Thermal stability of in situ synthesized (TiB + La2O3)/Ti composite

Thermal stability of in situ synthesized (TiB + La₂O₃)/Ti composite is investigated. The phase analysis is identified by X-ray diffraction. Microstructure of the melted and forged titanium matrix composites (TMCs) after heat treatment is characterized by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The room temperature tensile properties after an additional thermal exposure at 873 K, 923 K or 973 K for 100 h are tested. After the thermal exposure, the strength of specimen increases and ductility decreases. This is attributed to precipitation of ordered α₂ phase (Ti₃Al) and S₁ (silicide) in the titanium matrix composites after the thermal exposure.     

Microstructure and Properties of 3.5 vol.%TiBw/Ti6A14V Composite Tubes Fabricated by Hot-hydrostatic Extrusion

Tubes of 3.5 vol.%TiB whiskers reinforced Ti6AI4 V matrix composites(TiBw/Ti6AI4V) were successfully fabricated by a two-step hot-hydrostatic extrusion process:β extrusion at 1100 ℃ and subsequent near-βextrusion at 950 ℃.The dimensions of tubes were about 7 mm in diameter and 2 mm in thickness.A refined basket-weave structure in Ti6AI4 V matrix was achieved at ambient temperature after the extrusion process.Besides,the original network structure formed by TiB whiskers synthesized was broken,while the TiB whiskers were preferentially aligned in the extruding direction.Meanwhile,a fibrous texture was evolved finally,resulting from partial dynamic recrystallization during the β extrusion and the involvement of a phase during the near-β extrusion.The tensile and compressive tests results showed that both the strength and ductility of the tubes were significantly improved.In particular,the tubes exhibited good mechanical properties at elevated temperatures.     

Microstructure and Properties of 3.5 vol.% TiBw/Ti6Al4V Composite Tubes Fabricated by Hot-hydrostatic Extrusion

Tubes of 3.5 vol,% TiB whiskers reinforced Ti6Al4V matrix composites （TiBw/Ti6Al4V） were successfully fabricated by a two-step hot-hydrostatic extrusion process： （3 extrusion at 1100 ℃ and subsequent near-β extrusion at 950℃. The dimensions of tubes were about 7 mm in diameter and 2 mm in thickness. A refined basket-weave structure in Ti6Al4V matrix was achieved at ambient temperature after the extrusion process. Besides, the original network structure formed by TiB whiskers synthesized was broken, while the TiB whiskers were preferentially aligned in the extruding direction. Meanwhile, a fibrous texture was evolved finally, resulting from partial dynamic recrystallization during the β extrusion and the involvement of α phase during the near-β extrusion. The tensile and compressive tests results showed that both the strength and ductility of the tubes were significantly improved. In particular, the tubes exhibited good mechanical properties at elevated temperatures.     

Effects of extrusion and heat treatment on the microstructure and tensile properties of in situ TiBw/Ti6Al4V composite with a network architecture

In situ TiB whisker reinforced Ti6Al4V (TiBw/Ti64) composites with a network architecture were extruded and heat treated in order to further improve their mechanical properties. The microstructure results show that the equiaxed network architecture was extruded to column network architecture and TiB whisker to alignment distribution. The transformed β phase is formed and the residual stress generated during extrusion obviously decreases after water quenching and aging processes. The tensile test results show that the strength, elastic modulus and ductility of the composites can be significantly improved by the subsequent extrusion, and then, the strength can be further improved by water quenching and aging processes after hot extrusion deformation. The elastic modulus of the as-sintered composites with a novel network microstructure follows the upper bound of Hashin-Shtrikman (H-S) theory before extrusion, while that of the as-extruded composites with a column network microstructure agrees well with the prediction from Halpin-Tsai equation.     

Effects of sintering parameters on the microstructure and tensile properties of in situ TiBw/Ti6Al4V composites with a novel network architecture

In order to better understand the relationship of processing–structure–mechanical properties of in situ TiB whisker reinforced Ti6Al4V (TiBw/Ti64) composites with a novel network architecture, the effects of sintering parameters on the microstructure and tensile properties of the composites were investigated. TiB whiskers with the highest aspect ratio and the coarsest whiskers were obtained at 1100 °C and 1200 °C due to the skips of whisker growth speeds along the [0 1 0] direction and the [0 0 1] and [1 0 0] directions, respectively. Additionally, TiB whisker with a claw-like structure can be synthesized from one TiB₂ polycrystal parent. The quasi-continuous network architecture of TiBw/Ti64 composites can be achieved at higher sintering temperatures more than 1200 °C. The prepared composites with the quasi-continuous network architecture exhibit a superior combination of tensile properties.     

