Preparation of Fe-M Intermetallic/TiC-M_2O_3 Ceramic Composites from Ilmenite by SHS

Fe-Al intermetallic/TiC-Al2O3 ceramic composites were successfully prepared by selfpropagating high-temperature synthesis （SHS） from natural ilmenite, aluminium and carbon as the raw materials. The effects of carbon sources, preheating time and heat treatment temperature on synthesis process and products were investigated in detail, and the reaction process of the FeTiO3-Al-C system was also discussed. It is shown that the temperature and velocity of the combustion wave are higher when graphite is used as the carbon source, which can reflect the effect of the carbon source structure on the combustion synthesis; Prolonging the preheating time or heat treatment temperature is beneficial to the formation of the ordered intermetallics; The temperature and velocity of the combustion wave are improved, but the disordered alloys are difficult to eliminate with the preheating time prolonged. The compound powders mainly containing ordered Fe3Al intermetallic can be prepared through heat treatment at 750 ℃.

Properties of CNTs/MoSi2 composites prepared by spark plasma sintering

Molybdenum disilicide(MoSi_2) based composites with various contents of carbon nanotubes(CNTs) were fabricated by spark plasma sintering(SPS) in vacuum under a pressure of 25 MPa.The composites obtained under a sintering temperature of 1500 °C and time of 10 min exhibited optimum mechanical properties at room temperature in terms of fracture toughness and transverse rupture strength.MoSi_2 based composite with 6.0% CNTs(volume fraction) had the highest fracture toughness,transverse rupture strength and hardness,which were improved by about 25.7%,51.5% and 24.4% respectively,as compared with pure MoSi_2.A Mo_(4.8)Si_3C_(0.6) phase was detected in CNTs/MoSi_2 composites by both X-ray diffraction(XRD) method and microstructure analysis with scanning electron microscopy(SEM).It is believed that the fine grains and well dispersed small Mo_(4.8)Si_3C_(0.6) particles had led to a higher hardness and strength of CNTs/MoSi_2 composites because of their particle pullout,crack deflection and micro-bridging effects.

C/SiC/MoSi_2-SiC-Si multilayer coating for oxidation protection of carbon/carbon composites

C/SiC/MoSi2-SiC-Si oxidation protective multilayer coating for carbon/carbon （C/C） composites was prepared by pack cementation and slurry method. The microstructure, element distribution and phase composition of the as-received coating were analyzed by scanning electron microscopy （SEM）, energy dispersive X-ray spectroscopy （EDS） and X-ray diffraction （XRD）. The results show that the multilayer coating was composed of MoSi2, SiC and Si. It could effectively protect C/C composites against oxidation for 200 h with the mass loss of 3.25% at 1873 K in static air. The mass loss of the coated C/C composites results from the volatilization of SiO2 and the formation of cracks and bubble holes in the coating.

TiB_2-Cu基复合材料SHS工艺的正交优化

利用SHS技术制备了性能优良的TiB2 40 %Cu基复合材料。SHS工艺参数主要包括压坯相对密度、压坯轴向压力、预压力、延迟时间、高压压力、高温保压时间等 ,对这些参数和材料性能之间的关系进行了探讨。为了研究不同的延迟时间、高压压力和高温保压时间对材料致密度的影响 ,设计了一个三因素、三水平的正交试验。结果表明 ,TiB2 40 %Cu基复合材料的SHS最佳工艺参数为压坯相对密度为 5 9.0 %,压坯轴向压力为 1 3 5MPa,预压力为 1 8MPa ,延迟时间为 7s ,高压压力为 3 6 0MPa ,高温保压时间为 1 0s。

SHS制备纳米/微米结构抗弹陶瓷可行性研究

针对用于现代装甲防护的高性能抗弹陶瓷 ,以SHS冶金技术 ,通过材料原位合成手段并在大过冷条件下熔体发生共生共晶反应 ,快速一次性制备出自燃自组装、具有 1 3复合晶内型结构的纳米 /微米结构Al2 O3 ZrO2 共晶复相陶瓷。Vickers压痕试验发现该陶瓷具有较高的断裂韧性并表现出较强的塑变行为 ,SEM观察发现该复相陶瓷的裂纹扩展路径主要受纳米 /微米结构 (Al2 O3 +ZrO2 )共晶结构所控制 ,影响该陶瓷韧化行为的主要因素在于以“内晶型”纳米相所构成的独特的增韧机制。试验预示着采用SHS冶金技术 ,通过控制Al2 O3 /ZrO2 复相陶瓷成分和凝固行为 (如采用离心冷却控制技术 ) ,可以制备出用于现代装甲防护的高性能、低成本、高可靠性新型抗弹复相陶瓷面板。

