Inverse Hall–Petch behavior in diamantane stabilized bulk nanocrystalline aluminum

The mechanical behavior of bulk cryomilled aluminum stabilized by diamantane nanoparticles is compared with that of bulk cryomilled commercial purity (CP) aluminum with no diamantane. Bulk cryomilled aluminum was prepared by consolidating the cryomilled aluminum powders via hot isostatic pressing followed by trap extrusion (high strain extrusion) at room temperature. The results revealed that, while the grain size of cryomilled CP aluminum (155 nm) was larger than that of cryomilled aluminum containing diamantane (68–95 nm), the strength of the former is higher than of the latter. This finding is explained in terms of the occurrence of nanoscale softening, which signifies a transition from conventional Hall–Petch behavior (strengthening with decreasing grain size) to inverse Hall–Petch behavior (softening with decreasing grain size).