TiB2 Grain Growth Research Articles

Study on the microstructure and properties of TiB2-CoCrFeNiW0.2 metal ceramic composites prepared by pressureless sintering

Three types of TiB2-HEA ceramic composite were prepared using a non-equimolar ratio CoCrFeNiW0.2 high entropy alloy (HEA) as a binder, mechanical alloying (MA), and 1600 °C pressureless sintering (PS) processes. The phase analysis results indicate the presence of TiB2, FCC, and TCP phases in the TiB2-HEA composite. A detailed study was conducted on the microstructure of TiB2-HEA composite. The results indicate that there is a certain solid solubility between TiB2 and HEA. The Ti and B elements in TiB2 will enter HEA, and the elements in HEA will also enter TiB2. Along with the discovery of many lattice distortion regions in TiB2, a diffraction pattern of P-43m cubic structure was also observed in HEA. HEA has good wettability with TiB2, so it can not only inhibit the grain growth of TiB2, but also improve the performance of TiB2-HEA composite by utilizing the inherent excellent properties of HEA. There are both intergranular and transgranular fractures in the TiB2-HEA composite. The relative density, flexural strength, Vickers hardness, fracture toughness, and electrical resistivity of TiB2-30 wt%HEA are 99.7%, 680 MPa, 18.6 GPa, 6.4 MPa m1/2, and 5.7 × 10−7 Ω m, respectively.

Simultaneously enhancing the toughness and strength of cemented TiB2 nanocomposites through doping SiC whisker and graphene

The current study applies towards investigate the sintering behavior, microstructure as well as mechanical properties of SiC whisker (SiCw) and multi-layer graphene (MLG) reinforced cemented TiB2 nano-composites fabricated through two-step sintering (TSS), highlighting the critical roles of SiCw and MLG as exceptional strengthening/toughening phases. The sinterability of TiB2–TiC–Ni was somewhat improved as a function of MLG or SiCw incorporation. The addition of SiCw resulted in the grain growth of matrix grains, whereas MLG addition significantly impeded the grain growth of TiB2. TSS is effective in suppressing the grain growth coupled with improving the densification in comparison with conventional sintering (CS). The major toughening mechanisms for SiCw/TiB2–TiC–Ni are SiCw bridging, crack deflection and branching. The major toughening mechanisms for MLG/TiB2–TiC–Ni are MLG bridging/pullout as well as crack deflection/branching/arresting. MLG is a superior reinforcement in comparison with SiCw.

Influences of B4C content and particle size on the mechanical properties of hot pressed TiB2–B4C composites

ABSTRACT TiB2 ceramic has attracted many attentions due to its excellent performances both on mechanical and functional properties. However, it is still very difficult to obtain TiB2 bulks with full densification. Herein, TiB2–B4C composites were prepared via hot pressing at 1900°C under the pressure of 35 MPa with TiB2 and B4C as the raw materials. The influences of B4C content (0, 10 vol%, 20 vol% and 30 vol%) and particle size (0.50 µm, 3.12 µm and 7.09 µm) on the phase composition, relative density, microstructure and mechanical properties of TiB2–B4C composites were investigated. The using of 0.50 µm B4C brought more B2O3 which induced oriented growth of TiB2 grains and coarsened the obtained composites. When the raw B4C powders were 3.12 µm and 7.09 µm, the obtained composites with 20–30 vol% B4C could be fully densified. TiB2–30 vol% B4C composite prepared from 3.12 µm B4C powder resulted in the optimal mechanical properties. The bending strength, Vickers hardness, fracture toughness were 671 MPa, 27.93 GPa, and 3.91 MPa·m1/2, respectively. The excellent performance was attributed to its fine and homogenous microstructure.

