The long-term air oxidation behavior of in situ synthesized composites MoSi2-SiC with different volume fractions of SiC at 700 ℃ for 1000 h was investigated. The disintegration (pest) of the composites has not been observed after oxidation for 1000 h. The oxidation resistance of composites is significantly higher than that of monolithic MoSi2. The in situ synthesized composite MoSi2-30%SiC possesses higher oxidation resistance than the traditional composite with the same chemical composition fabricated by hot-pressing the mixture of commercial powders of MoSi2 and SiC. The oxide scale formed on the prepared composite is only composed of amorphous silica, therefore, the oxidation reaction of the materials may mainly occur between MoSi2 and O2. Silicon and molybdenum may simultaneously be oxidized at 700℃ for the composite, however, due to the faster volatilization of MoO3, thereby a thin continuous and dense amorphous SiO2 protective scale is rapidly formed on the composite surface, so the composite exhibits excellent long-term oxidation resistance.
Microstructure and tensile property of pure magnesium processed by cyclic extrusion-compression (CEC) at 350℃ for 2, 4 and 8 passes respectively, as well as for 2 passes at 250℃, 350℃ and 450℃ respectively are investigated. Results showed that the microstructure of pure magnesium is significantly refined and recrystallized after CEC for 2 passes at 350℃. With the increase of passes, the grain sizes of CECed pure magnesium showed no obvious differences. It was found by means of electron backscatter diffraction (EBSD) analysis that a texture with the basal plane of grains inclining 20o -40o to the extrusion direction developed during the CEC processing. With the increase of CEC passes, the maximum intensity of the texture and Schmid factor of basal slip system increased. Compare to that for 2 passes, the yield stress of pure magnesium after CEC for 8 passes at 350℃ decreases from 60 MPa to 41 MPa, and the elongation increases from 8% to 16.7%. The yield stress and grain size of the pure Mg processed by CEC for 2 passes at 250℃, 350℃and 450℃ follows the Hall-Petch relationship, which can be described as .
The corrosion behavior of the newly developed Al-Mg alloys was investigated in terms of their alloying element and thermo-mechanical treatments by means of intergranular corrosion test, exfoliation corrosion test, scanning electron microscopy and transmission electron microscopy. The results show that, with the increase of Mg content the mass loss of the alloys in concentrated nitric acid increased, correspondingly their intergranular corrosion resistance decreases. Zn addition to the Al-Mg alloys led to the formation of Mg32(Al, Zn)49 phase at the grain boundary, which dramatically increased the intergranular corrosion resistance of the alloys. The corrosion resistance of the alloys was also modified by thermo-mechanical treatment. Both intergranular corrosion resistance and exfoliation corrosion resistance of the alloys were dramatically increased by a proper post stabilizing treatment after cold rolling reduction. The residual stress, higher dislocation density and morphology of elongated grains after cold rolling reduction can lead to more continuous precipitation at the grain boundary, thus decrease the corrosion resistance of the alloys.
Composite sheets of carbon nanotubes-cellulose fibers were made by suction filtration method with cellulose fibers as matrix and graphitized CNTs as electroconductive agent. The products were characterized by scanning electron microscopy, shielding effectiveness, four-point probes, and while the effect of carbon nanotube content on the EMI shielding performance of the prepared composite sheets was investigated. The results show that the shape and conductivity is controllable for the composite sheets, which then exhibits good flexibility, electrical conductivity and EMI shielding effectiveness. CNTs were adsorbed on the cellulose fibers and formed a continuously interconnected conductive network. With the increasing amount of CNTs from 10% to 71%, the conductivity of the composite sheets increased from 9.92 S/m to 216.3S/m and correspondingly their EMI shielding effectiveness increased from 15dB to 45dB in the frequency range 175 MHz-1600 MHz.
