Microstructure Formed in Body Centred Cubic Metals by Laser Shock Processing

Abstract

This paper reports on a comparative study of the microstructural changes occurring within body centred cubic (bcc) metals (iron, molybdenum, and tantalum) by laser shock processing with different laser systems (titanium: sapphire laser and excimer laser). Using different complementary metallographic and electron microscopic investigation methods it is shown how the processing parameters (pulse duration and number of impacts) and the applied ablation mode (direct or confined ablation) influence the microstructure and hardness of these metals. The studies revealed that distinct as well as long range plastic deformation and hardening in iron, molybdenum, and tantalum can only be obtained by laser shock processing with nanosecond pulses in confined ablation mode using a thermo-protective coating. For all the metals investigated, slip and deformation twinning were observed to be active modes of deformation, though the microstructural changes and the degree of shock induced hardening depended on the material being treated. It is shown that the shock induced hardening increases with an increasing numbers of laser impacts. This strengthening is attributed to an increase in overall dislocation density.