Key Engineering Materials Vol. 351

Abstract: The microstructure characteristics of the spray-formed and melt-spun Al85Nd5Ni10 and Al89La6Ni5 alloys were studied. The spray forming process was demonstrated to produce a bulk scale hybrid composite consisting of amorphous and nanostructured phases directly without the need of an amorphous precursor. The spray-formed Al89La6Ni5 deposit (~1 mm in thickness) were partially amorphous, and the amorphous phase came from the undercooled liquid droplets upon deposition. The as-spray-formed Al85Nd5Ni10 deposit (~20 mm in thickness) was completely crystallized due to the devitrification of the retained amorphous phase to nano-scale secondary crystals upon deposition. Primary crystals (~1 μm) are dispersed uniformly in the bulk spray-formed amorphous/or partial amorphous composites and many distinctive deformation twins also are observed in the crystals, however, not twins found in the corresponding completely devitrified ribbon. This is mainly because of the stirring and impacting force offered by high velocity droplets during spray forming and the mismatch of thermal expansion coefficient between primary crystals and adjacent amorphous matrix.

Abstract: The effects of high-energy ball milling on SiC powders were studied using a planetary apparatus. Conditions to obtain nanostructured SiC powders with an average crystallite size of 4 nm were determined and powders were characterized by XRD, SEM and TEM analyses. This process was applied to prepare fine powders leading to dense SiC ceramics by sintering at 1900oC for 30 minutes under 30 MPa in argon.

Abstract: The nano-sized silica particulates reinforced poly(ether ether ketone) (PEEK) composites were fabricated by means of simple compression molding technique. The nano-sized silica, measuring 30 nm in size, was firstly modified by surface-pretreatment with stearic acid. The thermomechanical properties of the resulting PEEK/SiO2 nanocomposites were measured using dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA). The TMA results show that the coefficient of thermal expansion (CTE) becomes lowered when the content of the nanosilica increases. Furthermore, the CTE of the modified-silica filled PEEK nanocomposites shows higher CTE values, as compared with those of the unmodified counterparts. The dynamic modulus of the PEEK nanocomposites shows over 40% increment at elevated temperatures from 100-250oC, indicating the apparent improvement of elevated temperature mechanical properties.

Abstract: In the present study, Ti50Cu28Ni15Sn7 metallic glass and its composite powders reinforced with 4~12 vol% of SiC additions were successfully prepared by mechanical alloying. The as-milled Ti50Cu28Ni15Sn7 and composite powders were then consolidated by vacuum hot pressing into disc compacts with a 10 mm diameter and thickness of 2 mm. The structure of the as-milled powders and consolidated compacts was characterized by X-ray diffraction. While the thermal stability was examined by differential scanning calorimeter. In addition, the mechanical property of the consolidated bulk metallic glass and its composite was evaluated by Vickers microhardness tests. In the ball-milled composites, initial SiC particles were homogeneously dispersed in the Ti-based alloy glassy matrix. The presence of SiC particles did not dramatically change the thermal stability of Ti50Cu28Ni15Sn7 glassy powders. BMG composite with submicron SiC particles homogeneously embedded in a highly dense nanocrystalline/amorphous matrix was successfully prepared. A significant hardness increase with SiC additions was noticed for consolidated composite compacts.

Abstract: Materials characterization is a crucial issue in the development and application of new materials. Materials characterization aims to mine and acquire characteristic information and their evolution in the materials. It mainly includes three important topics which are microstructural characterization, properties characterization, and environmental degradation. In this paper, characterization techniques about these topics were discussed for C/SiC composites and a characterization system was preliminarily established. All these characterization research and their results further the better understanding of the relationship between microstructure and properties and of the failure mechanisms in the C/SiC composites.

Abstract: Copper and 2024 aluminum alloy were melt-infiltrated into porous β-SiC to form SiC/Cu and SiC/Al composites. The porous β-SiC was prepared using Moso bamboo as the bio-template and had structural characteristics of bamboo. The Cu infiltration occurred as low as 1100°C and became significant at 1200°C. After infiltration at 1300°C for 4 h, there was still ~5 % of residual porosity. For the composites with low degree of metal infiltration, the samples fractured like the bamboo-structured porous SiC. For the composites with high degree of infiltration, the sample behaved like monolithic copper. In the infiltration of Al alloy, infiltration occurred at 900°C. Higher Infiltration temperatures would result in significant formation of Al4C3, which gradually decomposed in air.

Abstract: Carbon fiber reinforced silicon carbide matrix composites (C/SiC) are promising candidate materials for high-temperature structural applications. However, in oxidizing environments the two main constituents, that is, carbon fiber and pyrolytic carbon interphase which bears and transfers loads respectively are susceptible to deplete rapidly for oxidation. In this paper, the oxidation behavior of carbon fiber and pyrolytic carbon were investigated by simulating environmental experiments and scanning electron microscopy. The reactivity discrepancy in the carbonaceous constituents and in the different zone of carbon fiber was discerned. After oxidation, the morphology of carbon phase broken before oxidation were compared with that of those broken after oxidation. Based on the microstructural model, the contrast results of morphology were well interpreted from the reactive preference and selectivity.