Effect of in situ synthesized TiB2 on the reaction between B4C and Al in a vacuum infiltrated B4C–TiB2–Al composite

The in situ reaction equation of B₄C and TiO₂ was identified using thermodynamic calculations and XRD analysis. The optimum presintering process was determined according to investigating the effect of presintering temperature on the flexural strength and porosity of the porous B₄C–TiB₂ preform. The effect of in situ synthesized TiB₂ on the reaction products and initial reaction temperature of B₄C and Al was discussed based on DSC and XRD analysis. The results showed that the in situ synthesized TiB₂ could effectively improve the mechanical properties of the B₄C–TiB₂–Al composite, elevate the initial reaction temperature of B₄C and Al, change the reaction products, and moderate the reaction of B₄C and Al. The mechanism of reaction between B₄C and Al was discussed.     

Current-activated pressure-assisted sintering (CAPAS) and nanoindentation mapping of dual matrix composites

Titanium boride (TiB_w) whiskers are currently recognized as one of the most compatible reinforcements for titanium (Ti) that have positively affected its wear resistance and stiffness. The fracture toughness and ductility have, however, been reported to deteriorate at increased TiB_w volume fractions, mainly due to the interlocking of these brittle TiB whiskers. This article investigates the processing of dual matrix Ti–TiB_w composites, where microstructures are generated consisting of TiB_w–Ti composite regions separated by a ductile, predominantly Ti, outer matrix. This microstructural design has the potential to prevent the continuous TiB_w interlocking over the scale of the composite (at high TiB_w volume fractions), and promote improved toughness of the material. The processing of these unique composites using current-activated pressure-assisted sintering (CAPAS) is discussed in this article. The effect of processing temperature on the microstructure and hardness of Ti–TiB_w dual matrix composites is also discussed, together with a simultaneous imaging and modulus-mapping nanoindentation technique used to characterize the composites     

Microstructure–mechanical properties relations in SiC–TiB2 composite

Densification and mechanical properties (fracture toughness, flexural strength and hardness) of SiC–TiB₂ composite were studied. Pressureless sintering experiments were carried out on samples containing 0–50 vol% of TiB₂ created by in situ reaction between TiO₂, B₄C and carbon. Al₂O₃ and Y₂O₃ were used as sintering additives to create liquid phase and promote densification at sintering temperature of 1940 °C. The sintered samples were subsequently heat treated at 1970 °C. It was found that the presence of TiB₂ serves as an effective obstacle to SiC grain growth as well as crack propagation thus increasing both strength and fracture toughness of sintered SiC–TiB₂ composite. The subsequent heat treatment of sintered samples promoted the elongation of SiC matrix and further improved mechanical properties of the composite. The best mechanical properties were measured in heat-treated samples containing 12–24 vol% TiB₂. The maximum flexural strength of ∼600 MPa was obtained in samples with 12 vol% TiB₂ whereas the maximum fracture toughness of 6.6 MPa m^1/2 was obtained in samples with 24 vol% TiB₂. Typical microstructures of samples with the mentioned volume fractions of TiB₂ consist of TiB₂ particles (<5 μm) uniformly dispersed in a matrix of elongated SiC plates.     

In situ TiB2 particulate reinforced near eutectic Al–Si alloy composites

In situ TiB₂ particulate reinforced near eutectic Al–Si alloy composites fabricated by the melt reaction composing (MRC) methods have been investigated. It has been shown that minute TiB₂ particles (less than 1 μm) uniformly distribute in the eutectic structure and they are interlaced with the coralline-like eutectic Si, while there are very few TiB₂ particles in α-Al. It has been also shown that in situ TiB₂ particles can enhance the tensile strength of the Al–Si alloy matrix. The strengthening effect increases with increasing TiB₂ content. The ultimate tensile strength (UTS) at room temperature of as-cast 6%TiB₂/Al–Si–Mg composite is 296 MPa, that is a 14.7% increase over the matrix, and its elongation at fracture is 5.5%. After heat-treatment (T6), the UTS of the composites reaches 384 MPa. The strengthening mechanism has been discussed.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Effect of shot peening on surface mechanical properties of TiB<Subscript>2</Subscript>/Al composite

