Fabrication of (TiB<Subscript>2</Subscript> − TiC)<Subscript>p</Subscript>/AZ91 magnesium matrix hybrid composite

Magnesium MMCs reinforced with TiB₂−TiC particulates were fabricated successfully via a master alloy route using a low cost Al-Ti-B₄C system as starting material system. Microstructural characterization of the (TiB₂−TiC)/AZ91 composite shows relatively uniform distribution of TiB₂ and TiC particulates in the matrix material. Moreover, the results show that the hardness and wear resistance of the composites are higher than those of the unreinforced AZ91 alloy.     

The sliding wear behavior of TiCp/AZ91 magnesium matrix composites

The AZ91 metal matrix composites (MMCs) reinforced with 5, 10 and 15 wt.% TiC particulates are fabricated by TiC_p–Al master alloy process combined with mechanical stirring. The effects of TiC particulate content, applied load and wearing time on the sliding wear behaviors of the composites were investigated using MM-200 wear testing apparatus. The results show that the wear resistance and friction coefficient of the composites increased and decreased with increase of the TiC particulate content, respectively. The wear volume loss and friction coefficient of the reinforced composites as well as the unreinforced AZ91 matrix alloy increased with increase of applied load or wearing time, but the increase rates of the reinforced composites in two performance is lower than those of the unreinforced AZ91 matrix alloy. Furthermore, the sliding wear behavior of the composites and the unreinforced AZ91 matrix alloy is characterized by ploughing, adhesion and oxidation abrasion.     

AZ91D/TiB2 Nanocomposites Fabricated by Solidification Nanoprocessing

AZ91D, as one of the most widely used casting magnesium alloys, still suffers from inadequate mechanical performances for various applications. Nanoparticles could be used to form high‐performance magnesium matrix nanocomposites. Among all nanoparticles, TiB₂ has great potentials to enhance the mechanical property of AZ91D. This paper studies the microstructures and mechanical property of AZ91D‐TiB₂ nanocomposites fabricated through solidification nanoprocessing. TiB₂ nanoparticles with a diameter of 25 nm are effectively fed into the AZ91D melt through a newly developed automatic nanoparticle‐feeding system. Ultrasonic cavitation is used to disperse these nanoparticles in AZ91D melt for casting. With 2.7 wt% (about 1.0 vol%) of TiB₂ nanoparticles addition, the mechanical property of AZ91D is much enhanced (by 21, 16, and 48% for yield strength, tensile strength, and ductility, respectively). Microstructural analysis with optical microscope, SEM, and S/TEM show that α‐Mg grain and a network of massive brittle intermetallic phase (β‐Mg₁₇Al₁₂) are simultaneously refined and modified. Further study suggests that the enhancement of mechanical properties of AZ91D is attributed not only to primary phase grain refinement, but also to the modification of intermetallic β‐Mg₁₇Al₁₂ by TiB₂ nanoparticles.     

Fabrication of TiC and TiB<Subscript>2</Subscript> locally reinforced steel matrix composites using a Fe–Ti–B<Subscript>4</Subscript>C–C system by an SHS-casting route

Steel matrix composites locally reinforced by in situ TiC and TiB₂ particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) in a Fe–Ti–B₄C–C system during casting. The locally reinforced steel matrix composites consist of three distinct regions: (i) a TiC and TiB₂ particulate-reinforced region, (ii) a transition region, and (iii) a steel matrix region. The TiC and TiB₂ particulates in the locally reinforced regions display a relatively uniform distribution, and their sizes decrease with the increase in Fe content from 10 wt.% to 40 wt.%. The wear resistance of the locally reinforced region of the steel matrix composites is much higher than that of the unreinforced steel matrix.     

