temperature.

Fabrication and cutting performance of reactively hot-pressed TiB2-TiC-SiC ternary cutting tool in hard turning of AISI H13 steel

TiB2-based ceramics are promising materials for fabrication of cutting tools, which are used in hard machining of difficult-to-cut materials. A TiB2-TiC-SiC ceramic cutting tool composite was fabricated via the reactive hot pressing (RHP) process. The microstructure and mechanical properties, as well as the strengthening and toughening mechanisms, were investigated. Elongated TiB2 grains with a diameter of 0.5–1 μm and an aspect ratio of 3–8 were in situ synthesized and uniformly distributed in the composite. The mechanical properties were tested, and the flexural strength was 768 MPa, the fracture toughness was 7.3 MPa m1/2, and the Vickers hardness was 21.8 GPa. The good mechanical properties were ascribed to the clean interface and toughening effect of the elongated TiB2 grains. The cutting performance including tool life and surface roughness, as well as tool failure mechanisms in hard turning of 55 HRC AISI H13 steel, was investigated. The effects of cutting parameters on tool life and surface roughness were studied. For the investigated range of cutting parameters, the optimal cutting speed, feed rate, and depth of cut were 100 m/min, 0.1 mm/rev, and 0.25 mm, respectively. Under this condition, the tool life and surface roughness were 45.4 min and 1.289 μm, respectively. The failure of the fabricated cutting tool was characterized by nose and flank face wear. The wear mechanisms consisted of adhesive wear, oxidation wear, and diffusion wear.

Effects of heat treatment on microstructure and high temperature tensile properties of TiBw/TA15 composite billet with network architecture

In situ 3.5vol% TiBw whisker reinforced TiBw/TA15 composite with a novel network architecture was successfully fabricated by reaction hot pressing (RHP) method. In order to improve the high temperature mechanical properties of the composite, solid solution and aging treatments with different parameters were carried out. After sintering, the composite formed a net-like microstructure. The strength of composite was improved by the effect of whisker-rich boundary and the interconnected whisker-lean matrix retained its ductility. The elastic modulus and hardness of different phases were tested by nanoindentation for better understanding the strengthening mechanism. The ultimate tensile strength at 600–700°C of the composite showed the highest value when the solution temperature is 1000°C followed by aging treatment, which was caused by the distribution of a few residual primary α phases on the boundary and in turn the increasing of volume fraction for the reinforcements. Furthermore, the size and fraction of fine α+β phases increased with increasing of aging temperature after 1000°C solid solution treatment, leading to the increasing of ductility and decreasing of strength for the composite tested at 600–700°C. The above results showed that the composite with novel network architecture could be strengthened by heat treatments at high temperature environment, e.g. the ultimate tensile strength could be improved more than 200MPa at 600°C, and more than 100MPa at 700°C.

Synthesis of MAX Phases in the Hf-Al-C System.

For the first time, MAX phases in the Hf-Al-C system were experimentally synthesized using reactive hot pressing. HfC was observed as the main competing phase. The lattice parameters of Hf2AlC and Hf3AlC2 were determined by Rietveld refinement based on the X-ray diffraction data. The atomic stacking sequence was revealed by high-resolution scanning transmission electron microscopy. Mixtures of 211 and 312 stacking were observed within the same grain, including 523 layers. This transition in atomic structure is discussed.

Controllable two-scale network architecture and enhanced mechanical properties of (Ti5Si3+TiBw)/Ti6Al4V composites.

Novel Ti6Al4V alloy matrix composites with a controllable two-scale network architecture were successfully fabricated by reaction hot pressing (RHP). TiB whiskers (TiBw) were in-situ synthesized around the Ti6Al4V matrix particles, and formed the first-scale network structure (FSNS). Ti5Si3 needles (Ti5Si3) precipitated in the β phase around the equiaxed α phase, and formed the secondary-scale network structure (SSNS). This resulted in increased deformation compatibility accompanied with enhanced mechanical properties. Apart from the reinforcement distribution and the volume fraction, the ratio between Ti5Si3 and TiBw fraction were controlled. The prepared (Ti5Si3 + TiBw)/Ti6Al4V composites showed higher tensile strength and ductility than the composites with a one-scale microstructure, and superior wear resistance over the Ti6Al4V alloy under dry sliding wear conditions at room temperature.

Reactive hot pressing of SiC-ZrC composites at low temperature

SiC-ZrC composites with relative density in excess of 99% were prepared by reactive hot pressing (RHP) of SiC and ZrH2 at 1800°C for 1h. The reaction between SiC and ZrH2 resulted in the formation of ZrC1-x. The formation process and densification behavior during RHP process were investigated. Low temperature densification of SiC-ZrC composites is attributed to the formed nonstoichiometric ZrC1-x and the removal of SiO2 impurity on the surface of SiC particles. As reinforced phase, ZrC1-x has inhibiting effect on the abnormal grain growth of SiC, resulting in homogeneous microstructure of fine SiC grains. Adding 10wt% ZrH2 to SiC, the formed SiC-4.62vol% ZrC composite exhibited better mechanical properties (Vickers hardness of 27.6±0.7GPa, flexure strength of 448±38MPa, fracture toughness of 6.0± 0.3MPa·m1/2, respectively) than monolithic SiC ceramic.

Microstructural development and mechanical properties of reactive hot pressed nickel-aided TiB2-SiC ceramics

TiB2-SiC composites with different amounts of Ni (0, 2 and 5wt.%) added as sintering aid were fabricated by reactive hot pressing (RHP). The mechanical properties were assessed under ambient conditions and the flexural strength was further tested in the temperature range of 700–1000°C. The microstructures of the composites were characterized by a scanning electron microscope (SEM), transmission electron microscope (TEM) and energy-dispersive spectrometer (EDS). The flexural strength degradation mechanism occurring at elevated temperatures was studied. Addition of a moderate amount of Ni led to an improvement of the mechanical properties at room temperature. For the investigated ceramic composites, TiB2-SiC-5 wt.% Ni sample showed significantly enhanced mechanical properties, i.e., a flexural strength of 1121±31MPa, a fracture toughness of 7.9±0.58MPa·m1/2, a hardness of 21.3±0.62GPa, and a relative density of 98.6±1.2%. Ni distributed along grain boundaries improved the interface strength. The improved fracture toughness was ascribed to crack deflection, grain rupture and crack shielding effect of Ni. A substantial strength degradation occurred at elevated temperatures, which was attributed to softening of the grain boundaries, surface oxidation and sliding of grain boundaries. The elastic modulus was found to decrease with increasing temperature.