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• The high rolling temperature would impede the DRX occurrence for these final-rolled sheets. • The reduction of the volume fraction of this β phases led to the highest peak-aging hardness in R-450 °C sheet. • The precipitate chains in R-385 °C + A sheets were beneficial for strength and elongation. • The traditional precipitation characteristic in R-450 °C + A sheet was bad for elongation but good for strength. • The peak-aging R-450 °C sheet showed the highest strength, i.e. UTS of 518±17 MPa and YS of 438±18 MPa. In this investigation, a high-strength Mg-12Gd-1.0Er-0.5Zr (wt.%) alloy sheet was produced by hot extrusion (HE) and subsequent hard-plate rolling (HPR) at different temperatures. The results indicate that the microstructures of these final-rolled sheets are inhomogeneous, mainly including coarse deformed grains and dynamic recrystallized (DRXed) grains, and the volume fraction of these coarse deformed grains increases as the rolling temperature increases. Thus, more DRXed grains can be found in R-385 °C sheet, resulting in a smaller average grain size and weaker basal texture, while the biggest grains and the highest strong basal texture are present in R-450 °C sheet. Amounts of dynamic precipitation of β phases which are mainly determined by the rolling temperature are present in these sheets, and its precipitation can consume the content of Gd solutes in the matrix. As a result, the lowest number density of β phase in R-450 °C sheet is beneficial to modify the age hardening response. Thus, the R-450 °C sheet displays the best age hardening response because of a severe traditional precipitation of β' (more) and β Η / β Μ (less) precipitates, resulting in a sharp improvement in strength, i.e. ultimate tensile strength (UTS) of ∼ 518 ± 17 MPa and yield strength (YS) of ∼ 438±18 MPa. However, the elongation (EL) of this sheet reduces greatly, and its value is ∼ 2.7 ± 0.3%. By contrasting, the EL of the peak-aging R-385 °C sheet keeps better, changing from ∼ 4.9 ± 1.2% to ∼ 4.8 ± 1.4% due to a novel dislocation-induced chain-like precipitate which is helpful to keep good balance between strength and ductility. [ABSTRACT FROM AUTHOR]

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]

The influence of primary and secondary extrusion on the microstructure and mechanical properties of the Mg-12Gd-2Er-0.4Zr alloy was investigated. After the primary extrusion, incomplete dynamic recrystallization (DRX) had led to the formation of non-homogenous grain structure with fine equiaxed grain and coarse grains, while the secondary extrusion refined the microstructure remarkably and further improved the mechanical properties. Moreover, the secondary phases were broken into small particles and distributed along the extrusion direction (ED). The secondary extruded (EX2) alloy had the smallest grain size ~3.0 µm and exhibited the best balance of strength and elongation (ultimate tensile strength (UTS) of 305 ± 5.7 MPa, tensile yield strength (TYS) of 258 ± 10 MPa, and elongation (EL) of 31 ± 2.4%). The excellent EL was mainly ascribed to the fully refined dynamic recrystallized (DRXed) grains after the secondary extrusion. Furthermore, the aged EX2 alloy (AEX2) exhibited an excellent balance of strength and elongation (TYS of 382 ± 11 MPa, UTS of 423 ± 6.2 MPa, and EL of 7.6 ± 0.9%). The alloy strengthening was mainly attributed to the refined grains and strong precipitation strengthening. [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]

This study investigates the effects of the extrusion ratio and annealing treatment on the microstructure, texture and mechanical properties of an as-extruded Mg -11Gd -4.5Y -1Nd -1.5Zn -0.5Zr (wt%) alloy. A high extrusion ratio (30:1) results in a homogeneous microstructure with fine dynamic recrystallized (DRXed) grains, while a low extrusion ratio (6:1) leads to a bimodal microstructure with un-DRXed regions and DRXed grains. The bimodal microstructure can be removed by subsequent annealing. This alloy contains several long-period stacking ordered (LPSO) and MgRE phases (RE: rare earth). The extrusion ratio and annealing process have negligible effects on the volume fraction of the LPSO phase but have significant effects on the MgRE phase. The volume fraction of the MgRE phase decreases as the extrusion ratio and annealing time increase. Cuboid precipitates form in the alloy extruded at low extrusion ratios after annealing. The alloy exhibits a bimodal texture with <0001> and < $$ 10\overline{1} 0 $$ > components. The presence of the <0001> component is determined by a critical grain size. The texture evolution (such as the degree of grain growth) is not influenced by the extrusion ratio, but it is affected by the annealing time, which is related to the MgRE phase rather than the LPSO phase. The grain refinement, the MgRE and LPSO phases, and the texture contribute to the alloy strengthening. Finally, a high-strength extruded Mg bar with a diameter of 32 mm (an extrusion ratio of 6:1) was successfully produced. [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]

