Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti–FeMo–B prepared by spark plasma sintering

The in situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering at 800–1200 °C under 20 MPa for 5 min. The effects of sintering temperature and reinforcement volume fraction on flexural strength, Young’s modulus and fracture toughness of the composites are investigated. The titanium matrix consists of -Ti and β-Ti phases, and the volume fraction of β-Ti increases with increasing sintering temperatures. The in situ synthesized TiB reinforcements are distributed randomly and uniformly in matrix. The transverse section of TiB has a hexagonal shape aligned along [0 1 0] direction, and the crystallographic planes of the TiB needles are always of the type . The 10 vol% TiB reinforced composite sintered at 1000 °C exhibits excellent mechanical properties. The flexural strength, Young’s modulus and fracture toughness of this composite are 1560 MPa, 137 GPa and 8.64 MPa · m^1/2, respectively.     

In situ synthesis mechanism and characterization of ZrB2–ZrC–SiC ultra high-temperature ceramics

The synthesis route, microstructure and properties of ZrB₂–ZrC–SiC composites prepared from a mixture of Zr–B₄C–Si powders by in situ reactive synthesis were investigated. The reactive path and synthesized mechanism of ZrB₂–ZrC–SiC composite were studied through series of pressureless heat treatments ranging from 800 °C to 1700 °C in argon. The in situ ZrB₂–ZrC–SiC composites were fabricated under different synthesis processing. The one with 88.4% relative density performed poorly in mechanical properties due to the occurring of self-propagating high-temperature synthesis (SHS). The fully dense ZrB₂–ZrC–SiC composite was obtained under the optimized synthesis processing without SHS reactions. Its Vickers hardness, flexural strength and fracture toughness were 20.22 ± 0.56 GPa, 526 ± 9 MPa and 6.70 ± 0.20 MPa m^1/2, respectively.     

SiC颗粒强韧化MoSi2复合材料

通过湿法混料和热压烧结工艺成功地制备了20vo1%SiC_P/MoSi₂复合材料,并测定了其显微组织和力学性能。结果表明:SiC_P/MoSi₂复合材料主要由MoSi₂和SiC颗粒组成,还有少量的Mo₅Si₃,致密度为92.3% 。与MoSi₂相比,其室温抗弯强度提高了30.6%,断裂韧性提高了53%,1200℃的抗压强度提高了44%,1400℃的抗压强度提高了53%;其硬度、弹性模量等性能有较大提高。在Al₂O₃和SiC对磨盘上表现出极其优异的耐磨性能。SiC颗粒对MoSi₂的室温增韧、高温增强效果显著。     

Effects of oxidation on surface stresses and mechanical properties of liquid phase pressureless-sintered SiC–AlN–Y2O3 ceramics

The understanding of the oxidation mechanism of 50 wt% SiC–50 wt% AlN composites obtained by means of pressureless sintering without the protective powder bed and with Y₂O₃ as sintering-aid were significantly improved by means of Raman spectroscopy. These analyses put in evidence that “amorphous carbon” started to be formed at 1300 °C as main effect of active oxidation of SiC. At higher temperature the crystallization process began and it was completed at 1500 °C when only graphite could be recognized. On the basis of these new evidences, oxidation effects on the mechanical properties of SiC–AlN–Y₂O₃ composites were defined. First of all, heat treatment in air was able to induce a compressive surface stress due to the volume gain associated to the oxidation of the intergranular phase. As a consequence apparent fracture toughness showed a value of 6.6 MPa m^1/2 after a heat treatment at 1300 °C, while at higher temperature effects of active oxidation caused a decreasing up to 4.7 MPa m^1/2. This toughening mechanism was also used to improve the resistance to thermal shock, which was evaluated by performing quenching tests. Furthermore, passive oxidation induced the healing of superficial flaws by means of the formation of -cristobalite. This phenomenon was assumed to be responsible for the increasing of the flexural strength.     

