Room Temperature Dynamic Strain Aging in Ultrafine-Grained Titanium

Dynamic strain aging (DSA) in coarse-grained (CG) titanium is usually observed at intermediate to high temperatures 473 K to 973 K (200 A degrees C to 700 A degrees C) and is characterized by serrations in the stress vs strain curves. In the present work, despite the absence of apparent serrations, ultrafine-grained titanium (UFG Ti) undergoes DSA at room temperature, exhibited through an abnormal increase in the elastic limit and negative strain rate sensitivity. This effect is observed at 293 K (20 A degrees C) in the strain rate interval of 10(-4) to 10(-2) s(-1), and at 203 K (-70 A degrees C) and 373 K (100 A degrees C) in a distinct strain rate range. Based on a calculated activation energy of 17.3 kJ/mol and microstructural observations by transmission electron microscopy, it is proposed that the dominant mechanism for DSA in UFG Ti involves interstitial solutes interacting with dislocations emitted from grain boundaries. The interstitials migrate from the grain boundaries along dislocation lines bowing out as they are emitted from the boundaries, a mechanism with a low calculated activation energy which is comparable with the experimental measurements. The dislocation velocities and interstitial diffusion along the dislocation cores are consistent.