Understanding the reaction mechanism of in-situ synthesized (TiC–TiB2)/AZ91 magnesium matrix composites

Magnesium matrix composites reinforced with a network of TiC and TiB₂ compounds have been successfully synthesized via an in-situ reactive infiltration technique. In this process, the ceramic reinforcing phases, TiC and TiB₂, were synthesized in-situ from the starting powders of Ti and B₄C without any addition of a third metal powder such as Al. The molten AZ91 magnesium alloy infiltrates the preform of 3Ti–B₄C by capillary forces. Furthermore, adding different weight percentages of MgH₂ powder to the 3Ti–B₄C preforms was used in an attempt to increase the Mg content in the fabricated composites. The results reveal a relatively uniform distribution of the reinforcing phases in the magnesium matrix with very small amounts of residual Ti, boron carbide and intermediate phases when they are fabricated at 900 °C for 1.5 h using a 3Ti–B₄C preform with 70% relative density. On the other hand, after adding MgH₂ to the 3Ti–B₄C preform, TiC_x and TiB₂ formed completely without any residual intermediate phases with the formation of the ternary compound (Ti₂AlC) at the expense of TiC. The percentage of reinforcing phases can be tailored by controlling the weight percentages of MgH₂ powder added to the 3Ti–B₄C preform. The results of the in-situ reaction mechanism investigation of the Ti–B₄C and Mg–B₄C systems show that the molten magnesium not only infiltrates through the 3Ti–B₄C preform and thus densifies the fabricated composite as a matrix metal, but also acts as an intermediary making the reaction possible at a lower temperature than that required for solid-state reaction between Ti and B₄C and accelerates the reaction rate. The investigation of the in-situ reaction mechanism with or without the addition of MgH₂ powder to the 3Ti–B₄C preforms reveals similar mechanisms. However, the presence of the MgH₂ in the preform accelerates the reaction resulting in a shorter processing time for the same temperatures.     

Effects of TiO2 coating on microstructure and mechanical properties of magnesium matrix composite reinforced with Mg2B2O5w

The interface between the reinforcement and the matrix is very important for metal matrix composites. The effects of TiO₂ coating on microstructure and mechanical properties of Mg₂B₂O₅w-reinforced AZ91D magnesium matrix composite fabricated by squeeze casting technique were studied. The results indicate the flexural strength and flexural modulus of Mg₂B₂O₅w/TiO₂/AZ91D composite is 40% and 35% up on that of Mg₂B₂O₅w/AZ91D composite. The reason is that the MgO interfacial product resulting from the reaction between the TiO₂ coating and the liquid Mg can enhance the interfacial bonding strength and increase the load transfer from the matrix to the reinforcement, which leads to higher mechanical properties of Mg₂B₂O₅w/TiO₂/AZ91D composite.     

Compressive Creep Behavior of TiC/AZ91D Magnesium-matrix Composites with Interpenetrating Networks

The 42.1 vol. pct TiC/AZ91D magnesium-matrix composites with interpenetrating networks were fabricated by in-situ reactive infiltration process. The compressive creep behavior of as-synthesized composites was investigated at temperature ranging from 673 to 723 K under loads of 95-108 MPa. For a comparative purpose,the creep behavior of the monolithic matrix alloy AZ91D was also conducted under loads of 15-55 MPa at 548-598 K. The creep mechanisms were theoretically analyzed based on the power-law relation. The results showed that the creep rates of both TiC/AZ91D composites and AZ91D alloy increase with increasing the temperature and load. The TiC/AZ91D composites possess superior creep resistance as compared with the AZ91D alloy. At deformation temperature below 573 K, the stress exponent n of AZ91D alloy approaches theoretical value of 5, which suggests that the creep process is controlled by dislocation climb. At 598 K, the stress exponentof AZ91D is close to 3, in which viscous non-basal slip deformation plays a key role in the process of creep deformation. However, the case differs from that of AZ91D alloy when the stress exponent n of TiC/AZ91D composites exceeds 9, which shows that there exists threshold stress in the creep process of the composites, similar to other types of composites. The average activation energies for the creep of the AZ91D alloy and TiC/AZ91D composites were calculated to be 144 and 152 k J/mol, respectively. The existence of threshold stress in the creep process of the composites leads to an increase in activation energy for creep.     

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.     

原位反应渗透法TiCp/Mg复合材料的制备和性能

利用Ti和C元素粉末间的原位放热反应合成TiCp,结合Mg熔体的自发渗透技术制备了TiCp/Mg以及TiCp/AZ91D两种镁基复合材料,观测了复合材料的相组成和原位反应生成物TiCp的形貌,研究了这两种镁基复合材料的常温压缩性能.结果表明,原位反应自发渗透技术制备的Mg基复合材料组织致密,增强相呈细小的颗粒状和互穿网片状,分布均匀.这是材料的压缩强度得到提高的原因.在常温以及应变速率为0.01 s-1的条件下,TiCp/Mg和TiCp/AZ91D镁基复合材料的压缩强度分别达到598和650 MPa.     

