Regular features in plastic-strain macrolocalization are examined at the parabolic stage of strain hardening in theÉ635 and Zircaloy-2 zirconium alloys. Instability of the plastic flow is observed, which is manifested as a periodic variation of space-time distributions of local strain as revealed by means of speckle interferometry. The data obtained are discussed within the framework of a synergetic model for the plastic flow evolution at the final stage. Introduction.Macrolocalization is known to accompany the plastic flow at all stages of the process [1, 2]. It has been found that both wave and stationary space-time patterns of plastic-strain localization can be observed during the deformation of single-crystal and polycrystalline samples. Emergence of either pattern is governed by the strain-hardening law acting at the corresponding stage of deformation [1,2].In the present work, we examine the regular features of development of plastic strain localization at the parabolic stage of the plastic flow and at the prefracture stage in samples of the commercial zirconium alloysÉ635 (an alloy contains by weight 1% Nb; 1.3% Sn; 0.4% Fe; rest Zr) and Zircaloy-2 (an alloy contains by weight 1.2% Sn; 0.5% Fe-Ni-Cr; rest Zr) traditionally used in manufacturing fuel-cell tubes for nuclear reactors [3].It was found previously that the flow curves of zirconium alloys immediately above the yield point can be fitted with parabolas σ ∼ ε n , where n is the parabolicity factor [4-6]. We will call this deformation stage the parabolic stage. The parabolic stage displays an intricate behavior and the corresponding strain curve can be divided into several segments approximated by parabolas with decreasing parabolicity factors running through the values form 0.1 to 0.7. The minimum value of n refers to the beginning of formation of a visible macroscopic neck. Of special interest here is the related evolution of the space-time patterns of plastic-strain localization [4][5][6]. For instance, a stable stationary strain localization pattern with a constant spatial period λ is typical only of strain patterns with parabolicity factors n 0.5 in the law σ ∼ ε n . A typical feature of segments with n < 0.5 in the parabolic strain curve is the motion of localized strain nuclei with a varying spatial period, which is accompanied by periodic strain accumulation at a localized nucleus that transforms to a neck by the end of the parabolic stage.Physical reasons for the observed evolution of localization at the final stage of the plastic flow in zirconium alloys remain unclear; therefore, strain-localization features resulting in the loss of stability of the plastic flow and in subsequent fracture call for a further study. The knowledge of the specific features of generation and development of plastic-strain localization finally resulting in fracture under plastic deformation is of considerable practical significance, in particular, in estimating the technological plasticity margin of zirconium alloys heavily strained in production of finished arti...

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