Microstructure and mechanical properties of hybrid hexagonal BN nanotube and nanoplatelet reinforced AZ91 Mg matrix composites

In this research, we report fabrication of AZ91 (Mg–Al–Mn–Zn) matrix composites reinforced with a hybrid of hexagonal boron nitride nanoplatelets (BNNP) and nanotubes (BNNT) using spark plasma sintering (SPS) and hot isostatic pressing (HIP). The resultant composites present dense microstructure featured by the refined α-Mg grains decorated with eutectic along the α-Mg grain boundaries, and trace amounts of in-situ interfacial-reaction products such as Mg3N2 and MgB2 are observed to serve as chemical anchors for stronger interface. More importantly, the AZ91 composites reinforced with a hybrid of 0.4 wt% BNNPs-0.1 wt% BNNTs displays ∼209 MPa of compressive yield strength, ∼329 MPa of ultimate compressive strength and ∼19.4% of elongation, achieving satisfactory balance of strength and ductility when compared to the AZ91 composite reinforced with sole BNNPs. This improvement can be attributed to the unique three-dimensional hybrid structure, where BNNPs maintain their high specific surface area, and the de-bundled BNNTs extend like tentacles, further contributing to their interfacial adhesion with the matrix. This research is expected to pave the way for the development of Mg-based composites with an optimal balance of strength and ductility.