Advanced Materials Research Vols. 105-106

Abstract: Ceramic composites containing BN and Al can be machined easily into complex shape, the hardness of the composites can also be improved by surface hardening process of in situ reaction between Al and BN. However, the reaction of Al and BN is a volume reduced one and the porosity will increase during the hardening treatment.. In order to solve this problem, a surface nitriding process was developed. Put the pre-sintering samples (with designed shapes) into a vacuum furnace in nitrogen atmosphere and reheated to high-temperature, so that the residual Al after pre-sintering would react with N2 completely, and change to AlN which has higher hardness. The phase transformation and microstructure were observed and the mechanical and other properties were also measured. The results showed that the surface nitriding process could increase the hardness, bending strength and density obviously. The porosity decreased sharply compared with the general hardening treatment.

Abstract: In this paper, XRD was engaged in studying phase transformation of quartz fibers, SEM was engaged in studying the surface micromorphology of quartz fibers heat treated at different temperatures, and the tensile strength was measured by a single fiber strength electronics instrument. The results indicate that surface infiltration agent have been iliminated after heat treatment at 500°C, and the tensile strength decreaced significantly. The higher the temperature was, the more the tensile strength decreaced. There were no significant phase transformation and no crystallization heat treatmented at 500~800°C. But there were some round and strip bulges, and scap defects on the surface. With temperature increasing,some scab defects and bulges began to flake off, and some new rifts and cracks were formed. This was one of the important factors that decreaced tensile strength markedly.

Abstract: Si3N4-SiC-C refractory composites were prepared under the condition of 1450°C × 3h with electrofoging anthracite, Si3N4, SiC as the raw material and the modification emulsified bitumen as the binder. The effects of the silicon and silica fine powder additives on the compressive strength of Si3N4- SiC-C refractory composites were studied. The results indicated that adding an appropriate ratio range of silicon or silica fine powder in the composites both can improve the compressive strength of the material.

Abstract: In this paper, negative thermal expansion coefficient eucryptite powders were prepared by sol-gel method using silica-sol as starting material. The raw blocks were obtained by dry pressing process after the powder was synthesized, and then the raw blocks were heat-treated at 600º, 1150º, 1280º, 1380º, 1420º and 1450°C, respectively. Variations of density, porosity and thermal expansion coefficient at different heat treatment temperatures were investigated. Phase transformation and fracture surface morphology of eucryptite heat-treated at different temperatures, respectively, were observed by XRD and SEM. The results indicate that, with the increasing heat- treatment temperature, the grain size and the bending strength increased, porosity decreased, thermal expansion coefficient decreased continuously. Negative thermal expansion coefficient of -5.3162×10-6~-7.4413×10-6 (0~800°C) was obtained. But when the heat-treatment temperature was more than 1420°C, porosity began to increase, bending strength began to decrease, which were the symbols of over-burning, while the main crystal phase didn’t change.

Abstract: Al–Si alloy-based composites are widely used in automotive, aerospace and mineral processing industries with improved properties. In situ techniques have been developed to fabricate Al–Si alloy-based composites, which can lead to better adhesion at the interface and hence better mechanical properties. Al–Si alloy reinforced with in situ TiB2 particles was synthesized successfully by using salt reaction route. These in situ composites have shown significant improvement in mechanical properties compared with the base alloy. The wear resistance of the alloy is also significantly improved with the addition of TiB2 particles. The hardness, strength and wear resistance are strengthened with increasing TiB2 content of the composites. TiB2 can act not only as a grain refiner for primary Al but also as a modifier of Si in eutectic mixture. Analysis of the worn surface of Al–Si/TiB2 composites tested under loads of 30 and 110N suggests that ploughing is predominant at lower loads and delamination is predominant at higher loads.

Abstract: TiC reinforced AZ91 magnesium matrix composites have been fabricated by a melt in-situ reaction spray deposition. The microstructures of spray-deposited alloys were studied by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The dry sliding wear behavior of the alloys was investigated by using a pin-on-disc machine under five loads, namely 10, 20, 30, 40 and 50 N. It has been found that the wear behavior of the alloys was dependent on the TiC content in the microstructure and the applied load. At a lower load (10 N), with increasing TiC content, the wear rate of the alloy was decreased, and the dominant wear mechanism was an oxidative mechanism. At a higher load (50 N), a spray- deposited AZ91/TiC composites exhibited superior wear resistance to the AZ91 magnesium alloy, the dominant wear mechanism was delamination.

