Effect of inclusions on cratering behavior in TiNi shape memory alloys irradiated with a low-energy, high-current electron beam

Abstract Irradiation of TiNi shape memory alloys with a microsecond low-energy (10–30 keV), high-current (10–25 kA) electron beam in surface melting modes induces microcrater formation, which limits the practical use of this method of surface modification. Potential centers of crater formation are inclusions, which are inherent to TiNi alloys. However, there is limited information about their role in crater formation and the nature of this phenomenon as applied to TiNi alloys. In this paper, the microstructural identification of inclusions was carried out in commercial (VIM) and precision TiNi alloys using TEM/SAED/EDS methods. The topographical features of microcraters formed by a single-pulse irradiation of TiNi samples near the surface melting threshold of the B2 matrix phase were identified. It was found by SEM/EDS-analysis, that microcraters are mainly nucleated on titanium oxycarbide TiC(O) (commercial alloy) and oxide Ti 4 Ni 2 O (precision alloy) inclusions; in both cases, the central region of microcraters is enriched with oxygen. The role of gradient capillary forces in the melt mass transfer (Marangoni effect), which determine the formation of microcraters, was analyzed. It was shown that thermocapillary forces play an insignificant role, while chemicapillary forces induced by decreasing a surface tension of the melt due to its oxygen enrichment can be sufficiently large to produce microcraters.