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

Abstract

The TiNi shape memory alloy was exposed to low-energy high-current pulsed electron beam treatment with surface melting at an energy density Es = 10 J/cm2, pulse duration τ = 20 μs 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 μm with a columnar B2 structure stabilized by TiO2 and Ti3Ni4 nanoparticles, and layer 3 of thickness up to 10 μm 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.