Abstract
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=150μs, number of pulses N=5) by a low-energy high-current (I=70A) electron beam with surface melting at electron beam energy densities E1=15J/cm2, E2=20J/cm2, and E3=30J/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≤20J/cm2, the layer that contains a martensite phase resides not on the surface but at some depth from it. In the NiTi specimens irradiated at E3=30J/cm2, 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≤20J/cm2 is high inhomogeneous residual stresses varying with depth from the irradiated surface.
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