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.     

Mechanical properties of 3D fiber reinforced C/SiC composites

High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm⁻³ after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m^1/2 and 12.0 kJ·m⁻², respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m⁻² measured by Charpy impact tests.     

Densification and strengthening of silver-reinforced hydroxyapatite-matrix composite prepared by sintering

Ductile-phase reinforcement of hydroxyapatite (HA) was achieved by addition of silver particulates (5–30 vol %) in HA powder and subsequent sintering of HA–Ag powder compacts. A composite made by sintering 10 vol % Ag and the balance HA at 1200 °C for 1 h in air had flexural strength of 75±7 MPa, which was almost double that of pure HA sintered under an identical condition. The density of HA-10 vol % Ag composite was 90±2% of the theoretical density (as calculated from the rule of mixture) and was lower than that (98.7±0.4%) of pure HA sintered at a similar condition. The X-ray diffraction pattern of the composite did not indicate any decomposition of HA or any reaction between HA and Ag. Ag in the composite melted during sintering, but, due to poor wetting, did not spread in between HA particles. The addition of Ag reduced densification and grain growth during sintering of HA–Ag composites. Indentation cracks in the composites went around Ag inclusions and often stopped at Ag inclusions. The increase in the flexural strength of the composites was thought to be due to crack-bridging and crack-arrest by silver particles.     

Microstructure and mechanical properties of an HfB2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB₂–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB₂ + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min⁻¹ heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO₂. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB₂. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.     

The effects of multiwalled carbon nanotubes on the hot-pressed 3 mol% yttria stabilized zirconia ceramics

The bulk composites of 3 mol% yttria stabilized zirconia ceramics reinforced by multiwalled carbon nanotubes were prepared by ball milling, spray-drying and hot-pressing processes. The effects of MWCNTs’ contents and heterocoagulation pretreatment on the mechanical properties of 3Y–ZrO₂/MWCNTs’ composites were investigated at room temperature. Experimental results showed that the heterocoagulation pretreatment played a vital role in homogeneous dispersion of MWCNTs in the ceramic matrix. The flexural strength of 989.8 ± 20.0 MPa and fracture toughness of 5.77 ± 0.06 MPa M^1/2 were obtained for the composite with 1.0 wt.% of MWCNTs’ content, which were 135.3 MPa (or 8.4%) higher in flexural strength and 0.92 MPa M^1/2 (or 21.1%) higher in fracture toughness than those of blank 3Y–ZrO₂, respectively. The mechanisms of strengthening and toughening of the composites could be attributed to the synergic effects of bridging, pulling out of MWCNTs and their promotive effects on the phase transformation of the ceramics.     

Microstructure and high temperature deformation characteristics of sol–gel derived aluminium titanate–mullite composites

Aluminium titanate (AT)–mullite composites with varying compositions were processed by sol–gel technique. The influence of mullite on the microstructure and creep deformation of AT–mullite composites was investigated. In the composites mullite addition was varied from 0 to 100 vol.%. The AT-80 vol.% mullite composite sintered at 1600 °C resulted in fine-grained microstructure with an average grain size of 2.5 μm. From the steady-state creep analysis of the different AT–mullite composites, the activation energies for the creep deformation and stress exponents were determined. The activation energies in the range 655–874 kJ mol⁻¹ were obtained for various the sol–gel derived AT–mullite composites. Similarly stress exponent values were found in the range 1.5–1.9.     

Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si₃N₄–SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄–SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10⁻³ at a resonant frequency of 14 GHz.     

Hot pressing of SiC-TiC composites

Sintering behaviour during hot pressing of SiC-TiC composite ceramics has been investigated with special emphasis on the effect of various processing parameters on the density and mechanical properties of the sintered body. At hot pressing temperatures greater than 2000° C, significant densification occurred in SiC-50 wt%TiC (–0.5 wt%B-1 wt%C) composites. The room temperature flexural strength of the sintered body increased with the hot pressing temperature up to 2000° C and reached a highest value of 710 MPa in accordance to the variation of density with temperature. In sintering of composites without additives, densification was enhanced with the addition of up to 25 wt%TiC, with relative densities higher than 98% observed when hot pressing at 2150° C for 2h.     

