Materials Science Forum Vols. 675-677

Abstract: In the environments of saturated limewater at 20±2°C and fly ash in cement mortars at 20±2°C, the hydration mechanism of fly ash is studied. The results indicate: under this test condition, the reaction speed of fly ash presents the degressive tendency basically within 90 days, and its chemical activation is an accumulation change process. The hydrated course of fly ash in the two environments is resembled, and only the age effectiveness is inconsistent. The test methods of Ka value and reaction degree can be used to analyze chemical activation of fly ash effectively. Rapid evaluation can be used on early activation of fly ash by Ka value and the later period one can be estimated by reaction degree.

Abstract: To confirm the sensitivity in the measurement of electrical properties affected by the nano structure of microwave AFM (M-AFM) probe, three kinds of M-AFM probe with a nano-slit on its tip in different width (75 nm, 120 nm and 160 nm) were investigated. Au and glass samples were measured by the probes working at a non-contact AFM mode. The M-AFM probe with the nano-slit having the width of 75 nm, by which the difference of the measured voltage between Au and glass samples is 55.1 mV, shows the highest sensitivity for detecting electrical properties of materials. As the result illustrated, the M-AFM probe with smaller width nano-slit on the tip can be considered to be an ideal nano structure.

Abstract: Carbon fiber-reinforced polymer (CFRP) material is increasingly being used for the repair of steel structures, mainly because of their superior mechanical properties. However, there is insufficient information on the environmental durability of CFRP-steel bond, especially in hygrothermal aging combined with sustained load. In this paper, 500 mm long І steel beam specimens reinforced with externally bonded wet-laid CFRP sheets are investigated. The specimens are subjected to a sustained four point flexural load of about 30% of initial ultimate load, placed into hygrothermal condition. The ultimate bear capacity and the failure characteristics of the CFRP and steel interface are studied after hydrothermal age and loading. The test results show that the coupled action of sustained load and hygrothermal aging reduces the bond strength of CFRP-steel, even for relatively short conditioning periods, and appears to significantly affect the failure characteristics of specimens.

Abstract: Laser additive direct deposition of metals is a new rapid manufacturing technology, which combines with computer aided design, laser cladding and rapid prototyping. The advanced technology can build fully-dense metal components directly from CAD files without a mould or tool. With this technology, a promising rapid manufacturing system called “Laser Metal Deposition Shaping (LMDS)” is being constructed and developed. Through the LMDS technology, fully-dense and near-net shaped metallic parts can be directly obtained through melting coaxially fed powder with a laser. Typically, the microstructure and mechanical properties of the as-formed thin-wall part were tested and analyzed synthetically. The results show that there are different grain categories distributing in certain position of thin-wall part, and the mechanical properties represent as anisotropy.

Abstract: By now, it is well-known that concrete will lose strength after exposure to elevated temperature. In this case, the damaged concrete is extremely vulnerable with respect to ingress of water and aggressive compounds. Therefore the potential for the protection of concrete from excessive ingress of water after exposure to high temperature, as for instance in an accidental fire, has been investigated. So far, surface impregnation of concrete with silane has been proved to be beneficial to reduce water penetration. In this contribution, surface impregnation with silane was applied on concrete exposed to elevated temperature. The efficiency of surface impregnation with respect to absorption of water and salt solutions by concrete with different levels of damage induced by elevated temperature has been investigated in particular. Results indicate that the increased water absorption of damaged concrete can be reduced significantly by surface impregnation. A reduction of more than 90 % can be achieved. The effective chloride barrier established by surface impregnation can help to extend the service life of fire-exposed concrete structures.

Abstract: The grain boundary character distributions (GBCDs) of two 304-type austenitic stainless steel samples, which are cold rolled with the same thickness reduction of 6% and then subjected to different annealing treatments are examined by Electron back-scatter diffraction (EBSD) techniques. The results indicate that, after 6% cold rolling, the two-step annealing (900°C/1 h+1050°C/5 min, Sample B) resulted in a quite different GBCDs compared with that obtained by single-step annealing (1050°C/5 min, Sample A) regardless of nearly the same fractions of S3 boundaries developed in the two samples. It was evidenced that large-sized cluster of grains interfaced by S3n boundaries (S3n CG) emerged in sample B, in which there are moderate preferential grain orientation distribution and {111} grain boundary plane distributions. Further discussions highlighted that, in the case of two-step annealing, the first annealing at relatively low temperature (900°C) for short time (1h) contributes a lot to the final optimization of GBCD and strain induced preferential boundary migration (SIBM) accompanied by multiple twinning via the migration and interactions between mobile S3n boundaries, especially incoherent S3 boundaries, might be the responsible mechanisms.

Abstract: Five-layered Al/Al-Cu functionally graded material (FGM) was fabricated by powder metallurgy technology. The microstructure and composition of the prepared specimen were studied. Vickers hardness, flexural strength and fracture surface morphology were also measured. The results showed that Al/Al-Cu graded material with dense structure and compositional continuous change was obtained by solution-precipitation method. The graded materials presented a compositional continuous change along the graded direction because of the diffusion effect, and the Vickers hardness was liner proportional to the distribution of Cu content. Compared with pure sintered Al, remarkable improvement on hardness and fracture strength was achieved due to the CuAl2 phase dispersively distributed in the matrix. With the increase of Cu content, the fracture mode changed from tough fracture to the tendency of brittle fracture.

Abstract: The flexural behavior of reinforced concrete (RC) members strengthened with postpoured Ultra High Toughness Cementitious Composites (UHTCC) was investigated in this paper. The flexural behavior, failure mode and crack propagation during loading process of composite specimens were studied, and their structural behavior was also compared to that of original members. The experimental results showed that post-poured UHTCC materials enhanced flexural bearing capacity and toughness of existing concrete members. And introducing UHTCC material into strengthening enabled the composite specimens sustain the loading at a larger deflection without failure. It also revealed that post-poured UHTCC layer dispersed larger cracks in upper concrete into multiple tightly-spaced fine cracks, which prolonged the appearance of harm surface cracks and improved the durability of existing structures.

Abstract: Rolling contact fatigue (RCF) properties of low-temperature bainitic microstructure in the surface layer of a low-carbon steel, obtained by means of carburization and successive lowtemperature austempering were studied. Comparisons were made with those properties of a martensite steel obtained by quenching and tempering. The results showed that the low-temperature bainitic steel exhibits an excellent RCF resistance and reaches a RCF life more than twice that of the martensite steel, presumably as a consequence of the very thin bainite plates (~80 nm in thickness) and the fine-scale dispersion of austenite between the plates. The apparently increased hardness in the surface layer may partly be the cause of the enhanced RCF resistance.

Abstract: Titanium-based coatings reinforced by in situ synthesized TiB and TiC were deposited on Ti6Al4V by laser cladding. The effects of Y2O3 on microstructure and mechanical properties of the coatings were investigated. The coating without Y2O3 is mainly composed of a-Ti cellular dendrites and an eutecticum in which a large number of coarse and fine needle-shaped TiB and a few equiaxial TiC particles are homogeneously embedded. A small amount of Y2O3 addition can refine the microstructure by transforming a-Ti grains from cellular dendrites to columnar or equiaxial crystals, and can increase the volume fraction of the reinforcements. The addition of Y2O3 can also increase microhardness and reduce the cracking sensitivity of the coating.