National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia:
M. G. Isaenkova, Professor, e-mail: isamarg@mail.ru
Yu. A. Perlovich, Leading Researcher
V. A. Fesenko, Researcher
Soe San Thu, Post-Graduate Student
M. M. Peregud, Senior Researcher

The creep of products, made of Zr-based alloys, determines the duration of their possible usage by preservation of initial structure and texture. This article is devoted to research of instability of substructure condition and texture of cladding tubes, made of Zr – 1 % Nb alloy, at their exploitation temperature. The noticeable, formerly unobserved varying of crystallographic α-Zr texture takes place by thermal creep tests of tubes under internal pressure at the temperatures of 300–450 ^оС. This varying consists of changes of integral texture Kearns parameters, surely influencing the radiation behavior of tubes. First of all, changes of the tube texture under creep tests are the result of plane twinning {1012}, leading to abrupt reorientation of α-Zr grains with turn of their basal axes by 85^o from tangential direction. This reorientation is the most intensive at the temperatures of 380–400 ^оС. Along with twinning, weakening the tangential component in tube texture, there is one more distinct tendency, which becomes apparent in tubes under creep tests. It consists in strengthening of the basal texture component by means of redistribution of basal axes to the radial direction as a result of more active slip by basal planes, favored by creep conditions. Besides, rotation of prismatic axes around the basal one testifies that the prismatic slip is among creep mechanisms. Greatest changes of the crystallographic texture are observed at outside layers of tested tubes. Thus, the thermal creep initiates the following deformation mechanisms: basal slip, prismatic slip and twinning. Their activity essentially depends on conditions of creep tests (temperature and stress).

1. Zaymovskiy A. S., Nikulina A. V., Reshetnikov N. G. Tsirkonievye splavy v atomnoy energetike (Zirconium alloys in nuclear power). Moscow : Energoizdat, 1994. 256 p.
2. Coleman C. E. et al. Mechanical properties of Zr – 2,5 % Nb pressure tubes made from electrolytic powder. Zirconium in the Nuclear Industry : 15^th International Symposium ASTM STP 1505. 2009. pp. 699–723.
3. Lichter B. D., Flanagan W. F., Lee D. N. The role of texture in stresscorrosion cracking of metals and alloys. Materials Science Forum. 2002. Vol. 408– 412. pp. 991–998.
4. Kim H. J., Kim T. H., Jeong Y. H. Oxidation characteristics of basal (0002) plane and prism (11–20) plane in HCP Zr. Journal of Nuclear Materials. 2002. Vol. 306. pp. 44–53.
5. Kocks U. F., Tome C. N., Wenk H.-R. Texture and Anisotropy. Preferred orientation in polycrystals and their effect on materials properties. Cambridge : Cambridge University Press, 1998. 676 p.
6.Borodkina M. M., Spektor E. N. Rentgenograficheskiy analiz tek stury metallov i splavov (X-ray analysis of texture of metals and alloys). Moscow : Metallurgiya, 1981. 272 p.
7. Isaenkova M. G., Perlovich Yu. A. Zakonomernosti teksturoobrazovaniya v tsirkonii pri plasticheskoy deformatsii i termoobrabotke (Regularities of texture-formation in zirconium in the time of plastic deformation and heat treatment). Moscow : Printing House of National Research Nuclear University MEPhI. 528 p.