Balanced strength and ductility in B<inf>4</inf>C/SiC/2024Al composite with multimodal grain structure achieved by multistep ball milling and powder metallurgy

A novel strategy of multistep ball milling was used to tune the multimodal grain structure in B4C microparticles and SiC nanowires hybrid reinforced 2024Al matrix composites (B4C/SiC/2024Al composites). During the ball milling process, SiC nanowires and 2024Al powders were added step by step, multimodal grain structure was achieved by different ball milling time. Then B4C microparticles were added and ball milled to achieve uniform dispersion of B4C microparticles and SiC nanowires. Multimodal grain structure was maintained in the subsequent preparation processes. Balanced strength and ductility were achieved in trimodal coarse grain (CG)/ fine grain (FG)/ ultrafine grain (UFG) structure, which exhibited yield strength of 616.7 ± 8.1 MPa, ultimate tensile strength of 714.6 ± 2.1 MPa and fracture elongation of 2.68 ± 0.12% with a comparable strength but doubled ductility as that in unimodal UFG structure. Significant toughening effect was achieved in the trimodal grain structure, which could be attributed to the hetero-deformation induced (HDI) hardening effect and the energy dissipation mechanism caused by the formation of a large number of SFs and the blunting effect of CG(FG) zones on crack tips. Therefore, multistep ball milling and powder metallurgy were confirmed an effective way to tune and fabricate multimodal grain structure with balanced strength and ductility in metal matrix composites.