Effect of Post-annealing on Grain Boundary of Nano-crystalline Cu Processed by Powder High-Pressure Torsion
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One of the critical microstructural factors that affect the properties of materials is the grain boundary (GB). Hence, GB engineering has been extensively attempted to improve bulk properties of conventional materials.[1,2] GBs comprise an extremely high volume fraction in ultrafine-grained (UFG)/nano-crystalline (NC) metallic materials, so their effect is much more significant in UFG/NC metals than that in coarse-grained metals. Various methods, such as nanopowder compaction, mechanical alloying, nanocrystallization of amorphous alloys, and severe plastic deformation (SPD), can be used to produce UFG/NC materials, and these processes result in various microstructures with different GB characteristics.[3–6] Also, depending on the SPD process used, different types of GB can be formed in EUN YOO YOON, Senior Researcher, is with the Materials Deformation Department, Korea Institute of Materials Science (KIMS), Changwon 641-831, Korea. DONG JUN LEE, Ph.D. Candidate, is with the Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Korea. LEE JU PARK, Principal Researcher, is with the Agency for Defense Development (ADD), Daejeon 305-152, Korea. SUNGHAK LEE and HYOUNG SEOP KIM, Professors, are with the Department of Materials Science and Engineering, POSTECH, and also with the Center for Advanced Aerospace Materials, POSTECH. Contact e-mail: [email protected]. kr MOHAMED IBRAHIM ABD EL AAL, Lecturer, is with the Mechanical Design and Production Engineering Department, Zagazig University, Zagazig, Egypt. Manuscript submitted February 20, 2014. Article published online July 17, 2014 4748—VOLUME 45A, OCTOBER 2014
UGF/NC materials, like high-angle or low-angle GBs and special or random GBs.[6–8] The most successful ‘‘top-down’’ approach to produce bulk UFG/NC metallic materials is SPD methods in which they are subjected to very large strains.[6,9–11] Among the various SPD methods, high-pressure torsion (HPT) is the most effective grain refinement method in that it provides the largest imposed strain.[10] Even though large strains are imposed during HPT, the grain size reaches limit due to the balance between grain subdivision and recovery under high strain.[12] However, in consolidated pure metallic powders, minimum grain size can be reduced and microstructural stabilization can be improved by combining ‘‘bottom-up’’-type powder metallurgy with HPT.[13–15] In our previous paper,[13] room temperature (RT) HPT-consolidated Cu showed high strength then the HPT-processed initial solid Cu. However, consolidated Cu showed few percentage of ductility. In this paper, to achieve both high strength and high ductility of Cu, we used a powder consolidation approach that consists of HPT followed by postannealing. The strength and ductility of the Cu produced using this approach was investigated using microtensile testing. Water-atomized Cu powders (purity 99.5 pct with diameter
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