Asymmetric tilt boundaries on conventional twin boundaries (TBs) are significant for understanding the role of twins on coordinating plastic deformation in many metallic alloys. However, the formation modes of many asymmetric tilt boundaries are hard to be accounted for based on traditional theoretical models, and the corresponding solute segregation is complex. Herein, atomic structures of a specific asymmetric boundary on {1012} TBs were reveled using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), molecular dynamics (MD) and density functional theory (DFT) simulations. Reaction between <a₆₀>_M dislocations and the {1012} TB can generate a ~61°/25° asymmetric tilt boundary. The segregation of Gd and Zn atoms is closely related to the aggregated <a> dislocations and the interfacial interstices of the asymmetric tilt boundary, which is energetically favorable in reducing the total system energy.

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.