Bending properties and fracture mechanism of C/C composites with high density preform

C/C composites with banded structure pyrocarbon were fabricated by fast chemical vapor infiltration（CVI）,with C3H6 as carbon source,N2 as carrier gas,and three-dimensional（3D） 12K PAN-based carbon fabric with high density of 0.94 g/cm3 as preform.Experimental results indicated that the fracture characteristics of C/C composites were closely related to the frequency of high-temperature treatment（HTT） at the break of CVI process.According to the load？displacement curves,C/C composites showed a pseudoplastic fracture after twice of HTT.After three times of HTT,load？displacement curves tended to be stable with a decreasing bending strength at 177.5 MPa.Delamination failure and intrastratal fiber fracture were observed at the cross-section of C/C composites by scanning electronic microscope.Because the content of pyrocarbon and fibers has a different distribution in layers,the C/C composites show different fracture characteristics at various regions,which leads to good toughness and bending strength.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Multiscale SiC_p Hybrid Reinforced 6061 Aluminum Matrix Composites

The performance of solid solution aging treatment on aluminum matrix composites prepared by powder metallurgy and reinforced with 6061 aluminum alloy powder as matrix;meanwhile, nano silicon carbide particles(nm Si Cp), submicron silicon carbide particles(1 μm Si Cp) and Ti particles were studied. The Al/Si Cp composite powder was prepared by high-energy ball milling, and then cold-pressed, sintered, hotextruded, and then heat-treated with different solution temperatures and aging times for the extruded composites. Optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy(EDS), X-ray diffractometer(XRD) and extrusion testing were used to analyze and test the microstructure and mechanical properties of aluminum matrix composites. The results show that after the multi-stage solid solution at 530 ℃×2 h+535 ℃×2 h+540 ℃×2 h, the particles are mainly equiaxed grains and uniformly distributed. There is no reinforcement agglomeration, and the surface is dense and the insoluble phase is basically dissolved. In the matrix, the strengthening effect is good, and the hardness and compressive strength are 179.43 HV and 680.42 MPa, respectively. Under this solution process, when the aluminum matrix composites are aged at 170 ℃ for 10 h, the hardness and compressive strength can reach their peaks and increase to 195.82 HV and 721.48 MPa, respectively.

Comparative Study on Microstructure and Mechanical Properties of Coarse-grained WC-based Cemented Carbides Sintered with Ultrafine WC or (W+C) as Additives

The effects of ultrafine WC(WC_(UF),0.5μm) or W(1μm) and C(0.3μm)(W+C)_(UF) additives on the densification,microstructure and mechanical properties of coarse-grained cemented carbides were compared systematically.Overall,the cemented carbides with WC_(UF)/(W+C)_(UF) additives are almost fully densification to be higher than 99%,and the average grain size is kept above 2.8μm.The WC_(UF) additive assists grains to(truncated)trigonal prism shape by two dimensional(2D) growth,whereas the(W+C)_(UF) additive assists grains to rounded shape by three dimensional(3D) growth,lowers WC contiguity and increases face-centered-cubic Co.The hardness and bending strength of(75WC_(C)-15WC_(UF))-10Co are 86.6 HRA and 2 272 MPa,respectively,both higher than those of(75WC_(C)-15(W+C)_(UF))-10Co,which could be ascribed to the enhanced densification and unblemished grains.However,the fracture toughness of the(75WC_(C)-15(W+C)_(UF))-10Co is 23.5 MPa·m^(1/2),higher than that of the(75WC_(C)-15WC_(UF))-10Co due to the uniform WC-Co structure and flexible binder phase.

