61 ISSN 1063-7850, Technical Physics Letters, 2017, Vol. 43, No. 1, pp. 61–63. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.I. Betekhtin, A.G. Kadomtsev, M.V. Narykova, O.V. Amosova, V. Sklenicka, 2017, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2017, Vol. 43, No. 1, pp. 38–44. Defect Structure and Mechanical Stability of Microcrystalline Titanium Produced by Equal Channel Angular Pressing V. I. Betekhtin a *, A. G. Kadomtsev a , M. V. Narykova a , O. V. Amosova a , and V. Sklenicka b a Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia b Institute of Physics of Materials, Academy of Sciences of the Czech Republic, 61662 Brno, Czech Republic *e-mail: vladimir.betekhtin@mail.ioffe.ru Received July 16, 2016 Abstract—It is established that increases in nanoporosity and the proportion of high-angle grain boundaries in the process of equal-channel angular pressing are the main structural factors leading to reduction in mechanical stability (durability) of microcrystalline titanium during long-term tests under creeping condi- tions. DOI: 10.1134/S1063785017010047 The wide area of application of titanium is evoking increased interest in the study of the structure pecu- liarities of this metal in its high-strength microcrystal- line state formed as a result of severe plastic deforma- tion (SPD). It is known that the high mechanical properties after SPD are determined basically by the size of the grains and condition of their boundaries. Reduction of the size of grains during SPD leads to an increase in the volume fraction of their boundaries, where high concentrations of defects (dislocations, vacancies, nanopores, etc.) and high internal stresses are localized [1, 2]. Because of this, the nano- and microcrystalline metals produced by SPD are inher- ently nonequilibrium, and so the problem of their mechanical stability, especially during prolonged stress, is important both for fundamental research and in terms of application [3]. In [4, 5], it was shown that a significant influence on the mechanical stability is exerted by two structural factors: nanoporosity formed during SPD and high-angle boundaries (ϕ > 15°) causing a high level of internal stress. In this paper, we investigate the contribution of these two factors to mechanical stability (durability during the test in the creeping mode) of microcrystalline titanium produced by equal channel angular pressing (ECAP). VT1-0 titanium with an impurity content ≈0.3% was selected for the study. The ECAP was performed by route B c with cyclic rotation of the workpiece around the axis of the channel by 90° after each cycle and the angle of intersection of the channels of 120° at 673 K [6]. For mechanical testing, samples were used that had been prepared after different numbers of ECAP passes. The samples had a length of the uniformly deformable part of 15 mm with a cross-sectional area of 3 × 2 mm 2 . The prepared samples were tested at T = 673 K and σ = 15 MPa until rupture, and the time to failure (durability) was determined. Additionally the micro- hardness and its variation depending on the number of passes in ECAP were determined on initial (before testing in creep conditions) samples. The density of the samples and its change during ECAP caused by pore formation, among other things, were determined by triple hydrostatic weighing. The parameters of the pores were found with the help of a modified method of X-ray scattering in the field of ultralow angles using a high (1.5 GPa) hydrostatic pressure for identifying the void nature of the scatter- ing inhomogeneities [7]. The sizes of grains and their distribution by disordering were determined using transmission and scanning electron microscopy and backscattering of electrons. Consider the obtained experimental data. It was established that the densities of samples of titanium in the original (before ECAP) state and after two, four, and eight passes were 4.5127 ± 0.0003, 4.5117 ± 0.0005, 4.5060 ± 0.0006, and 4.5100 ± 0.0005 g/cm 3 , respectively. Thus, there is a clear trend toward growth “loosening” of the titanium (determined by the level of nanoporosity, among other factors) with an increase in the number of passes. The effect of high hydrostatic pressure, as studies have shown, leads to a significant increase of density. For example, after four passes in ECAP, the density increased from 4.5065 to 4.5100 g/cm 3 because of the applied hydrostatic pres- sure. The action of hydrostatic pressure allowed us to identify the nature of the increased intensity of the small angle scattering that occurs after ECAP (Fig. 1, curves 1 and 2). It is seen that the intensity of the scat- tering is markedly reduced after the action on samples