Functionally graded structure of a nitride-strengthened Mg_(2)Si-based hybrid composite

The ex-situ incorporation of the secondary SiC reinforcement,along with the in-situ incorporation of the tertiary and quaternary Mg_(3)N_(2) and Si_(3)N_(4) phases,in the primary matrix of Mg_(2)Si is employed in order to provide ultimate wear resistance based on the laser-irradiation-induced inclusion of N_(2) gas during laser powder bed fusion.This is substantialized based on both the thermal diffusion-and chemical reactionbased metallurgy of the Mg_(2)Si–SiC/nitride hybrid composite.This study also proposes a functional platform for systematically modulating a functionally graded structure and modeling build-direction-dependent architectonics during additive manufacturing.This strategy enables the development of a compositional gradient from the center to the edge of each melt pool of the Mg_(2)Si–SiC/nitride hybrid composite.Consequently,the coefficient of friction of the hybrid composite exhibits a 309.3%decrease to–1.67 compared to–0.54 for the conventional nonreinforced Mg_(2)Si structure,while the tensile strength exhibits a 171.3%increase to 831.5 MPa compared to 485.3 MPa for the conventional structure.This outstanding mechanical behavior is due to the(1)the complementary and synergistic reinforcement effects of the SiC and nitride compounds,each of which possesses an intrinsically high hardness,and(2)the strong adhesion of these compounds to the Mg_(2)Si matrix despite their small sizes and low concentrations.

Preparation and oxidation property of ZrB_2-MoSi_2/SiC coating on carbon/carbon composites

To improve the oxidation resistance of carbon/carbon composites,ZrB2-MoSi2/SiC coating on the carbon/carbon substrate was prepared.The inner coating of SiC was prepared by pack cementation and the outer coating of ZrB2-MoSi2 was prepared by slurry painting.The phase compositions and microstructures of the coating were characterized by XRD and SEM,respectively.The preparation and the high temperature oxidation property of the coated composites were investigated.The results show that the outer coating of carbon/carbon composites is composed of ZrB2,MoSi2 and SiC phases.The mass losses of the ZrB2-MoSi2/SiC coated samples with SiC nano-whiskers after 30 h and 10 h of oxidation at 1 273 K and 1 773 K were,respectively,5.3% and 3.0%.The ZrB2-MoSi2/SiC coated samples exhibit self-sealing performance and good oxidation resistance at high temperature.

Anti-oxidation properties of ZrB_2 modified silicon-based multilayer coating for carbon/carbon composites at high temperatures

To improve the anti-oxidation ability of silicon-based coating for carbon/carbon （C/C） composites at high temperatures, a ZrB2 modified silicon-based multilayer oxidation protective coating was prepared by pack cementation. The phase composition, microstructure and oxidation resistance at 1773, 1873 and 1953 K in air were investigated. The prepared coating exhibits dense structure and good oxidation protective ability. Due to the formation of stable ZrSiO4-SiO2 compound, the coating can effectively protect C/C composites from oxidation at 1773 K for more than 550 h. The anti-oxidation performance decreases with the increase of oxidation temperature. The mass loss of coated sample is 2.44% after oxidation at 1953 K for 50 h, which is attributed to the decomposition of ZrSiO4 and the volatilization of SiO2 protection layer.

Hot deformation and processing maps of Al_2O_3/Al composites fabricated by flake powder metallurgy

The deformation behaviors of Al2O3/Al composites were investigated by compressive tests conducted at temperature of 300-450 °C and strain rates of 0.001-1.0 s-1 with Gleeble-1500 D thermal simulator system. The results show that the flow stress increases with increasing strain rate and decreasing temperature. The hyperbolic sine constitutive equation can describe the flow stress behavior of Al2O3/Al composites, and the deformation activation energy and constitutive equations were calculated. The processing maps of Al2O3/Al-2 μm and Al2O3/Al-1 μm composites at strain of 0.6 were obtained and the optimum processing domains are in ranges of 300-330 °C, 0.007-0.03 s-1 and 335-360 °C, 0.015-0.06 s-1 for hot working, respectively. The instability zones of flow behavior can also be recognized by the maps.

