Femtosecond laser-driven shock hardening of iron and aluminum

We found that surface layer around 10 um becomes harder than the parent material of iron and aluminum compressed by the femtosecond laser-driven shock wave. Femtosecond laser pulses with wavelength of 800 nm and pulse width of 130 fs were irradiated to the surface of pure iron with purity of 99.99 {\%} and pure aluminum with purity of 99.999 {\%}, respectively, varing the pulse energy. Hardness was measured using nanoindentation. We found that dense dislocation network exists in the hardened regions as a result of the TEM observation. We suggest that rapidly cooling in the femtosecond laser-driven shock compressed region prevent the dislocation network from recovering.