Effect of mechanical alloying on sinterability and phase evolution in pressure-less sintered TiB2‒TiC ceramics

Phase relation and microstructure evolution in the pressure-less sintered TiB2‒TiC ceramics preceded with mechanical alloying were systematically studied by a combination of SEM analysis. WC debris from milling balls promotes sintering by dissolving into the TiC phase to achieve dense microstructures at 1600 °C. Variation of W solution in TiC grains exposes two types of core‒rim structures, with no or more W in dark and white cores respectively but with common medium W in both rims. Diminishing white-cores reveal an exchange reaction between WC and TiC via mechanical alloying to form the Ti1-zWzC phase prior to sintering. The dark-cores inherit from the as-milled TiC power to further enable the reprecipitation of rims from a mixed liquid-phase, which facilitated also the anisotropic growth of TiB2 grains. The dark-cores grow persistently in the second-step at 2000 °C enabled by this liquid-phase, which coarsens the TiB2 grains too. With more alloyed phase, sintering was insufficient at 1500 °C with only the surface fluidity from the primary powders, and the second-step sintering increased the fluidity in the liquid-phase to fully densify the binary microstructure. Re-distribution of the alloyed W by two-step sintering rationalizes the evolution process of the binary microstructures and leads to better understanding of the mechanical behaviors.

Influence of Si addition on structure and properties of TiB2-Si nanocomposite coatings deposited by high-power impulse magnetron sputtering

TiB2 coatings with various Si doping levels were fabricated using magnetron sputtering from TiB2 and Si targets. The chemical and phase composition, microstructure, mechanical properties, and high-temperature oxidation resistance of the coatings were investigated using different techniques, such as electron probe microanalysis (EPMA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nanoindentation, and thermogravimetric analysis. The results revealed that the microstructure of the TiB2 coating with and without Si doping changed from an apparent columnar structure to a densely stacked structure, and finally to a nearly featureless planar structure. A certain amount of added Si limits the growth of TiB2 grains and favors the formation of a nanocomposite structure with nanocrystalline TiB2 grains embedded in an amorphous Si phase matrix, as shown by XPS and TEM results. The hardness reached a maximum value of 35.2 GPa at 4.5 at% Si and then linearly decreased upon further increasing the Si content, owing to the excessively amorphous phase in coatings. However, it is worth noting that the high-temperature oxidation resistance of the coatings was improved by Si addition, due to their compact homogeneous structure that hinders the diffusion of oxygen. The results indicate that the hardness and oxidation resistance of the coatings can be simultaneously improved by the addition of an appropriate Si content.

Microstructure and mechanical properties of TiB2-40 wt% TiC composites: Effects of adding a low-temperature hold prior to sintering at high temperatures

TiB2–40 wt% TiC composites were prepared by pressureless sintering. Prior to the conventional heating at 2000 °C for 1 h for final densification, addition of a low temperature hold at 1500 °C for 4 h was found effective in densification of the composite, refining the microstructure and promoting the anisotropic growth of TiB2 grains. The TiB2-TiC composite prepared by two-step sintering had better mechanical properties with flexural strength of 776 MPa and fracture toughness of 7.43 MPa m1/2, in comparison with the values of 419 MPa and 4.81 MPa m1/2 for composite by conventional one-step sintering.

Formation and properties of TiB2-Ni composite ceramics

An analysis of physical and chemical processes occurring during hot pressing of the 95 wt. % TiB2–5 wt. % NiCl2By powder mixture in the temperature range 1800–2000 °C has been performed by X–ray diffraction, scanning electron microscopy, an electron-probe microanalysis. It has been established that, in the process of heat treatment, sintering, TiB2grain growth, diffusion of boron and titanium into nickel layers, and the formation of NixByTiz layers between TiB2 grains occur. These layers act as a grains binder TiB2. It is shown that the drilling of the obtained high-strength ceramics can be performed by laser machining.

