The phase transformation temperatures of pseudoelastic NiTi alloy were defined by differential scanning calorimetry. The effect of the stress range on the functional properties of the NiTi alloy under uniaxial tension in ice water at a temperature of 0 0 С was studied. The cylindrical specimens with 4 mm in diameter and a gage length of 12.5 mm were tested under static and cyclic loading with frequency of 0.5 Hz. All cyclic tests were performed under the crosshead displacement controlled condition on the STM100 machine. At the temperature above Af the effect of the cyclic loading on the maximum stress in general could be characterized by several stages: strengthening, softening, stabilization and rapid decrease of the maximum stress, which is caused by the initiation and macrocrack growth. With the increase of the maximal stress in the first cycle from 509 MPa to 605 MPa, the strain range also increases.

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A B S T R A C T This paper presents the results of investigation of a nuclear reactor pressure vessel steel 15Kh2MFA of two strength levels under cyclic loading. The mechanism of microcrack formation on the surface and in the bulk of 15Kh2MFA steel under cyclic deformation was investigated. Analysis of the specimen surfaces has shown that microcracks are caused by cyclic sliding in grains most favourably oriented with respect to the direction of the maximum shear stresses. Transmission electron microscope investigations show that microcracks in the material inside the grains are formed mainly along the band-type dislocation structure parallel to the dislocation subboundary. During cyclic deformation, the dislocation density on the subboundaries increases, in the local areas the dislocation density becomes limiting and it reaches the plasticity limit and causes microcrack formation. The interrelation of the average length of microcracks and their surface density with the energy density of inelastic deformation has been found. N O M E N C L A T U R Eb = Burger's vector d = distance between subboundaries F n = specimen surface area under investigation k = number of microcracks K = coefficient depending on the subgrain shape l = average microcrack length l i = microcrack length N = number of cycles N = life fraction N f = macrocrack-initiation life q = microcrack density R hkl = distance between reflection (hkl) and the central reflection R ε = strain ratio β = a parameter δ = elongation r = distance between reflections (hkl) W = inelastic strain energy density W = inelastic strain energy density per cycle at N = 0.5 W r = inelastic strain energy density ε a = total strain amplitude Correspondence: P. V. Yasniy.

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