The influence of shot peening on the surface mechanical properties of the TiB₂/6351Al composite has been investigated. The microstructures were determined by X-ray diffraction line profile analysis. The results showed that the increment of hardness was about 50% in the top surface layer. The matrix proof stress σ _0.2 of the shot peened surface had been increased by 27% and the whole strength increment was about 21% by considering the contribution of the reinforcements. The domain size and the dislocation density in the strengthened surface were 55 nm and 3.67 × 10¹⁵ m⁻², respectively. The mechanical properties improvement of the modified surface was partially due to the reinforcements but mainly due to the fine domains, high value of dislocation density induced by shot peening.     

Fracture toughness assessment of silicon carbide-based ceramics and particulate-reinforced composites

The fracture toughness of sintered silicon carbide (α-SiC) and silicon carbide reinforced with particulate titanium diboride (TiB₂/SiC) has been evaluated using specimens in bending containing chevron notches and through-thickness precracks at ambient and elevated temperatures in air and in vacuum. Fracture toughness values measured from through-thickness precracked test-pieces are lower at all test temperatures. The particulate reinforcement has been shown to toughen the matrix significantly at room temperature only. At the test temperature of 1200°C the difference in toughness between the two materials is reduced and increasing the temperature to 1600°C further reduces this difference, to the extent that the two materials have values of fracture toughness which are indistinguishable. This provides strong evidence that the dominant toughening mechanism in the composite is the effect of thermal residual stresses which are relieved as the temperature is increased. Fractographic observations suggest that the bonding between the SiC and TiB₂ particulate is relatively weak because interfacial decohesion of particles is observed at all test temperatures. Nevertheless, surface roughness measurements indicate that there may also be a contribution to the toughness from increased crack deflection in the composite material at room temperature only.     

Processing and Characterization of In Situ (TiC–TiB2)p/AZ91D Magnesium Matrix Composites

In this paper, a practical and cost‐effective processing route, in situ reactive infiltration technique, was utilized to fabricate magnesium matrix composites reinforced with a network of TiC–TiB₂ particulates. These ceramic reinforcement phases were synthesized in situ from Ti and B₄C powders without any addition of a third metal powder such as Al. The molten Mg alloy infiltrates the preform of (Ti_p + B₄C_p) by capillary forces. The microstructure of the composites was investigated using scanning electron microscope (SEM)/energy dispersive X‐ray spectroscopy (EDS). The compression behavior of the composites processed at different conditions was investigated. Also, the flexural strength behavior was assessed through the four‐point‐bending test at room temperature. Microstructural characterization of the (TiB₂–TiC)/AZ91D composite processed at 900 °C for 1.5 h shows a relatively uniform distribution of TiB₂ and TiC particulates in the matrix material resulting in the highest compressive strength and Young's modulus. Compared with those of the unreinforced AZ91D Mg alloy, the elastic modulus, flexural and compressive strengths of the composite are greatly improved. In contrast, the ductility is lower than that of the unreinforced AZ91D Mg alloy. However, this lower ductility was improved by the addition of MgH₂ powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates.     

Adiabatic shear failure of high reinforcement content aluminum matrix composites

Dynamic failure behaviors of high reinforcement content TiB₂/Al composites were experimentally investigated using split Hopkinson pressure bar (SHPB). The TiB₂/Al composites showed high flow stresses and good plastic deformation ability at high strain rates. Adiabatic temperature rise decreased the flow stresses of TiB₂/Al composites, which was verified by the prediction of Johnson–Cook model. While the predictions by Cowper–Symonds model exhibited obvious strain hardening characteristic, the values of which were much higher than those of the Johnson–Cook model and the experimental. The composites were failed macroscopically in brittle fracture and some phase transformation bands were found on the shearing surfaces. The dynamic failure behavior of TiB₂/Al composites was predominated by the formation of adiabatic shear bands.     

