Gradient changes in structural condition of the B2 phase of NiTi surface layers after electron-beam treatments

Abstract

Structural conditions of the B2 phase of the Ti49.5Ni50.5 alloy surface layers before and after electron-beam treatments (pulse duration τ = 150 μs, number of pulses n = 5, beam energy density E ≤ 20 J/cm2) were studied by X-ray diffraction analysis. Analysis of the X-ray patterns demonstrates that surface layers modified by electron beam treatment contain phase with B2surf structure. It is revealed that the lattice parameter of the B2surf phase in the surface (modified) layer is also higher than the lattice parameter of the B2 phase in the underlying layer (aB2 = 3.0159±0.0005). The values of lattice parameter of phase B2surf amounted aB2surf = 3.0316±0.0005 A and aB2surf = 3.0252±0.0005 A, for the specimens after electron-beam treatment at E1 = 15 J/cm2 and E2 = 20 J/cm2, respectively. Inflated lattice parameters aB2surf are associated with changes in the chemical composition and the presence of residual stresses in the surface region of the samples after electron-beam treatments.