This research addresses fundamentals regarding the temperature-dependent composition of η phase in Al-Zn-Mg-Cu alloys under high pressure torsion (HPT) to uncover kinetics and thermodynamics governing the η phase composition evolution. Two-stage HPT experiments are deployed to study the stability of the high-temperature η under low-temperature processing. The first-stage HPT on a solid-solution-treated Al-Zn-Mg-Cu alloy (AA7075) at 200 ℃ for 10-revolutions is to get the high-temperature η, and the second-stage HPT is performed at either 100 ℃ or RT. Atom probe tomography reveals that the high-temperature η phase becomes chemically unstable during the low-temperature processing and its chemistry evolves towards low-temperature values. Surprisingly, a high-Mg Al-Mg phase is, for the first time, observed to form in the alloy under the 2nd-stage HPT processing, in correlation with the decomposition of the high-temperature η phase under the low-temperature processing. Unlike the 1st-stage HPT processed alloy, the 2nd-stage HPT processed alloy develops the segregation of Zn with other elements at grain boundary, accompanied with the chemical modification of the high-temperature η phase. The thermodynamics driving η phase composition variation with temperature provides the Al alloys with temperature variable for multi-stage treatments to engineer precipitation for optimised properties. [Display omitted] [ABSTRACT FROM AUTHOR]

• A high-performance Mg-Li alloy with a TYS of 202 MPa is produced. • The studied alloy consists of deformed α-Mg grains and recrystallized β-Li grains. • Three Mg 3 RE variants form in the alloy and affect the mechanical properties differently. • A novel island-shape β 1R phase is firstly observed in the studied alloy. • The formation mechanism and strengthening mechanism of β 1R phase are clarified. In this study, a high-performance Mg–8Gd–2Y–1Zn–6Li (wt%) alloy having a TYS of 202 ± 2.6 MPa, UTS of 243 ± 2.1 MPa and EL of 10.7 ± 2.3% was successfully fabricated by casting, heat treatment and hot extrusion. The effects of intermetallic compounds on microstructure and mechanical properties were deeply investigated. The results show that the studied alloy consists of coarse deformed α-Mg grains with (0001) basal texture, fine recrystallized β-Li grains with (002) texture and three Mg 3 RE phase variants, which are bulk Mg 3 RE phase with a size range of 10–30 µm, spot Mg 3 RE phase with a size range of 1–2 µm and spherical Mg 3 RE phase with a size range of 0.3–1 µm, respectively. The spherical Mg 3 RE phase has a strong dispersion strengthening effect, while the bulk Mg 3 RE phase acts as crack resources and deteriorates the ductility. A novel island-like β 1 R phase (FCC, a = 0.78 nm) is found for the first time in the Mg-Gd-Li based alloys. It has a coherent (1 ̅ 12) β 1 R // (10 1 ̅ 0) α interface at the broad facet and a semi-coherent (1 ̅ 11) β 1 R // (11 2 ̅ 0) α interface at the end facet. The β 1 R phase transforms from β H phase at the semi-coherent interface by atomic shear of RE and Mg atoms. The β 1 R phase provides a significant precipitation strengthening effect to the studied alloy, while the dissolution of β 1 R phase results in the age-softening phenomenon. The high-performance of the studied alloy is mainly attributed to the α/β duplex structure, three Mg 3 RE phase variants, and β 1 R phase. [ABSTRACT FROM AUTHOR]

