Mesomechanism of steel fiber reinforcement and toughening of reactive powder concrete

Reactive powder concrete (RPC) is a novel cement-based composite material with ultra-high strength. Embedding a certain amount of short steel fibers in the matrix can improve the RPC's toughness and overcome the disadvantage of high brittleness. In this paper, a number of direct uniaxial tension tests have been carried out with '8-shape' RPC200 specimens. The bond-slip process, mesoscopic structural variation and mechanical characteristics of a fiber pullout of the matrix have been investigated using the real-time SEM loading system and CCD observation techniques. The influence of the volume of embedded short steel fibers in matrix on the mesoscopic morphology of attachments on the surface of a pulled individual fiber, the initial cracking force, the ultimate pullout force, interfacial bond strength and the pullout rupture energy have been analyzed. A general formulation relating these quantities to the volume of fibers in matrix has been proposed. The components comprising the interfacial bond strength have been outlined. In addition, the contribution that fibers make to enhance and toughen the reactive powder concrete has been discussed. It is shown that there exists an optimal threshold of fiber volume ρv, opt =1.5% at which the bond performance of a fiber pullout of RPC behaves best.     

High temperature plastic deformation behavior of non-oriented electrical steel

High temperature plastic deformation behavior of non-orientated electrical steel was investigated by Gleeble 1500 thermo-mechanical simulator at strain rate of 0.01−10 s⁻¹ and high temperature of 500–1 200 °C. The stress level factor (a), stress exponent (n), structural factor (A) and activation energy (Q) of high temperature plastic deformation process of non-orientated electrical steel in different temperature ranges were calculated by the Arrhenius model. The results show that, with dynamic elevation of deformation temperature, phase transformation from α-Fe to γ-Fe takes place simultaneously during plastic deformation, dynamic recovery and dynamic recrystallization process, leading to an irregular change of the steady flow stress. For high temperature plastic deformation between 500 and 800°C, the calculated values of a, n, A, and Q are 0.039 0 MPa⁻¹, 7.93, 1.9×10¹⁸ s⁻¹, and 334.8 kJ/mol, respectively, and for high temperature plastic deformation between 1 050 and 1 200 °C, the calculated values of a, n, A, and Q are 0.125 8 MPa⁻¹, 5.29, 1.0×10²⁸ s⁻¹, and 769.9 kJ/mol, respectively. Foundation item: Project(2005038560) supported by the Postdoctoral Foundation of China; Project(05GK1002-2) supported by Key Program of Hunan Province     

A study on the bonding structure of CaO−SiO2 slag by means of molecular dynamics simulation

The investigation results of the bonding structure of CaO−SiO₂ slag by means of molecular dynamics simulation are presented. The characteristics of partial radial distribution functiong _ij(r) are in good agreement with the measurement of X-ray diffraction, and the variation ofQ _n with different SiO₄ tetrahedra following the change ofX _CaO is consistent with the results of Raman spectroscopy. The partial vibrational density of states Γ_Si(ω) shows that two bands appear in the range of 636–737 cm⁻¹ and 800–1200 cm⁻¹ respectively which are also consistent with Raman spectroscopy. Project supported by Shanghai Natural Science Foundation.     

Use of double edge-cracked Brazilian disk geometry for compression-shear fracture investigation of rock

A new specimen geometry-the double edge-cracked Brazilian disk and a relevant fracture analysis by weight function method are proposed for the investigation of rock fracture caused by compression-shear loading. Not only can the mixed mode fracture with any ratio of KI /KII be achieved, but also the pure mode n crack extension can be obtained. The combined mode fracture analysis for this geometry shows that diametral compression in the far-field can induce a compression-shear stress state in the singular stress field ahead of crack tips. Experimental investigations conducted on marble specimens show that the pure mode [I crack extension can be obtained when the dimen-sionless crack length a>0. 7 and the inclined crack angle 5°≤ψ≤40°. Normalized mode I and mode II stress intensity factors decrease from -0. 45 and 2. 47 at ψ= 5° to - 1. 65 and 1. 52 at ψ=40°, respectively. The strains at three points of specimen are also measured in order to investigate the influence of stress singularity on initi     

