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Session Chairperson: N.J. Kim, Center for Advanced Aerospace Materials, POSTECH, Pohang, 790-784, Korea
CREEP BEHAVIOR OF ALUMINUM STRENGHENING WITH VERY HIGH VOLUME FRACTIONS OF SUBMICRON ALUMINA PARTICLES: C. Verdon, D.C. Dunand, Department of Materials Sciences and Engineering, Room- 8-328, Massachusetts Institute of Technology, Cambridge, MA 02139
High-temperature mechanical properties of dispersion strengthened cast aluminum containing 0.3 µm alumina particles were investigated. Compression creep tests were performed on samples containing three different volume fractions of oxide dispersoids, i.e. 25%, 33% and 43%, in the temperature range between 450°C and 600°C. Both as-cast materials, whose grain size in the range of centimeter, and extruded materials, which exhibit submicron grains, were tested. In the coarse-grained materials, the deformation is controlled by dislocation creep, whereas the deformation of the finegrained materials is controlled either by diffusional creep or by dislocation creep, depending on temperature and stress. The high values of the apparent stress exponents and of the apparent activation energy indicate a threshold stress for both creep regimes. Experimental results are discussed with respect to existing dispersion strengthening models.
STRUCTURE AND PROPERTIES OF Si PARTICULATE REINFORCED ALUMINUM MATRIX COMPOSITES: S.J. Song, D.H. Kim, Dept. of Metallurgical Engineering, Yonsei University, 134 Shinchondong, Seodaemunku, Seoul, 120749, Korea; J.S. Kim, Materials Engineering & Test Dept., Hyundai Motors Company, 700 Yangchungdong, Jungku, Ulsan, 681791, Korea
To obtain a homogeneous distribution of fine Si particles in aluminum matrix, Si particulate reinforced aluminum matrix composites have been processed by using P/M method. 20-40µm size Si particulates and Al alloy powders were mixed, degassed and extruded at 350°C for pure Al matrix composites and at 400°C for 2024 and 6061 matrix composites. Wear properties of the composites have been discussed in terms of the observed microstructural characteristics and physical properties such as tensile properties and thermal expansion coefficients.
THE DEVELOPMENT OF LIGHT COMPOSITE MATERIALS WITH LOW COEFFICIENT OF THERMAL EXPANSION: M. Smagorinslci, S. Grenier, P.G. Tsantrizos, PERMA, 1744 William, Montreal, Quebec, Canada, H3J IR4
The aerospace industry uses several types of Al-based composite materials. The main criteria for their application are light weight, high strength and hardness, low CTE, and high dimensional stability (capability to keep dimensions unchanged under thermal load). Theoretical formulas were developed to calculate the CTE of binary composites, as well as the ideal diameter of the strengthening particles for composites working under stringent physical engineering conditions. The calculations which were performed permitted to select the volume fraction and the dimension of the reinforcing particles which offer the most potential. Based on these calculations, composites containing 40% of reinforcing particles were selected and produced using the Vacuum Plasma Spraying (VPS) method. The VPS netshape forming of composites was shown to possess distinct advantages over conventional manufacturing processes. The VPS technique can be used to fabricate advanced composites with complex shapes, high density (9798% of the theoretical density) and, uniform particles distribution of various ceramic materials (size and composition). Plastic deformation and heat treatment result in further density improvement, which in turn significantly improves the mechanical properties of the composites.
MICROSTRUCTURES AND MECHANICAL PROPERTIES OF DIE CAST SiCp/Al COMPOSITES: Tae-Won Lee, Jun-Ho Seo, Chi-Hwan Lee, Department of Metallurgical Engineering, Inha University, Inchon, Korea
This study was focused to investigate the effect of die casting parameters on the microstructure and mechanical properties of SiCp (10 and 20 vol%)/Al composites. Die casting was performed using the mold preheated at 130°C under the pressure of 916 kg/m3 and pouring temperature of 650~700°C. In this work, twostage injection system, which was composed of slow and high speed injections, was used to prevent the input of air during injection. The speeds of slow and high speed injections were 0.3~0.6 and 1.4 m/s, respectively. It was found that the SiC particles were homogeneously distributed in refined Al matrix. This results from the rapid mixing of Al and SiC particles during injection and the low segregation due to fast solidification rate. These microstructures provided good roomtemperature mechanical properties.