Low-cycle fatigue behavior of in situ TiB2/Cu composite prepared by reactive hot pressing

Copper-based composite reinforced with in situ TiB₂ particulates was prepared through reactive hot pressing of Ti, B and Cu powders. The formation of in situ TiB₂ particulates was verified by the X-ray diffraction technique. Tensile test showed that fine TiB₂ particulates were very effective to increase the tensile and yield strengths of copper at the expense of tensile ductility. Strain-controlled low-cycle fatigue measurements demonstrated that the in situ TiB₂/Cu composite exhibited essentially stable cyclic stress response behavior under small total strain amplitudes of 0.1–0.3%. However, this composite exhibited slight cyclic hardening under strain amplitude of 0.4%. Such cyclic hardening was more pronounced at a total strain amplitude to 0.6% due to the formation of dislocation cells and networks. Finally, the fatigue life data of the in situ TiB₂/Cu composite can be described by the Coffin–Manson equation.     

Microstructure and mechanical behavior of AlSiCuMgNi piston alloys reinforced with TiB2 particles

AlSiCuMgNi piston composites reinforced with in-situ TiB₂ particles were fabricated by mixing salts reaction process successfully. Microstructures of the composites were observed by mean of scanning electron microscope (SEM) and transmission electron microscope (TEM). X-ray diffraction (XRD) was used to identify the phases in the composites. TiB₂ reinforcement grows in equiaxed or near equiaxed shape and the interfaces between reinforcements and matrix are clear. Compared with the matrix alloys, the composites show an obvious aging peak and an incubation time in the hardness. The aging is accelerated in the composites reinforced with TiB₂. At room temperature, the ultimate tensile strength (UTS) of the composites increases as the percentage of TiB₂ reinforcement increases. When the temperature is beyond 250°C, the ultimate tensile strength of the piston composites decreases sharply. The fracture surfaces of the piston composites are analyzed.     

In situ reacted TiB2-reinforced mullite

In situ formation of TiB₂ in mullite matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed and pressureless-sintered samples, in addition to TiB₂, TiC was also found to be dispersed phases in mullite matrix. However, in the case of pressurelesssintered samples, mullite/TiB₂ composite with 98% relative density can be obtained through a preheating step held at 1300 °C for longer than 3 h and then sintering at a temperature above 1600 °C. Hot-pressed composite containing 30 vol% TiB₂ gives a flexural strength of 427 MPa and a fracture toughness of 4.3 MPam^1/2. Pressureless-sintered composite containing 20 vol% TiB₂ gives a flexural strength of 384 MPa and a fracture toughness of 3.87 MPam^1/2.     

Synthesis and characterization of Al (Al2O3–TiB2/Fe) nanocomposite by means of mechanical alloying and hot extrusion processes

Synthesis and characterization of Al–(Al₂O₃–TiB₂/Fe) nanocomposite by means of mechanical alloying and hot extrusion processes was the goal of this study. For this regards, mechanical alloying was done in two steps; formation of Al₂O₃–TiB₂/Fe reinforcements and preparation of Al-base nanocomposite. Results showed that Al₂O₃–TiB₂/Fe nanocomposite powders can synthesis by mechanical alloying and subsequent heat treatment at 700 °C. Hot extrusion of powder samples lead to preparation of fully dense Al-base nanocomposite. With increasing the amount of complex reinforcements, the compression strength was increased and reached to 560 MPa. Consolidated samples show good ductility related to the nature of Al₂O₃–TiB₂/Fe reinforcements.     

In situ reacted TiB2-reinforced alumina

In situ formation of TiB₂ in Al₂O₃ matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed samples, in addition to TiB₂, TiC and Al₂TiO₅ were also found to be dispersed phases in Al₂O₃ matrix. However, in the case of pressureless-sintered samples, pure Al₂O₃/TiB₂ composite with > 99% relative density can be obtained through a preheating step held at 1300°C for longer than 30 min and then sintering at a temperature above 1500°C. Pressureless-sintered composite containing 20vol% TiB₂ gives a flexural strength of 580 MPa and a fracture toughness of 7.2 MPa m^1/2.     