Effect of(Cr,V)_(2)(C,N)on the Microstructure and Mechanical Properties of WC-10Co Cemented Carbides

WC-10Co cemented carbides with finer WC and narrower grain size distributions are produced by using(Cr,V)_(2)(C,N)as grain growth inhibitors.As a result,with the increase of(Cr_(0.9),V_(0.1))_(2)(C,N)and(V_(0.9),Cr_(0.1))_(2)(C,N),the grains size of WC and mean free path of Co phase decrease,and adjacency of WC increases.Refinement and homogenization of grains enhance the transverse rupture strength(TRS)and the hardness.Meanwhile,the deflection and bridging of cracks keep the fracture toughness at a respectable level.The WC-10Co-0.6(Cr_(0.9),V_(0.1))_(2)(C,N)-0.025(V_(0.9),Cr_(0.1))_(2)(C,N)cemented carbides exhibit excellent comprehensive mechanical properties with the TRS of 4602.6 MPa,hardness of 1835 kg/mm^(2),and fracture toughness of 10.39 MPa·m^(1/2),respectively.However,the large pores are caused by excess N larger than 0.03 wt%and deteriorates the mechanical properties.We provide a new approach to WC-Co cemented carbides preparation with a narrow grain size distribution by adding novel grain growth inhibitors.

Fracture mechanism of 2D-C/C composites with pure smooth laminar pyrocarbon matrix under flexural loading

Using natural gas as carbon source, 2D needle felt as preform, 2D-C/C composites were prepared by thermal gradient chemical vapor infiltration. Their microstructures were observed under polarized light microscope （PLM） and scanning electron microscope （SEM）, and the flexural behaviors before and after heat-treatment were studied with a universal mechanical testing machine. The fracture mechanism of the composites was discussed in detail. The results show that, carbon matrix exhibits pure smooth laminar （SL） characteristic including numerous wrinkled layered structures and some inter-laminar micro-cracks. With the decreasing density, the strength of the composites decreases and the toughness increases slightly; after 2500 °C heat-treatment, the inter-laminar micro-cracks in matrix increase, the strength decreases, and the toughness obviously increases. The fracture mode of the composites changes from brittle to pseudo-plastic characteristic due to more crack deflections in SL matrix.

Microstructure evolution,mechanical properties and fracture behavior of Al-xSi/AZ91D bimetallic composites prepared by a compound casting

In this paper,the effect of the Si content on microstructure evolution,mechanical properties,and fracture behavior of the Al-xSi/AZ91D bimetallic composites prepared by compound casting was investigated systematically.The obtained results showed that all the Al-xSi/AZ91D bimetallic composites had a metallurgical reaction layer(MRL),whose thickness increased with increasing Si content for the hypoeutectic Al-Si/AZ91D composites,while the hypereutectic Al-Si/AZ91D composites were opposite.The MRL included eutectic layer(E layer),intermetallic compound layer(IMC layer)and transition region layer(T layer).In the IMC layer,the hypereutectic Al-Si/AZ91D composites contained some Si solid solution and flocculent Mg_(2)Si+Al-Mg IMCs phases not presented in the hypoeutectic Al-Si/AZ91D composites.Besides,increasing Si content,the thickness proportion of the T layer increased,forming an inconsistent preferred orientation of the MRL.The shear strengths of the Al-xSi/AZ91D bimetallic composites enhanced with increasing Si content,and the Al-15Si/AZ91D composite obtained a maximum shear strength of 58.6 MPa,which was 73.4% higher than the Al-6Si/AZ91D composite.The fractures of the Al-xSi/AZ91D bimetallic composites transformed from the T layer into the E layer with the increase of the Si content.The improvement of the shear strength of the Al-xSi/AZ91D bimetallic composites was attributed to the synergistic action of the Mg_(2)Si particle reinforcement,the reduction of oxidizing inclusions and the ratio of Al-Mg IMCs as well as the orientation change of the MRL.

Microstructure and mechanical properties of 3D fine-woven punctured C/C composites with P_yC/SiC/TaC interphases

