Assessment of different processing strategies to fabricate bulk Mg-Fe nanocomposites

Abstract

Recent observations indicate that very high deformation temperatures (673 K corresponding to 0.7 of Mg's melting point) by high pressure torsion (HPT) are mandatory to successfully synthesize bulk Mg-Fe nanocomposites with homogenous microstructure and exceptionally high strength (2.2 GPa Hardness). This spurs the development of new fabrication strategies to further strengthen the material while maintaining microstructural homogeneity for lower processing temperatures (T < 673 K). In this regard the present work deals with various strategies of severe plastic deformation at temperatures below 673 K including ball milling the Mg50Fe50 powder followed by HPT, HPT at 673 K followed by HPT at room temperature, inter-stage annealing during deformation, and changing the geometry of HPT anvils. Microstructural analysis using a scanning electron microscope and results of microhardness tests revealed that although abovementioned strategies accelerated microstructural refinement in Mg-Fe composites, strain localization cannot be suppressed. The strain localization and consequently inhomogeneous microstructure can be attributed to the immediate strengthening of basal texture intensity in Mg phase at low temperature driven by shear strain. Hence, the lack of structural reproducibility and hardness values less than 2.2 GPa show that all strategies are not sufficiently effective to synthesize bulk Mg50Fe50 nanocomposites. Only a high deformation temperature is the main key to fully transform a coarse structure into a homogeneous nano-structure with exceptional strength and good reproducibility.

Keywords

• Hcp/bcc nanocomposites
• Magnesium
• Iron
• Severe plastic deformation
• High-pressure torsion