OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 4 2024 а b c d Fig. 4. SEM images of Ni-Ti alloy specimens with the results of EDS analysis before (a, b) and after UV laser treatment for 300 s (c), 600 s (d) titanium in the TiNi alloy, which has a higher electronegativity and thus is considerably more chemically reactive in the presence of oxygen. Titanium is more prone to lose electrons and form oxides than iron, chromium, and nickel found in stainless steel. Additionally, titanium can lead to the formation of more stable oxides, such as TiO2, compared to the typical oxides formed in stainless steel. The oxide fi lm of TiO2 also exhibits a more ordered and compact structure than the oxides formed on stainless steel surfaces, such as chromium oxides. Figs. 4, d and 5, d illustrate that after 600 s of treatment, changes occur in the morphology of the TiNi and steel surfaces, leading to the formation of distinct surface textures on both materials. A crack network is observed on the TiNi surface, and the microcracking of a thin surface layer during long-term laser treatment is likely a consequence of the infl uence of the heat-aff ected zone caused by local heating during laser treatment, followed by rapid cooling after the end of treatment. This phenomenon is associated with the thermal gradient and stresses generated as a result of the rapid cooling of the treated material’s surface. Microcracking can also be caused by the diff erence in the coeffi cients of linear thermal expansion of the base material and the metal oxide formed on the metal surface during laser treatment. In contrast to the TiNi alloy, a granulated structure is developed on the surface of the steel specimens after 600 s of UV laser treatment. In [31], it was reported that similar granulated structures were observed on the surface of AISI 316L steel when subjected to laser irradiation at a wavelength of λ = 532 nm and a laser radiation power density of 1.1 J/cm². The formation of these structures was attributed to the rapid solidifi cation of the molten zone after ablation. The formation of such a structure on the steel surface under UV laser exposure can also be caused by thermal processes, such as melting and evaporation of the material. Diff erent surface morphology after UV laser treatment under identical conditions for TiNi and steel specimens is related to its diff erent thermophysical and chemical properties. Microcracking on the
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