Surface microstructure and B2 phase structural state induced in NiTi alloy by a high-current pulsed electron beam

In the work, we studied structural phase states in surface layers of electron beam-irradiated nickel-titanium (NiTi) alloy depending on beam energy density. The surface of NiTi specimens was exposed to pulsed irradiation (pulse duration τ = 150 μs, number of pulses N = 5) by a low-energy high-current (I = 70 A) electron beam with surface melting at electron beam energy densities E₁ = 15 J/cm², E₂ = 20 J/cm², and E₃ = 30 J/cm². The surface layer structure was examined by X-ray diffraction analysis and transmission electron microscopy. It is found that in the NiTi specimens irradiated at E ≤ 20 J/cm the layer that contains a martensite phase resides not on the surface but at some depth from it. In the NiTi specimens irradiated at E = 30 J/cm, the entire modified surface zone is characterized by a two-phase state in which the B19′ phase dominates over the B2 phase. It is supposed that a barrier to B2 → B19′ martensite transformation in the melted NiTi layer irradiated at E ≤ 20 J/cm² is high inhomogeneous residual stresses varying with depth from the irradiated surface.