Strengthening and toughening mechanisms of Mg matrix composites reinforced with specific spatial arrangement of in-situ TiB2 nanoparticles

To improve the strength and toughness of Mg matrix composites, we propose a novel strategy to successfully develop Mg matrix composites reinforced with alternating fiber-like nanoparticle-rich (NPR) zones. A combination of in-situ casting and hot thermomechanical processing approach is employed to achieve this unique microstructure, and the influence of spatial arrangement of TiB2 nanoparticles on the microstructure, mechanical behavior, strengthening and toughening mechanisms are investigated in detail. The results show that the designed nanocomposite exhibits alternative fiber-like NPR zones and nanoparticle-free (NPF) zones, leading to the typical bimodal structure. Meanwhile, EBSD analyses reveal that the fiber-like NPR zones have much higher density of geometrically necessary dislocations than NPF zones. Accordingly, the hardness and elastic modulus in fiber-like NPR zones are enhanced significantly, acting as "hard" units, while NPF zones with high plasticity factor acting as "soft" regions. As a consequence, superior combination of tensile strength (388 MPa) and ductility (10.1%) is realized in TiB2/AZ91 nanocomposite when compared to AZ91 matrix. The strengthening mechanisms of the nanocomposite are evaluated quantitatively, and found that the fiber-like arrays significantly enhanced the load-bearing capacity of TiB2 nanoparticles. Moreover, grains in NPF zones can deform easily, acting as strain bearing soft units, coupling with cracks deflection caused by the "hard" NPR zones, both of which are responsible for toughening nanocomposite.