Microstructure formed in body centred cubic metals by laser shock processing

This paper reports on a comparative study of the microstructural changes oc curring within body centred cubic (bcc) metals (iron, molybdenum, and tanta lum) by laser shock processing with different laser systems (titanium:sapph ire laser and excimer laser). Using different complementary metallographic and electron microscopic investigation methods it is shown how the processi ng parameters (pulse duration and number of impacts) and the applied ablati on mode (direct or confined ablation) influence the microstructure and hard ness of these metals. The studies revealed that distinct cis well as long r ange plastic deformation and hardening in iron, molybdenum, and tantalum ca n only be obtained by laser shock processing with nanosecond pulses in conf ined ablation mode using a thermoprotective coating. For all the metals inv estigated, slip and deformation twinning were observed to be active modes o f deformation, though the microstructural changes and the degree of shock i nduced hardening depended on the material being treated. It is shown that t he shock induced hardening increases with an increasing numbers of laser im pacts. This strengthening is attributed to an increase in overall dislocati on density.