Materials Science Forum Vols. 620-622

Abstract: The effects of the introduction of a buffer layer between the bond and top coats on the indentation stress-strain behavior and the contact damage were investigated in air-plasma sprayed (APS) zirconia (ZrO2)–based thermal barrier coatings (TBCs). The microstructure is relatively continuous in the TBC system with the buffer layer, showing Zr, Ni, Cr, and Mg elements between the top and bond coats, whereas the Zr element suddenly disappears by passing the interface between the top and bond coats. The TBC system with the buffer layer shows less strain than that without the buffer layer in the higher stress regions above about 1.3 GPa, while both TBC systems become soft by forming the top coat in the lower stress regions compared with the substrate. The stress–strain curve in both TBC systems is dependent on the dwell time of thermal exposure condition. The TBC system with the buffer layer shows the lower stress-strain curves than that without the buffer layer in thermal cycles with the relatively short dwell time of 1 h, showing the reverse trend with the relatively long dwell time of 10 h. Subsurface damage in substrate is reduced at both indentation loads of P = 500 N and P = 2000 N by introducing the buffer layer, independent of thermal exposure. Therefore, the TBC system with the buffer layer is more efficient in protecting the substrate from contact environments than that without the buffer layer, showing cracking or delamination between the top coat and the buffer layer in the TBC system with the buffer layer.

Abstract: The erosion wear characteristics of SiC particles reinforced steel matrix surface composite were studied by self-made slurry erosion wear test apparatus with quartz slurries, and contrasted with that of plain carbon steel, high chromium cast iron and low chromium cast iron. The results indicated that the erosion wear rate of the four tested materials has the different changes not only with the quartz particles sizes in slurry, but also with the erosion angle of slurry. The erosion wear rate of SiC particle reinforced steel matrix surface composite is the lowest in four kinds of tested materials under the test conditions. And that of high chromium cast iron is less than that of low chromium cast iron under the same test conditions. The SiC particles embedded the steel matrix could prevent the steel surface from wear brought by quartz particles in slurry erosion wear, and protect the steel matrix, so that the erosion wear resistance of steel matrix could be increased.

Abstract: Bulk Fe-80at%Ni melts were undercooled by using cyclic superheating and glass slag purification technique, and the maximum undercooling 340 K could be achieved. The microstructures of Fe-80at%Ni alloys were observed by means of optical microscope (OM). The phase composition was identified by X-ray diffraction (XRD) analysis. The magnetic properties of Fe-80at%Ni alloys were measured by vibrating sample magnetometer (VSM) with a DC M-H analyzer. The results showed that there was only single γ-(Fe, Ni) phase existing in undercooled Fe-80at%Ni alloys. Two grain refinements and one grain coarsening were observed in the undercooling range from 28 K to 340 K. The first grain refinement could be ascribed to dendrite-remelting, and the second to recrystallization induced by the stress originating from rapid solidification. The grain coarsening could be considered as a result of solid-state grain coalescence. The measurement of soft magnetic properties showed that the grain size D decreases with an increase of undercooling, the maximum Ms is 109.98emu/g, corresponding to minimum grain size 42.9μm or undercooling 210 K, and the coercive force Hc is in proportion to the reciprocal of grain size D-1.

Abstract: To investigate the possibility and reaction route of fabrication high purity Ti3SiC2 powders using coarse reactants by a relatively simpler method, Ti/Si/TiC mixtures (74μm in average size) added proper amount of aluminum were heated in vacuum sintering furnace with different sintering scheme. The effects of Al addition and temperature on the purity of resultant Ti3SiC2 powder were also examined. The results showed that over 99wt.% content Ti3SiC2 powders could be obtained at optimal temperature 1410°C-1420°C holding 90-120min with 0.10-0.20 Al addition (molar ratio). XRD, SEM and EDS results indicated that Ti5Si3 and TiCx were the main intermediate phases which redounded to synthesize Ti3SiC2 powder subsequently.

Abstract: The mechanical properties of two high manganese steels with manganese contents (15% and 25%) were investigated during tensile tests at room temperature. The results indicated that the strengths of steels were a little low and the elongations were improved greatly with increasing the manganese content. Stress fluctuations were found during tensile tests of the Fe-15Mn steels, and as the strain increased the range of stress fluctuations became wider. The strain hardening exponent n of samples changed with strain in a parabola model. The microstructures before and after deformation were investigated and the results showed that phase transformation of γ (fcc) →ε (hcp) and γ (fcc) →ε (hcp) →α (bcc) induced plasticity occurred in the Fe-15Mn steels. However, in the Fe-25Mn steels, at the beginning stage of the deformation, phase transformation induced plasticity played an important role, and with the increase of deformation, the main mechanism was twinning induced plasticity.

