Materials Science Forum Vols. 539-543

Abstract: The oxide layer that usually covers the surface of liquid aluminum and its alloys, is one of the main factors that hinders infiltration of these alloys into graphite particle compacts. The oxide film increases the threshold pressure for infiltration and the porosity of the resulting composites is large because the wetting at the metal/carbon interface is reduced. Infiltrating graphite compacts with tin requires, however, a much lower pressure, less than half of that required to infiltrate the eutectic Al-12Si alloy. As the surface tension of tin is half that of the Al-12Si alloy, this result indicates that wetting at the Sn/C interface is slightly better. As a result, porosity in the infiltrated samples is reduced. In order to reduce the threshold pressure and improve the properties of Al-Si/graphite composites, a novel method has been used in this work that consists in placing a thin film of tin at the compact end through which infiltration takes place. During the infiltration process the graphite particles are firstly infiltrated by tin, which is pushed by the aluminum alloy, thus avoiding the oxidation of the latter. The method proved to be very effective in reducing the threshold pressure, while keeping almost constant the infiltration rate.

Abstract: Multi-layered composite sheet materials with nominal composition of Ti-46Al-9Nb (at.%) were successfully processed from Ti, Al and Nb elemental foils using the cold roll bonding technique. To promote the formation of intermetallic compounds in these composites, annealing at 600°C was employed for specimens subjected to various amounts of reduction. The microstructures and phases that formed after cold rolling, the first annealing stage, and the second annealing stage were characterized using scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometer (EDS), transmission electron microscopy (TEM), and x-ray diffraction (XRD). Good bonding was achieved for all rolled samples with a threshold reduction in thickness of about 35% in the first rolling pass. No new phases were formed in the cold rolling stage. Annealing stage did promote the formation of the TiAl3 and NbAl3 phases at the interfaces.

Abstract: The wear behavior of unreinforced as well as feldspar particles reinforced copper alloy (phosphor-bronze) composites was studied as a function of sliding speed and applied loads under unlubricated conditions. The content of feldspar particles in the composites was varied from 1- 5% by weight in steps of 2%. A pin-on-disc wear tester was used to evaluate the wear rate. Loads of 20-160 N in steps of 20 N and speeds of 1.25, 1.56, and 1.87 m/s were employed. The results indicated that the wear rate of both the composites and the alloy increased with increase in load and sliding speed. However, the composites exhibited lower wear rate than the alloy. It was found that above a critical applied load, there exists a transition from mild to severe wear both in the unreinforced alloy and in the composites. But the transition loads for the composites were much higher than that of the alloy. The transition loads increase with the increase in weight % of feldspar particles, but decreases with the increase in sliding speeds.

Abstract: The effects of V substitution for Cr to the sintering behavior of Cr containing Mo2NiB2 ternary boride base cermets and Mn addition to the mechanical properties and microstructure of the Cr and V containing cermets were investigated by using Ni-5.0B-51.0Mo-(17.5-x)Cr-xV (mass%) and Ni-5.0B-51.0Mo-12.5Cr-5.0V-(0-1.5)Mn (mass%) model cermets. 10mass%V substitution for Cr in the Cr containing cermets markedly improved transverse rupture strength and hardness from 2.27 to 2.94GPa and from 85.3 to 87.2RA respectively and refined the microstructure by retarding the progress of sintering especially at liquid phase sintering stage. Small amount of Mn addition to the Cr and V containing Mo2NiB2 base cermets significantly improved the sinterability and increased the mechanical properties of the cermets.

Abstract: FeAl-TiB2 composites have been combustion synthesized from mixtures of Fe, Al, Ti and B powders. When the powder mixture was heated in vacuum to approximately 900 K, an abrupt increase in temperature was observed, indicating that the combustion synthesis reactions occurred in the powder mixture. X-ray diffraction analyses revealed that the combustion-synthesized sample consisted of only FeAl and TiB2. Metallographic investigations using a scanning electron microscope revealed that fine TiB2 particles were dispersed in FeAl matrix phase. As the volume fraction of the TiB2 particles increased from 0.3 to 0.8 by controlling the powder mixture composition, the average TiB2 particle size increased to 1 to 7 μm and the average Vickers hardness of the composites increased from 800 to 1600. This method has been applied to the fabrication of some other ceramic particle dispersed metal matrix composites such as Fe-TiC and FeAl-TiC systems.

