Influence of laser irradiation parameters on the ultrafine-grained Ti-45Nb alloy surface characteristics

In the present study surface alterations of the high pressure torsion (HPT)-deformed Ti-45Nb (mass%) alloy induced by laser irradiation treatment are presented. The alloy was irradiated in air, argon, and nitrogen atmosphere using the Nd:YAG laser. Laser-induced surface modifications were analyzed by field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), profilometric analysis, and Vickers microhardness measurements. Alloy irradiation under a low laser output energy and a low number of accumulated pulses resulted in the formation of shallow surface craters. An increase of these irradiation parameters caused the appearance of more pronounced surface craters with microcracks and hydrodynamic effects in the form of wave-like structures, ripples, and solidified droplets and consequently caused an increase in the surface roughness. The highest value of the surface roughness of 3.062 Im was attained during the alloy irradiation in argon atmosphere under 15 mJ with 150 accumulated pulses. Interaction of the laser beam with the alloy surface was accompanied by plasma formation and surface ablation due to the irradiation energy absorption. The maximal ablated target material volumes of 5.5 x 10(14) nm(3) and 2.6 x 10(14) nm(3) were observed after the irradiation under 15 mJ with 150 accumulated pulses in argon and air atmosphere, respectively. Moreover, attainment of high temperatures at the alloy surface induced its chemical composition alterations. Formation of mixed Ti- and Nb-oxide surface film and absence of nitride particles at the alloy surface was detected irrespective of the irradiation atmosphere. Irradiation in air resulted in the appearance of the most pronounced surface oxide layer with the highest microhardness value which can in great merit influence an increase of the alloy bio-integration abilities and its tribo-corrosion resistance.