Mg-RE (RE = rare earth) based alloys generally exhibit outstanding mechanical properties. However, their high-strength seems to be unexplained using classic strengthening mechanisms in some cases. Herein, a Mg-13Gd-0.4Zr (wt%) alloy that was fabricated by a conventional differential thermal extrusion plus artificial aging treatment exhibits ultra-high yield strength over 510 MPa in both tension and compression. Characterizations using Cs-corrected scanning transmission electron microscopy (STEM) show two unusual microstructures in non-recrystallized grains as: a large density of basal stacking faults (SFs) and profuse distortion areas (DAs). Atomic-resolution STEM imaging indicates that basal SFs are consisted of two types of intrinsic SFs, namely I₁ and I₂, and DAs are self-assembled by 〈c〉 and 〈c + a〉 screw partials. Their strengthening mechanisms are analogous to grain boundary strengthening and dispersion strengthening, respectively, contributing satisfactory yield-strength increments of ~46 MPa and ~76 MPa, respectively. Moreover, DAs improved aging hardening by inducing novel clusters at DA-related boundaries, or increasing the number density of β_H’ precipitate and promoting their distribution along a certain direction. This work supplements the strengthening mechanisms in traditional high-strength Mg-RE(-Zr) based alloys and provides novel insights in the development of ultra-high-strength Mg alloys.

This work reports an exceptional reversed yield strength asymmetry at room temperature for a rare-earth free magnesium alloy containing a mass of fine dispersed quasicrystal (I-phase) precipitates. Although exhibiting traditional basal texture, it owns an exceptional CYS/TYS as high as ~1.17. Electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) examinations indicate pyramidal < c + a > and prismatic < c > dislocations plus tensile twinning being activated after immediate yielding in compression while basal and non-basal < a > dislocations in tension. I-phase particles transferred the concentrated stress by self-twinning to provide the driving force for tensile twin initiating in neighboring grains, thereby significantly increasing the critical resolved shear stress of tensile twinning to possibly the level of pyramidal < c + a > slip, finally leading to the dominance of pyramidal < c + a > slip plus tensile twinning in texture grains. This results in a higher contribution on yield strength by ~55 MPa in compression than in tension, which reasonably agrees with the experimental yield strength difference (~38 MPa). It can be concluded that I-phase particles influence deformation modes in tension and in compression, finally result in reversed yield strength asymmetry.