  We review the results of our research on damage mechanisms in materials
irradiated with femtosecond free-electron-laser (FEL) pulses. They were
obtained using our hybrid approach, XTANT (X-ray-induced Thermal And Nonthermal
Transitions). Various damage mechanisms are discussed with respect to the pulse
fluence and material properties on examples of diamond, amorphous carbon, C60
crystal, and silicon. We indicate conditions: producing thermal melting of
targets as a result of electron-ion energy exchange; nonthermal phase
transitions due to modification of the interatomic potential; Coulomb explosion
due to accumulated net charge in finite-size systems; spallation or ablation at
higher fluences due to detachment of sample fragments; and warm dense matter
formation. Transient optical coefficients are compared with experimental data
whenever available, proving the validity of our modeling approach. Predicted
diffraction patterns can be compared with the results of ongoing or future FEL
experiments. Limitations of our model and possible future directions of
development are outlined.
