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In this study, we successfully developed a low RE containing Mg-3Y-2Gd-1Nd-0.5Zr (wt%) alloy with high strength-ductility synergy by combined processes of hot extrusion, hot rolling and ageing. This alloy exhibits an excellent strength-ductility balance (UTS of 345 ± 2.0 MPa, TYS of 301 ± 5.0 MPa and EL of 9.2 ± 1.9%), which is better than that of many Mg-RE wrought alloys with higher RE concentration and even comparable to that of 6061 Al wrought alloy. A long-range chain-like structure consisting of β′ phase, β H phase, β M phase and zig-zag atomic columns is observed for the first time in the studied alloy. The combined process of hot extrusion and hot rolling boosts the formation of deformed grains and low angle grain boundaries, and makes the deformed grains dominate in the alloy strengthening. Under this circumstance, the following ageing generates a novel heterogeneous structure comprising the long-range chain-like structure with broad interparticle spacing and the spacious precipitate-free zones in the deformed grains, which plays a key role in the concurrent strengthening and toughening of the alloy. The present study demonstrates that the deformed grains with long-range chain-like structures and precipitate-free zones is desirable microstructure for the low RE containing Mg alloys to achieve high strength-ductility synergy. [ABSTRACT FROM AUTHOR]

• β F ′ precipitate acts as the nucleate site for β 1 precipitation. • β 1 is transformed from β F ′ by short-range solute diffusion, atomic shuffle and shear mechanism. • The transformation from β ′ to β 1 was not observed in the present work. The formation mechanism of β 1 phase has been a controversy in age-hardenable magnesium-rare earth alloys for over two decades. Its nucleation and growth were not clearly illustrated so far due to the lack of direct experimental evidence. In the present work, we unveil the formation mechanism of β 1 phase based on the in-situ and ex-situ observations using high-angle annular dark-field scanning transmission electron microscopy. The direct evidences reveal that β 1 phase is transformed from β F ′ phase by short-range solute diffusion, atomic shuffle coupled with shear mechanism. The β F ′ precipitate firstly converts to intermediate precipitate via short-range solute diffusion in its zig-zag array on (10 1 ¯ 0) α plane, and then transforms into β 1 precipitate via atomic shuffle in the modified β F ′ lattice and shear of (10 1 ¯ 0) α zig-zag array by ∼5° with respect to [ 11 2 ¯ 0 ] α orientation. These findings shed light on the formation mechanism of β 1 phase, and hopefully address the long-standing controversy. [Display omitted] [ABSTRACT FROM AUTHOR]

The microstructure evolution and mechanical properties of as-extruded Mg-11.5Gd-4.5Y-0.3Zr (wt%) alloy with Zn and Nd additions were investigated. The addition of Zn inhibits the dynamic recrystallization (DRX) due to the presence of the long-period stacking ordered (LPSO) phase. The addition of Nd promotes the precipitation of the Mg 5 RE (RE: rare earth) phase. The existence of the densely distributed Mg 5 RE phase before hot extrusion promotes the DRX in subsequent hot extrusion process and leads to grain refinement. The increase in the number of Mg 5 RE phase particles degrades the mechanical properties of the resultant alloy. After hot extrusion, the studied alloys exhibit a bimodal microstructure consisting of fine dynamic recrystallized (DRXed) grains of several microns and strongly textured course un-DRXed grains. The as-extruded Mg-11.5Gd-4.5Y-1.5Zn-0.3Zr alloy exhibits an excellent balance of strength and ductility (tensile yield strength of 371 ± 3.0 MPa and elongation of 7.2 ± 0.8%). The alloy strengthening is attributed to the bimodal microstructure, the Mg 5 RE and LPSO phases, and the basal texture. The tensile yield strength of the as-extruded Mg-11.5Gd-4.5Y-1.5Zn-0.3Zr alloy can be further increased to 425 ± 2.5 MPa by precipitation hardening with the T5 treatment. [ABSTRACT FROM AUTHOR]

An as-extruded Mg-11.5Gd-4.5Y-1.5Zn-0.4Zr (wt %) alloy with an excellent strength-ductility balance has been successfully developed by pre-annealing and hot extrusion. The effects of pre-annealing on microstructure and mechanical properties have been studied. Results show that the Mg 5 RE (RE: rare earth) particles, which are generated by pre-annealing treatments, are cracked during hot extrusion, and resulting Mg 5 RE fragments not only enhance the recrystallization of particle simulated nucleation (PSN), but also improve the continuous dynamic recrystallization (C-DRX) by promoting the grain subdivision. After hot extrusion, the studied alloy exhibits a bimodal microstructure consisting of fine DRXed grains with relatively random orientations and coarse un-DRXed grains with strong basal texture. Increasing the pre-annealing duration raises both the quantity of Mg 5 RE particles and the fraction of DRX, thereby decreasing the strength but increasing the ductility. With pre-annealing for 1 h the studied alloy achieves the best strength-ductility balance with tensile yield strength (TYS) of 377 ± 1.2 MPa and elongation to failure (EL) of 10.8 ± 2.0%. Further pre-annealing degrades the ductility due to the excessive Mg 5 RE particles. The alloy strengthening is attributed to the bimodal microstructure, Mg 5 RE and long-period stacking ordered (LPSO) phases, solute segregated SFs, and texture. [ABSTRACT FROM AUTHOR]