Effect of different reinforcements on composite-strengthening in aluminium

In the development of metal-matrix composites, reinforcements of aluminium and its alloys with ceramic materials has been pursued with keen interest for quite sometime now. However, a systematic comparison of the effect of different reinforcements in powder-processed aluminium and its alloys is not freely available in the published literature. This study examines the influence of SiC, TiC, TiB₂ and B₄C on the modulus and strength of pure aluminium. B₄C appears slightly superior as a reinforcement when comparing the effect of SiC, TiC, B₄C and TiB₂ on specific modulus and specific strength values of composites. However, TiC appears to be a more effective reinforcement, yielding the best modulus and strength values among those considered in this study. The differences in thermal expansion characteristics between aluminium and the reinforcements do not seem to explain this observation. The other advantage of TiC is that it is economically a more viable candidate as compared to B₄C and TiB₂ for reinforcing aluminium alloys. It is suggested that the superior effect of TiC as a reinforcement is probably related to the high integrity of the bond at the Al-TiC interface.     

TiB2 and TiC stainless steel matrix composites

Stainless steel matrix composites reinforced with TiB₂ or TiC particulates have been in situ produced through the reactive sintering of Ti, C and FeB. X-ray diffraction analysis confirmed the completion of reaction. The TiB₂, TiC and steel were detected by X-ray diffraction analysis. No other reaction product or boride was found, indicating the stability of TiB₂ and TiC in steel matrix. The SEM micrographs revealed the morphology and distribution of in situ synthesized TiB₂ and TiC reinforcements in steel matrix. During sintering the reinforcements TiB₂ and TiC grew in different shapes. TiB₂ grew in hexagonal prismatic and rectangular shape and TiC in spherical shape.     

The relationship between TiB<Subscript>2</Subscript> volume fraction and fatigue crack growth behavior in the in situ TiB<Subscript>2</Subscript>/A356 composites

The relationship between TiB₂ volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB₂ particles has been investigated. The mechanisms of crack propagation in the TiB₂/A356 composites were also discussed. The results show that the 3 vol% TiB₂/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB₂/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB₂/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB₂ volume fraction. Fractographies reveal that an increase in TiB₂ volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB₂ particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB₂ particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB₂ increases with the increase in TiB₂ volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB₂ particles propagate and coalesce, and then accelerates the crack growth in TiB₂/A356 composites.     

Effect of tungsten addition on the microstructure and tensile properties of in situ TiB2/Fe composite produced by vacuum induction melting

In situ TiB₂ particulate reinforced Fe-based composite was produced by vacuum induction melting (VIM) technique. The effect of tungsten element on the microstructure and tensile properties of the composite was investigated. The results show that the tungsten can dissolve into the TiB₂ particulates and the segregation of TiB₂ is reduced. Meanwhile, with the addition of 3.0 wt.% tungsten, the composite is solid strengthened and an optimal tensile property can be obtained. The yield strength (YS), ultimate tensile strength (UTS) and elongation to rupture (Er) of the composite reach as high as 360 MPa, 690 MPa and 18.5%, respectively. The fracture morphologies also indicate that the addition of 3.0 wt.% tungsten results in the increase of plastic fracture.     

Shock Wave Fabricated Ceramic-Metal Nozzles

Shock compaction was used in the fabrication of high temperature ceramic-based materials. The materials' development was geared towards the fabrication of nozzles for rocket engines using solid propellants, for which the following metal-ceramic (cermet) materials were fabricated and tested: B₄C-Ti (15 vol.-%), B₄C-Al, and TiB₂-Al, with an Al content typically between 15–20 vol.-%. Here, the B₄C-Ti was only shock-compacted, while the other two cermets were shock compacted followed by melt infiltration with Al.The materials were subjected to gradually more severe testing conditions. Slabs of the materials were first tested for thermal shock resistance in an acetylene flame, followed by testing in the exhaust gas stream of a rocket propellant, and thereafter as a cylindrical insert in a nozzle of TZM alloy. The B₄C-Ti composite showed erosion and cracking after the first test in the propellant flame, while the B₄C-Al composite failed the insert tests. The TiB₂-Al composite performed well under all conditions. A venturi nozzle of that material was formed during compaction. This real, shaped nozzle was shown to function well, even during repeated 3–6 s tests. This could be explained by the resistance of TiB₂ to molten Al, the high thermal conductivity of the TiB₂-Al cermet and the in situ formation of a protective layer, consisting mainly of Al₂O₃.