激光熔化沉积制备TiB+TiC/Ti6Al4V复合材料微观组织研究

激光熔化沉积制备的钛合金微观组织中常出现异常粗大的柱状晶粒,限制了其在复杂承力结构件方面的应用。为降低原始晶粒的尺寸,提高合金强度,本文基于原位自生反应原理,在Ti6Al4V粉末中添加少量的颗粒增强体B₄C得到混合粉末,并通过激光熔化沉积工艺制备出熔覆层以及多层钛基复合材料(TMC)。利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)和显微硬度仪等测试手段研究了B₄C的添加对Ti6Al4V合金微观组织的影响规律,并对其作用机制进行了分析。研究表明:B₄C的添加降低了原始β晶粒的尺寸,并强化了合金基体。当添加1wt.% B₄C颗粒时,晶粒的外延生长得到有效抑制,原始β晶粒开始出现柱状晶向等轴晶转变(CET)的趋势,柱状晶粒尺寸由原始的平均600 μm减小到50 μm。同时,B₄C与钛基体发生原位反应形成的混杂增强相TiB和TiC富集在晶界,构成三维网状结构,不仅限制了晶内α相的生长,同时也起到了第二相强化的作用,使基体的硬度较基材提高了15 %以上。     

Research on heat treatment of TiBw/Ti6Al4V composites tubes

Heat treatment with different parameters were performed on the hot-hydrostatically extruded and swaged 3.5 vol.% TiBw/Ti6Al4V composites tubes. The results indicate that the primary α phase volume fraction decreases and transformed β phase correspondingly increases with increasing solution temperatures. The α + β phases will grow into coarse α phases when the aging temperature is higher than 600 °C. The hardness and ultimate tensile strength of the as-swaged TiBw/Ti6Al4V composite tubes increase with increasing quenching temperatures from 900 to 990 °C, while they decrease with increasing aging temperatures from 550 to 650 °C. A superior combination of ultimate tensile strength (1388 MPa) and elongation (6.1%) has been obtained by quenching at 960 °C and aging at 550 °C for 6 h. High temperature tensile tests at 400–600 °C show that the dominant failure modes at high temperatures also differ from those at room temperature.     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

Microstructure evolution and mechanical properties in ultrafine grained Al/TiC composite fabricated by accumulative roll bonding

In this study, the influence of nano-TiC particle on microstructure development and mechanical properties of Al/TiC composite fabricated by accumulative roll bonding (ARB) was considered to investigate. Microstructural characterization by electron backscatter diffraction (EBSD) system proved that the grain size decreased to around 200 nm and the TiC reinforcement particles were uniformly distributed in the Al matrix by 7-cycle of the ARB process. It is also found that presence of the TiC particles could accelerate grain refinement. Uniaxial tensile test exhibited that yield and ultimate tensile strength significantly improved more than four times in the 7-cycle ARB processed Al/TiC composite compared with the annealed aluminum specimen which used as the starting material. In addition, the obtained results demonstrated that adding the TiC reinforcement particles could improve the yield strength of the 7-cycle ARB processed Al sheet about 40 percentage.     

Influence of TiBw volume fraction on microstructure and high-temperature properties of in situ TiBw/Ti6Al4V composites with TiBw columnar reinforced structure fabricated by pre-sintering and canned extrusion

Tailoring TiBw volume fraction was utilized in TiBw/Ti6Al4V composites to pursue performance optimization for potential high-temperature applications. Detailed investigations focused on the influence of TiBw volume fraction on microstructure and its correlations with mechanical properties mainly at high temperatures ranging from 500 °C to 700 °C. Highly aligned TiB whiskers along extrusion direction caused the formation of TiBw columnar reinforced structure, which was composed of ductile TiBw-poor interior regions and hardened TiBw-rich boundaries. Refined prior β phases arising from dynamic recrystallization led to the size reduction in α colonies. This tendency commonly accompanied with the formation of equiaxed α phase was evidently enhanced by the restriction of increasing TiB whiskers at the boundaries. Much better strengthening of TiB whiskers was achieved on the premise of good interfacial bonding with Ti matrix at high temperatures, and the strengthening depended linearly on their volume fractions, with strength increments of about 44.3 MPa/vol.% at 500 °C, and 27.5 MPa/vol.% at 600 °C, yet merely 6.3 MPa/vol.% at 700 °C. Attributed to the softening of Ti matrix and the crack retardation of ductile TiBw-poor regions by raising temperature, enhanced ductility was negatively correlated with TiBw volume fraction below 600 °C, but positively at 700 °C under the possible grain boundary sliding inspired by ample equiaxed α phase.     

