Investigation of tensile deformation behaviour and mechanical properties of TiNi shape memory alloy after high-current electron beam surface modification

Abstract

The impact of the pulse number at low-energy High-Current Pulsed Electron Beam (HCPEB) treatment at constant energy density ES upon the deformation behavior of TiNi alloy, its inelastic properties and fracture pattern under quasistatic uniaxial tension are described. It is shown that inelastic properties of the TiNi alloy can be kept at the initial level whereas ductility and ultimate strength can be increased when the following parameters of low-energy high-current pulsed electron beam treatment are used: pulse duration τ = 2−2.5 µs, maximum electron energy 25 keV, energy density ES = 3.8 ± 0.7 J/cm2 as well as the pulsed irradiation mode and optimal number of irradiation HCPEB pulses (n) are taken. The HCPEB modification of the TiNi surface layer results in the increase of martensite yield plateau length, which is 15-30 % larger than one in the unirradiated TiNi samples. The strength properties decrease of the HCPEB-modified TiNi alloy at n=15 due to the formation of a columnar structure in the matrix B2-phase. After the HCPEB treatment at n = 5 these parameters are varied due to the change of the chemical composition, namely, the nickel depletion of the B2-phase in the surface layer. The mentioned variation of the chemical composition of the matrix B2-phase is responsible for the increase in the temperature of the martensite transformations.