Fractal nature of the dislocation structure of low-alloyed steel after controlled rolling

Abstract

Methods of x-ray diffractometry and transmission electron microscopy are used to study the crystallographic texture and dislocation structure of low-alloyed steel after controlled rolling. It is demonstrated that the formation of crystallographic texture and the corresponding dislocation structure are interrelated. These processes are a manifestation of different sides of one phenomenon of developed plastic strain. The fractal dimensions of cellular dislocation structure boundaries of the main texture components in deformed steel are determined using the multiple grid method.Plastic deformation is a relaxation process with strain localization on different scales in the form of dislocation pileups and dipoles, slip bands, dislocation tangles, cellular and band structures, etc. [1,2]. One structure type is transformed into another through the structural instability when the boundaries between individual substructure elements are smeared [3]. Under further strain, a new regular structure is formed after the structural instability. Moreover, this process occurs cooperatively and self-consistently. The above-listed structures result from the stress relaxation in the process of load removal [4]. A distinguishing feature is that the dislocation structure differs for different crystallite texture components [5]. Thus, as demonstrated recently for molybdenum deformed single crystals [6], structures observed on different scales (macro-, meso-, and microscales) are self-similar. This is indicative of the fractal nature of the structure formed under plastic strain. One of the basic parameters influencing the fractal system properties is its fractal dimension that does not coincide with the dimension of space in which the fractal is formed [7].The present paper is aimed at determining the fractal dimension of boundaries of dislocation structures formed after rolling of low-alloyed steel with the body centered cubic (BCC) lattice.We investigated samples of low-alloyed steel (Fe-0.11% C-1.50% Mn-0.38% Si) prepared by controlled rolling [8]. The temperature of controlled rolling termination t c.r was 650°C. As demonstrated in [9], the strain texture in this case was sufficiently uniform over the sheet cross section and had {001}<110> and {111}<110> orientations with volume contents of ~70 and 30%, respectively, that is, it was the rolling texture typical of BCC metals [10]. The thickness of steel sheets after rolling was 20 mm.The crystallographic texture was studied by the method of x-ray transmission electron microscopy with construction of direct pole figures in the rolling plane and reciprocal pole figures in the direction normal to the sheet plane, in the rolling direction, and in the transverse sheet direction, respectively [10]. Micrographs were recorded with a DRON-3M diffractometer using K α magnesium radiation. Before recording, sample surfaces were chemically polished to remove the distorted layer to depths as great as 0.2 mm. Samples prepared from small-sized recrystallized filings were used as reference textureless ...

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