SiO_2对重力分离-SHS法制备Al_2O_3/Fe复合管组织的影响

采用重力分离-自蔓延高温合成(Self-propagating High-temperature Synthesis,简称SHS)法制备了Al2O3/Fe陶瓷内衬复合管。分析了添加剂SiO2对复合陶瓷层的相组成、陶瓷层的表面质量以及对Al2O3/Fe过渡层的影响。XRD分析表明,陶瓷层主晶相为-Al2O3、FeAl2O4,加入SiO2后可形成Al2SiO5硅线石。SEM观察发现,SiO2的加入量过多会降低过渡层的质量,使过渡层变薄,结合形式由冶金结合变为机械结合。

Self-Propagating High-Temperature Reductive Synthesis of TiB_2-Al_2O_3 Composite Powders

TiB2-Al2O3 composite powders were produced by self-propagating high-temperature synthesis（SHS） method with reductive process from B2O3-TiO2-AI system. X-ray diffraction（XRD） and scanning electron microscopy（SEM） analyses show the presence of TiB2 and Al2O3 only in the composite powders produced by SHS. The powders are uniform and free-agglomerate. Transmission electron microscopy （TEM） and high resolution electron microscopy （HREM） observation of microstructure of the composite powders indicate that the interfaces of the TiB2-Al2O3 bond well, without any interfacial reaction products. It is proposed that the good interfacial bonding of the composite powders can be resulted from the TiB2 particles crystallizing and growing on the Al2O3 particles surface with surface defects acting as nucleation centers.

Thermochemical Evaluation of Combustion Synthesis of Al_2O_3/B_4C Composite

The equilibrium phase and adiabatic temperature for combustion synthesis of Al_2O_3/B_4C employing Al, B_2O_3 and C as starting materials is analyzed by both conventional and CALPHAD method. The adiabatic temperature calculed by CALPHAD method is significantly lower than that obtained by conventional method. The CALPHAD calculation also reveals that the equilibrium phases presented at the adiabatic temperature are different to the desired composites. The adiabatic temperature in this system can be lowered by introducing Al_2O_3 as diluents. The maximum amount of Al_2O_3 that can be added to the system while maintain a self-sustaining combusion mode is 1.3 mol.

SHS-熔铸工艺制备MoSi_2-Fe原位复合材料的研究

以MoO3-Al-Si-Fe为反应体系,提出了制备MoSi2-Fe原位复合材料的SHS-熔铸工艺。分析了体系的SHS反应热力学、SHS过程中Mo-Si-Fe合金熔体的形成及冷却速度对熔体凝固组织的影响。结果表明,利用所提出的SHS-熔铸工艺可实现MoSi2-Fe复合材料的原位合成与液态成形一体化,制备的复合材料由原位合成的MoSi2和Fe组成,Fe以粘结相分布在MoSi2的边界上;提高熔体的冷却速度,可明显降低复合材料组织中MoSi2的晶粒尺寸。

Self-propagating High-temperature Synthesis, Microstructure and Mechanical Properties of TiC-TiB_2-Cu Composites

TiC-TiB2-Cu composites were produced by self-propagating high-temperature synthesis combined with pseudo hot isostatic pressing using Ti, B4C and Cu powders. The microstructure and mechanical properties of the composites were investigated. The X-ray diffraction （XRD） and scanning electron microscopy （SEM） results showed that the final products were only TiC, TiB2 and Cu phases. The clubbed TiB2 grains and spheroidal or irregular TiC grains were found in the microstructure of synthesized products. The reaction temperature and grain size of TiB2 and TiC particles decreased with increasing Cu content. The introduction of Cu into the composites resulted in a drastic increase in the relative density and flexual strength, and the maximum values were obtained with the addition of 20 wt pct, while the fracture toughness was the best when Cu content was 40 wt pct.

Effect of MoSi_2 content on dielectric and mechanical properties of MoSi_2/Al_2O_3 composite coatings

Molybdenum disilicide （MoSi2） sheath and aluminum oxide （Al2O3） core blended powders were fabricated by spray drying. A derived coating material was produced for the application as microwave absorbers using the as prepared powders by atmospheric plasma spray （APS） technology. The effects of MoSi2/Al2O3 mass ratio on the dielectric and physical mechanical properties of the composite coatings were investigated. When the MoSi2 content of the composites increases from 0 to 45%, the flexure strength and fracture toughness improve from 198 to 324 MPa and 3.05 to 4.82 MPa-m1/2 then decline to 310 MPa and 4.67 MPa-m1/2, respectively. The dielectric loss tangent increases with increasing MoSi2 content, and the real part of permittivity decreases conversely over the frequency range of 8.2-12.4 GHz. These effects are due to the agglomeration of early molten MoSi2 particles and the increase of the electrical conductivity with increasing MoSi2 content.

