Fabrication of 3D Interconnected Fe<sub>80</sub>Cr<sub>20</sub>-Mg Ultrafine-grained Heterostructure Composite Materials Using High-pressure Torsion: Effects of Sample Thickness

This study investigates the refinement of 3D interconnected Fe<sub>80</sub>Cr<sub>20</sub>-Mg heterostructure composite material fabricated via Liquid Metal Dealloying (LMD) followed by High-Pressure Torsion (HPT) process. The LMD process was executed using two distinct thicknesses of the (Fe<sub>80</sub>Cr<sub>20</sub>)<sub>70</sub>Ni<sub>30</sub> precursor alloy, specifically 0.8 mm and 1.5 mm, which were immersed in an 800 °C Mg melt for two hours. Due to immiscibility and spinodal decomposition between the precursor alloy and the Mg metal melt, Ni was solely extracted from the alloy, leading to self-organization of the Fe<sub>80</sub>Cr<sub>20</sub> structure. The fabricated composite exhibited a heterostructure of 3D interconnected bcc Fe<sub>80</sub>Cr<sub>20</sub> and <i>hcp</i> Mg phases. ImageJ analysis determined that, at a thickness of 0.8 mm, the average sizes of the Fe<sub>80</sub>Cr<sub>20</sub> and Mg phases were 6.43 µm and 2.24 µm, respectively, while at a thickness of 1.5 mm, their sizes were 5.56 µm and 2.56 µm. However, no significant microstructural differences were observed between the different thicknesses of the precursor alloys after the LMD process. The composites underwent HPT treatment at a pressure of 6 GPa, involving various rotational cycles on 10 mm diameter discs. During the severe deformation process, a distinct thickness-dependent response was evident: the composite with a 0.8 mm thickness exhibited minimal plastic deformation due to the presence of a dead metal zone. In contrast, the 1.5 mm thick sample demonstrated notable microstructural alterations. The HPT process effectively changed the grain structure, reducing sizes from micrometers to lamellar ultrafine grains. This microstructural evolution increased the hardness, which escalated from an initial 89 HV to 180 HV after 60 rotations.