In situ polymerized nanocomposites: Polystyrene/CNT and Poly(methyl methacrylate)/CNT composites

Carbon nanotubes (CNTs) were incorporated into polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices via in situ emulsion and emulsion/suspension polymerization methods. The polymerizations were carried out using various initiators, surfactants, and carbon nanotubes to determine their influence on polymerization and on the properties of the composites. The loading of CNTs in the composites varied from 0 to 15 wt.%, depending on the CNTs used. Morphology and dispersion of the CNTs were analyzed by transmission and scanning electron microscopy techniques. The dispersion of multi-walled carbon nanotubes (MWCNT) in the composites was excellent, even at high CNT loading. The mechanical properties, and electrical and thermal conductivities, of the composites were also analyzed. Both electrical and thermal conductivities were improved.     

Investigation of carbon nanotube reinforced aluminum matrix composite materials

We have increased the tensile strength without compromising the elongation of aluminum (Al)–carbon nanotube (CNT) composite by a combination of spark plasma sintering followed by hot-extrusion processes. From the microstructural viewpoint, the average thickness of the boundary layer with relatively low CNT incorporation has been observed by optical, field-emission scanning electron, and high-resolution transmission electron microscopies. Significantly, the Al–CNT composite showed no decrease in elongation despite highly enhanced tensile strength compared to that of pure Al. We believe that the presence of CNTs in the boundary layer affects the mechanical properties, which leads to well-aligned CNTs in the extrusion direction as well as effective stress transfer between the Al matrix and the CNTs due to the generation of aluminum carbide.     

Synthesis of uniformly dispersed carbon nanotube reinforcement in Al powder for preparing reinforced Al composites

In order to obtain homogeneously dispersed carbon nanotube (CNT) reinforcement with well structure in Al powder, a novel and simple approach was developed as a means of overcoming the limits of traditional mixing methods. This process involves the even deposition of Ni catalyst onto the surface of Al powder by impregnation route with a low Ni content (0.5 wt.%) and in situ synthesis of CNTs in Al powder by chemical vapor deposition. The in situ synthesized CNTs with well-crystallized bamboo-like structure in the composite powders can obviate the reaction with Al below 1000 °C. The feasibility of fabricating CNT/Al composites with high mechanical properties using the as-prepared composite powders was proved by our primary test, which indicated that the compressive yield stress and elastic modulus of 1.5 wt.%-CNT/Al composites synthesized by hot extrusion are 2.2 and 3.0 times as large as that of the pure Al matrix.     

Friction stir welding of thick plates of aluminum alloy matrix composite with a high volume fraction of ceramic reinforcement

The microstructure and mechanical properties of joints conducted by friction stir welding, FSW, at different rotational speeds in thick plates of a composite material with a high volume fraction of reinforcement, namely 2124Al/25vol%SiC_p, are studied. Original particle-free regions vanish during the stirring process, leading to a homogeneous particle distribution. Occasional breakage of some large particles occurs. Tunnel defects appear at low rpm, and disappear at high rotational speeds. The size of the thermo mechanically affected zone, TMAZ, increases with increasing rpm. Ductility of the welds in the range of 10–15% is achieved in compression tests whereas a rather brittle behavior is obtained in tension. A strength difference, SD, effect between compression and tensile test is obtained. This accounts for the little detrimental effect of the FSW process on the matrix–reinforcement interface. The SD effect is attributed to the presence of a microscopic residual stress.     

Fabrication of (TiB/Ti)-TiAl composites with a controlled laminated architecture and enhanced mechanical properties

The(TiB/Ti)-TiAl composites with a laminated structure composing of alternating TiB/Ti composite layers,α₂-Ti₃Al interfacial reaction layers of andγ-TiAl layers were successfully pre pared by spark plasma sintering of alternately stacked Tib₂/Ti powder layers and TiAl powder layers.And the influence of thickness ratio of Tib₂/Ti powder layers to TiAl powder layers on microstructure evolution and mechanical properties of the re sulting(TiB/Ti)-TiAl laminated composites were investigated systemically.The results showed that the thickening ofα₂-Ti₃Al layers which originated from the reaction of Ti and TiAl was significantly hindered by introducing Tib₂particles into starting Ti powders.As the thickness ratio of Tib₂/Ti powder layers to TiAl powder layers increased,the bending fracture strength and fracture toughness at room temperature of the final(TiB/Ti)-TiAl laminated composites were remarkably improved,especially for the(TiB/Ti)-TiAl composites prepared by Tib₂/Ti powder layers with thickness of 800μm and TiAl powder layers with thickness of 400μm,whose fracture toughness and bending strength were up to 51.2 MPa·m^1/2and 1456 MPa,respectively,293%and 108%higher than that of the monolithic TiAl alloys in the present work.This was attributed to the addition of high-performance network TiB/Ti composite layers.Moreover,it was noteworthy that the ultimate tensile strength at 700℃of(TiB/Ti)-TiAl composites fabricated by 400μm thick Tib₂/Ti powder layers and 400μm thick TiAl powder layers was as high as that at 550℃of network TiB/Ti composites.This means the service temperature of(TiB/Ti)-TiAl laminated composites was likely raised by 150℃,meanwhile a good combination of high strength and high toughness at ambient tempe rature could be maintained.Finally,the fracture mechanism of(TiB/Ti)-TiAl laminated composites was proposed.     