The 3D fine-woven punctured C/C-(PyC/SiC/TaC)composites,composed of PyC/SiC/TaC interphases and pyrocarbon (PyC)matrix,were synthesized by isothermal chemical vapor infiltration(ICVI)methods.The alternating layers and the structure of these composites were examined by polarized light microscopy(PLM),X-ray diffractometry(XRD)and scanning electron microscopy(SEM).It is found that the PyC matrix has rough laminar(RL)structure,the TaC layer has NaCl-type cubic structure,and the SiC layer has few wurtzite type 10H-SiC besidesβ-SiC structure.The effects of fiber coating and the bulk density on the tensile and flexural properties of composites along X or Y and Z direction were investigated.It is shown that fiber coated 3D woven punctured C/C composites have good tensile and flexural strength,and the maximum of flexural strength is 375 MPa in X or Y direction at density of 1.89 g/cm 3 ,which is about three times higher than that of samples without TaC/SiC fiber coating.The flexural strength and bending strength increase with increasing the density of the composites.The analysis of fracture surfaces reveals that fibers and fiber bundles are pulled out in composites,indicating that the composite exhibits a non-linear failure behavior through propagation and deflection of the cracks.

Effect of extrusion on the microstructure and mechanical properties of a low-alloyed Mg-2Zn-0.8Sr-0.2Ca matrix composite reinforced by TiC nano-particles

A low-alloyed Mg-2Zn-0.8Sr-0.2Ca matrix composite reinforced by TiC nano-particles was successfully prepared by semi-solid stirring under the assistance of ultrasonic,and then the as-cast composite was hot extruded.The results indicated that the volume fraction of dynamical recrystallization and the recrystallized grain size have a certain decline at lower extrusion temperature or rate.The finest grain size of~0.30μm is obtained in the sample extruded at 200℃ and 0.1 mm/s.The as-extruded sample displays a strong basal texture intensity,and the basal texture intensity increases to 5.937 mud while the extrusion temperature increases from 200 to 240℃.The ultra-high mechanical properties(ultimate tensile strength of 480.2 MPa,yield strength of 462 MPa)are obtained after extrusion at 200℃ with a rate of 0.1 mm/s.Among all strengthening mechanisms for the present composite,the grain refinement contributes the most to the increase in strength.A mixture of cleavage facets and dimples were observed in the fracture surfaces of three as-extruded nanocomposites,which explain a mix of brittle-ductile fracture way of the samples.

Effect of VC/Cr_3C_2 on microstructure and mechanical properties of Ti(C,N)-based cermets

Effects of VC/Cr3C2 on the microstructure and mechanical properties of Ti（C,N）-based cermets were studied. The microstructure was investigated by means of optical microscopy, X-ray diffractometry as well as scanning electron microscopy in combination with energy dispersive spectrometry. Mechanical properties, such as transverse rupture strength, hardness and fracture toughness, were measured. The results show that there are black core-grey rim structure and white core-grey rim structure in the microstructure. The grains become fine due to the VC/Cr3C2, and the grains of cermet added with 0.75VC/0.25Cr3C2 are refined most remarkably. The black core becomes finer with the increase of VC addition and rim phase becomes thicker with the decrease of Cr3C2 addition. The porosity increases with the increase of VC addition in VC/Cr3C2. Compared with the cermet free of VC/Cr3C2, the transverse rupture strength and hardness of cermets with VC/Cr3C2 are both improved, and the maximum values are both found for the cermet with 0.25VC/0.75Cr3C2. The fracture toughness can be effectively promoted by adding VC/Cr3C2 with an appropriate ratio of VC to Cr3C2, and the maximum value is found for the cermet with 0.5VC/0.5Cr3C2.

Effect of CuO addition on microstructure and properties of aluminum composites produced by quick spontaneous infiltration process

Al matrix composites with a high volume fraction of reinforcements were fabricated with a compact of Al-Ti-B4C powder mixtures by quick spontaneous infiltration(QSI) process in an aluminum melt. Given the exothermal nature of the reaction between CuO and Al, a certain amount of CuO addition to the Al-Ti-B4C system dramatically increases the adiabatic temperature and thereby enables the complete combustion reaction in an aluminum melt(about 1,173 K). After the QSI process, the compact fabricated with CuO retains its original shape and the obtained composite exhibits sound microstructure containing reaction products of TiB2, Al3BC and B4C. The formation of such reinforcements when adding CuO contributes to enhancing the properties of the composites that show far superior hardness and elastic modulus of 3.03 GPa and 158.9 GPa, respectively, with lower coefficient of thermal expansion(9.44 ppm?K-1) compared to those with no CuO addition.

