Materials Science Forum Vols. 534-536

Abstract: The increasing demand of PM parts for automobile and aerospace applications has caused a strong development of the aluminium based metal matrix composites (MMCs).Aluminium alloys are one of most widely used materials as matrix in MMCs, both in research and development as well as in industrial applications. The main reason is the combination of good specific properties, the first requirement in most applications, and the competitive price compared to other low density alloys such as Mg or Ti alloys. In the present work, the influence of the ceramic reinforcement addition to a 2xxx series aluminium alloy is studied. Several percentages of TiCN have been added to the Al-Cu alloy using PM techniques, in order to analyze its influence on the liquid phase sintering process and also on the final properties of the material. The materials have been uniaxially compacted using two different pressures to determine the optimal pressing condition. They have been subsequently sinterized in a N2-H2 atmosphere. Analysing the results obtained, it can be concluded that the addition of TiCN allows a better distribution of the liquid phase up to the 10 %wt, where the optimal results are observed: For higher quantities of reinforcement, elemental copper is observed in the microstructure with the consequent decrease in densification.

Abstract: Investigation results of the heat treatment effect on the corrosion resistance of the EN AW-AlCu4Mg1 (A) aluminium alloy base composite materials reinforced with the Ti(C,N) particles with varying volume fractions are presented. Examinations were made of the EN AW-Al Cu4Mg1(A) aluminum alloy, and also of the composite materials with the matrix from this aluminium alloy. It was found out, basing on own research, that corrosion wear after the corrosion tests of the composite materials with the addition of 5% of the Ti(C,N) particles is smaller compared to the pure aluminium alloy. Precipitation hardening causes improvement of the corrosion resistance of the investigated composite materials and - like in the state before the heat treatment, materials with 5% portion of the Ti(C,N) reinforcement ratio are characteristic of more advantageous features compared to the material without the reinforcement.

Abstract: The aging behavior of sintered Al composites with various ceramic contents was investigated. 2xxx series blended powder was used as the starting powder. Ceramic contents were 0 wt.% and 5 wt.%. The blended powders were compacted at 250 MPa. The sintering process was performed at 620oC for 60 min in a N2 atmosphere. After sintering, each part was solution-treated at 518oC for 60 min and aged at 180oC. The Rockwell hardness value at the peak aging time increased with increased ceramic contents. However, the peak aging time at maximum hardness was reduced with increased ceramic contents. The changes in aging behavior may have been caused by the acceleration of diffusion due to the strain induced by differences in the thermal expansion coefficients of the matrix and the ceramic phases.

Abstract: Electronic packaging involves interconnecting, powering, protecting, and cooling of semiconductor circuits for the use in a variety of microelectronic applications. For microelectronic circuits, the main type of failure is thermal fatigue, owing to the different thermal expansion coefficients of semiconductor chips and packaging materials. Therefore, the search for matched coefficients of thermal expansion (CTE) of packaging materials in combination with a high thermal conductivity is the main task for developments of heat sink materials electronics, and good mechanical properties are also required. The aim of this work is to develop copper matrix composites reinforced with carbon nanofibers. The advantages of carbon nanofibers, especially the good thermal conductivity, are utlized to obtain a composite material having a thermal conductivity higher than 400W/mK. The main challenge is to obtain a homogeneous dispersion of carbon nanofibers in copper. In this paper, a technology for obtaining a homogeneous mixture of copper and nanofibers will be presented and the microstructure and properties of consolidated samples will be discussed. In order to improve the bonding strength between copper and nanofibers, different alloying elements were added. The microstructure and the properties will be presented and the influence of interface modification will be discussed.

Abstract: Mechanical properties of 7xxx series Al metal matrix composite (MMC) powders containing different amounts of ceramic were investigated. The ceramic contents of the starting powders were 5 wt.% or 10 wt.%. 7xxx Al blended powder was used for comparison. The powders were uniaxially cold compacted using a cylindrical die with a compacting pressure of 250 MPa and were sintered at 620oC in a dry N2 atmosphere for 60 min. The heating rate was varied up to 100oC/min. For the heat treatment, sintered parts were solution treated at 475oC and aged at 175oC. Compression tests were conducted to reveal the effect of Al2O3 particle content on the mechanical properties of the composites. Fractography was examined using a scanning electron microscope.