Microstructure and properties of MoSi2–MoB and MoSi2–Mo5Si3 molybdenum silicides

MoSi₂-based intermetallics containing different volume fractions of MoB or Mo₅Si₃ were fabricated by hot-pressing MoSi₂, MoB, and Mo₅Si₃ powders in vacuum. Both classes of alloys contained approximately 5 vol.% of dispersed silica phase. Additions of MoB or Mo₅Si₃ caused the average grain size to decrease. The decrease in the grain size was typically accompanied by an increase in flexure strength, a decrease in the room temperature fracture toughness, and a decrease in the hot strength (compressive creep strength) measured around 1200 °C, except when the Mo₅Si₃ effectively became the major phase. Oxidation measurements on the two classes of alloys were carried out in air. Both classes of alloys were protected from oxidation by an in-situ adherent scale that formed on exposure to high temperature. The scale, although not analyzed in detail, is commonly recognized in MoSi₂ containing materials as consisting mostly of SiO₂. The MoB containing materials showed an increase in the scale thickness and the cyclic oxidation rate at 1400 °C when compared with pure MoSi₂. However, in contrast with the pure MoSi₂ material, oxidation at 1400 °C began with a weight loss followed by a weight gain and the formation of the protective silica layer. The Mo₅Si₃ containing materials experienced substantial initial weight losses followed by regions of small weight changes. Overall, the MoB and Mo₅Si₃ additions to MoSi₂ tended to be detrimental for the mechanical and oxidative properties.     

Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane

Three-dimensional braided carbon fiber reinforced silicon carbide composites (3D-B C_f/SiC) were prepared through eight cycles of vacuum infiltration of polycarbosilane (PCS) and subsequent pyrolysis under an inert atmosphere. The influences of heating rate and pyrolysis temperature on the microstructure and mechanical properties of C_f/SiC were discussed. It was found that the heating rate had great effect on the mechanical properties of C_f/SiC composites. With the increase of heating rate, the density of C_f/SiC composites increased and the interfacial bonding was weakened. As a result, the flexural strength of C_f/SiC was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15 °C/min. The results showed that the flexural strength of the C_f/SiC composites fabricated at a heating rate of 15 °C/min could be increased from 480 to 557 MPa if the pyrolysis temperature of the sixth cycle was elevated from 1200 to 1600 °C, which was also attributed to the desirable interfacial structure and increased density. When tested at 1300 °C in vacuum, the C_f/SiC showed higher flexural strength (680 MPa) than that (557 MPa) at room temperature.     

Improvements in mechanical properties of TiB2 ceramics tool materials by the dispersion of Al2O3 particles

TiB₂–Al₂O₃ composites with Ni–Mo as sintering aid have been fabricated by a hot-press technique at a lower temperature of 1530 °C for 1 h, and the mechanical properties and microstructure were investigated. The microstructure consists of dispersed Al₂O₃ particles in a fine-grained TiB₂ matrix. The addition of Al₂O₃ increases the fracture toughness up to 6.02 MPa m^1/2 at an amount of 40 vol.% Al₂O₃ and the flexural strength up to 913.86 MPa at an amount of 10 vol.% Al₂O₃. The improved flexural strength of the composites is a result of higher density than that of monolithic TiB₂. The increase of fracture toughness is a result of crack bridging by the metal grains on the boundaries, and crack deflection by weak grain boundaries due to the bad wetting characters between Ni–Mo and Al₂O₃.     

Mechanical properties of vanadium beryllide,VBe12

The mechanical properties of VBe₁₂, both at room and elevated temperatures (up to 1200°C), have been measured. Room-temperature properties, including Young's modulus, flexural strength, and fracture toughness are reported. The material behaved elastically at room temperature but became plastic at temperatures above 1000°C. Creep properties of VBe₁₂ were also studied in temperature ranges from 1000–1200°C and applied stress ranges from 33–58 MPa. At low strain rates (approximately < 10^–5s^–1), the stress exponent was about 4, suggesting deformation was controlled by dislocation climb. Microstructural examination indicated that fracture was initiated from grain boundaries subjected to tensile stresses. The creep behaviour of VBe₁₂ is briefly compared with that of other intermetallics.     

Fracture behavior and mechanical properties of Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) alloy at room temperature

The microstructure, mechanical properties and fracture behavior of gravity die cast Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) (WNG421) alloy are studied at room temperature in different thermal conditions, including as-cast, solution-treated and different aging-treated (both isothermal and two-step aging) conditions. The results indicate that WNG421 alloy shows different behaviors of crack initiation and propagation in different thermal conditions during tensile test at room temperature. After pre-aged at 200 °C for 5 h, the hardness of WNG421 alloy first reduces and then increases when secondary aged at 250 °C (two-step aging). The peak hardness and corresponding tensile strength of the two-step aged alloy both increases compared with those in 250 °C isothermal peak-aged condition. Tensile strength of WNG421 alloy at room temperature in low temperature (200 °C) isothermal peak-aged condition is much higher than that in high temperature (250 °C) isothermal peak-aged condition.     