The 6061-T6 aluminium alloy was modified by multi-pass friction stir process with air or water cooling. Corrosion behavior of the processed zone was investigated by immersion test and electrochemical corrosion test, as well as optical microscopy, SEM and TEM. The results show that the grain size of stirred zone was successfully refined and its corrosion resistance became better obviously after the base metal was modified by FSP with different cooling methods. Compared with the base metal, the nugget zone exhibited higher free-corrosion potential, smaller corrosion current density and greater corrosion impedance. Compared to FSP with water-cooling, the nugget zone prepared by FSP with air-cooling showed better corrosion resistance.
The corrosion wear properties of NiAl-2.5Ta-7.5Cr-x(0, 1)B alloys in 5% H2SO4 solution have been investigated at room temperature. The results show that in comparison with NiAl-2.5Ta-7.5Cr alloy, NiAl-2.5Ta-7.5Cr-1B alloy exhibited better corrosion resistance to 5% H2SO4 solution with passive current density 0.299(μAcm-2)and free corrosion potential -0.213 V. For which the key factor is that a passive film could form on the surface of NiAl-2.5Ta-7.5Cr-1B alloy. The corrosion wear rate of NiAl-2.5Ta-7.5Cr-1B alloy decreased 2-8 times compared with NiAl-2.5Ta-7.5Cr alloy. The abrasion erosion mechanism for NiAl-7.5Cr-2.5Ta-1B alloy was dominated by the synergistic effect of corrosion wear and protectiveness of passive film. While that for NiAl-7.5Cr-2.5Ta alloy was mainly corrosion wear and abrasive wear.
The high temperature mechanical properties of P-contained high strength interstitial free (IF) steel were investigated using Gleeble3500 thermal simulation testing machine, in terms of zero ductility temperature (ZDT), zero strength temperature (ZST) and the relationship of tensile strength with temperature. The fractured surfaces at different tensile temperatures were characterized by means of scanning electron microscope. The phase transition temperature interval was deduced by the THERMO-CALC software. The results show that the ZDT and ZST of the steel are 1420℃ and 1445℃, respectively. The brittleness temperature interval Ⅰ is from 1400℃ to the melting point, and there is no the brittleness temperature interval Ⅲ. The transverse cracks on the surface of casting blank did not occur during the straightening process. The tensile strengths decrease with the increasing temperature in the test temperature range, and which are lower than 5.3 MPa above 1300℃. The result of THERMO-CALC calculation shows that, with the specimens cooling from 500℃ to 400℃, Fe3P precipitates out in the α-Fe matrix, which may results in cold short. The transverse cracks on the continuous casing (CC) slab could dramatically be reduced by Hot-charging Technology.
The effect of coupling agent silane Si69 on properties of nanocomposites of nature rubber with different sized nano-silica (15, 30 and 80 nm)was investigated in terms of curing performance, mechanical properties, Mullins effect, Payne-effect, loss factor, the dynamic heat build-up and dynamic mechanical property etc. while taking the same composites without coupling agent as comparison. Results show that with the addition of the coupling agent Si69, the dispersity of nano-silica in the rubber and the binding of nano-silica with the rubber matrix were improved; the mechanical properties of nano-silica/nature rubber composite were effectively enhanced and the positive sulfuration time t90 was shortened. It is noted that the smaller size of the nano-silica is, the better positive effect on the performance and mechanical properties of the composite can be obtained. Due to the addition of the coupling agent Si69, the Payne-effect, loss factor and the dynamic heat build-up can be reduced for the composites with nano-silica of 15 nm and 30 nm, respectively, but is not obvious for the ones with 80 nm silica.
Interlaminar shear properties along directions of 0 ° and 90° of resin composites reinforced with multilayer-connected biaxial weft knitted(MBWK) fabrics of carbon fibers, were investigated by shear test. The test composites involve three, four and five layer-connected MBWK fabrics respectively. It was found that the shear strength increased with the increasing fiber volume fraction. The variation of the interlaminar shear load-deflection curves tended to be linear. When the load reached the maximum value the load-deflection curve dropped suddenly, and the composites showed brittle fracture. The analysis of fracture surface showed that the structure of reinforcement has significant effect on shear properties of MBWK fabrics reinforced composites. The shear failure mode of the composites was delaminating. Meanwhile, the cracking preferred mainly along the delaminating at the interface between the fabric groups.
