ZrB2 nanoparticles were used to modify a selected solidification processed Mg-RE alloy to give it ultrahigh strength (tensile yield strength > 400 MPa). This approach did not involve time consuming and therefore cost incurring stages such as (1) ingot solutionizing and quenching prior to hot extrusion as well as (2) thermal aging beyond 24 h after hot extrusion. Rather, the ZrB2 nanoparticle induced finer LPSO phase (nano-LPSO-layer) formation due to nano-surface effects and the consequent nucleating effects of the fibrous LPSO ends during hot extrusion resulted in the formation of nanograins. Alternatively, free zirconium from ZrB2 nanoparticles reacting with the magnesium matrix may have had a significant nanoscale grain refining effect on the alloy. During the 24 h period of lower temperature (200 A degrees C) thermal aging in this study, the LPSO phase formed in nanograins containing sufficient dissolved Gd, Y, and Zn, this being nano-LPSO-grain formation which "auto-locked" the nanoscale grain size during thermal aging due to the thermal stability of the high melting point rare earth containing LPSO phase. Compared to the surrounding alloy matrix, the nano-LPSO-grain cluster with random grain striation orientation was more robust. This was confirmed by the observation of predominantly non-basal or aOE (c) c+a > type dislocations requiring higher CRSS around as well as within the room temperature tensile deformed nano-LPSO-grains. The LPSO phase generally constricted the flow of dislocations during deformation. The nano-LPSO-layer also acted as finely divided nanoscale reinforcement for the alloy matrix, including nanoscale strengthening of selected micrograin boundaries by bridging. The higher robustness of the nano-LPSO-grain cluster (and nano-LPSO-layer), good stress transfer characteristics across the nano-LPSO-grain boundary (and nano-LPSO-layer-alloy matrix interface), and nanoscale bridging across selected micrograin boundaries by nano-LPSO-layers contributed to the ultra-high strength characteristic (tensile yield strength > 400 MPa) of the selected Mg-RE alloy.