Self-propagating high temperature synthesis of MoSi_2 matrix composites

MoSi2 is presently regarded as the most important material for electrical heating and as one with huge potential for high temperature structural uses. MoSi2 and MoSi2 matrix composites were prepared by self-propagating high temperature synthesis （SHS）. Pure MoSi2 was obtained and a compound of MoSi2 and WSi2was synthesized in the form of predominant solid solution （Mo,W）Si2. By adding aluminum of 5.5 at.% to Mo-Si, the crystal structure of MoSi2 changed into a mixture of tetragonal Cllb MoSi2and hexagonal C40 Mo（Si,Al）2. The （Mo,W）Si2-Mo（Si,Al）2-W（Si,Al）2 composite materials were synthesized by adding aluminum of 5.5 at.% to Mo-W-Si. However, if the amount of the added aluminum was not larger than 2.5 at.%, it did not have any significant effect. SHS is an effective technology for synthesis of MoSi2 and MoSi2 matrix composites.

TOUGHENING OF IN SITU SYNTHESIZED MoSi_2-SiC COMPOSITE AT ROOM TEMPERATURE

The in situ synthesized MoSi2-SiC composite is proved to be of higher fracture toughness than the monolithic MoSi2. The TEM and HREM study reveals that the interface between MoSi2/SiC is of direct atomic bonding without any amorphous glassy phase, such the SiO2 structure. Based on the fractography and the observation of crack propagation path from indentation, it is concluded that the toughening of such composite at room temperature can be attributed to the high interfacial binding energy, the refinement of the MoSi2 matrix and the deflection and bridging behavior in the crack propagation.

Fabrication of Titanium Diboride-Cu Composite by Self-High Temperature Synthesis plus Quick Press

Titanium diboride based composites, good candidates for contact materials, have high hardness, Young＇s modulus, high temperature stability, and excellent electrical, thermal conductivity. However a good interface of TiB2/Cu is very difficult to achieve for oxidation of TiB2. To avoid this oxidation behavior, the in situ combusting synthesis technology, SHS, was used to prepare TiB2/Cu composite. The characters of Ti-B-xCu SHS were studied in detail, such as combustion temperature, products phases and grain size. Based on the experimental results a proper technology way of self-high temperature synthesis plus quick press （SHS/QP） was determined and compact TiB2/Cu composites with relative density over than 97 pct of the theoretical were fabricated by this method. The properties and microstructures of these TiB2 based composites were also investigated.

Microstructure, sintering behavior and mechanical properties of SiC/MoSi_2 composites by spark plasma sintering

SiC/MoSi2 composites were synthesized at different temperatures by spark plasma sintering using Mo, Si and SiC powders as raw materials. The phase composition, microstructure and mechanical properties of the as-prepared composites were investigated and the sintering behavior was also discussed. Results show that SiC/MoSi2 composites are composed of MoSi2, SiC and trace amount of Mo4.8Si3C0.6 phase and exhibit a fine-grain texture. During the synthesis process, there was an evolution from solid phase sintering to liquid phase sintering. When sintered at 1600 °C, the SiC/MoSi2 composites present the most favorable mechanical properties, the Vickers hardness, bending strength and fracture toughness are 13.4 GPa, 674 MPa and 5.1 MPa·m^1/2, respectively, higher 44%, 171%, 82% than those of monolithic MoSi2. SiC can withstand the applied stress as hard phase and retard the rapid propagation of cracks as second phase, which are beneficial to the improved mechanical properties of Si C/MoSi2 composites.

Fabrication of MoSi_2-SiC Composites by a Spark Plasma Sintering Method

MolybdenUmdisilicidepeoSi,)hashighmeltingpoint（2293K）,relativelylowdensity（6.249/cm3）,abrittle-to-ductiletemper;tabout1173K[l]andoutstandingoxidationresistance[2M].Forthisreason,thedisilicidematrixcompositesarebeingconsideredasoneofthemoStPromisingc...