Dry sliding wear behavior of AA6061/ZrB_2 in-situ composite

The dry sliding wear behavior of AA6061/ZrB2 in-situ composite prepared by the reaction of inorganic salts K2ZrF6 and KBF4 with molten aluminum was investigated.An attempt was made to develop a mathematical model to predict the wear rate of AA6061/（0-10%） ZrB2 in-situ composites.Four-factor,five-level central composite rotatable design was used to minimize the number of experiments.The factors considered are sliding velocity,sliding distance,normal load and mass fraction of ZrB2 particles.The effect of these factors on the wear rate of the fabricated composite was analyzed and the predicted trends were discussed by observing the wear surface morphologies.The in-situ formed ZrB2 particles enhance the wear performance of the composite.The wear rate of the composite bears a proportional relationship with the sliding velocity,sliding distance and normal load.

Properties, Phases and Microstructure of Microwave Sintered W-20Cu Composites from Spray Pyrolysis-continuous Reduction Processed Powders

The effects of microwave sintering on the properties, phases and microstructure of W-2OCu alloy, using composite powder fabricated by spray pyrolysis-continuous reduction technology, were investigated. Compared with the conventional hot-press sintering, microwave sintering to W-2OCu composites could be achieved with lower sintering temperature and shorter sintering time. Furthermore, microwave sintered W-Cu composites with high densification, homogenous microstructure and excellent properties were obtained. Microwave sintering could also result in finer microstructures. ：~

Effects of in-situ ZrB_2 particle on grain refinement of ZrB_2/AZ91D magnesium matrix composite

In-situ ZrB2/AZ91D magnesium matrix composite was successfully synthesized with AI/K2ZrF6＋NH4BF4 by means of Direct Melt Reaction. The fabricated ZrB2/AZ91D magnesium matrix composite through direct melt mixing method was investigated. Results from X-ray diffraction （XRD） and energy dispersive X-ray spectroscopy （EDS） confirmed the existence of ZrB2 particles in the AZ91D alloy, and most ZrB2 particles were in the size range of just several microns, some even to 100 nm. The cast specimens were studied through corrosion testing and heat treatment. The average grain size of AZ91D decreased markedly from about 250 pm to 50 IJm. In addition, the shape and size of the ,β-MglTAI12 phase as well as the morphologies of primary a-Mg in the magnesium matrix composite were greatly changed. The network structure of the β-MglTAI12 phase was broken into small blocks and the size of a-Mg decreased significantly.

Preparation of ZrB_2-SiC Powders via Carbothermal Reduction of Zircon and Prediction of Product Composition by Back-Propagation Artificial Neural Network

Phase pure ZrB2-SiC composite powders were prepared after 1 450℃/3 h via carbothermal reduction route,by using ZrSiO4,B2O3 and carbon as the raw materials.The influences of firing temperature as well as the type and amount of additive on the phase composition of final products were detailedly investigated.The results indicated that the onset formation temperature of ZrB2-SiC was reduced to 1 400℃by the present conditions,and oxide additive（including CoSO4·7H2O,Y2O3 and TiO2）was effective in enhancing the decomposition of raw ZrSiO4,therefore accelerating the synthesis of ZrB2-SiC.Moreover,microstructural observation showed that the as-prepared ZrB2 and SiC respectively had well-defined hexagonal columnar and fibrous morphology.Furthermore,the methodology of back-propagation artificial neural networks（BP-ANNs）was adopted to establish a model for predicting the reaction extent（e g,the content of ZrB2-SiC in final product）in terms of various processing conditions.The results predicted by the as-established BP-ANNs model matched well with that of testing experiment（with a mean square error in 10^（-3） degree）,verifying good effectiveness of the proposed strategy.

Joining and machining of(ZrB2-SiC)and(Cf-SiC)based composites

Filler materials of(ZrB_2-SiC-B_4C-YAG) composite were developed for gas tungsten arc welding(GTAW) of the ZrB_2-SiC and Cf-SiC based composites to themselves and to each other. Reaction with filler material,porosity and cracks were not observed at weld interfaces of all the joints. Penetration of filler material in to voids and pores existing in the Cf-SiC composites was observed. Average shear strength of 25.7 MPa was achieved for joints of Cf-SiC composites. By incorporation of Cf-SiC(CVD) ground short fibre reinforcement the(ZrB_2-SiC-B_4C-YAG) composite was machinable with tungsten carbide tool. The joint and machined composites were resistance to oxidation and thermal shock when exposed to the oxy-propane flame at 2300℃ for 300s. The combination of(ZrB_2-SiC-B_4C-YAG) and Cf-SiC based composites can be used for making parts like thermal protection system or nozzles for high temperature applications.

