Room Temperature Dynamic Strain Aging in Ultrafine-Grained Titanium
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NTRODUCTION
THE yield point phenomenon was reported in titanium as early as 1953[1,2] and since then has been extensively investigated in various grades with grain sizes varying from 1 to 25 lm.[3–26] In addition to the observation of yield point and Lu¨ders band formation caused by static strain aging, other responses which can be ascribed to dynamic strain aging (DSA) include serrations in the flow curve (Portevin-Le Chaˆtelier effect), enhancement of the work hardening rate, humps in the temperature dependence of yield strength, reduction in ductility (blue brittleness effect), and abnormally low strain rate sensitivity.[7–15,18,22–26] Strong DSA effects in coarse-grained titanium extend from as low as 473 K to 973 K (200 C to 700 C). Most results were obtained for commercial purity grades with 2000 ppm of interstitial solutes.[9,12,18,22] However, even in highpurity Ti containing only 150 ppm of interstitials, DSA still occurs (although drastically reduced).[12] An activation energy of 172 kJ/mol for the onset of serrations in CG Ti[12] was found to be close to that for C and N lattice diffusion. This corroborates the mechanisms of dislocation pinning or dragging by interstitials (Cottrell atmosphere).[13,18,22] FELIPE PERISSE´ D. LOPES, Graduate Student, and SERGIO N. MONTEIRO, Professor, are with the Materials Science Department, Military Institute of Engineering, IME, Rio de Janeiro Brazil. CHIA HUI LU, Graduate Student, SHITENG ZHAO, PhD Student, and MARC A. MEYERS, Distinguished Professor, are with the University of California, San Diego, CA. Contact e-mail: mameyers@ eng.ucsd.edu Manuscript submitted January 22, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
In recent years, investigations on the mechanical behavior of ultrafine-grained titanium (UFG Ti) processed by severe plastic deformation (SPD) have received considerable attention due to its potential applications, primarily in the biomedical field. However, these studies have either ignored[27–41] or merely superficially discussed[42–44] the possible occurrence of DSA. Serrations in stress–strain curves were never analyzed in the UFG Ti processed by equal channel angular pressing (ECAP)[27,28,31–33,35,36,38–41] or equal channel angular extrusion (ECAE),[34,37] even though they were observed. Some room temperature (RT) tensile stress– strain curves for UFG Ti processed by high pressure torsion (HPT)[29,30] display a serrated plastic flow. Blum and Breutinger[42] investigated the deformation kinetics of ECAPed UFG Ti and attributed a work softening at 723 K (450 C) to DSA. Wang et al.[43] also discussed the possibility of DSA in ECAPed UFG Ti compressed at RT at quasi-static (104 to 101 s1) and dynamic (103 to 104 s1) strain rates. Stress–strain curves depicted by Wang et al.[43] reveal a jerky flow, which was not mentioned by the authors. Moreover, they report an abnormal strain hardening behavior at large strains and dynamic strain rates. Contrary to Blum and Breutinger,[42] Wang et al.[43] concluded that such behavior might be associated with twinni
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