Abstract: The paper described the detailed design and production method of the FRP bar and the anchorage device, and it also introduced the test method of mechanical properties on the tensile strength, elastic modulus and the rate of elongation of the FRP bar. Moreover, it conducted statistical analysis into test results and put forward the mechanical property index for structural design of the FRP bar concrete. Test results indicated that stress-strain relation of the FRP bar presented linear variation. FRP bar is superior to steel bar concerning high tensile strength-quality ratio and high durability, the elastic modulus and the rate of elongation of the FRP bar are also less than those of the steel bar, it is practicable to use FRP bar as a new type of tensile material in concrete structures. And then, it put forward design ideas of the hybrid FRP bar by a series of analysis on the hybrid effect and the ductility of the hybrid FRP bar.

Abstract: The effects of bond coat nature in thermal barrier coating (TBC) systems on the delamination or fracture behavior of the TBCs with different bond coats prepared using two different processes—air plasma spray (APS) and high velocity oxyfuel (HVOF)—were investigated by cyclic thermal fatigue tests. The TBCs with the HVOF bond coat were delaminated or fractured after 3–6 cycles, whereas those with the APS bond coat were delaminated after 10 cycles or show a sound condition. These results indicate that the TBC system with the APS bond coat has better thermal durability than the system with the HVOF bond coat under long-term cyclic thermal exposure. The hardness values of the TBCs (top coats) in both systems are dependent on applied loads, irrespective of the hardness of the bond coats and the substrate. The values are not responded to the bond coat nature or the exposure time. Thermally grown oxide (TGO) layers in both cases consist of two regions with the inner TGO layer containing only Al2O3 and the outer TGO layer of mixed-oxide zone containing Ni, Co, Cr, Al in Al2O3 matrix. The outer TGO layer has a more irregular shape than the inner TGO layer, and there are many pores within the outer layer. At failure, the TGO thickness of the TBC system with the HVOF bond coat is 9–13 m, depending on the total exposed time, and that of the TBC system with the APS bond coat is about 20 m. The both TBC systems show the diffusion layer on the side of substrate in the interface between the bond coat and the substrate. The relationship between the delamination or fracture behavior and the bond coat nature has been discussed, based on the elemental analysis and microstructural evaluation.

Abstract: Porous asphalt is one of the most functional structures for the surface layer of asphalt pavement, which has already shown excellent noise reduction property for its 20% air voids in the structure. However, mechanical degradation is also monitored because of the high air voids, for example, permanent deformation in high service temperature and cracking in low service temperature. The Tafpack Super (TPS) additive was used in this study and expected to improve the resistance to permanent deformation and low-temperature cracking of porous asphalt mixture. Unconfined static uniaxial creep test and three-point bending test were conducted to study the effects of TPS additive on the permanent deformation and anti-cracking property of porous asphalt mixture, respectively. Experimental results showed that the TPS additive decreased the creep deformation and increased the creep stiffness modulus in uniaxial creep test. TPS additive had little effect on the brittleness temperature porous asphalt mixtures as shown in three-point bending testing. However, the flexural strength-strain response curve indicated that with the increase of TPS additive, porous asphalt mixtures exhibited better resistance to cracking. Porous asphalt mixtures with TPS additive indicate excellent temperature susceptibility and can be applied to a wider field of road engineering.

Abstract: One micro-powder rubber (styrene butadiene rubber-SBR) at three concentration levels are used for manufacturing polymer modified asphalt with a mixing technique. The effects of concentration of modifiers and mixing temperature on the mechanical properties of the compounds are investigated. Based on a limited study, test results indicate that micro-powder rubbers have positive effect on the performance of asphalt in low temperature. Soft point, penetration and ductility show that the ideal percentage of rubber additive (7 wt. %) and preparing temperature (210°C) are brought forward. The fluorescence microscopy test indicates that rubbers are homogeneous dispersed in asphalt and have a little dissolving at the surface of the ultra-fine rubbers. The difference between soft point values in the high temperature storage stability test is only 0.2~0.5°C, which proves that modified asphalts have good storage stability. Complex modulus master curves are drawn to analyze and compare the rheological properties of various modified asphalts. Results indicate that modified asphalt binders have more excellent properties at both high and low temperatures, compared with original asphalt.

Abstract: Specimens of SAF2507 super-duplex stainless steel were heated at 920°C with different aging time. The phase transformation and development of microstructure in the materials were investigated by color optical microscopy, scan electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. The hardness was tested by HB-3000B hardness tester. The facts that sigma phase precipitated right after 2 min. At 920°C, the grain boundaries to be the preferential precipitation sites and the sites changed from ferritic-austenitic phase interface into the inside of ferritic phase was found. Increase in aging time is proportional to the increase in sigma phase precipitation mass was most frequently observed in specimens. For phase precipitation, the values of hardness of the super-duplex stainless steel was enhanced evidently.