Ultrafine nanocrystalline microstructure in Mg[sbnd]B alloy for ultrahigh hardness and good ductility

Abstract

Mg alloys are remarkably light but lack the strength offered by other structural materials. They can be traditionally strengthened by introducing hard reinforcements, but such method usually compromises ductility (toughness). An ingenious structural design could provide a way of retaining lightness while adding hardness of Mg. Here, we focus on B as light-weight dopant and develop new MgB alloy with small B content addition of 6.6 at.% using non-equilibrium magnetron sputtering technology. The special nanostructure with ultrafine Mg(B) interstitial solid solution grains (mean diameter: ~5 nm) embedded into narrow amorphous shell (~2 nm thick) is highlighted. The resulting binary alloy has substantially improved hardness from 2.6 ± 0.2 to 8.5 ± 0.4 GPa and elasticity modulus from 89.9 ± 4.7 to 150.3 ± 6.3 GPa, without much compromising the good ductility of pure Mg, as a consequence of the synergistic actions from almost-dislocation-free refined grains and narrow amorphous matrix. This newly simple approach can overcome the long-standing challenge of trade-off between hardness and toughness, and can offer a widely applicable route to high-performance light-metal materials for both structural and nonstructural applications.