Tape Casting of Al_2O_3-TiB_2/Al_2O_3-Nano-TiC Multilayer Composites

Ceramic tapes, containing Al2O3-25 wt pct TiB2(B) and Al2O3-25 wt pct nano-TiC (c), have been obtained by tape casting process. Numerous tapes (about 60～80 tapes) were prepared by stacking in turn the composition (B) and (C), laminating under 10 MPa pressure, eliminating the solvent and burning out the polymer additives. The final green bodies were hot pressed at 1750℃ and 30 MPa. The composite has a bending strength of 568 MPa and a fracture toughness of 5.8 M Pa·m1/2. SEM analysis exhibits that Al2O3 particle growth was inhibited by TiC particles in C. but TiB2 particles could not hinder Al2O3 growth in B. The curves of GTA indicates that all organic additives could be removed completely above 600℃

Microstructures and Dry Sliding Wear Properties of ZrB_2/A356 Composites Synthesized by Magneto-chemistry in situ Reaction

In situ A356-x%PVF （particle volume fraction） ZrB2 （ x=1, 3, 5） composites were prepared via magneto-chemistry in situ reaction and the dry sliding wear properties of the composites were investigated. The experimental results show that ZrB2 reinforcement particle is obtained and its morphology mainly present in spherical and regular hexagon. Wear test results show that the values of wear weight loss of the composites decrease with the increase of value under a given sliding time and a certain load of 60 N. Especially, when x=5, the weight loss of the as-prepared composite is 43.1 mg, which is only 36.4% to that ofA356 alloy, 118 rag. The wear mechanism is changed from adhesion wear to adhesion wear and abrasive wear and then to abrasive wear with the increase of x value.

ON ZrO_2-NiCr COMPOSITE POWDER FOR SPRAYING

Composite powder prepared of calcium oxide stabilzed ZrO2(CSZ) and NiCr by vacuum sintering is studied,and the effect of additive TiO2 on its properties is discussed. The morphology and phases of the powder were measured and determined by SEM. EPMA, XRD methods and the testing of flowability, bulk density and microhard-ness. The results show that the metallic components in the powder are in homogeneous distribution , the flowability, bulk density and microhardness are superior to the ZrO2-NiCr powder prepared by mechanically mixing. All components in the coating made of the composite powder are well-distributed because of avoiding segregation in the middle of mixing. It proves that the composite powder is an excellent material for plasma spraying.

低成本TiH_(2)粉末制备Ti/(TiB+TiC)复合材料的组织与性能

使用低成本的TiH_(2)粉末代替纯钛粉,通过添加B4C原位生成TiB和TiC两种增强相,经过真空无压烧结及热挤压工艺制备出具有优异力学性能的Ti/(TiB+TiC)钛基复合材料,分析了制备工艺和增强相对复合材料组织与性能的影响。结果表明,TiH_(2)粉末具有较好的烧结活性,脱氢烧结样品的相对密度可达97.7%;经热挤压工艺,相对密度进一步提升到99.9%,接近于全致密。增强相TiB为短纤维状,TiC为颗粒状,均匀分布在等轴α-Ti基体中,能抑制等轴晶的长大,细化晶粒。热挤压工艺能进一步细化晶粒,使组织更加均匀致密,挤压态钛基复合材料具有高硬度和良好的强塑性匹配。TiH_(2)+4%B4C(体积分数)挤压态复合材料维氏硬度Hv0.3310,屈服强度683 MPa,抗拉强度851 MPa,断后伸长率15.1%。

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.

In-situ Preparation of Al/TiAl_3(P) Composite by Direct Reaction between Molten Al and TiO_2 Powder

TiAl3 particle reinforced pure Al composite has been made by direct reaction among molten Al, TiO2 and a flux. The composite exhibits high Synthetically properties. The strength and hardness are higher than those of pure Al matrix by 71.5% and 134% respectively However, the elongation is 20.36%, slightly lower than that of the Al matrix.

Combustion Synthesis of C/MgAl_(2)O_(4)Composite Powders and Their Microstructure Evolution

This work aims at solving the problems of difficult dispersion,easy oxidation and high cost of nano carbon during application,carbon/magnesium aluminate spinel(C/MgAl_(2)O_(4))composite powders were prepared using MgC2O4·2H2O,MgO2,Al2O3 powder,and Al powder as raw materials by combustion synthesis.The results indicate that with the maximum MgC2O4·2H2O addition of 33.34 mass%,the prepared powder contains 1.17 mass%of carbon and carbon distributes among spinel grains.The MgAl_(2)O_(4)spinel shows both granular and rod-like morphologies.The granular MgAl_(2)O_(4)spinel is generated from mutual diffusion between MgO and Al2O3;while the rod-like MgAl_(2)O_(4)spinel is mainly formed by the vapor-solid growth mechanism from Mg vapor and Al2O3.

