Observation of ex-situ microstructure relaxation of non-conventional misorientations post femtosecond laser shock exposure in cp-Ti

Abstract

The effect of shock passage in commercially pure titanium (cp-Ti) with minimal macroscopic plastic strain was investigated in the present study. For the first time, ex-situ residual strain relaxation in a shocked material was recorded post-shock exposure. Femtosecond laser was used to produce a shock in cp-Ti. Post-shock investigation was carried using EBSD, TEM imaging, and TEM OIM. Uniform but low misorientations were observed inside the grains. Lamella like structures and nano grain pockets were present in the material. Observed misorientations of the remnant nanograins correspond to the multiple twinned region of twins with low twinning shear. However, a large fraction of misorientation pertains to the double twinned structure. Lattice misorientation build-up was absent in the microstructure, in spite of a large number of defects observed. This is confirmed by the GND component analysis, indicating that the macroscopic plastic strain is minimal. The lattice displays alternating curvature which cancels out over long distances, which indicates the presence of SSDs. Ex-situ strain relaxation was observed in the TEM sample resulting in bending of the TEM lamella about the triple point junction of grain. This resulted in the reverting back of defects to the parent grain, which is similar to defect annihilation on shock unloading as reported in previous MD simulations. A schematic spanning the microstructural changes before and after shock passage, and showing the effect of further strain relaxation on the microstructure is shown.