High temperature flexural strength and fracture toughness of AlN with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic

The effects of temperature on the fast fracture behavior of aluminum nitride with 5 wt% Y₂O₃ ceramic were investigated. Four-point flexural strength and fracture toughness were measured in air at several temperatures (30–1,300 °C). The flexural strength gradually decreased with the increase of temperature up to 1,000 °C due to the change in the fracture mode from transgranular to intergranular, and then became almost constant up to 1,300 °C. Two main flaw types as fracture origin were identified: small surface flaw and large pores. The volume fraction of the large pores was only 0.01%; however, they limited the strength on about 50% of the specimens. The fracture toughness decreased slightly up to 800 °C controlled by the elastic modulus change, and then decreased significantly at 1,000 °C due to the decrease in the grain-boundary toughness. Above 1,000 °C, the fracture toughness increased significantly, and at 1,300 °C, its value was close to that measured at room temperature.     

Mg0.27Al2.58O3.73N0.27透明陶瓷机械性能评价

MgAlON透明陶瓷具有优良的光学与机械性能, 在军用和民用领域均具有重要的应用潜力。本研究表征并评价了Mg_0.27Al_2.58O_3.73N_0.27透明陶瓷的机械性能, 实验测定了Mg_0.27Al_2.58O_3.73N_0.27透明陶瓷的维氏硬度、断裂韧性以及室温和高温断裂强度, 并使用Weibull函数对室温断裂强度测试结果进行统计。结果表明: 该透明陶瓷特征断裂强度为255 MPa、断裂韧性为2.56 MPa/m^1/2、杨氏模量为288 GPa、硬度为15.1 GPa。样品的Weibull模数为4.5, 1200℃下样品断裂强度达125 MPa。同时还测定了不同载荷下Mg_0.27Al_2.58O_3.73N_0.27透明陶瓷的硬度与不同载荷加载速率下的断裂强度。研究表明MgAlON光学透明陶瓷机械性能介于AlON和MgAl₂O₄之间。     

The affect of densification and dehydroxylation on the mechanical properties of stoichiometric hydroxyapatite bioceramics

This paper reports the effects of processing densification on the mechanical properties of hydroxyapatite bioceramics. Densification of synthetic hydroxyapatite is conducted in the range 1000-1300 °C. X-ray diffraction and SEM microscopy are used to check the microstructure transformations. Vickers hardness, toughness and Young's modulus are analyzed versus the density and grain size. The sintering temperature and the particle size influence strongly the densification and the resulting mechanical properties. In addition, the critical sintering temperature appears around 1200 °C and the declined strength at the temperature up to 1200 °C is found sensitive to the dehydroxylation process of hydroxyapatite.     

Fracture and flexural characterization of SiCw/SiC composites at room and elevated temperatures

The fracture and flexural behaviour of monolithic SiC and SiC-whisker reinforced SiC composites (SiC_w/SiC) has been investigated at room and elevated temperatures. Flexure and fracture tests were conducted in a four-point beam configuration at 23 °C, 800 °C and 1200 °C to study the effects of whisker reinforcements especially in respect of mechanical and thermal stability at high energy environments. Flexural strengths and fracture toughness data within the test temperature range are presented in graphical as well as in Weibull form, and experimental observations are analysed and discussed. Increase in flexural strength as well as in fracture toughness has been observed with the whisker reinforcement. However, it was found that the trend discontinues after a certain range of temperature. Post-failure analyses have been performed with the scanning electron microscope (SEM). Formation of glass phase has been observed at the whisker/matrix interface and the crack growth was found to be shifting from intergranular to transgranular with the rise in temperature. Effects of whisker reinforcement and the degradation of flexural and fracture properties at elevated temperature are investigated. Ultrasonic velocity measurements have been performed through the thickness of the untested as well as fractured specimen, and the variation in the sonic wave velocity is discussed in this paper.     

Effects of weak boundary phases (WBP) on the microstructure and mechanical properties of pressureless sintered Al2O3/h-BN machinable composites

Al₂O₃/h-BN machinable composites were fabricated by pressureless sintering at 1750 °C for 2 h in nitrogen atmosphere. The relative density of the sintered composites decreased, while the porosity increased with increasing h-BN content. By adding 20 vol.% h-BN to the composites, the porosity increased up to 16.7%. The effects of weak boundary phases (WBP), including h-BN and pores, on the microstructure, mechanical properties and machinability of the composites were investigated. The flexural strength, fracture toughness, Young's modulus and hardness of the composites decreased with increasing WBP content. When WBP content increased up to 18.8 vol.%, the composites can be machined easily by cemented carbide drills. Furthermore, the machining mechanisms of the composites were investigated using Hertzian contact tests.     