Al content dependence of reaction behaviors and mechanism in combustion synthesis of TiB2–AlN-based composites

Abstract The TiB2–AlN and TiB2–AlN–TiN ceramic matrix composites were successfully produced by self-propagating high-temperature synthesis (SHS) reactions from the Al–Ti–BN compacts through the manipulation of the Al reactant content. The effects of the Al content on the adiabatic and practical maximum combustion temperatures, wave velocity, phase composition and microstructure of the synthesized products, and reaction mechanism were investigated in detail. Thermodynamically and practically, the reaction was most exothermic for the Al content being 35.5 wt.% in the reactants to produce only the dual TiB2–AlN phases. Kinetically, the reaction commenced with the formation of Al3Ti, and then proceeded with the interactions of the Al–Ti intermetallics/melts with BN to yield the AlN and TiB2 phases. The substantial reaction between the elemental Ti and BN into formation of TiN took place only in the low Al content samples. On the other hand, the presence of considerable amount of the TiN grains effectively inhibited the abnormal growth of the TiB2 grains, leading to very fine microstructures; whereas, the grain growth of TiB2 and AlN as well as their inhomogeneous distribution was enhanced by the increase in the Al reactant, while TiN was completely inhibited from development.

Polycarbosilane을 이용한 TiB2-SiC 세라믹의 형성

The formation of TiB2-SiC ceramics from TiB₂-polycarbosilane (PCS) mixtures was investigated. The powder mixture of TiB₂ with PCS was pressed at 300℃ with 200MPa and sintered at 1700~2000℃ for 1 h in a flowing Ar atmosphere. The sintered density of TiB₂ with PCS is 93.7% after sintering at 2000℃ for 1 h, which is slightly smaller than that of the specimen without PCS. The microstructure of TiB₂ with PCS consists of small and uniform TiB2 particles with well dispersed SiC particles derived from PCS. It is believed that the addition of PCS was effective to suppress the grain growth of TiB₂.

Polycarbosilane 첨가에 의한 TiB2의 소결

The effect of SiC additive on the densification behavior and microstructural change of TiB₂ ceramics was investigated. The infiltration and direct mixing with polycarbosilane (PCS), which was converted to SiC by pyrolysis during sintering, were used for the addition of SiC. The addition of Fe enhanced the densification of TiB₂, but which resulted in exaggerated grain growth. However, the addition of PCS enhanced the densification and suppressed the exaggerated grain growth of TiB₂. Moreover, the addition of PCS using direct mixing was more effective for suppress of grain growth as compare to the addition of PCS using infiltration.

TiB<sub>2</sub>-B<sub>4</sub>C Composites Pressureless-Sintered Using Ni and C as Densification Aids

TiB2-B4C composites were fabricated by pressureless sintering at 1600°C using Ni and C as densification aids. X-ray diffraction revealed the presence of TiB2, B4C and NiB. Up to 20vol% B4C additive content, dense composites with -94% of the theoretical density were obtained. The dispersion of B4C particles inhibited the grain growth of TiB2 and improved the fracture toughness and bending strength. The maximum fracture toughness and bending strength of the composites were 6.1MPa⋅m1/2 and 530MPa, respectively, in contrast with 5.1MPa⋅m1/2 and 380MPa for a sintered body without B4C. The crack deflection by B4C particles was considered to be the main toughening mechanism for this composite system. With increasing B4C additive content, the thermal expansion coefficient of the composites decreased and the electrical resistivity increased. The relation between the resistivity and composition was discussed based on the effective medium theory.

Effects of SiC Addition on the Mechanical Properties and Sinterability of TiB<sub>2</sub>-(2mol% Y<sub>2</sub>O<sub>3</sub>-ZrO<sub>2</sub>) Composite

