Experimental Comparison of the Effects of Nanometric and Micrometric Particulates on the Tensile Properties and Fracture
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PARTICULATE-REINFORCED Al matrix composites (PR AMCs) are emerging as potential advanced composites and, therefore, are possible attractive alternatives to the existing, monolithic, high-strength Al alloys and titanium alloys for many applications. This has become possible because of their superior characteristics, such as higher stiffness, superior strengthto-weight ratio, improved wear resistance, increased creep resistance, low coefficient of thermal expansion, improved high-temperature properties, and high workability, as well as their ability to be synthesized using conventional processing routes (casting and powder metallurgy).[1–8] Several microstructural variables control and contribute to the improvements in the properties of particulate-reinforced metal matrix composites (PR MMCs). These include the matrix alloy, the morphology, size, and volume fraction of the reinforcement particulate; the material processing technique; and the heat treatment applied.[1,3,9–13] The size of the reinforcement particulate usually varies from a few ADNAN AHMED, Visiting Research Associate, ANDREW J. NEELY and KRISHNA SHANKAR, Senior Lecturers, are with the School of Engineering and Information Technology, University of New South Wales at the Australian Defence Force Academy, Canberra 2600, Australia. Contact e-mail: [email protected] Manuscript submitted May 24, 2010. Article published online October 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
micrometers to several hundreds of micrometers in the composites based on micrometric-scale reinforcements. This size is considered large (1 to 100 lm); it makes a negligible contribution to strengthening by Orowan inhibition of dislocation motions. In addition, because the volume fraction is relatively high (10 pct to 40 pct), the load transfer from the matrix to the reinforcement is significant. Micrometric-scale particulate reinforcements are effective in strengthening monolithic pure metals and alloys. Therefore, the materials composed of such reinforcements are considered as a relatively established field of materials research and are being produced commercially.[3,14–20] However, the quest for better materials has inspired researchers to discover the immense potential of nanometric-scale reinforcements to advance the material attributes of the composites to new levels. The concept of ceramic nanocomposites was first suggested by Niihara.[21] A major purpose of producing PR MMCs is to increase the stiffness and the strength of the matrix alloy. Recently, it has been discovered[22–24] that the nanometric-scale reinforcements are a better choice than the traditional reinforcement type (micrometric-scale reinforcement) for developing novel composites with enhanced tensile strength. Among Al alloys, it is more beneficial to reinforce a high-strength alloy (e.g., Al 7xxx series alloys), because it possesses a higher strength-to-weight ratio than a low- or a mediumstrength alloy. Several investigations have also reported improvements in the tensile properties of Al 7xxx seri
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