Influence of sintering temperatures on hardness and Young''s modulus of tricalcium phosphate bioceramic by nanoindentation technique

Nanoindentation experiments on tricalcium phosphate (TCP) bioceramic sintered at different temperatures were performed with a Berkovich indenter for determining hardness and elastic modulus from load and displacement data. The hardness and Young's modulus increased with the increase of sintering temperature up to 1300 °C, but the Young's modulus decreased with the further increase of sintering temperatures at 1400 and 1500 °C. X-ray diffraction (XRD) results showed that the transformation β→-TCP happened when the sintering temperature reached around 1400 °C, which contributed to the decreases of modulus at 1400 and 1500 °C. Scanning electron microscopy (SEM) results showed that the sintering effect was improved with the increase in sintering temperature.     

Fracture of hot-pressed alumina and SiC-whisker-reinforced alumina composite

The flexural strength of hot-pressed alumina and SiC-whisker-reinforced alumina composite were evaluated as a function of temperature (20 to 1400° C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 800° C, in which the failure mode was a mixture of transgranular and intergranular crack propagation. In this region, the alumina composite showed significantly higher fracture strength and toughness compared to polycrystalline alumina. Above 800° C, both materials (alumina and alumina composite) displayed a decreasing fracture strength due to the presence of subcritical or slow crack growth which occurred intergranularly. Flexural stress rupture evaluation in the temperature range 600 to 1200° C has identified the stress levels for time-dependent and time-independent failures.     

Strength and toughness of beryllium-niobium intermetallic compounds

Bend and compression strengths, fracture toughness, and high-temperature microhardness of Be---Nb intermetallic compounds were measured at temperatures up to 1200 °C. Be₁₂Nb and Be₁₇Nb₂ materials exhibited brittle behavior at temperatures below 1100 °C in bending and below 800 °C in compression. Hot isostatically pressed (HIP) Be₁₂Nb had the highest low-temperature strengths (250 MPa in bending and 2750 MPa in compression) resulting from its greater fracture toughness (K_IC = 4 MPa m^1/2) compared with the other Be---Nb materials, vacuum hot pressed (BHP) Be₁₂Nb, and HIP Be₁₇Nb₂, which had . Measured strengths for the HIP Be₁₂Nb were more than twice that measured for the VHP Be₁₂Nb or for HIP Be₁₇Nb₂. The HIP Be₁₂Nb also exhibited good high-temperature mechanical properties, having a bend strength of 250 MPa at 1200 °C, compared with less than 100 MPa for the VHP Be₁₂Nb. However, intergranular embrittlement was observed at intermediate temperatures, reducing the HIP Be₁₂Nb bend strength and fracture toughness below those measured for the other materials. HIP Be₁₇Nb₂ exhibited poor low-temperature properties, but high-temperature bend strengths of 740 MPa at 1100 °C and 400 MPa at 1200 °C were measured. Strength in compression was similar for all materials above 800 °C, decreasing sharply to about 600 MPa at 1000 °C and to 200 MPa at 1200 °C. Microhardness and indentation creep tests also revealed similar high-temperature behavior among the materials. Power-law creep exponents ranging from 4.1 to 6.6 and activation energies of 220–290 kJ mol⁻¹ were measured for the beryllides, with the HIP Be₁₂Nb having the highest activation energy for creep.     

First investigations on twin-rolled Zr59Cu20Al10Ni8Ti3 bulk amorphous alloy by mechanical spectroscopy

Zr₅₉Cu₂₀Al₁₀Ni₈Ti₃ is one among compositions of ZrCu-based alloys giving bulk amorphous material by cooling from the melt. Twin-roll casting enabling samples suitable for our inverted torsion pendulum has been processed in strips of about 0.60 mm thick.

Low temperature IF measurements have been conducted on a specimen from room temperature to −120 °C at different heating and cooling rates. IF spectra exhibit peaks at around −40 °C (cooling) and −10 °C (heating) which are sensitive to heating rates and to the number of cycle (heating and cooling). DSC measurements have also been performed to help interpret the phenomena linked to the IF peaks.     

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.     

Densification and grain growth during pressureless sintering of TiO2 nanoceramics

Anatase titania nanopowders with mean particle sizes of 7, 15, 26 and 38 nm synthesized by sol–gel method were used to sinter bulk TiO₂ nanoceramics. The relative densities and average grain sizes of the TiO₂ nanoceramics were studied as a function of the compaction pressure on green sheet, sintering temperature, and mean particle size of the starting TiO₂ nanopowders. The relative density of the TiO₂ nanoceramics increases rapidly and average grain size increases slowly with increasing sintering temperature below 800 °C. Sintering at higher temperatures above 800 °C enhances the densification of the TiO₂ nanoceramics and leads to a increase of the grain size. Bulk TiO₂ nanoceramics with an average grain size of less than 60 nm and relative density over 95% was obtained by a phase-transformation-assisted pressureless sintering at a relatively low temperature (800 °C).