The Mg-8Gd-3Er-0.5Ag-0.5Zr (wt.%) alloy fabricated by hot-extrusion + stamping exhibits an exceptional aging hardening response, with the yield strength being improved by >200 MPa. Traditional extrusion microstructure was observed in the as-extruded sample, with the elongated non-recrystallized grains showing typical (1010) fiber texture. After stamping, almost all non-recrystallized grains were twinned mainly following the (1012) twin orientation. This significantly changed precipitate morphologies formed during the following aging, from the well-known granular β′ precipitate into net-work β′ + β′_H structure, with the granular β′ precipitates being connected by chain-like β′_H precipitates. Additionally, a novel fault was found in the β′ precipitates, whose formation is highly related to the metastable I₂-type stacking fault. This new precipitation structure means the effective interparticle spacing being approximately zero, and the faults in the β′ precipitates can not only enhance strength of precipitates but also could efficiently impede dislocation motion, thus resulted in positive contribution on alloy’s yield strength. This work provides new insights in developing high-strength Mg alloys by modifying precipitation structure along with the inner faults in precipitation.

The age-hardening behavior and precipitation evolution of an isothermal aged Mg−5Sm−0.6Zn−0.5Zr (wt.%) alloy have been systematically investigated by means of transmission electron microscopy (TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 ℃ for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β₀^′ precipitate (orthorhombic, a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ ” precipitate (hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ ” precipitate is more thermostable than prismatic β₀^′ precipitate in the present alloy. β₀^′ precipitates gradually coarsened and were even likely to transform into β₁ phase (face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged 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.

Instantaneous reactions of Al, Mn, Zn, Zr and Y with Ni by mixing the prepared Mg-8Al-0.4Mn, Mg-6Zn-2Y-0.5Zr and Mg-0.6Ni melts were investigated in this work to reveal the underlying mechanisms of their effects on the removal of Ni impurity. The results indicate three Ni-containing intermetallics, namely Al₄NiY, Al₄Ni(Y,Zr) and Al₃₁Ni₂Mn₆. The former two phases present lath-like and have a relatively larger size (> 20 µm in length) than the latest one which is granular with the diameter of ~120 nm. This illustrates that Al and Y(/Zr) can efficiently remove Ni by forming Al₄NiY or Al₄Ni(Y,Zr) which would precipitate to the bottom of the melt. Furthermore, adding Y into Mg-Al based alloys can simultaneously remove Fe and Ni, which contributes their excellent corrosion resistance. Finally, this paper proposes two methods helped to efficiently remove Ni for both Mg-Al based alloys and Al-free Mg alloys, and both of them are also benefit to improve alloys’ strength.