Physical and Technical Institute of the National Academy of Sciences of Belarus, Minsk, Republic of Belarus:
A. I. Pokrovsky, Cand. Eng., Associate Professor, Head of the Laboratory of High Pressures and Special Alloys, e-mail: art@phti.by

The goal of this work is to investigate the microstructure, fractograms and morphology of graphite inclusions in ductile cast iron, to prove their plastic flow and to propose a mechanism of plastic deformation of spherical graphite inclusions during pressure forming. Fractographic studies revealed two different morphological types of inclusions after hot forming: the central core and the tail part, which differ in strength. Scanning electron microscope studies of the surface of deformed cast iron after electroetching permitted evaluating the shape and morphology of graphite inclusions and prove their integrity. A two-stage mechanism of plastic deformation of graphite inclusions is proposed. At the first stage, which applies to a reduction ratio of up to 50%, only the outer layers (flakes) of the graphite inclusion are involved in the deformation process. Layer-by-layer exfoliation of flakes occurs exclusively from the surface of the spherulite. The inner core retains a radial-sectoral structure. At a reduction ratio above 60%, at first the above described process occurs, which is accompanied by displacement of the scales into tail protrusions. At the second stage of deformation, the mechanism changes dramatically. It consists in sliding of prismatic sectors (blocks) of graphite relative to each other, wedging out of the central ones from those sectors of the sphere that are located along the application of the load. At the same time, transverse sectors move closer to each other. Since the deformation process takes place at a high temperature, about 950 °С, and is very short, lasting about 1 s, then, after the completion of deformation, interatomic bonds are restored along the shortest interatomic distances, and the inclusion retains its integrity.
The work was carried out at the Physical and Technical Institute of the National Academy of Sciences of Belarus (Minsk) within the framework of the State Scientific Research Institute "Metallurgy", task No. 2.01.

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