Abstract: The B4C/BN composites were fabricated by hot-pressing process. The microstructure, mechanical properties and oxidation resistances of the B4C/BN composites were investigated. It was shown that the h-BN particles were distributed in the B4C ceramics matrix. The mechanical properties of the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increasing content of h-BN. The mechanical properties of the B4C/BN nanocomposites were significantly improved in comparison with the B4C/BN microcomposites. The oxidation processes were performed at 1000oC, 1100oC, 1200oC, 1300oC for 20h. The oxidation curves of the B4C monolith, the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increase of oxidation temperature and oxidation time. The specimen’s weight and the oxidation resistance decreased gradually with the increase of oxidation temperature and oxidation time. The specimens remained good oxidation resistance at 1000oC; the oxidation resistance decreased remarkably at 1300oC. The decreasing specimen’s weight was attributed to the evaporation of B2O3 which produced by oxidation process of B4C and h-BN. The phase composition and microstructure of specimen’s surface after oxidation process were investigated by XRD and SEM.

Abstract: After preparing samples (3mm long×4mm wide×36mm high) of Al2O3-MgAlON composites and sintered at 1500°C for 2 h in N2 atmosphere, samples’ cracks were carved by a Vickers hardometer’s pressing head on the center of the sample surface (4 mm×36 mm). Subsequently, the cracks were healed at 1000°C-1550°C for 6 h respectively. Effects of healing temperature on sample’s strength, crack healing dynamics and its molecular dynamics simulation were investigated. The results suggested that: the optimum range of cracks healing temperature was 1300°C-1550°C, and the healing process accelerated at 1300°C, meanwhile, the strength of samples increased significantly. Cracks completely healing finished at 1550°C. The dynamics equation of crack healing was lnν = -Q/kT+lnC. Through characterizing the crack healing rate with the recovering rate of sample’s strength, the diffusion activation energy Q = 4.264 × 10-30 J•K-1 and diffusion constant C=7.359 were claimed. The result of the molecular dynamics simulation suggested that cracks healing process was caused by diffusion could be divided into five stages: passivation of crack tips, formation of salient island, crack shrinkage, generation of secondary crack, and complete healing.

Abstract: Flow control devices made of refractories are established in the tundish to modify the flow characteristics. In practical producing, the metallurgical function of flow control devices is weakened because of refractories corrosion. In this subject, an index named refractory wear factor which is used to evaluate the refractory wear was defined at first. And then, the characteristics of molten steel flow and the distribution of temperature and the refractory wear factor in a one-strand tundish equipped with flow control devices of a turbulence inhibitor, a weir and a dam were numerically simulated using the commercial CFD software. At last, the influence on refractory wear factor was analyzed. The following results were obtained. The high-speed molten steel is mainly confined to the injecting zone, especially to the turbulence inhibitor. The walls around the entering zone tundish and the weir wall faced to the flow direction have higher temperature than other walls. The highest value of refractory wear factor is focused on the inhibitor and bottom wall near the outlet. In order to keep the synchronization for the tundish, the refractories used in those zones should have higher anti-wear properties.

Abstract: Rod-shaped oxide eutectic ceramics fabricated through SHS process can acquire outstanding mechanical properties. First, Four-phase model method was used to study effective disturbance strain of rod-shaped oxide eutectic ceramics produced by nano fibers with inter-phases, then average strain of rod-shaped oxide eutectic ceramics aroused thermal inconsistency is obtained. The effective thermal expansion longitudinal strain and transverse strain of rod-shaped oxide eutectic ceramics are determined by the average strain. Rod-shaped oxide eutectic ceramics is transverse isotropy and has two independent thermal expansion coefficients. The results show that the thermal expansion coefficients of rod shaped oxide eutectic ceramics are dependent on the diameter of nano-fiber. The thermal expansion coefficients will decrease when the diameter of nano-fiber decrease.