ISSN 1063-7850, Technical Physics Letters, 2011, Vol. 37, No. 10, pp. 977–979. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.I. Betekhtin, A.G. Kadomtsev, V. Sklenicka, M.V. Narykova, 2011, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2011, Vol. 37, No. 20, pp. 75–79. 977 ^ At present, equal-channel angular pressing (ECAP) is among the most widely used methods of obtaining an ultrafine-grained (UFG) structure in metals and alloys. Metallic materials processed by this method possess improved mechanical properties, including increased yield strength and microhardness. These characteristics usually increase with the number of ECAP passes [1, 2]. However, these UFG materials may exhibit a different behavior in long-term tests, e.g., being strained under creep conditions, whereby a decrease in the durability is observed for the samples with increasing number of ECAP passes [3]. In our previous investigation [4], the observed behavior was attributed predominantly to the ECAP-induced increase in the nanoporosity of samples. 1 The aim of the present investigation was to eluci- date the nature of a complex dependence of the dura- bility of a metal on the number of ECAP passes for UFG aluminum. The experiments were performed with the samples of high-purity aluminum (99.99% Al), which were subjected to ECAP (from 1 to 12 passes) by analogy with the experiment described in [4]. Then, a half of the samples were tensile tested under creep conditions in a neutral medium. The other half of the ECAP-pro- cessed samples were subjected to hydrostatic compres- sion at 1.4 GPa and then creep-tested in the same regime as the first half. Finally, the structural charac- teristics of all tested samples were determined by microscopy, X-ray diffraction, and dilatometry. The aim of the treatment by hydrostatic pressure was as follows. As is known [7], a hydrostatic pressure 1 The formation of nanopores and microcracks in the course of metal straining under ECAP conditions was recently also reported in [5, 6]. within 1–10 GPa does not change the dislocation and block structure (at least, in aluminum and, probably, other fcc metals). At the same time, this treatment leads to a significant decrease in the porosity of sam- ples [4]. Thus, it is possible to separate factors influ- encing the mechanical properties of a material. Figure 1 shows the characteristic curves of creep for Al samples after (a) two and (b) four ECAP passes, without (curve 1) and with (curve 2) subsequent hydrostatic compression at 1.4 GPa. As can be seen, the additional high-pressure treatment significantly influences the creep resistance and produces an increase in the durability of samples and a decrease in their creep rate. Note also that the compression- induced increment in the durability sharply drops in the samples with increased number of ECAP passes. Thus, it is necessary to explain two phenomena: (i) general increment in the durability of pressure- treated samples and (ii) a decrease in this effect with increasing number of ECAP passes. As for the phenomenon of general increase in the durability of samples upon the high-pressure treat- ment, there are all grounds to believe that it is due to an increase in the density (and the corresponding decrease in nanoporosity). Indeed, it was found that the porosity of samples upon ECAP was within 0.1– 0.3%, which was related primarily to the presence of nanopores with diameters ~30 nm. The compression to 1–1.4 GPa decreases the dimensions of pores and reduces the porosity by a factor of 2–3. As is known, the porosity affects almost all mechanical properties of materials, especially the characteristics of creep and long-term durability [8]. For this reason, it was natural to suggest that it was a decrease in the porosity that accounted for the increment in durability of pressure- treated samples. Effect of Hydrostatic Pressure on Defect Structure and Durability of Ultrafine-Grained Aluminum V. I. Betekhtin*, A. G. Kadomtsev, V. Sklenicka, and M. V. Narykova Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia Institute of Physics of Materials, Academy of Sciences of Czech Republic, 61662 Brno, Czech Republic *e-mail: Vladimir.Betekhtin@mail.ioffe.ru Revised manuscript received June 9, 2011 Abstract—The influence of hydrostatic pressure on the structure and durability of ultrafine-grained alumi- num obtained equal-channel angular pressing (ECAP) has been studied. It is established that the experimen- tally observed initial growth in the durability of ECAP-processed metal is due to the healing of nanopores, whereas a decrease in this growth with increasing number of passes is caused by the related increase in the fraction of large-angle grain boundaries. DOI: 10.1134/S106378501110018X ^