The magnesium matrix composite reinforced with 3 vol. % of short carbon fibers (Csf), fabricated, under industrial conditions, by the stir casting method, was applied to obtain composite bars by two extrusion methods: the novel method of cold severe plastic deformation with a forward-backward rotating die (KoBo) and conventional extrusion at 400 °C. The effect of Mg(α) grain refining, as well as fibers behavior and phenomenon at the fiber-matrix interface, was examined by optical microscopy,

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... nning electron microscopy with energy dispersive spectroscopy and scanning-transmission electron microscopy methods. The Mg(α) grain quantitative characteristics revealed a decrease of the equivalent diameter from 219 ± 76 μm (as-cast) to 24 ± 10 μm and 0.89 ± 0.35 μm (the hot-extruded and KoBo-processed, respectively). In addition, due to the KoBo application, except for the Csf orientation that was parallel to the extrusion direction, an effect of fibers fragmentation on the length of few Csf diameters was detected. No significant changes in the Csf-matrix interface (besides those between new carbon surfaces) formed by fibers fragmentation, and the matrix created by extrusion were detected. A comparison of the mechanical properties of the Mg-Csf composite showed that the KoBo method ensured a spectacular increase in strength and plasticity.