The spinel cobalt chromium zinc ferrites (Co0.7Cr0.1Zn0.2Fe2O4) and composites of polyaniline(PANI)-Co0.7Cr0.1Zn0.2Fe2O4 were prepared by polyacrylamide gel and in situ polymerization method, respectively, and then the synthesized materials were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FT-IR), their magnetic properties were measured using vibrating sample magnetometer (VSM) and their microwave absorbing capability was investigated by wave-guide method. The results show that the spinel Co0.7Cr0.1Zn0.2Fe2O4 ferrites and the composites PANI-Co0.7Cr0.1Zn0.2Fe2O4 are acquired. Because a small amount Co2+ ion of the octahedron ferrite is replaced by Cr3+ ions, the lattice constant of the Co0.8Zn0.2Fe2O4 ferrite reduces from 0.8409 nm to 0.8377 nm. The saturation magnetization (Ms), remanent magnetization (Mr) and coercive force (Hc) of the PPy-Co0.7Cr0.1Zn0.2Fe2O4 composites are 8.80 emug-1, 3.14 emug-1 and 37.22 kAm-1, respectively, which are smaller than that of the Co0.7Cr0.1Zn0.2Fe2O4 ferrite. In a measuring frequency range of 5.0-20.0 GHz, two peak values of reflection loss for the composites PPy-Co0.7Cr0.1Zn0.2Fe2O4 appear at 14.1 GHz and 17.9 GHz with -13.17 dB and -15.36 dB, respectively, which is obviously higher than those of the Co0.7Cr0.1Zn0.2Fe2O4 ferrite.
The in vitro biocompatibility (i.e. the cell proliferation and apoptosis of L929 cells) of 4 cast dental alloys (CoCrMo+1%Cu, CoCrMo+2%Cu, CoCrMo+3%Cu and CoCrMo) were evaluated by CCK-8 and Annexing-V/PI double marking methods to provide a biology reference in the clinical application of prosthodontics. Results show that the in vitro biocompatibility of 4 alloys decreases according to following sequence: CoCrMo+1%Cu, CoCrMo+2%Cu, CoCrMo and CoCrMo+3%Cu. However, the cytotoxicity grades of the 4 alloys are all ranked in level one, implying they are good enough in vitro biocompatibility.
Core/shell structured composite abrasives with solid-silica (SSiO2) cores and mesoporous-silica (MSiO2) shells were synthesized using tetraethylorthosilicate as silica source, ammonia solution as catalyst and cetyltrimethylammonium bromide as sacrificial template. The prepared abrasives were characterized by small angle XRD, FESEM, HRTEM, FTIR, TGA, nitrogen adsorption-desorption isotherm and the corresponding pore size distribution. The results show that the MSiO2 shells (70-80 nm in thickness) with radial pores are evenly coated on the external surfaces of the SSiO2 cores (210-230 nm).The BET surface area of the prepared abrasives is 558.2 m2/g. The mesochannels (2-3 nm in pore size) in the (MSiO2) shell are perpendicular to the surface of the SSiO2 core. The topography, roughness and profile curve of silicon wafers with a SiO2 surface film of ca 1200 nm were investigated by AFM before and after chemical-mechanical polishing in the presence of abrasives. By comparison with the conventional abrasives SSiO2, the composite abrasives are in favor of decreasing the surface roughness and increasing the material removal rate of the polished wafers. The MSiO2 shells of the prepared composite abrasives may be contribute to optimize the real interfacial contact between abrasives and wafer surface through certain mechanical and/or chemical effects.