Deep understanding of typical CNT morphology on the microstructure and mechanical properties of 2D carbon/carbon composites

For the inadequate interlaminar strength of 2D carbon/carbon(C/C)composite,in-situ grown carbon nanotubes(CNTs)reinforcing strategy was put forward to strengthen the interlaminar matrix at the nanoscale and inhibit the interlaminar cracking.CNT morphology is an essential factor in influencing the enhancement effect.Herein,the influence of in-situ grown CNT morphology on the microstructure and mechanical properties of C/C composite was deeply studied.The radially-aligned straight CNTs could induce the formation of highly-ordered pyrolytic carbon(PyC),while PyC in randomly-distributed curved CNTs concentrated area exhibits an isotropic structure.Further,radially-aligned straight CNTs show better improvement on the flexural and shear strength of C/C composites.According to the fine structural characterization and finite element simulation,the influence mechanism of CNT morphology was revealed.CNT morphology can influence the stress distribution in the PyC protective layer,and compared with radially-aligned straight CNTs,randomly-distributed curved CNTs induce higher tensile stress in the PyC protective layer,which has a detrimental impact on the flexural and shear properties of C/C com-posite.This work provides novel insights into the effect of CNT morphology on the microstructure and mechanical properties of C/C composites,which gives a basis for the structural design and preparation of CNTs reinforced C/C composites.

DIELECTRIC PROPERTIES OF NANO Si/C/N COMPOSITE POWDER AND NANO SiC POWDER AT HIGH FREQUENCIES AND MICROSTRUCTURE CHARACTERIZATION

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me 3Si) 2NH) (Me:CH 3) and SiH 4 C 2H 2 respectively by a laser induced gas phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured between 8 2GHz and 12 4GHz. The real and imaginary parts of the complex permittivities of nano Si/C/N composite powder are much higher than those of nano SiC powder. The SiC microcrystalline in the nano Si/C/N composite powder dissolved a great deal of nitrogen. The local structure around Si atoms changed by introducing N into SiC. Carbon atoms around Si were substituted by N atoms. So charged defects and quasi free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. The high ε″and loss factor tgδ(ε″/ε′) of Si/C/N composite powder were due to the dielectric relaxation.

MECHANICAL PROPERTIES AND MICROSTRUCTURES OF Ti(C,N) BASED CERMETS

ＭＥＣＨＡＮＩＣＡＬＰＲＯＰＥＲＴＩＥＳＡＮＤＭＩＣＲＯＳＴＲＵＣＴＵＲＥＳＯＦＴｉ（Ｃ，Ｎ）ＢＡＳＥＤＣＥＲＭＥＴＳ￥ＬｉｕＮｉｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅａｎｄＥｎｇｉｎｅｅｒｉｎｇ，ＡｎｈｕｉＩｎｓｔｉｌｕｔｅ...

Mechanical properties and microstructure of TRIP steels produced using TSCR process

C-Si-Mn TRIP steels were produced using the thin slab casting and rolling （TSCR） process under simulation in laboratory. The results of tensile tests show that the yield strength, tensile strength, and the total elongation of the experimental TRIP steels are 430 MPa, 610 MPa, and 28.4%, respectively. Optical microscopy, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM） were employed to identify the microstructures of the TRIP steels. The final microstructures consist of ferrite, bainite, and retained austenite. The results of quantitative color metallography show that the fraction of the retained austenite is about 5.8%.

CHEMICAL COMPOSITION AND MICROSTRUCTURE OF Ti(C,-N)-TiN COATING

ＣＨＥＭＩＣＡＬＣＯＭＰＯＳＩＴＩＯＮＡＮＤＭＩＣＲＯＳＴＲＵＣＴＵＲＥＯＦＴｉ（Ｃ，Ｎ）－ＴｉＮＣＯＡＴＩＮＧＯＭａ，Ｌｉｕｙｉｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅａｎｄＥｎｇｉｎｅｅｒｉｎｇ，ＣｅｎｔｒａｌＳｏｕｔｈＵｎ...