Physical simulation of hot deformation of TiAI based alloy

In order to establish a model between the grain size and the process parameters, the hot deformation behaviors of Ti-49.5Al alloy was investigated by isothermal compressive tests at temperatures ranging from 800 to 1 100 ℃ with strain rates of 10^-3-10^-1 s^-1. Within this range, the deformation behavior obeys the power law relationship, which can be described using the kinetic rate equation. The stress exponent, n, has a value of about 5.0, and the apparent activation energy is about 320 J/mol, which fits well with the value estimated in previous investigations. The results show that, the dependence of flow stress on the recrystallized grain size can be expressed by the equation: σ = K1 drex^-0.56. The relationship between the deformed microstructure and the process control parameter can be expressed by the formula: lgdrex= -0.281 1gZ 3.908 1.     

Superplasticity of Mg-5.8Zn-1Y-Zr alloy sheet fabricated by combination of extrusion and hot-rolling processes

Superplastic behaviors of quasicrystal phase containing Mg-5.8Zn-1Y-0.48Zr alloy sheets fabricated by combination of extrusion and hot-rolling processes have been investigated at temperature ranging from 623 to 753 K and at the strain rates ranging from 10-4 to 10-2 s-1 by uniaxial tensile tests. An excellent superplasticity with the maximum elongation to failure of 1020% was obtained at 753 K and the strain rate of 1.04×10-3 s-1 and its strain rate sensitivity, m, is as high as up to 0.75. The microstructure was stable during superplastic deformation due to the uniformly distributed fine quasicrystal particles. In addition, micro-cavities and their coalescences were observed in the superplastic deformation of the ZW61 magnesium alloy. Grain boundary sliding (GBS) was considered to be the main deformation mechanism during the superplastic deformation. Dislocation creep controlled by atom diffusion through grain boundaries or interior grains is suggested mainly to accommodate the GBS in super-plastic deformation.     

Stability, detonation properties and pyrolysis mechanisms of polynitrotriprismanes C6H6-n（NO2）n （n=1-6）

To further test whether polynitriprismanes are capable of being potential high energy density materials (HEDMs), extensive theoretical calculations were carried out to investigate on a series of polynitrotriprismanes (PNNPs): C₆H_6−n (NO₂) _n (n=1−6). Heats of formation (HOFs), strain energies (SE), and disproportionation energy (DE) were obtained using B3LYP/6-311+G(2df, 2p)//B3LYP/6-31G* method by designing different isodesmic reactions, respectively. Detonation properties of PNNPs were obtained by the well-known KAMLET-JACOBS equations, using the predicted densities (ρ) obtained by Monte Carlo method and HOFs. It is found that they increase as the number of nitro groups n varies from 1 to 6, and PNNPs with n≥4 have excellent detonation properties. The relative stability and the pyrolysis mechanism of PNNPs were evaluated by the calculated bond dissociation energy (BDE). The comparison of BDE suggests that rupturing the C-C bond is the trigger for thermolysis of PNNPs. The computed BDE for cleavage of C-C bond (88.5 kJ/mol) further demonstrates that only the hexa-nitrotriprismane can be considered to be the target of HEDMs.     

Study on the thermal stability and electrical properties of the high-k dielectrics (ZrO2)x(SiO2)1?x

(ZrO₂) _x (SiO₂)_1−x (Zr-Si-O) films with different compositions were deposited on p-Si(100) substrates by using pulsed laser deposition technique. X-ray photoelectron spectra (XPS) showed that these films remained amorphous after annealing at 800°C with RTA process in N₂ for 60 s. The XPS spectra indicated that Zr-Si-O films with x=0.5 suffered no obvious phase separation after annealing at 800°C, and no interface layer was formed between Zr-Si-O film and Si substrate. While Zr-Si-O films with x >0.5 suffered phase separation to precipitate ZrO₂ after annealing under the same condition, and SiO₂ was formed at the interface. To get a good interface between Zr-Si-O films and Si substrate, Zr-Si-O films with bi-layer structure (ZrO₂)_0.7(SiO₂)_0.3/(ZrO₂)_0.5(SiO₂)_0.5/Si was deposited. The electrical properties showed that the bi-layer Zr-Si-O film is of the lowest equivalent oxide thickness and good interface with Si substrate. Supported by the National Nature Science Foundation of China (Grant No. 60636010) and the National Basic Research Program of China (“973” Program) (Grant No. 2004CB619004)     