The influence of nano/micro hybrid structure on the mechanical and self-sensing properties of carbon nanotube-microparticle reinforced epoxy matrix composite

Nano/micrometer hybrids are prepared by chemical vapor deposition growth of carbon nanotubes (CNTs) on SiC, Al₂O₃ and graphene nanoplatelet (GNP). The mechanical and self-sensing behaviors of the hybrids reinforced epoxy composites are found to be highly dependent on CNT aspect ratio (AR), organization and substrates. The CNT–GNP hybrids exhibit the most significant reinforcing effectiveness, among the three hybrids with AR1200. During tensile loading, the in situ electrical resistance of the CNT–GNP/epoxy and the CNT–SiC/epoxy composites gradually increases to a maximum value and then decreases, which is remarkably different from the monotonic increase in the CNT–Al₂O₃/epoxy composites. However, the CNT–Al₂O₃ with increased AR ⩾ 2000 endows the similar resistance change as the other two hybrids. Besides, when AR < 3200, the tensile modulus and strength of the CNT–Al₂O₃/epoxy composites gradually increase with AR. The interrelationship between the hybrid structure and the mechanical and self-sensing behaviors of the composites are analyzed.     

Enhanced strength and excellent transport properties of a superaligned carbon nanotubes reinforced copper matrix laminar composite

Superaligned carbon nanotube (SACNT) reinforced copper matrix laminar composites have been fabricated by means of the traditional copper sulfate electroplating process. The mechanical properties and transport properties of the Cu/SACNT composites with different SACNT content have been studied systematically, and the experimental results show that the as-prepared composites possess a better comprehensive performance than pure copper. The simple rule of mixtures (ROM) has been used to estimate the potential maximum properties of the Cu/SACNT composites. The Cu/SACNT composite is considered to be a promising material for electronics and communications applications.     

Fabrication of aluminum matrix composites with enhanced mechanical properties reinforced by in situ generated MgAl2O4 whiskers

Aluminum matrix composite reinforced by in situ generated single crystalline MgAl₂O₄ whiskers was fabricated by chemical synthesis method in an Al-Mg-H₃BO₃ system. A large number of MgAl₂O₄ whiskers were generated during the sintering process and distributed homogeneously in the Al matrix. The whiskers penetrate into the matrix grains to form the framework of the materials, leading to an incredible increase in mechanical properties of the composites. The generation mechanism of the MgAl₂O₄ whiskers was also discussed.     

Assessment of tensile behaviour of an Al–Mg alloy composite reinforced with NiAl and oxidized NiAl powder particles helped by nanoindentation

AA5056 matrix composites have been reinforced with as-received and oxidized NiAl particles and their nanohardness investigated as a function of distance to reinforcement. Results indicate that a non-heat treatable aluminium matrix, as is the present case, does not require that the intermetallic particles are surrounding by a protective Al₂O₃ layer to avoid reactions at matrix-reinforcement interfaces. On the other hand, the quality of the matrix-reinforcement bonding has been quantified by the reinforcement influence distance, defined as the distance from the particle at which the nanohardness of the matrix drops to its asymptotic value.     

Deformation behavior of aluminum alloy matrix composites reinforced with few-layer graphene

Microstructure and mechanical properties of aluminum alloy 2024 (Al2024)/few-layer graphene (FLG) composites produced by ball milling and hot rolling have been investigated. The presence of dispersed FLGs with high specific surface area significantly increases the strength of the composites. The composite containing 0.7 vol.% FLGs exhibits tensile strength of 700 MPa, two times higher than that of monolithic Al2024, and around 4% elongation to failure. During plastic deformation, restricted dislocation activities and the accumulated dislocation at between FLGs may contribute to strengthening of Al2024/FLG composites.