Compressive behaviour of SiC/ncsc reinforced Mg composite processed through powder metallurgy route

In this present work nano coconut shell charcoal (ncsc) and silicon carbide (SiC) particulates were reinforced with AZ31B Mg alloy and suitable magnesium composite was developed by using the powder metallurgy technique followed by hot extrusion. Density measurement of the Mg composites revealed that the addition of ncsc significantly improved the density of the composites and porosity measurement showed minimal porosity. The microstructure of the composites showed even distribution of the ncsc in the AZ31B/3SiC Mg composite. The compressive and impact behaviour of the samples were characterized, the results showed that on increasing the weight percentage of ncsc in AZ31B/3SiC/0.5ncsc Mg composites the mechanical properties such as ultimate compressive strength, 0.2% yield strength, ductility and impact strength decreased. The scanning electron microscope (SEM) analysis of fractured surface of AZ31B Mg alloy and AZ31B/3SiC/0.5ncsc Mg composites showed quasi-cleavage fracture. The presence of ncsc above 0.5 wt% composites revealed mixture of quasi cleavage planes and some dimples.     

粉煤灰漂珠粒径对粉煤灰漂珠/AZ91D镁合金复合材料阻尼性能的影响

通过搅拌铸造法向半固态AZ91D镁合金中添加粉煤灰漂珠（FAC）制备了FAC/AZ91D镁合金复合材料,研究了FAC粒径对该复合材料阻尼性能的影响。结果表明:FAC/AZ91D镁合金复合材料的阻尼性能明显优于基体材料,在FAC含量相同时,FAC的粒径越大,其阻尼性能越好。室温下FAC对提高FAC/AZ91D镁合金复合材料的阻尼性能起重要作用,FAC附近的基体产生了高密度的位错,形成了塑性区。室温下FAC粒径越大,在其附近产生的塑性区越大,阻尼性能越好。随温度的升高,FAC/AZ91D镁合金复合材料的阻尼性能迅速提高。位错、晶界以及FAC和基体之间的界面运动是提高阻尼性能的关键。      

In situ synthesis method and damping characterization of magnesium matrix composites

Magnesium matrix composites reinforced with 8 wt% TiC was in situ synthesized using remelting and dilution technique. X-ray diffraction analysis revealed the presence of TiC phase in sintered block and magnesium matrix composites. Uniform distribution of fine TiC particulates in matrix material was obtained through microstructure characterization. The results of damping characterization revealed that damping capacity of materials is independent of frequency, but dependent on strain and temperature. There were damping peak in damping–strain curve, which is due to the foul and tangle of dislocations. There were two damping peaks at damping–temperature curve under respective temperature of 130 °C and 240 °C. The former damping peak of magnesium matrix composites is due to dislocation motion, and the latter is due to interface and grain boundary sliding. Generally, damping capacity of magnesium matrix composites is higher than that of AZ91 magnesium alloy, which is due to the addition of TiC particulates.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Effect of different types of nano-size oxide particulates on microstructural and mechanical properties of elemental Mg

In the present study, magnesium based composites were fabricated with three different types of 1.1 volume percent nanosize oxide particulate reinforcements (i.e., Al₂O₃, Y₂O₃ and ZrO₂) using blend-press-sinter methodology avoiding ball milling. Microstructural characterization of the materials revealed reasonably uniform distribution of nano-reinforcement, significant grain refinement and the presence of minimal porosity. Mechanical properties characterization revealed that the incorporation of nano-sized oxide particulates as reinforcement led to a simultaneous increase in hardness, 0.2% yield strength, UTS and ductility of pure magnesium. The results further revealed that the 0.2% yield strength, UTS and ductility combination of the magnesium containing nano-size Al₂O₃ remained higher when compared to high strength magnesium alloy AZ91 reinforced with much higher amount of micron size SiC particulates. An attempt is made in the present study to correlate the effect of different types of nano-sized oxide particulates on the microstructural and mechanical properties of magnesium.     