A type of recurring relation on sequences and efficient decoding of a class of algebraic-geometric codes (II)

For a class of algebraic-geometric codes, a type of recurring relation is introduced on the syndrome sequence of an error vector. Then, a new majority voting scheme is developed. By applying the generalized Berlekamp-Massey algorithm, and incorporating the majority voting scheme, an efficient decoding algorithm up to half the Feng-Rao bound is developed for a class of algebraic-geometric codes, the complexity of which isO(γo ₁ n ², wheren is the code length, and γ is the genus of curve. On different algebraic curves, the complexity of the algorithm can be lowered by choosing base functions suitably. For example, on Hermitian curves the complexity isO(n^7/3). Project supported by the National Natural Science Foundation of China (Grant Nos. 69673025 and 69673016).     

Critical driving force for martensitic transformation fcc (γ)→hcp(ε) in Fe-Mn-Si shape memory alloys

By the application of Chou's new geometry model and the available data from binary Fe-Mn, Fe-Si and Mn-Si systems, as well as SGTE DATA for lattice stability parameters of three elements from Dinsdale, the Gibbs free energy as a function of temperature of the fcc(γ) and hep(ε) phases in the Fe-Mn-Si system is reevaluated. The relationship between the Neel temperature of the γ phase and concentration of constituents in mole fraction, is fitted and verified by the experimental results. The critical driving force for the martensitic transformation fcc (γ)→ hep (ε), △ G_C~(γ→ε), defined as the free energy difference between γ and ε phases at M_s of various alloys can also be obtained with a known M_s. It is found that the driving force varies with the composition of alloys, e. g. △ G_C~(γ→ε) = - 100.99 J/mol in Fe-27.0Mn-6.0Si and △ G_C~(γ→ε) = - 122.11 J/mol in Fe-26.9Mn-3.37Si. The compositional dependence of critical driving force accorded with the expression formulated by Hsu of the     

Synthesis and oxidation behavior of boron-substituted carbon powders by hot filament chemical vapor deposition

Boron-substituted carbon powder, B _x C_1−x with x up to 0.17, has been successfully synthesized by hot filament chemical vapor deposition. The boron concentration in prepared B _x C_1−x samples can be controlled by varying the relative proportions of methane and diborane. X-ray diffraction, transmission electron microscopy, and electron energy loss spectrum confirm the successful synthesis of an amorphous BC₅ compound, which consists of 10–20 nm particles with disk-like morphology. Thermogravimetry measurement shows that BC₅ compound starts to oxidize approximately at 620°C and has a higher oxidation resistance than carbon. Supported by the National Natural Science Foundation of China (Grant Nos. 10474083, 50472051, 50532020, 50672081) and the National Basic Research Program of China (Grant No. 2005CB724400)     

A new method of characterizing equivalent strain for equal channel angular processing

In order to establish the quantitative relationship between equivalent strain and the performance index of the deformed material within the range of certain passes for equal channel angular processing (ECAP), a new approach to characterize the equivalent strain was proposed. The results show that there exists better accordance between mechanical property (such as hardness or strength) and equivalent strain after rolling and ECAP in a certain range of deformation amount, and Gauss equation can be satisfied among the equivalent strain and the mechanical properties for ECAP. Through regression analysis on the data of hardness and strength after the deformation, a more generalized expression of equivalent strain for ECAP is proposed as: ɛ=k ₀exp[−(k ₁ M−k ₂)²], where M is the strength or hardness of the material, k ₁ is the modified coefficient (k ₁∈(0, 1)), k ₀ and k ₂ are two parameters dependent on the critical strain and mechanical property that reaches saturation state for the material, respectively. In this expression the equivalent strain for ECAP is characterized novelly through the mechanical parameter relating to material property rather than the classical geometry equation. Foundation item: Projects(50471102, 50671089) supported by the National Natural Science Foundation of China     

