The case of the manifestation of a cross-sectional constriction for extremeky thin bands of alloy steels (0.05 turn) subjected to tension at high temperatures (1023-1173 K) is discussed. Tile assumption that this effect is generated by the superposition of deformation and collective recrystallization during rupture is made.

At the present time, it is considered that when thin bands of even very plastic materials are subjected to tension, dimensional brittleness develops and they rupture without the formation of a stationary neck essentially with a zero constriction [1]. This is governed, in our opinion, by the unique strain and stress state for the small thickness of the specimens; this leads to slowing of strain localization, a sharp reduction in the local-strain volume, and, as a result, to brittleness [2]. Nevertheless, the presence of a neck during the tensioning of thin bands (a thickness of less than 0.05 ram, precision foil) formed from steels Khl5Yu5 and Kh23Yu5 was unexpected. These steels exhibit rather high plasticity at room temperature; under tension, however, no stationary neck is observed. A well defined neck was observed only with respect to the specimen's width during the high-temperature testing of tension specimens 700 mm long and 25 mm wide on a vertical tension machine with a grip-displacement rate of 4 ram/rain. The experiments were conducted at 293, 773, 873, 973, 1073, and 1173 K. The band experienced brittle fracture right up to 773 K, and the fracture surface was perpendicular to the direction of tensioning. A crack, and in certain cases cracks, appeared on the edge of the specimen; one of them propagated quite rapidly through its entire section. There was no constriction of the specimen (Fig. la). Rupture of the band occurred differently at temperatures above 873 K: a clearly expressed constriction with a magnitude to 30% appeared on attainment of the maximum force. The constriction propagated over much of the specimen's length, as a result of which the "volume of locally deformed metal" can attain significant values; interestingly, the surface of the ruptured band smooths over: it does not remain plane, but works itself into unique "wrinkles," which propagate over a rather large part of the length in the direction of tensioning (Fig. lb). One develops the impression that the directions of maximum tensile deformations between which the specimen is "crimped" are manifested in the metal. It is conceivable that this phenomenon is associated with nonuniformity of the specimen's attachment in the grips, when its individual surfaces do not contact the planes of the metal's "fixity," or lose contact with them during plastic flow. Pronounced fluctuations in thickness across the specimen's section, which develop in connection with nonuniform deformation, are also observed. The strength and plasticity characteristics of the steels vary in a similar manner, differing only in terms of numeric indicators (Figs. 2 and 3). At temperatures to 1023 K, the ultimate strength is higher for the steel Khl5Yu5 specimens; at higher temperatures, this indicator is higher for the steel Kh23Yu5 specimens. Naturally, the plasticity of the specimens varies in the opposite manner. At 1023 K, the plasticity is similar for both steels. It should be remembered that from 873 K, the process of primary, and, predominant collective recrystallization begins to take place vigorously in both steels. Collective recrystallization develops especially acutely at temperatures above 1073 tC It is precisely in this temperature interval (1073-1173 K) that local plastic flow propagates through the largest volume of metal; as microscopic examinations indicate (Fig. 4), the structure outside the region of localization consists of grains highly extended in the direction of tension (which coincides with the band's direction of rolling) at points of its relatively moderate occurrence. At 1173 K, however, rather large polyhedral grains that had grown visibly during collective recrystallization, which accumulated