Abstract: The creep behavior of particulate Al-5vol.% SiC composite was investigated. The composite powder was produced by mechanical milling and hot extruded at 450°C at the ratio of 16:1. A creep test was carried out at a constant load at 598, 648, and 673 K. Using the existing equations proposed for the steady-state creep of metals, the threshold stress and the stress exponent of the creep as a function of temperature were determined. The stress exponent was found to be 3 at the temperature of 673 K and 8 at the lower temperatures, i.e. 598 and 648 K. The dependency of the threshold stress to temperature obeys the Arrhenius relationship with the energy term of 29 kJ mole-1.

Abstract: The preparation of Ti50Cu28Ni15Sn7 metallic glass composite powders was accomplished by the mechanical alloying of a pure Ti, Cu, Ni, Sn and carbon nanotube (CNT) powder mixture after 8 h milling. In the ball-milled composites, the initial CNT particles were dissolved in the Ti-based alloy glassy matrix. The thermal stability of the amorphous matrix is affected by the presence of the CNT particles. Changes in Tg and Tx suggest deviations in the chemical composition of the glassy matrix due to a partial dissolution of the CNT species in the amorphous phase. The bulk metallic glass composite was successfully prepared by vacuum hot pressing the as-milled CNT/ Ti50Cu28Ni15Sn7 metallic glass composite powders. A significant hardness increase with the CNT additions was observed for the consolidated composite compacts.

Abstract: In this study, aluminum-based composites reinforced with various amounts of α-Si3N4 were produced by powder metallurgy (P/M). The machinability properties of MMCs were determined by means of cutting forces and surface roughness. Machining tests were carried out by using PCD and K10 cutting tools. Increasing of Si3N4 volume fraction in the matrix resulted in a decrease of the surface roughness and turning forces. PCD cutting tools showed better cutting performance than K10 tools. Surface roughness and turning forces were decreased significantly by PCD tool.

Abstract: The composite used in this paper was prepared by hot-pressing ball-milled Mg alloy powders, in which NiTi shape memory alloy fibers in a row were sandwiched. The microstructure were examined by an optical microscope, scanning electron microscope, X-ray diffraction, and transmission electron microscope to measure its microhardness and density. It is shown that the composite consisted of a homogenous matrix with uniformly distributed NiTi shape memory alloy fibers, where recrystallization took place in the Mg alloy matrix that was subjected to plastic deformation. It is known that plastic deformation is beneficial to the refinement of the grains, that is an adequate bonding forms between the matrix and fibers, the density of the composite increases after the hot-forging; its tensile strength increases significantly because of the grain refinement; the hot-forging process improves the properties of the NiTi shape memory alloy fibers reinforced Mg matrix composite.

Abstract: Sintered composites of Al-8wt%Cu-10vol%SiCp were deformed by repressing or equal channel angular pressing(ECAP) at room temperature, 500°C and 600°C. Their microstructures and transverse rupture properties were compared. Repressing produced more densification than ECAP but ECAP resulted in much higher strengths than repressing. In both cases, the transverse rupture strengths, after deformation at room temperature and 500°C, are much lower than those deformed at 600°C, despite of only slight differences in density. Fractured SiC particles were observed after the deformation, very frequently at room temperature, less frequently at 500°C and rarely at 600°C. Some interfacial de-bonding between the matrix and SiC particles were also observed particularly in the ECAPed specimens. The higher bend strengths and less SiC fracturing at 600°C are attributable to the presence of an Al-Cu liquid phase during deformation. ECAP at 500°C and 600°C caused recrystallization and produced very fine grain structures. Grain refinement, as well as enhanced bonding between particles due to high shear deformation associated with ECAP, is considered the major reason for the much higher strengths. The employment of copper coated SiC instead of bare SiC particles for preparing the composites was found not beneficial neither for minimizing the particle fracturing nor for improving the properties.