Microstructure and mechanical properties of ultrafine-grained titanium processed by multi-pass ECAP at room temperature
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ARTICLE Microstructure and mechanical properties of ultrafine-grained titanium processed by multi-pass ECAP at room temperature using core–sheath method Alireza Derakhshandeha) and Hamed Shahmir School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran 11155-4563, Iran
Mahmoud Nili-Ahmadabadi School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran 11155-4563, Iran; and Center of Excellence for High Performance Materials, School of Metallurgy and Materials, University of Tehran, Tehran 11155-4563, Iran (Received 3 May 2018; accepted 27 July 2018)
A commercially pure titanium (CP-Ti) of grade 1 as a hard-to-deform material was processed successfully by ECAP processing up to four passes at room temperature via the core–sheath method using a die with an internal channel angle of 90°. The simulation and analytical calculations demonstrated that imposed back pressure on the core was increased at each pass due to strain hardening of sheath metal (AISI 1015 steel) during deformation which prevented damage accumulation and crack initiation at a high number of passes. The scanning electron microscopy and transmission electron microscopy observations of ECAP-processed Ti revealed a severely deformed microstructure which consisted of a high dislocation density and an average grain size of ;250 nm. Mechanical properties of four-pass ECAP-processed CP-Ti showed a substantial enhancement of ultimate tensile strength up to 890 MPa associated with a reasonable elongation to failure of 15.3%.
I. INTRODUCTION
Excellent corrosion resistance, significant biocompatibility, and low Young’s modulus have made CP-Ti as one of the most appropriate materials for biomedical applications.1 Although the mechanical properties of CP-Ti is relatively low when compared with its alloyed counterparts such as Ti–6Al–4V, which has also been proven to be toxic to human body,2–4 but it has been demonstrated that this deficiency can be overcome through grain refinement by severe plastic deformation (SPD).5–7 Considering various SPD methods, ECAP has been known specifically viable since large samples can be readily processed to produce ultrafine-grained (UFG) bulk materials.8 Earlier investigations reported that grain refinement of CP-Ti via ECAP not only enhances the strength but also increases corrosion resistance,9 decreases wear rate,10 elevates fatigue behavior,11,12 and improves its biocompatibility.13–15 However, this material is classified as hard-to-deform materials due to hcp crystal structure and restricted slip systems, which affect the capability of its processing by ECAP at room temperature.16 As a result, ECAP processing of CP-Ti is conducted at a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.291 J. Mater. Res., 2018
elevated temperatures17–19 or implemented by dies with internal channel angles, u, of higher than 90° at room temperature.20,21 ECAP processing at elevated temperatures has been proven to
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