Influence of the structural state and crystallographic texture of Zr-2.5% Nb alloy samples on the anisotropy of their thermal expansion

Abstract

• The thermal linear expansion coefficient of Zr-2.5%Nb channel tubes has been studied. • Crystallographic texture before and after phase transformation has been investigated. • The influence of initial crystallographic texture and structural-phase state on dimension changes after dilatometric measurements has been established. • The physical processes occurring in Zr-alloys during heating and cooling at 20-1200° C temperature interval has been explained. Zirconium remains the main structural material for thermal reactors due to the small capture cross section of thermal neutrons. In the period of stricter safety requirements for reactors with a simultaneous increase in operating parameters, predicting the behavior of the material in emergency situations has gained greater urgency. In this work, we measured the temperature dependence of the thermal expansion of samples cut from thick-walled tubes made of Zr-2.5% Nb alloy that were deformed and annealed in different modes. The analysis of the physical processes responsible for the shaping of the material is carried out. It was found that the anisotropic change in the linear dimensions of cubic samples both during heating and cooling is due to a change in the zirconium content in the β-phase, phase transformations α + β-Zr → α + β-Nb → β → α + β-Zr, as well as the preferred orientation of the grains of the α-phase, characterized by high anisotropy of linear expansion. It is shown that the expansion of the investigated samples upon heating in the α -phase is determined exclusively by the integral texture parameters of Kearns, and the coefficients of thermal linear expansion (TLEC) in different directions vary over a wide range from 3⋅10 -6 to 12⋅10 -6 K −1 . During cooling at the stage of the reverse phase transformation of the β-phase into α, an increase in the size of the cubic samples in the tangential direction up to 2.3 % and a decrease in the radial direction to 1.3 % are noted, which is associated with the orientation of the α-phase grains formed during cooling in the β -matrix and anisotropy of the TLEC of the α -grains. It is shown that the orientation of the α-phase grains in the reverse β → α transformation is determined not only by the orientation of the basal axes in the initial material, but also by the spatial distribution of the prismatic axes relative to the external directions in the tube. Significant size fluctuations in different directions of the samples obtained using the technology of manufacturing channel tubes for CANDU reactors should lead to the development of significant macrostresses both during heating and cooling of the tube. The structural state of the samples deformed or annealed at 400–530 °C does not significantly affect the temperature dependence of the TLEC, but is manifested only in some of its features. In this case, completely recrystallized samples, i.e., with a recrystallization texture, in which the < 11 2 - 0 > directions are oriented along the tube axis, are characterized by a significantly lower variation in dimensions in different directions in the temperature range 20–1200 °C.