Influence of screw extrusion on the atomic order in constructional steel

The atomic order in low-carbon steel strained by screw extrusion making the microstructure finer and changing the phase ratio in the material is studied by X-ray diffraction analysis. The steel is found to consist of multiscale structural fractions: a finely crystalline (300–600 $\mathring{\mathrm{A}}$) $\alpha$-Fe matrix, nanodimensional (180–250 $\mathring{\mathrm{A}}$) Fe$_3$C and Fe$_3$Si or Fe$_5$Si$_3$ phases, and a fraction with randomly arranged atoms. A second-order order-disorder phase transition is revealed with order prevailing in the longitudinal $(\parallel)$ section of the samples and disorder prevailing in their cross $(\perp)$ section. Sets of “extended” and “compressed” planes are found to coexist in the structure for the first time, with extended planes prevailing in the $(\parallel)$ section and compressed planes prevailing in the $(\perp)$ one.