Thermodynamic prediction ofM s in Fe−Mn−Si shape memory alloys associated with fcc (γ) → hcp (ε) martensitic transformation

By means of X-ray diffraction profile analysis of three different composition Fe−Mn−Si alloys, the relationship between stacking fault probabilityP _sf with the concentrations of constituents in alloys, 1/P _sf =540.05+23.70× Mn wt%-138.74×Si wt%, was determined. According to the nucleation mechanism by stacking fault in this alloy, the equation between critical driving force †G _c andP _sf †G _c=67.487+0.177 5/P _sf (J/mol), was made. Therefore, the relationship between critical driving force and compositions was established. Associated with the thermodynamic calculation, theM _s of fcc (γ)→ hcp(ε) martensitic transformation in any suitable composition Fe−Mn−Si shape memory alloys can be predicted and results seem reasonable as compared with some experimental data. Project supported by the National Science Foundation of China (Grant No. 59671023).     

The Effect of Bi2O3 on the Structure and Microwave Dielectric Properties of Ba6-3xNd8＋2xTi18O54 System（x=2/3）

The structures and dielectric properties of Ba6-3xNd8＋2xTi18O54 system（x=2/3） doped with different contents of Bi2O3, whose final molecular formula is Ba6-3x（Nd1-yBiy）8＋2xTi18O54 were investigated. It is indicated that the dielectric constant increases greatly whereas Q value（f0=4 GHz） decreases with the increase of Bi2O3 content. However, the temperature coefficient could be controlled below 0±30×10^-6/℃ in the experiment. These phenomena are related to the appearance of a new phase, Bi4Ti3O12, which has high dielectric constant. Also, that Bi^3＋（0.13 nm） substitutes for Nd^3＋（0.099 5 nm） will increase the unit cell volume, which will lead to the enlargement of the octahedron B site occupied by Ti^4＋. So the spontaneous polarization of Ti^4＋ ions will be strengthened. Besides, Bi^3＋ will fill up some vacancies which Ba^2＋ or Nd^3＋ ions leave in two A1 sites and four A2 sites. More positive ions polarize, which also contributes to higher dielectric constant. The samples got with the optimium properties are sintered at 1 200 ℃ for 4 h, when y=0.25, ε≈110, Q≈5 400（f0=4 GHz）, TCC=-4.7×10^-6/℃; When y=0.3, ε≈120, Q≈5 000（f0=4 GHz）, TCC=-24×10^-6/℃.     

Flow stress behavior of Cu13Zn alloy deformed at elevated temperature

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Novel asymmetrical twin-core photonic crystal fiber for gain-flattened Raman amplifier

A novel asymmetrical twin-core photonic crystal fiber was proposed, whose effective overlap core area A _eff can be designed to synchronize the variation of Raman gain coefficient with respect to frequency. This fiber possesses a higher and flatter Raman gain efficiency coefficient curve r _R=g _R/A _eff over a specified band of wavelength than a conventional fiber. Therefore, it is a good candidate of gain medium for a flat, broad gain band fiber Raman amplifier. It was numerically demonstrated that for the Raman gain efficiency r _R, relative fluctuations of less than 2.2% and 5.7% are achievable in the C (1530−1565 nm) band and L (1565−1625 nm) band, respectively. Supported by the National Natural Science Foundation of China (Grant Nos. 60588502, 60607005, 60877033), the Science and Technology Bureau of Sichuan Province (Grant No. 2006z02-010-3) and the Youth Science and Technology Foundation of UESTC (Grant No. JX0628)     

Novel wide band gap alloyed semiconductors, <Emphasis Type="Italic">x</Emphasis>(LiGaO<Subscript>2</Subscript>)<Subscript>1/2</Subscript>-(1−<Emphasis Type="Italic">x</Emphasis>)ZnO,and fabrication of their thin films