Effects of copper addition on the sintering behavior and mechanical properties of powder processed Al/SiC<Subscript><Emphasis Type="Bold">p</Emphasis></Subscript> composites

The effect of copper addition on powder processed Al-10 vol% SiC composites was studied in regards to their sintering responses. Copper was mixed with aluminum powder either as elemental powders or as the coated layer on SiC particles. After sintering at 600°C for 1 h, Al-SiC composites with no copper addition showed little densification. It also demonstrated very low bend strengths of 49 and 60 MPa, indicating poor bonding between the powders in the sintered composite. The addition of 8% copper to the Al/SiC system effectively improved the sintering response, producing over 95% theoretical density, a bend strength of 231 MPa with the copper coated SiC, and a 90% density with over 200 MPa bend strength with the admixed copper.The as-sintered microstructures of the Al–SiC composites clearly revealed particle boundaries and sharp pores, indicating that only a limited neck growth occurred during sintering. In the case of Al–Cu–SiC composites, however, a liquid phase was formed and spread through particle boundaries filling the interfaces or voids between SiC particles and the matrix powders. The coated copper on SiC particles produced a somewhat better filling of the interface or voids, resulting in a little more densification and better sintered strength. Since the solubility of copper in aluminum is less than 2% at the sintering temperature, the alloying of copper in the aluminum matrix was limited. Most of the copper added was dissolved in the liquid phase during the sintering and precipitated as CuAl₂ phase upon cooling.     

Hot-pressed ZrB2–SiC–YSZ composites with various yttria content: Microstructure and mechanical properties

ZrB₂–10 vol%SiC–20 vol%YSZ composites were prepared by hot-pressed sintering with yttria content ranging from 2 mol% to 8 mol% in YSZ. The phase constitution, microstructure and mechanical properties of the composites were found to be strongly dependent on the yttria content. The average grain size became bigger for the composites with higher yttria content. When the yttria content was below 3 mol%, there is no cubic zirconia in the polished surface of composites, and the flexural strength of the composites was above 740 MPa. With the increase in yttria content, the fracture toughness fell down from 6.4 MPa m^1/2 to 5.6 MPa m^1/2. Vickers’ hardness of the hot-pressed composites varied above 18 GPa without obvious effect of the yttria content.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.     

Partially alloyed filler sheet for brazing of Ti and its alloys fabricated by spark plasma sintering method

Partially alloyed filler metals in the form of powders and laminated foils were used for the brazing of Ti and Ti alloys to lower the manufacturing cost. In this study, by using a raw elemental powder mixture, a multi-component filler sheet with a nominal composition of 37.5Ti–37.5Zr–15Cu–10Ni was fabricated using a Spark Plasma Sintering (SPS) machine in the temperature range from 650 °C to 785 °C for 1 min. As the sintering temperature was increased from 650 °C to 750 °C, the bending strength of the sheets tended to rise, but the bending strength at 785 °C was drastically reduced. The melting range of the sheets became similar to that of the as-cast alloy. The sheets sintered at 750 °C showed the highest bending strength of 259 MPa, which was much higher than that of the as-cast material, and the melting range of this sheet was from 800 °C to 852 °C. The relatively high strength of the sheet was due to the remaining elemental powders such as Ti or Zr, but the brittle intermetallics, such as Ti₂Cu and (Ti,Zr)₂Ni Laves phases, formed in the sheet during the sintering process deteriorated its mechanical strength. The partially developed eutectic phase between the remaining Ti or Zr powder caused the sheet to exhibit melting behavior similar to that of the as-cast alloy. The brazability of the sheet sintered at 750 °C was examined with commercially pure Ti at 870 °C for 5–60 min. The tensile strength of the Ti joint brazed for 30 min was 431 MPa, which was close to that of the base metal.     

Enhanced properties of alumina–aluminium titanate composites obtained by spark plasma reaction-sintering of slip cast green bodies

Full dense alumina + 40 vol.% aluminium titanate composites were obtained by colloidal filtration and fast reaction-sintering of alumina/titania green bodies by spark plasma sintering at low temperatures (1250–1400 °C). The composites obtained had near-to-theoretical density (>99%) with a bimodal grain size distribution. Phase development analysis demonstrated that aluminium titanate has already formed at 1300 °C. The mechanical properties such as Vickers hardness, flexural strength and fracture toughness of bulk composites are significantly higher than those reported elsewhere, e.g. the composite sintered at 1350 °C show values of about 24 GPa, 424 MPa and 5.4 MPa m^1/2, respectively. The improved mechanical properties of these composites are attributed to the enhanced densification and the finer and more uniform nanostructure achieved by non-conventional fast sintering of slip-cast dense green compacts.     

