Processing Pure Ti by High-Pressure Torsion in Wide Ranges of Pressures and Strain
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INTRODUCTION
TO achieve ultra-fine-grained materials with grain sizes in the nanometer or submicrometer level, great attention has been allocated to the application of severe plastic deformation (SPD).[1–6] Grain refinement using the SPD leads to an increase in the strength with reasonable ductility without addition of alloying elements. Pure Ti is a typical material in SPD-related research because of its biomedical application.[7–9] Different SPD methods have been used for processing of Ti. They are high-pressure torsion (HPT),[10–12] equalchannel angular pressing (ECAP),[13,14] ECAP followed by HPT,[15,16] ECAP followed by cold rolling,[17–19] accumulative roll bonding,[20] multidirectional forging (MDF),[21] and hydrostatic extrusion.[22,23] In the HPT method, a thin disc sample is placed between two anvils under a high pressure and intense shear strain is introduced by rotating the two anvils with respect to each other. To the best of the authors’ knowledge, little is understood regarding the influence of the applied pressure on the mechanical properties and microstructures in HPT-processed Ti. The pressure for the HPT is normally in the range of several giga-pascals (GPa), but few experiments have been attempted with pressures higher than 8 GPa, except the application of 20 GPa for Fe.[24] Pure Ti may transform from an a phase with the hcp crystal structure to an x phase with the simple KAVEH EDALATI, Graduate Student, and ZENJI HORITA, Professor, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan. Contact e-mail: [email protected] EIICHIRO MATSUBARA, Professor, is with the Department of Materials Science and Engineering, Faculty of Engineering, Kyoto University, Kyoto 606-8501, Japan. Manuscript submitted October 4, 2008. Article published online June 30, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
hexagonal structure.[25–28] This phase transformation was observed using a diamond anvil cell at pressures greater than 2.9[27] to 11 GPa.[28] Errandonea et al.[29] showed that such a pressure range for initiating the phase transformation is attributed to the presence of nonhydrostatic conditions arising from the difference in pressurizing media. The existence of a b phase with the bcc structure in Ti was also theoretically predicted at pressures higher than 36 GPa, but this transformation has never been observed experimentally.[30,31] A survey of the literature concludes that, in spite of the numerous studies concerning the effect of pressure on the phase transformations in pure Ti, there are limited works on the role of strain for the pressure-induced phase transformations in Ti.[32,33] Kilmametov et al.[32] reported that the formation of an x phase occurred in Ti even under a pressure of 3 GPa when HPT was used. Furthermore, it was reported that the fraction of x phase increases with increasing pressure from 3 to 6 GPa and with increasing shear strain. All these results suggest the importance of not only pressu
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