Mechanical Characterization of Impact-Induced Dynamically Recrystallized Nanophase

Dynamic failure of impact-loaded structures is often caused by dynamic shear localization, that is also known as adiabatic shear banding (ASB). While ASB has long been thought to be triggered by thermal softening, another potent softening mechanism has been recently identified in which islands of dynamically recrystallized nanograins nucleate and coalesce, ultimately leading to fracture. However, the exact nature and extent of the softening has not yet been characterized experimentally. Ti-6Al-4V is chosen as a model material to study the influence of impact-induced dynamic recrystallization (DRX) on the subsequent quasistatic flow properties through a systematic combination of dynamic tests up to a predefined level of strain followed by static testing to fracture. With the dynamic prestrain, the subsequent quasistatic yield strength of the material increases, while the strain-hardening capacity decreases noticeably once the relative dynamic prestrain level exceeds 0.5. Those observations, which are supported by transmission-electron-microstructural characterization, confirm not only the early formation of dynamically recrystallized islands reported by D. Rittel [Phys. Rev. Lett. 101, 165501 (2008)]PRLTAO0031-900710.1103/PhysRevLett.101.165501, but mostly the influence this sparse phase has on the bulk mechanical response. In that respect, the present experiments confirm previously reported trends for other bulk nanograined materials, namely, elevation of the yield stress, significant drop in the strain hardening, and enhanced tendency for shear localization. The first two effects are clearly observed for the sparse islands of DRX that form in the bulk impacted material and allow for future modeling of the response of such hierarchical microstructures composed of both ultrafine and coarse grains.