Mechanical Behavior of a Tri-modal Al Matrix Composite
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Mechanical Behavior of a Tri-modal Al Matrix Composite Jichun Ye, Bing Q. Han, Feng Tang and Julie M. Schoenung Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, CA 95616-5294 USA ABSTRACT Mechanical milling at cryogenic temperatures (cryomilling) was applied to fabricate a composite powder with 20 wt. % B4C (submicron-to-several microns in size) in a nanocrystalline (NC) 5083 Al matrix. A uniformly blended powder with 50 wt. % cryomilled composite powder and 50 wt. % coarse-grained (CG) 5083 Al powder was degassed, cold isostatic pressed (CIPped) and extruded to form a composite with 10 wt. % B4C, 50 wt. % CG 5083 Al and balance NC 5083 Al. This tri-modal material was then tested for mechanical behavior under compressive and tensile load conditions at various temperatures. The composite exhibited an extremely high yield stress at room temperature, but limited ductility. Although the composite lost its strength at elevated testing temperatures rapidly, the retained strength was still much higher than that of the conventional 5083 Al. The composite exhibits its highest ductility of 26% at 200°C under tensile load. In compression, it plastically deformed uniformly at all the elevated temperatures (≥373 K) and did not fracture even when the deformation exceeded 30%. The microstructure of this composite, including the distribution of each phase, the grain sizes of the Al matrix, the interfaces between these three phases, and the fracture surfaces were characterized using transmission electron microscopy (TEM) and optical microscopy (OM) techniques. The relationship between the microstructures and mechanical properties was discussed. INTRODUCTION Particulate reinforced aluminum matrix composites have been identified as attractive structural materials, due to lightweight, high strength, high modulus, good performance at high temperature, excellent fatigue resistance, creep resistance and abrasion resistance and ease of fabrication [1]. B4C is a good reinforcing candidate for the aluminum matrix composite, ranking third in hardness (just after diamond and cubic boron nitride) and having a low density of 2.51 g/cm3 (lighter than Al) [2]. Besides the reinforcing materials, the microstructure of the matrix is also important to the overall properties of the composites, such as the grain size of the matrix material [1]. It is well known that, for a monolithic metal alloy, the strength of the alloy increases with the decreasing of the grain size [3]. Thus, if a nanostructured Al matrix is achieved, the strength of the composite material can be further improved. The Al based composite with the nanostructured Al matrix reinforced by the hard B4C particles can be fabricated via cryomilling (mechanical milling at cryogenic temperature) [4, 5]. Furthermore, cryomilling has been demonstrated to provide a homogeneous distribution of the B4C in the Al matrix and a good interface between the B4C and the Al matrix [4, 5]. However, the B4C/Al nanocomposite fabricated by cryomilling a
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