The True Origin of Ductile Fracture in Aluminum Alloys
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INTRODUCTION AND BACKGROUND
MONOTONIC fracture in metals begins with the initiation of microvoids induced by the damage initiation at particles.[1] The fracture is then terminated by the formation of a new surface, which is called a fracture surface, through the growth[2] and coalescence[3] of the microvoids. The process, which is called ductile fracture, is common and occurs in almost all structural metallic materials[4,5] A great deal of study has been undertaken to obtain a complete understanding of this process from various viewpoints ranging from the nanoscopic level, where the dislocation theory applies, to the meso- and macroscopic levels, where continuum mechanics is required. Today, this process is taken for granted in our understanding of the fundamental strength of materials, and appears in every textbook dealing with the field.[4,5] It would not be an exaggeration to say that modern structural materials have been developed by HIROYUKI TODA, Professor, is with the Department of Mechanical Engineering, Kyushu University, 744, Motooka, Nishiward, Fukuoka 819-0395, Japan. Contact e-mail: [email protected] HIDEYUKI OOGO, Graduate Student, and MASAKAZU KOBAYASHI, Associate Professor, are with the Department of Mechanical Engineering, Toyohashi University of Technology, 1-1, Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan. KEITARO HORIKAWA, Associate Professor, is with the Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan. KENTARO UESUGI, AKIHISA TAKEUCHI, and YOSHIO SUZUKI, Researchers, are with the Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan. MITSURU NAKAZAWA, Graduate Student, and YOSHIMITSU AOKI, Associate Professor, are with the Electronics and Electrical Engineering, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan. Manuscript submitted July 3, 2013. Article published online October 2, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
controlling material characteristics, taking into account the above mentioned fracture mechanism.[6] In terms of the damage initiation at particles, it is well known that ordinary modes of damage initiation involve either fracture of secondary-phase particles dispersed in the metals or debonding at the metal/particle interface.[1] Although microvoids can nucleate at slip band intersections in titanium alloys and Incoloy,[7] this is not typical microvoid nucleation mechanism. The damage initiation at particles has been extensively investigated in both experimental and modeling studies to date. It has been reported that microvoid nucleation is strongly influenced by particle radius, particle shape, and particle volume fraction.[7] According to Gangulee et al., particle fracture increases with d1/2/f1/3, where d is the particle diameter, and f is the particle volume fraction. Experimental evidence also suggests that particle damage occurs first at larger particles
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