High temperature mechanical retention characteristics and oxidation behaviors of the MoSi2（Cr5Si3）–RSiC composites prepared via a PIP-AAMI combined process

In the present paper,MoSi2(Cr5Si3)–RSiC composites were prepared via a combination of precursor impregnation pyrolysis(PIP) and MoSi2-Si-Cr alloy active melt infiltration(AAMI) process. Composition, microstructure, mechanical retention characteristics, and oxidation behaviors of the composites at elevated temperature were studied. X-ray diffraction(XRD) pattern confirms that the composites mainly compose of 6 H–SiC, hexagonal MoSi2, and tetragonal Cr5Si3. Scanning electron microscopy(SEM) image reveals that nearly denseMoSi2(Cr5Si3)–RSiC composites exhibiting three-dimensionally(3D) interpenetrated network structure are obtained when infiltrated at 2173 K, and the interface combination of the composites mainly depends on the composition ratio of infiltrated phases. Oxidation weight gain rate of the composites is much lower than that of RSiC matrix, where MoSiCr2 possesses the lowest value of 0.1630 mg×cm-2, about 78% lower than that of RSiC after oxidation at 1773 K for 100 h. Also, it possesses the highest mechanical values of 139.54 MPa(flexural strength σf and RT) and 276.77 GPa(elastic modulus Ef and RT), improvement of 73.73% and 29.77% as compared with that of RSiC, respectively. Mechanical properties of the composites increase first and then decrease with the extension of oxidation time at 1773 K, due to the cooperation effect of surface defect reduction via oxidation reaction and thermal stress relaxation in the composites, crystal growth, and thickness increase of the oxide film. Fracture toughness of MoSiCr2 reaches 2.24 MPa·m1/2(1673 K), showing the highest improvement of 31.70% as compared to the RT value.

Fabrication and fracture analysis of MoSi_2 matrix composites by mechanical alloying

Four composites, MoSi 2+ZrO 2, MoSi 2+ZrO 2(Y 2O 3), MoSi 2+ZrO 2+SiC and MoSi 2+ZrO 2(Y 2O 3)+SiC are fabricated by mechanical alloying. It is clear that cracks produced on the MoSi 2 matrix composites during hardness testing belong to the Palmquist crack system. The value of highest fracture toughness of MoSi 2+ZrO 2+SiC composite is 7.58?MPa·m 1/2 , which is nearly three times that of monolithic MoSi 2. This can be attributed to well distributed ZrO 2 and SiC particles along the boundaries of very fine MoSi 2 grains.

High temperature creep behavior of in-situ synthesized MoSi_2-30%SiC composite

The compressive creep behavior at 1?200 ～ 1?400?℃ of an in situ synthesized MoSi 2 30%SiC (volume fraction) composite and a traditional PM MoSi 2 30%SiC (volume fraction) composite is investigated. The creep rate of the in situ synthesized MoSi 2 30%SiC (volume fraction) composite is about 10 -7 s -1 under stress of 60 ～ 120?MPa, and significantly lower than that made by PM method above 1?300?℃. The reason is that the interface between SiC particle and MoSi 2 matrix in in situ synthesized SiC p/MoSi 2 is of direct atomic bonding without any amorphous glassy phase, such as SiO 2 structure. Creep deformation occurs primarily by dislocation motion and the dislocations have Burgers vectors of the type of <110> and <100>.

Property and Oxidation Behaviours of (Mo,Cr)Si2 + ZrO2 Composite Produced by Pressure-Less Sintering

A composites of (Mo0.9Cr0.1)Si2 + 15vol% ZrO2 was prepared with powder metallurgy and Pressure- Less Sintering (PLS) method, aiming at applications of high temperature structural materials. Mechanical properties of the composites were assessed with hardness, indentation fracture toughness Kc and KIC tested using SEVNB, flexure strength at room temperature and 1200?C, and isothermal oxidation at 1400?C. The results showed that the native silica oxide and molybdenum-oxides on the silicide feedstock surface were significantly reduced in terms of Cr-alloying. (Mo0.9Cr0.1)Si2 and its composite also exhibited improved sinterability and grain growth, owing to the presence of (Cr, Mo)5Si3 at grain boundaries. Fracture toughness of the composite was increased by a factor of 1.6 to that in the monolithic silicide. Mechanical property of the composite at high temperature was not affected by Cr addition. However, the high temperature oxidation resistance was greatly improved in the (Mo0.9Cr0.1)Si2 + 15vol% ZrO2 compared with the non Cr-alloyed counterpart. The Cr-alloying effects on the microstructure, fracture behaviour, and high temperature oxidation resistance were discussed.