Mitigating the strength-ductility inversion in B₄C/Al composites by constructing heterogeneous structure

Heterogeneous B4C/Al composites with soft and hard zones (SZs and HZs) were fabricated through a new approach combining segmented ball milling with pressure infiltration. The segmented ball milling was implemented in two steps, which included high-speed ball milling in the first step (step I) and low-speed ball milling in the second step (step II). It indicated that with the increase of step I ball milling time, the width of SZs reduced. The heterogeneous composite with an SZ width of similar to 2.71 mu m (Hetero-2 mu m) exhibited a 161% increase in elongation compared to the homogeneous composite, with almost no decrease in ultimate tensile strength. Due to the mechanical incompatibility between the SZs and HZs, geometrically necessary dislocations (GND) accumulated near the interface. However, the length of GND pile-ups was finite, resulting in the presence of non-interface-affacted zones (NIAZ), which was harmful to the strength. The high strength of Hetero-2 mu m was attributed to the reduction of NIAZ width. Furthermore, the uniform distribution of plastic strain and some external toughening mechanisms, such as crack proliferation and deflection, were believed to be the reason for the ductility enhancement.