Oxide semiconductor alloys of x(LiGaO₂)_1/2-(1−x)ZnO were fabricated by the solid state reaction between β-LiGaO₂ and ZnO and rf-magnetron sputtering. For the solid state reaction, the wurtzite-type single phases were obtained in the composition range of x⩽0.38. The formation range of the alloys was wider than that of the (Mg_1−x Zn _x )O system, because the β-LiGaO₂ possesses a wurtzite-derived structure and approximately the same lattice constants with ZnO. The electrical resistivity and energy band gap of the 0.38(LiGaO₂)_1/2-0.62ZnO alloyed ceramic were 0.45 Ωcm and 3.7 eV, respectively, at room temperature. For the alloying by sputtering, the films consisting of the wurtzite-type single phase were obtained over the entire composition range of x(LiGaO₂)_1/2-(1−x)ZnO. The energy band gap was controllable in the range from 3.3 to 5.6 eV. For the as-deposited film fabricated using the 0.4(LiGaO₂)_1/2-0.6ZnO alloyed ceramic target, the energy band gap was 3.74 eV, and the electrical resistivity, carrier density and the Hall mobility at room temperature were 3.6 Ωcm, 3.4×10¹⁷ cm⁻³ and 5.6 cm² V⁻¹ s⁻¹, respectively.     

Synthesis and characterization of Nd doped M-type hexagonal barium ferrite ultrafine powders

Ｂｅｃａｕｓｅｐｅｒｍａｎｅｎｔｂａｒｉｕｍｆｅｒｒｉｔｅｍａｔｅｒｉａｌｐｏｓｓｅｓｓｅｓｒｅｌａｔｉｖｅｌｙｈｉｇｈｓａｔｕｒａｔｉｏｎｍａｇｎｅｔｉｚａｔｉｏｎ ,ｇｒｅａｔｃｏｅｒｃｉｖｅｆｏｒｃｅａｎｄｈｉｇｈｍａｇｎｅｔｉｃａｎｉｓｏｔｒｏｐｙｆｉｅｌｄａｓｗｅｌｌａｓｅｘｃｅｌｌｅｎｔｃｈｅｍｉｃａｌｓｔａｂｉｌｉｔｙａｎｄｃｏｒｒｏｓｉｏｎｒｅｓｉｓｔａｎｃｅ ,ｉｔｈａｓｂｅｅｎｗｉｄｅｌｙｕｓｅｄａｓｔｒａｄｉｔｉｏｎａｌｐｅｒｍａｎｅｎｔｍａｇｎｅｔｓ,ｈｉｇｈ ｄｅｎｓｉｔｙｍ…     

Ball-milling-induced polytypic transformation of 6H-SiC→3C-SiC

The results of X-ray diffraction (XRD) and high resolution electron microscopy (HREM) show that ball milling at room temperature can induce the polytypic transformation of 6H-SiC→3C-SiC. HREM study reveals that a large number of partial dislocations which play an important role in the transformation can be introduced into SiC crystals during BM by the instant and repeated collisions between balls and powder. The phase transformation follows the route: 6H= (3~ ,3~-)→(4~ ,2~-)→(5~ , 1~- )→(6~ ,0~- ).     

Evaluation of Fracture Toughness of Al-Cu-Fe-B Quasicrystal

1　IntroductionTheapplicationsofthequasicrystals (QCs)haveal waysbeenemphasized ,astheyhaveaseriesofunusualphysicalandchemicalproperties (forexample ,highhard ness ,lowsurfaceenergyandfrictionalcoefficient) ,result ingfromitsaperiodicalstructure .Manystudies[1-3 ] indi catedthatmostQCsareverybrittleatroomtemperatureandveryeasilytobefracturedevenifhavingverysmalldeformation .Athightemperature ,Al Cu TMQCs (TM =Fe B ,Cr ,Si)showahighplasticityaftercompressiontesting .ForAl Cu FeQCs ,asm…