Ceramic TiB2-19.0-19.5wt% (2mol% Y2O3-ZrO2) composites with 2.5-5.0wt% SiC were fabricated by hot isostatic pressing (HIP) with and without a glass encapsulation. The effects of SiC addition on the mechanical properties and siterability of TiB2-20wt% (2mol% Y2O3-ZrO2) HIP'ed bodies were investigated. The addition of SiC improved the sinterability of TiB2-20wt% (2mol% Y2O3-ZrO2) appreciably. Composites of TiB2-19.0-19.5wt% (2mol% Y2O3-ZrO2) with 2.5-5.0wt% SiC densified up to 96.7% of their theoretical densities by vacuum sintering at 1700°C for 14.4ks. These sintered bodies almost fully densified by capsule-free-HIP. On the other hand, TiB2-20wt% (2mol% Y2O3-ZrO2) densified up to 63.7% by the same vacuum sintering. The addition of SiC was also effective in retarding the grain growth of TiB2 and ZrO2. The grain sizes of TiB2 and ZrO2 of TiB2-19.0-19.5wt% (2mol% Y2O3-ZrO2)-2.5-5.0wt% SiC HIP'ed bodies were about 1.5 and 1.2μm, respectively. They were about 2/3 of those of TiB2-20wt% (2mol% Y2O3-ZrO2) HIP'ed bodies. The three-point bending strengths of TiB2-19.0-19.5wt% (2mol% Y2O3-ZrO2)-2.5-5.0wt% SiC glass-encapsulated-HIP'ed bodies were 1240MPa, which is slightly higher than that of TiB2-20wt% (2mol% Y2O3-ZrO2) HIP'ed bodies. Vickers hardness and fracture toughness were not affected by the addition of 2.5-5.0wt% SiC. The mechaical properties of TiB2-(2mol% Y2O3-ZrO2) HIP'ed bodies were controlled by the monoclinic ZrO2 ratio in ZrO2 and monoclinic ZrO2 content in HIP'ed bodies, as we have reported in the previous paper. The addition of SiC affected them only slightly, so that the mechanical properties were also little affected.

ＴｉＢ２基焼結体に関する研究

It was difficult to produce dense and high strength TiB2 ceramics by using hot pressing. This paper described densification and strengthening mechanisms by using of three kinds of additives. First additives are CoB, FeB, Ni4B3 and NiB. Each additive makes dense TiB2 ceramics. The densification mechanism is explained as activated sintering and pressure effects. The ceramics which have higher transverse rupture strengths and lower porosities can be produced by adding the second each additive of VB2, NbB2, TaB2, CrB2, MnB2, MoB2, Mo2B5 and W2B5. The higher transverse rupture strength has been caused mainly by suppressing grain growth of TiB2 with the solid solution between TiB2 and the hexagonal boride such as TaB2, and caused secondarily by the lattice strain of TiB2.Hence, it is expected that TiB2 ceramics having the higher mechanical shock resistance can be made from the raw materials with smaller particle size of TiB2. However, fine raw powders gave more pores, because of oxidizing during mixing. Final additives are cubic carbides such as TiC, ZrC and HfC. When the each additive adds to TiB2-1%CoB-6%TaB2 mixed powder, the ceramics have the smaller particle size and the lower porosity.This lower porosity, for example, may be formed by a solid solution of M(C, O) and deoxidation of the raw TiB2 powder.

Microstructure of TiB2liquid phase sintered with Ni3Al

TiB2is a ceramic material which possesses exceptional thermal, chemical, and electrical properties. Commercial powders can be formed into bulk pieces by hot pressing at temperatures near 2000°C. However, this produces excessive grain growth which can lead to cracking and low mechanical integrity due to the relatively large anisotropic thermal expansion of the TiB2grains. Liquid phase sintering can be used to produce structural forms of TiB2by hot pressing at temperatures near 1450°C. This process limits the TiB2grain growth but introduces an intergranular phase. Previous work used Ni as a binder and under most hot-pressing conditions this resulted in the formation of a Ni3B intergranular phase. Additional binders are being considered to understand the phase development during liquid-phase sintering and to improve the properties of the bonded TiB2forms.The present research concerns the use of Ni3Al as a binder for TiB2. The Ni3Al powder was a physical mixture of 32 wt % NiAl and 68 wt % of a solid solution of 95 at. % Ni with 5 at. % Al powders.