Impact of electron beam surface modification on deformation behavior and fracture properties of TiNi shape memory alloy

The study deals with the impact of the pulse number at low-energy High-Current Pulsed Electron Beam (HCPEB) treatment at constant energy density E-S upon the deformation behavior of TiNi alloy, its inelastic properties and fracture pattern under quasistatic uniaxial tension. It is shown that inelastic properties of the TiNi alloy under study can be kept at the initial (constant) 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 tau = 2-2.5 mu s, maximum electron energy 25 key, energy density E-S = 3.8 0.7 J/cm(2) 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 under uniaxial static tension results in the increase of martensite yield plateau length Delta epsilon(M), which is 15-30% larger than one in the unirradiated TiNi samples. The reasons of different impact of the HCPEB irradiation on strength and elastoplastic properties of TiNi alloy (with regard to the n) are discussed. The main reason for the strength properties decrease of the HCPEB-modified TiNi alloy at n = 15, 32 is attributed to the formation of a columnar structure in the matrix B2-phase with a particular crystalline lattice orientation (< 110 > B2) in the columnar B2 grains. Whereas 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. The latter results in a more complete realization of the mechanisms of inelastic strain accumulation induced by these transformations, as well as the accumulation of larger plastic strain in the 'soft' martensitic phase.