Effects of TiB2 Content on the Microstructures and Mechanical Properties of Low-Cobalt Ti（C,N）-TiB2 Cermets Fabricated by Vacuum Hot Pressing

Ti(C,N)-TiB2 cermets were fabricated from Ti(C,N), TiB2, Co and WC powder mixtures via a vacuum hot pressing process. The influence of TiB2 content on their microstructures and mechanical properties was investigated. As a result of the elevated TiB2 contents, two types of corerim microstructures were present in the Ti(C,N)-TiB2 cermets, and remarkably improved mechanical properties were achieved. With the increase of TiB2 content, the flexural strength, fracture toughness and hardness of the Ti(C,N)-TiB2 cermets first increased, and then decreased, while their relative density consistently decreased. Attributed to an integration of the intergranular and intrangranular fracture behaviors, the Ti(C,N)-TiB2 cermets with 20 wt% TiB2 content exhibited the best overall properties with the relative density, hardness, fracture toughness and flexural strength at 99.3%, 1 995 HV, 7.92 MPa·m1/2 and 1 114 MPa, respectively. The underlying mechanism for their enhanced properties was studied in detail.

Effect of Yttrium on Mechanical Properties and Microstructure of Ceramets

The effect of rare earth element yttrium on the mechanical properties and microstructure of Ti(C, N) based ceramets have been studied in this paper. The experimental results have shown that the appropriate addition of rare earth element yttrium can increase the bending strength and hardness, and the reason is mainly due to the reaction of element yttrium with impurity clement sulphur forming the compound particles, thus cleaning the interfaces of ceramics-ceramics and ceramics-metals, and increasing the interfacial bonding strength.

Effect of cooling rate on microstructure and mechanical properties of CB2 tempered martensitic steel

The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings.The 9Cr-1.5Mo-1Co cast steel,referred to as CB2,is one of the most promising alloys for various cast components to be used under ultrasupercritical conditions.In this study,HRTEM,SEM,and XRD methods were used for qualitative and quantitative analyses of contents,phases,and sizes of the inclusions and precipitates,as well as microstructure observation of the tempered martensitic steel at different cooling rates.Traditional tensile tests were conducted to characterize the material mechanical properties.Results show that the size of the boron nitride and precipitate,the width of the martensite lath and the content of theδ-ferrite are reduced as the cooling rate increases from 5-8℃·min^-1(CB2-S steel)to 50-60℃·min^-1(CB2-F steel).The precipitates are mainly composed of M23C6 and a small amount of M3C.The average diameters of the M23C6 precipitates in CB2-F and CB2-S are 202 nm and 209 nm,respectively.The inclusions are mainly composed of BN,Al2O3 and MnO2,and the inclusion ratios are 0.1969%for the CB2-F and 0.06556%for CB2-S steel.The average martensite lath widths of CB2-F and CB2-S steels are 289 nm and 301 nm,respectively.Furthermore,the M3C having a diameter of about 150 nm and a thickness of 20 nm is observed in theδ-ferrite of the tempered CB2-S steel.The presence of theδ-ferrite reduces the precipitation strengthening and dislocation density in CB2-S steel.In addition,the lower cooling rate stabilizes theδ-ferrite structure at room temperature.

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.

SiC/Si-W-Mo coating for protection of C/C composites at 1873 K

In order to prevent carbon/carbon composites from oxidation at 1873 K, an efficient oxidation protective SiC/Si-W-Mo coating was prepared by a two-step pack cementation technique. The microstructures and the phase composition of the as-received multi-coating were examined by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. It is seen that the compact multi-coating is composed of α-SiC, Si, and （WxMO1-x）Si2. Oxidation test shows that, after oxidation at 1873 K in air for 102 h and thermal cycling between 1873 K and room temperature for 10 times, the weight loss of the SiC/Si-W-Mo coated C/C composites is only 0.26%. The invalidation of the multi-coating is attributed to the formation of penetrable cracks in the coating. 2008 University of Science and Technology Beijing. All rights reserved.