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[References: 47 tit.]

Studies were carried out in this work to establish the influence of thermal cyclic and mechanical cyclic loads that simulated the factors of operational impact on structural stability and crystal lattice dilatation of zirconium alloys. It is shown that during cold cyclic deformation at the first moment of loading the Zr–2.5Nb alloy is characterized by significant instantaneous deformation. The effect of anomalous thermal deformations of the zirconium alloy crystal lattices, which are typical for phase transitions of type I and type II, was detected in the operating temperature range (250–350?°C). The absence of phase transformation in this temperature range allows us to suggest the existence of a causal relationship between the anomalous thermal expansion effects of zirconium crystal lattices with orientation changes and phase transformations in grain boundaries. The performed calculations showed the presence of an anomalous change in the crystallite size distribution at a given temperature. The coincidence that temperatures were correlated with size effects and phenomena such as blistering and flecking has been established. The hypothesis of grain-boundary transformations is indirectly confirmed by the results of the second thermal cycle, during which a second jump of the crystal lattice thermal deformations, not related to structural phase transitions, was detected in the range of 550–650?°C.