Peak-aged microstructures in Ho-dominant magnesium alloys: Higher precipitate number density and finer size distribution

Magnesium–rare-earth (Mg–RE) alloys are promising lightweight structural materials, but the precipitation-strengthening behavior of many RE elements beyond conventional Gd/Y/Nd-based systems remains insufficiently understood. In this study, Ho-dominant Mg–Ho–Y and Mg–Ho–Gd alloys were designed using a combined-RE alloying strategy, with the well-established Mg–Gd–Y alloy used as a reference. Their age-hardening behavior, room-temperature tensile properties, and peak-aged precipitation microstructures were systematically investigated. The results show that peak-aged Mg–Ho–Y/Gd alloys achieve strength levels close to those of Mg–Gd–Y, although with lower elongation and a longer time to peak hardness. In the peak-aged condition, the dominant strengthening phase remains β′, but its morphology and size distribution differ markedly from those in conventional Mg–Gd–Y alloys. In particular, the Ho-dominant alloys exhibit a substantially higher number density and finer size distribution of β′ precipitates, with the most pronounced refinement observed in Mg–Ho–Gd. First-principles calculations combined with classical nucleation theory indicate that Ho-containing β′ precipitates exhibit more negative formation enthalpies and relatively low elastic strain energies, leading to a stronger thermodynamic tendency for β′ nucleation. These findings demonstrate that Ho-dominant combined-RE alloying is an effective strategy for regulating β′ precipitation in Mg–RE alloys and provides a new route for achieving refined and high-density precipitate distributions.