Development of Al 6063–TiB2 in situ composites

In situ TiB₂ reinforced Al 6063 composites have been successfully synthesized through the chemical reaction between Al–10%Ti and Al–3%B master alloys in the Al 6063 alloy using liquid metallurgy route. The amount of TiB₂ formed in the composite is estimated using gravimetric analysis. Mechanical properties in terms of microhardness, ultimate tensile strength and modulus of elasticity have been improved by 21%, 47% and 65% respectively in comparison with matrix alloy. Further, ductility in terms of percentage elongation of the composites was found to increase by about 368% when compared with the matrix alloy. The improvement in ductility may be associated with the grain refinement of the composite with an increase in the content of Al–3%B master alloy.     

Fabrication of TiB2 particulate reinforced magnesium matrix composites by powder metallurgy

Magnesium metal matrix composites (MMCs) reinforced with 10, 20 and 30 vol.% TiB₂ particulates, respectively, were fabricated by powder metallurgy. The microstructure, porosity, hardness and abrasive wear behavior of the composites were evaluated. Microstructural characterization of Mg MMCs showed generally uniform reinforcement distribution. As compared with pure Mg, the hardness (HB) values of Mg MMCs reinforced with 10, 20 and 30 vol.% TiB₂ particulates were increased by 41%, 106% and 181%, respectively. The abrasive wear tests showed that the wear resistance of Mg MMCs is increased with the increasing of the reinforcement volume fraction. This was due to the strong particulate-matrix bonding and high hardness of the TiB₂ particulate.     

Fabrication of TiB2/TiC composites by the directional reaction of titanium with boron carbide

Dense TiB₂/TiC composites were fabricated by the directional reaction of molten titanium with boron carbide preform. The reaction between pure molten titanium and boron carbide preform could not progress due to reaction choking. However, when a few weight per cent of nickel were added in the titanium, the reaction progressed continuously and resulted in TiB₂/TiC composites. A gradient of grain sizes was observed in the reaction products. The processing temperature affected the microstructure of the reaction products rather than the reaction rate. The degree of grain-size gradient in the reaction product increased with the processing temperature.     

Interfacial failure behavior of PyC-Cf/AZ91D composite fabricated by LSEVI

Carbon fiber reinforced AZ91D matrix composites with pyrolytic (PyC) coating deposited on fiber surface (PyC-C_f/AZ91D composites) have been fabricated by Liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI). Interfacial microstructure and failure behavior of the composites were investigated. Instead of interfacial reaction products, block-shaped interfacial precipitates Mg₁₇Al₁₂ were detected at the interface, which indicates that interfacial reaction was restrained by LSEVI and PyC coating. Nano-MgO was detected at the interface. Interfacial failure behavior of the PyC-C_f/AZ91D composites, which was the failure between PyC coating and AZ91D alloy due to the mismatch of thermal expansion and relatively poor bonding, was proposed. Fracture surface of the PyC-C_f/AZ91D composites was characterized by fibers pulling-out tests. PyC coating served not only as protection to the fibers, but also an adjustment of the interface of the composites.     

Multi-phase aluminide-based composites – fabrication,microstructure and properties

Microstructures of Al45–Mo25–Zr25–Ge5 (in at.%, AMZG) alloy, produced by reaction hot pressing of elemental powder mixtures, have shown co-existence of AlMo₃, Al₃Mo₈, ZrAl₂, Zr₂Al, MoGe₂ and ZrGe₂. In addition, its composites were fabricated through addition of micro-sized TiC, partially stabilized zirconia (PSZ-ZrO₂) or SiC particulates into the pulverized multi-phase aluminide powders. The presence of SiC particulates showed a much less significant contribution to the strength/toughness enhancement of AMZG alloy, due to the existence of residual porosity and weak interfacial bonding. In contrast, the other two composites were superior in both flexural strength and fracture toughness to the AMZG multi-phase alloy, which is derived from the contribution of uniformly distributed and well-embedded harder particulates and the constrained plastic deformation of the matrix. The addition of hard ceramic particles simultaneously yielded higher bulk Vickers hardness. The toughness enhancement in the composites was attributed to the increased tortuousity by crack deflection, branching and bridging. Moreover, the transformation of tetragonal zirconia particles into the monoclinic form might also partially contribute to the toughness enhancement in the AMZG/ZrO₂ composite.