Microstructure and mechanical properties of nanocrystalline high strength Al–Mg–Si (AA6061) alloy by high energy ball milling and spark plasma sintering

In the present paper, the microstructure and mechanical properties of nanostructured Al–Mg–Si based AA6061 alloy obtained by high energy ball milling and spark plasma sintering were reported. Gas atomized microcrystalline powder of AA6061 alloy was ball milled under wet condition at room temperature to obtain nanocrystalline powder with grain size of 30 nm. The nanocrystalline powder was consolidated to fully dense compacts by spark plasma sintering (SPS) at 500 °C. The grain size after SPS consolidation was found to be 85 nm. The resultant SPS compacts exhibited microhardness of 190–200 HV_100 g, compressive strength of 800 MPa and strain to fracture of 15%.     

Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide–oxide ceramic composite at 1200 °C

The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide–oxide ceramic composite was evaluated in laboratory air at 1200 °C. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tensile tests conducted at loading rates of 0.0025 and 25 MPa/s revealed a strong effect of rate on the stress–strain behavior as well as on the ultimate tensile strength (UTS), elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses 25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress–strain behavior was an artifact of incomplete processing of fibers in the as-received material. The unusual material response in the 0–30 MPa stress range was further investigated in creep tests conducted with the applied stresses ≤26 MPa. Negative creep (i.e. decrease in strain under constant stress) was observed. Porosity measurements indicate that a decrease in matrix porosity and matrix densification may be taking place in the N720/A composite exposed to 1200 °C at stresses <30 MPa for prolonged periods of time.     

Low-temperature compaction of Ti–6Al–4V powder using equal channel angular extrusion with back pressure

Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride–dehydride Ti–6Al–4V powder at temperatures ≤400 °C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100–400 °C, and in excess of 98% at 400 °C with applied back pressure ≥175 MPa. ECAE compaction at 20 °C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 °C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 °C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412 HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure.     

炭纤维针刺预制体增强C/SiC复合材料的制备与性能研究

以炭纤维复合网胎针刺织物为预制体, 采用“化学气相渗透法+先驱体浸渍裂解法”(CVI+PIP)混合工艺, 制备了C/SiC陶瓷复合材料; 研究了针刺预制体的致密化效率以及复合材料的微观结构和力学性能, 并与目前常用的三维编织C/SiC复合材料和预氧丝针刺织物增强C/SiC复合材料进行了对比. 结果表明, 针刺预制体的致密化效率明显高于三维编织预制体, 在相同致密工艺条件下, 炭纤维针刺织物增强复合材料和预氧丝针刺织物增强复合材料的密度分 别达到2.08和2.02g/cm³, 而三维编织预制体增强复合材料的密度仅为1.81g/cm³. 炭纤维针刺复合材料的力学性能高于预氧丝针刺复合材料, 弯曲强度和剪切强度分别达到237和26MPa.     

Improvement of SiCf/SiC density by slurry infiltration and tape stacking

SiC fiber-reinforced SiC–matrix ceramic composites (SiC_f/SiC) were fabricated by vacuum infiltration of a SiC slurry into Tyranno™-SA grade-3 fabrics coated with a 200 nm-thick pyrolytic carbon (PyC) layer followed by hot pressing using a transient eutectic-phase. The density of the composite was improved using a special infiltration apparatus with a pressure gradient and alternating tape insertion between fabrics. Their overall properties were compared with those of monolithic SiC and composite containing chopped fibers. Although the density of the composites decreased with increasing fiber fraction, SiC_f/SiC containing 50 vol.% fibers had a density of 3.13 g/cm³, which is the highest reported thus far. The composites containing continuous fibers had a maximum flexural strength of 607 MPa and a step increase in the stress–displacement behavior during the three-point bending test due to fiber reinforcement, which was not observed in the monolith.     

Structure and oxidation resistance of plasma sprayed Ni–Si coatings on carbon steel

Ni–Si coatings consisting of mainly NiSi₂ and NiSi were deposited on a carbon steel by air plasma spraying. Isothermal oxidation tests of the carbon steel substrates with the Ni–Si coatings at 500–800 °C have been carried out. The result indicated that a protective SiO₂-based oxide scale was formed on the surface of the coatings after oxidation. On the other hand, during oxidation, phase transformation occurred among the NiSi₂, NiSi and Ni₂Si phases constructing the Ni–Si coatings. This was caused by the extraction of silicon from the silicides and the reformation of silicides at the silcide/Si-blocks interface. Above 700 °C, the outward diffusion of iron and carbon became very fast and consequently decarburization happened at the coating/substrates interface, which induced the formation of pores in the substrates near the interface. In addition, grain boundary oxidation of Cr in the steel substrate was observed above 700 °C.