Microstructure and Mechanical Properties of Copper Processed by Twist Extrusion with a Reduced Twist-Line Slope
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DEVELOPMENT of industrially compatible severe plastic deformation (SPD) methods is a challenging and appealing issue in the modern field of structural materials science. The interest in SPD stems from unique properties that were exhibited by ultrafine-grained materials produced by SPD methods.[1–4] To date, there have been a no. of SPD processes developed at the laboratory scale; however, most of them prove difficult to be commercialized. Twist extrusion (TE) was introduced[5] as an attempt to scale-up the process of high-pressure torsion. Being promising in the commercial sense,[6] TE enjoyed some interest, which is reflected in continuous research on the subject[7–13] as well as in the emergence of new SPD methods inspired by the concept of TE.[14–18] In the quest to incorporate TE into the industry, such issues as endurance of the tool are becoming important. Of special concern are twist dies, the most expensive tool for TE. Since the main deformation events during TE occur at the transients between the twist channel and the straight (entry and exit) channels of the die (Figure 1), one can MARAT I. LATYPOV and DONG JUN LEE, Ph.D. Candidates, and HYOUNG SEOP KIM, Professor, are with the Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] EUN YOO YOON, Senior Researcher, formerly with the POSTECH, Pohang, Korea, is now with the Light Metal Division, Materials Deformation Department, Korea Institute of Materials Science (KIMS), Changwon 642-831, Republic of Korea. ROMAN KULAGIN, Researcher, and YAN BEYGELZIMER, Professor, are with the Donetsk Physics & Engineering Institute of the National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine. MAJID SEYED SALEHI, Researcher, is with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., 11365-9466, Tehran, Iran. Manuscript submitted July 8, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
expect that the maximum stresses r of the tool will concentrate around the transient regions. According to the Coffin–Manson relation, the no. of cycles to failure is proportional to rk, where k > 1 is a material constant, being k = 2 for tool steels.[19] In the first approximation, stresses, r, in the die during TE are directly proportional to von Mises strain per pass, e0. It, therefore, follows that the no. of passes, N, that the die can endure is N ~ (e0)2. On the other hand, the no. of passes, n, necessary to accumulate a certain von Mises strain, e(n), in a metallic bar by TE is n ~ (e0)1. From the above relations, one can estimate the amount of bars that can be processed (to a given von Mises strain, e(n)), in a single TE die without failure of the die as N= ¼ 1= : n e0
½1
In TE, von Mises strain per pass, e0, depends on the die geometry. One of the important geometrical features of the TE die is the slope of the twist line, b (Figure 1). The angle b depends on the angle between entry and exit channels, a; the radius of the die profile, R; and the l
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