ZrB2加入量对BN-AlN-TiB2复相陶瓷性能的影响

以h-BN、TiB2、AlN、Y2O3粉为主要原料,研究了ZrB 2的加入量(添加质量分数分别为1%、2%、3%、4%和5%)对BN-AlN-TiB 2复相陶瓷显气孔率、抗弯强度和电阻率的影响,并用XRD和SEM分别表征了试样的物相组成和显微结构。结果表明:随着ZrB2添加量(w)从1%增加到5%,BN-AlN-TiB2复相陶瓷的显气孔率显著降低,抗弯强度显著提高,而电阻率明显降低。当ZrB2加入量(w)为5%时,BN-AlN-TiB2复相陶瓷的各项性能较佳。

二步煅烧法制备高热稳定性无定形SiO_(2)-Al_(2)O_(3)复合粉体

采用二步煅烧法制备了高热稳定性无定形SiO_(2)-Al_(2)O_(3)复合粉体。首先以十二水合硫酸铝铵和葡萄糖为原料混合煅烧制备无定形Al_(2)O_(3)前驱体,然后以正硅酸乙酯形式向无定形Al_(2)O_(3)中引入适量的SiO_(2),煅烧后即可获得具有高热稳定性的无定形SiO_(2)-Al_(2)O_(3)复合粉体。当SiO_(2)引入量为26%(质量分数)时,粉体经950℃下煅烧4 h仍为无定形结构;对前驱体进行研磨后,粉体热稳定性进一步提升,在1 020℃下煅烧4 h仍为无定形结构。研究表明,SiO_(2)引入量和前驱体混合均匀性对复合粉体的高温热稳定性有明显影响。

Fabrication of ZrC/ZrB2/C Composites Having High Mechanical Properties at Elevated Temperatures

Ultra-high temperature ceramics (UHTCs) are most recently getting much attention for structural parts of hypersonic missiles with their cruising speed of more than Mach 5. Most of the UHTCs are poor sinterability carbides, nitrides, and borides. Therefore, they have been studied and developed for a long time. However, there are still many problems to solve. In this paper, based on the solid-state reaction presented as an equation of (x + y)·ZrC + 2·y·B → x·ZrC + y·ZrB2 + y·C, three-phase ZrC/ZrB2/C composites have been fabricated from ZrC and amorphous B powders using pulsed electric-current pressure sintering at 1373 to 2173 K for 6.0 × 102 s under 50 MPa in a vacuum. ZrC/ZrB2/C = 30/70/C~70/30/C vol% composites with the relative densities Dr of 96.6 to 98.7% were obtained at 2073 K. The 60/40/C vol% composite revealed high bending strength σb (554 MPa), Vickers hardness Hv (19.2 GPa) and moderate fracture toughness KIC (5.25 MPa·m1/2) at room temperature. Furthermore, all composites showed elastic deformation up to 1873 K and revealed σb more than 600 MPa at this temperature, in addition, some composites showed higher σb than 900 MPa at the same temperature. These high mechanical behaviors are discussed with those of the simple binary ZrC/ZrB2 composites which were fabricated under the same conditions except for their starting materials. The best mechanical properties of binary composites were σb (474 MPa), Hv (18.5 GPa), and KIC (4.45 MPa·m1/2) at room temperature, and σb of 400 - 700 MPa at 1873 K. Overall, three-phase composites, nevertheless including soft carbon, have higher mechanical properties than the binary composites.

碳热还原氮化硅藻土制备Si_(2)N_(2)O/SiC复合粉体

氧氮化硅(Si_(2)N_(2)O)是性能优良的耐火材料和高温结构材料,具有优异的抗蠕变性、耐腐蚀性和抗氧化性等优点。本工作利用硅藻土作为硅源,乙炔炭黑作为还原剂,通过碳热还原氮化法在硅藻土表面原位合成Si_(2)N_(2)O/SiC复合粉体。采用X射线衍射(XRD)分析、扫描电子显微镜(SEM)和N_(2)吸附/脱附等温线对样品进行表征。结果表明,在原料配比m(硅藻土)∶m(乙炔炭黑)=1∶1条件下,1 450℃煅烧4 h后,SiO_(2)完全转变成Si_(2)N_(2)O和β-SiC物相,样品整体呈球状形貌,大颗粒周围分布大量片层状小颗粒,并存在介孔结构。Si_(2)N_(2)O/SiC复合粉体作为性能优良的高温结构材料,有望在结构复合材料中得到广泛的应用。