  Modification of titanium microstructure after propagation of a melting shock
wave (SW) generated by a femtosecond laser pulse is investigated experimentally
and analyzed using hydrodynamic and atomistic simulations. Scanning and
transmission electron microscopy with analysis of microdiffraction is used to
determine the microstructure of subsurface layers of pure titanium sample
before and after modification. We found that two layers of modified titanium
are formed beneath the surface. A top surface polycrystalline layer of
nanoscale grains is formed from a shock-molten layer via rapid crystallization.
In a deeper subsurface layer, where the shock-induced melting becomes
impossible due attenuation of SW, recrystallization of plastically deformed
titanium leads to grain size changes in comparison with intact titanium.
Molecular dynamics simulation of single-crystal titanium reveals that the SW
front continues to melt/liquefy even after its temperature drops below the
melting curve $T_m(P)$. The enormous shear stress generated in a narrow SW
front leads to collapse/amorphization of the crystal lattice and formation of a
supercooled metastable melt. Such melt crystallizes in an unloading tail of SW
until its temperature becomes higher than $T_m(P)$ due to a rapid pressure
drop. Later, crystallization of the subsurface molten layer will continue after
the heat leaves it. After the shear stress drops below $\sim 12$GPa within the
SW front, such the cold mechanical melting ceases giving place to the
shock-induced plastic deformations. The depth of modification is limited by SW
attenuation to the Hugoniot elastic limit, and can reach several micrometers.
The obtained results reveal the basic physical mechanisms of surface hardening
of metals by ultrashort laser pulses.