FABRICATION AND TENSILE PROPERTIES OF SiCP/AZ91 Mg COMPOSITES BY HOT EXTRUSION: Doo-Myun Lee, Chi-Hwan Lee, Department of Metallurgical Engineering, Inha University, Inchon, Korea
The tensile properties and microstructural evolution of hot-extruded SiCp/AZ91 Mg matrix composites have been investigated as functions of extrusion parameters and SiC particulate size. Also, the effect of SiC particulates on. the grain size of matrix in the composites was studied. The AZ91 Mg alloy powders prepared by wet attrition milling from Mg machined chips were hotpressed with and without SiC particulates, hot-extruded and then solutiontreated. The microstructural observation revealed that both the composites and Mg alloy have equiaxed grains due to the dynamic recrystallization during hot extrusion. The tensile strength of the both alloys increased with increasing extrusion ratio, and the strength of the composites were higher than that of the Mg alloy without reinforcement. It was found that the tensile strength of the both materials decreased after solutiontreatment; the decrease in tensile strength of the composites was considerably smaller than that of the Mg alloy. The grain growth of the matrix in the composites was inhibited by the introduction of the SiC particulates, resulting in the improvement in the yield strength of the composites.
IMPROVED HIGH CYCLE FATIGUE PROPERTIES OF TITANIUM-BASED PARTICULATES COMPOSITES: I. Hagiwara, S. Emura, Y. Kawabe, National Research Institute for Metals 1-2-1 Sengen, Tsuluba 305, Japan
The titaniumbased particulates composites are known to exhibit superior physical and mechanical properties compared to the unreinforced alloy. In cases where TiB or TiC is used as the reinforcing particulates, it has been reported that the fatigue fracture originates neither from the interior of particulate nor from the particulate/matrix interface, put rather from the matrix. Therefore it is suggested that the high cycle fatigue strength of the composite is strongly dependent on the matrix microstructure. In the present work, the effect of the matrix microstructure on the high cycle fatigue strength was studied for blended elemental P/M Ti6Al2Sn4Zr2Mo/lOTiB and Ti6Al1.7feO.lSi/10 TiB composites. It was found that the composites with a fine acicular - two-phase microstructure showed an improved high cycle fatigue strength over those for conventionally processed composites with colony matrix microstructure.
INFLUENCE OF FIBRE STRENGTH DISTRIBUTION ON THE FATIGUE BEHAVIOUR OF A TiB2 Is/SCS-6 COMPOSITE: J. Liu, J.G. Pursell, P. Bowen, IRC for High Performance Materials/School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B 15 2TT, UK
The fatigue behaviour of silicon carbide fibre reinforced titanium matrix composites (TMCs) has been the subject of numerous studies. Bridging is found to he the dominant mechanism leading to the decrease in the crack growth rate us the fatigue crack grows. However, once the bridging fibres fail, the benefit of bridging will diminish. The influence of statistical fibre strength distribution on matrix cracking in fibre composites has been modeled by assuming the strength of the fibres falls in a twoparameter Weibull distribution. However, recent studies have shown that the fibre strength in TMCs is bimodal. The aim of this paper is to study the effects of fibre strength, and especially of those fibres with lower Weibull modulus. on the fatigue behaviour of a Ti,321s/SCS6 composite. First, single fibre tensile tests were conducted to evaluate the fibre strength distribution in both asreceived uncl fatigued specimens. The fracture surfaces of the fibres showing different strengths were examined using a SEM to distinguish lowstrength fibres fractographically. Fatigue tests were also conducted on single edge notched specimens. Different maximum loads and notch sizes were employed so that different fibre stresses were obtained. Acoustic emission, and direct current potential drop techniques were used to detect fibre failure and to monitor the crack growth. respectively. After the fatigue tests, the percentage of lowstrength fibres in the specimens was again estimated fractographically. The influence of fibre distribution was also modeled by characterizing the fibre strengths using two twoparameter Weibull distributions. The results show that the percentage of the lowstrength fibres plays an important role in determining the fatigue behaviour of the composite. Under identical loading condition, specimens with more lowstrength fibres are. of cause, more likely to fail. and this has been quantified.
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