Thermally stable and strong bulk Mg–MgO in situ nanocomposites by reactive cryomilling and high-pressure consolidation

Abstract

Nanoparticles have great potentials to improve the strength of metal matrix composites, but unfortunately, they tend to grow at high temperatures and are difficult to disperse uniformly with a high content, limiting the improvement in thermal stability and mechanical properties. Here we show the synthesis and performance of Mg–MgO in situ nanocomposites with a fraction of up to 40 vol% MgO nanoparticles. Our synthetic strategies include reactively cryomilling Mg with oxygen and subsequently consolidating the cryomilled powders under a high pressure of 6 GPa. Dispersed MgO nanoparticles with a fine particle size of 7.8 ± 1.7 nm are mainly situated at grain boundaries and exhibit a strong interfacial bonding with Mg matrix. Because of the strong Zener pinning effect of in situ formed MgO nanoparticles, the thermal stability is largely enhanced from ~ 100 °C for nanocrystalline Mg to 400 °C for Mg–10vol%MgO. The high thermal stability of Mg–MgO enables us to consolidate the cryomilled powders at a high temperature of 500 °C under a pressure of 6 GPa and achieve bulk Mg–MgO nanocomposites with a high compressive yield strength: 562 and 688 MPa for Mg–10vol%MgO and Mg–20vol%MgO, respectively. Meanwhile, the room-temperature hardness of the Mg–MgO nanocomposites increases linearly with the content of MgO nanoparticles and reaches 3.65 GPa for Mg–40vol%MgO. Furthermore, the MgO nanoparticles significantly improve the high-temperature hardness of nanocrystalline Mg.