Effects of grain size and pressing speed on the deformation mode of commercially pure Ti during equal channel angular pr
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12/6/03
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Communications Effects of Grain Size and Pressing Speed on the Deformation Mode of Commercially Pure Ti during Equal Channel Angular Pressing INYOUNG KIM, JONGRYOUL KIM, DONG HYUK SHIN, and KYUNG-TAE PARK Equal channel angular pressing (ECAP), at present the most prominent severe plastic deformation technique for producing porosity-free, ultrafine grained structure, is now applied to most metals and alloys regardless of crystal structure.[1,2,3] Particularly, for hexagonal close-packed (hcp) metals and alloys, it is noticeable that large plastic strain can be accumulated via ECAP in spite of their inherent lack of plastic deformability own to their insufficient slip systems.[4–9] In this study, a deformation mode transition associated with microstructural evolution in commercially pure titanium (CPTi) during ECAP was investigated with two main motivations: (a) to elucidate the accommodation mechanism of a large strain induced by ECAP in Ti alloys, and (b) to optimize the processing conditions of ECAP to fabricate ultrafine grained Ti alloys. The relative importance of the deformation modes in hcp metals, i.e., slip and twinning, is known to strongly depend on external conditions such as temperature, strain rate, and strain and internal conditions such as grain size. For example, Okazaki and Conrad[10] formalized the critical twinning stress for Ti as 0 C3T C2 n s s0T C1 a b C4T kT d1/2 e
[1]
1/2
sO kT d
where s is the twinning stress, sOT is the frictional stress, ˙ O is the reference strain rate, ˙ is the strain rate, is the strain, d is the grain size, T is the temperature, and C1, C2, C3, C4, s0, n, and kT are constants. Therefore, in the present investigation, the effect of the initial grain size and the pressing speed (related to the strain rate) on the deformation mode transition between twinning and slip in CP-Ti during ECAP was investigated. A CP-Ti with an average initial grain size of 30 mm was used as the starting material. Its composition was Ti-0.07 pct Fe-0.009 pct N-0.012 pct H-0.01 pct O in weight percent. In order to control the initial grain size, and remove the residual stress and texture of as-received material, the CPTi bars were annealed at two different annealing conditions, 1073 K for 1 hour and 873 K for 4 hours, under argon atmosphere. As shown in Figure 1, the average linear intercept grain sizes of the annealed bars were 100 and 20 mm, INYOUNG KIM, Graduate Student, JONGRYOUL KIM, Associate Professor, and DONG HYUK SHIN, Professor, are with the Department of Metallurgy and Materials Science, Hanyang University, Ansan, 425-791, Korea. Contact e-mail: [email protected] KYUNG-TAE PARK, Associate Professor, is with the Division of Advanced Materials Science and Engineering, Hanbat National University, Taejon, 305-719, Korea. Manuscript submitted September 4, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Optical micrographs of CP-Ti annealed at two different conditions: (a) 1073 K for 1 h and (b) 873 K for 4 h.
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