Structural decomposition and phase changes in TiNi surface layer modified by low-energy high-current pulsed electron beam

The TiNi shape memory alloy was exposed to low-energy high-current pulsed electron beam treatment with surface melting at an energy density E-s =10 J/cm(2), pulse duration tau = 20 mu s and number of pulses n = 1, 10, 25. The structure and phase state of modified surface layer of TiNi were analyzed in detail by methods of AES, XRD and TEM/EDS/SAED in cross-section geometry. It was established that modified TiNi surface region reveals three layers: upper layer 1 of thickness 5-7 nm which comprises TiO2 and Ti3Ni4 nanoparticles, layer 2 of thickness 1.5-5 mu m with a columnar B2 structure stabilized by TiO2 and Ti3Ni4 nanoparticles, and layer 3 of thickness up to 10 mu m which is represented by B2 structure with a high dislocation density and strain-induced B19' martensite. It was considered how the number of pulses at a constant energy density influences its structural characteristics, and discuss the role of oxygen in stabilizing the columnar structure of its melted and rapidly solidified surface layer. (C) 2019 Elsevier B.V. All rights reserved.