Abstract: Copper-based high strength nanofilamentary wires reinforced by Nb nanofilaments are prepared by severe plastic deformation (repeated hot extrusion, cold drawing and bundling steps) for the winding of high pulsed magnets. The effects of microstructure refinement on the plasticity mechanisms were studied via nanoindentation, in-situ deformation in TEM and under neutron beam: all results evidence size effects in each nanostructured phase of the nanocomposite wires, i.e. single dislocation regime in the finest regions of the Cu matrix and whisker-like behaviour in the Nb nanofilaments. The macroscopic high yield stress is thus the results of the combination of the different elastic-plastic regimes of each phase that include size effects.

Abstract: Room-temperature dry sliding wear behavior of hot-pressure sintered monolithic Co, Co- 20 wt.% CuSn and Co-20 wt.% WC composites were investigated. Wear tests of the materials were carried out using a pin-on-disk wear tester at various loads of 10N-100N under a constant sliding speed condition of 0.38m/s against glass (83% SiO2) beads. Sliding distances were varied with a range of 100m-600m. A scanning electron microscopy was used to examine worn surfaces, cross sections, and wear debris. X-ray diffraction (XRD) was utilized to identify phases of the specimen and wear debris. All specimens exhibited low friction coefficients ranging from 0.12 to 0.4. The sintered Co exhibited distinctive wear that was characterized by shallow dug canals on worn surface, a very thin detaching surface layer, and fine debris. Thermal transformation of the Co specimen from ε (hcp) phase to α (fcc) phase occurred during the wear of the Co, which was inferred from XRD analysis of the wear debris. The transformation was suggested to cause the thin detaching surface layer and the fine wear debris of the sintered Co. The wear of the Co-CuSn composite proceeded by shear deformation of the CuSn particles, while WC particles of the Co-WC composite sustained most of the applied load, which resulted in the low wear rate with fine wear debris of the Co-WC composite.

Abstract: Composites materials consisting of pure copper reinforced with 1 vol.% of NbC were prepared by the powder metallurgy route to determine the influence of the milling process on the mechanical and electrical properties. For comparative purpose different milling times at four different rotational speeds were used. The resulting powders were consolidated by hot uniaxial pressing under 90MPa for 2h at 923K to obtain materials with a fine microstructure without residual porosity. It was found that the microstructure and properties of composite materials could be principally related to the amount of Fe, Cr, C and O incorporated as impurities during the milling process. Therefore, the rotational speeds used for milling has an important influence on the properties of the final product. A lower energy-ball milling is accompanied by a smaller amount of impurities (Fe, C and O) incorporated during milling. Composites materials combine electrical conductivity above 40% IACS with high strength. A detailed microstructural analysis by scanning and transmission electron microscopy and X ray diffraction showed that these properties are related not only to NbC particles, but also to the presence of very fine particles of carbides and oxides.

Abstract: Due to excellent oxidation and corrosion resistance at elevated temperature, oxide ceramic could be the preference served at high temperature oxidizing atmosphere over a long period of time. In recent years, alumina-based eutectic in situ composite prepared by various solidification techniques, which has superior properties even close to the melting point about 2100K, has been paid much attention. In this paper, Al2O3/YAG/ZrO2 ternary eutectic and hypoeutectic ceramics are prepared from melt by laser zone-remelting technique, the rapid solidification characteristic and the mechanical property of the composites are investigated. The results show that: (1) Compared to sintered composite with the same composition, laser zone-remelted Al2O3/YAG/ZrO2 eutectic in situ composite has different microstructure showing fine interpenetrated network with Al2O3, YAG and ZrO2 phases continuously intergrown, while none of pores, grain boundaries and amorphous phases is found. (2) The scanning rate and the power density of the laser beam has strong effect on the microstructure morphology. When the power density is determined, the eutectic spacing is reduced with the scanning rate increased. The characteristic eutectic spacings and phase sizes of YAG and Al2O3 are about 2~3μm, and the characteristic dimension of ZrO2 is less than 1μm. (3) The hardness and the room-temperature fracture toughness of Al2O3/YAG/ZrO2 eutectic are respectively Hv=16.7±2.0 GPa and KIC=8.0±2.0 MPa.m1/2, and those of hypoeutectic are respectively Hv=15.8±2.0 GPa and KIC=3.9±1.0 MPa.m1/2.

Abstract: The crystal structure of metastable phase in Ag added Al-Mg-Si alloy was investigated by comparing the β’-phases in Al-Mg-Si alloy without Ag, using images of high resolution transmission electron microscope (HRTEM), selected area electron diffraction patterns (SADPs) and an energy dispersive X-ray spectroscopy (EDS). SADPs and HRTEM images obtained from metastable phase in the Ag added Al-Mg-Si alloy showed similar to those of β’-phase in Al-Mg-Si alloy without Ag and had different lattice spacings because of the effect of Ag. According to our careful analysis on obtained HRTEM images and SADPs, it includes more complicated crystal lattice of distorted hexagons.