A high strength Mg-11.7Gd-4.9Y-0.3Zr (wt%) alloy with a weak tension-compression yield asymmetry has been successfully developed by pre-deformation annealing, hot extrusion and ageing. The effects of pre-deformation annealing on the microstructure evolution and mechanical properties are studied. The results reveal that pre-deformation annealing generates a large number of Mg 5 RE (RE: rare earth) phase particles and raises the fraction of dynamic recrystallization (DRX). The preformed Mg 5 RE particles not only enhance the DRX by particle simulated nucleation (PSN), but also facilitate the continuous DRX (C-DRX) by promoting the grain subdivision during hot extrusion. Without pre-deformation annealing, the as-extruded alloy exhibits a high tensile yield strength (TYS) of 376 ± 9.6 MPa but a low elongation to failure (EL) of 4.3 ± 0.1% due to the bimodal microstructure consisting of coarse un-DRXed grains with strong basal texture and fine DRXed grains with weak basal texture. After T5 treatment, the TYS further increases to 500 ± 5.5 MPa, whereas the EL reduces to 2.7 ± 0.4%. An excellent balance of strength and ductility (TYS of 343 ± 0.2 MPa and EL of 9.3 ± 0.9%) can be realized by pre-deformation annealing for 1 h due to the raised fraction of DRX and the weakened basal texture. T5 treatment further increases the TYS to 446 ± 3.8 MPa but reduces the EL to 3.0 ± 0.2%. The studied alloy exhibits good compressive performance, resulting in a weak tension-compression yield asymmetry. The grain refinement, Mg 5 RE and β ′ phases, and solute-segregated SFs contribute to the alloy strengthening. [ABSTRACT FROM AUTHOR]

• Li addition overcomes the strength-ductility trade-off of precipitation-strengthened Mg-Gd alloy. • Li addition into Mg-Gd alloy reduces its density and increases its specific yield strength. • Li addition change the dominant phase from β ′ to β H − I I ′ in the peak-aged Mg-Gd-Li alloy. • β , β 1 R , β H − I I ′ phases and Li clusters offer better combined strengthening effect than β ′ phase. • Li addition enhances the activity of non-basl dislocations to accommodate the strain. The ambient strength-ductility trade-off has been a long-standing dilemma in metallic alloys, in particular Mg alloys. Here we report a new strategy to solve such a strength-ductility dilemma in precipitation-strengthened Mg-Gd alloys via Li addition. Different from the strengthening of traditional β ′ phase in Mg-7Gd (wt%) alloy, 1 wt% Li addition to this alloy not only boosts the precipitation of different sized β , β 1 R , β H − I I ′ phases and Li clusters to offer better combined strengthening effect, but also enhances the activity of dislocations to accommodate the strains during plastic deformation. Consequently, both the ambient tensile yield strength and ductility are simultaneously improved as compared to Mg-7Gd (wt%) alloy. Moreover, Li addition brings a reduction in density, in turn increasing the specific yield strength. The present strategy with Li addition offers a new insight into the development of Mg alloys with high strength-ductility synergy and with high specific yield strength. [Display omitted] [ABSTRACT FROM AUTHOR]

Mg-xGd-3Y-yNd-0.5Zr alloys (x = 6, 8, 10; y = 0.5, 1 wt%) were prepared by hot extrusion, pre-annealing and ageing treatments. After hot extrusion, the alloys exhibit a bimodal structure consisting of coarse deformed grains with strong basal texture and fine recrystallized grains with weak < 0001 > texture. The increases of Gd and Nd contents in the alloys effectively refine the grains, weaken the texture, promote the precipitation of fine Mg 5 RE (RE: rare earth) particles and enhance the age-hardening response. The pre-annealing treatments generate a large number of plate-like Mg 5 RE particles in grain interiors and the bulk Mg 5 RE particles at the grain boundaries. These Mg 5 RE particles crack into small fragments during hot extrusion, and hence promote the dynamic recrystallization and further refine the recrystallized grains by pinning the grain boundaries. The ductility and strength of the alloy are simultaneously improved due to the pre-annealing treatment prior to the hot extrusion. In the studied alloys, the peak-aged Mg-10Gd-3Y-0.5Nd-0.5Zr alloy exhibits the best mechanical performance with tensile yield strength of 415 MPa and elongation to failure of 4.6 %. The alloy strengthening is mainly attributed to the bimodal structure, Mg 5 RE phase at grain boundaries and β ′ phase in the grain interiors. • The tensile strength of the alloys was significantly improved with increasing Gd and Nd contents. • The peak-aged GWN1030K alloy has TYS of 415 MPa and EL of 4.6 %. • The ductility and strength of GWN830K alloy are simultaneously improved by pre-annealing treatment. • Pre-existing particles promote the DRX of the alloys due to the PSN mechanism. [ABSTRACT FROM AUTHOR]