This study investigates the effects of extrusion ratio and temperature on the microstructure and mechanical properties of as-extruded Mg-11.5Gd-4.5Y-(1Nd/1.5Zn)-0.3Zr (wt %) alloys. After hot extrusion the studied alloys exhibit a bimodal microstructure consisting of fine dynamic recrystallized (DRXed) grains with relatively random orientations and coarse un-DRXed grains with strong basal texture. The increase of extrusion ratio promotes the DRX, increases the volume fraction of Mg 5 RE phase, and refines the DRXed grains, whereas the increase of extrusion temperature decreases the volume fraction of Mg 5 RE phase and coarsens the DRXed grains. The increase of extrusion temperature suppresses the DRX of Mg-11.5Gd-4.5Y-0.3Zr (GW) and Mg-11.5Gd-4.5Y-1Nd-0.3Zr (GWN) alloys, but it has a limited effect on that of Mg-11.5Gd-4.5Y-1.5Zn-0.3Zr (GWZ) alloy. The increase of extrusion ratio improves the mechanical properties of GWZ and GWN alloys, while it deteriorates the mechanical property of GW alloy. The increase of extrusion temperature leads to a decreased strength and increased ductility of the studied alloys. The change of mechanical properties is a result of the competition between the "strengthening effect" of DRXed grains and the "hardening effect" of un-DRXed grains in the changing bimodal microstructure. The Mg 5 RE phase also contributes to the alloy strengthening, but the extensive Mg 5 RE precipitates acting as the crack resources are detrimental to the ductility, especially for the GWN alloy. With the optimum extrusion condition (temperature of 450 °C and ratio of 20:1) the GWZ alloy exhibits the best mechanical performance, which is superior to that of its competitor 6000 series aluminum alloys. • The studied Mg-Gd-Y-Zn-Zr alloy has a superior strength to 6000 series Al alloys. • Both Mg5RE phase and 18R-LPSO phase enhance the DRX behaviour. • The effect of bimodal microstructure on the mechanical properties was studied. • Three Mg–Gd based alloys were extruded using different extrusion parameters. • A big database (microstructural and mechanical) was obtained by quantitative statistics. [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]

The Mg-12Gd-2Er-0.4Zr (GE122K) is fabricated by double extrusion, and the effects of grain refinement and precipitate evolution on the mechanical performance of the extruded alloys are deeply analyzed. Scanning electron microscope results spectacle that some bulky precipitates in the single extruded alloy are fragmented to fine precipitates during double extrusion and distribute linearly along the extrusion direction (ED). The fine precipitates exhibit the strongest dispersion strengthening effect, whereas the bulky precipitates act as crack resources and decrease the elongation. After double extrusion, a refined grain structure with an average grain size of 2.7 µm is achieved, which is attributed to dynamic recrystallization (DRX). Moreover, double extrusion boosts the content of rare-earth elements into the matrix by dissolving the fine precipitates. During aging, extensive β′ precipitates form in the double extruded alloy than in the single extrude alloy. As a result, refined grains and strong precipitation strengthening make a significant contribution to a high yield strength (YS) of 422 ± 1.59 MPa with an elongation (EL) of 6.5 ± 0.89% in the double extruded+peak-aged alloy (ACX2). The possible strengthening mechanisms are discussed, and it is found that grain refinement and precipitate strengthening are the main contributors to the high strength of the GE122K alloy. Fabrication of Mg-12Gd-2Er-0.4Zr (wt%) alloy via double extrusion+aging with excellent mechanical performance. [Display omitted] • A facile and robust fabrication mechanism is adopted to attain an excellent balance of yield strength and elongation. • Grain refinement and precipitates have different effects on the strength of Mg-12Gd-2Er-0.4Zr alloy. • The β′ precipitate strengthening is the dominate strengthening mechanism during aging treatment of the extruded alloys. [ABSTRACT FROM AUTHOR]

• Rolling temperatures led to great variations in microstructure, aging behaviors and mechanical properties. • Dislocations left after rolling at 300 °C led to the formation of a novel chain-like precipitates. • Chain-like precipitates were composed of dominate β ', β ′ F phases et al. and improve age-hardening response greatly. • The good strength-ductility balance was mainly ascribed to the bimodal microstructure and chain-like precipitates. The sheet rolled at 300 °C exhibited the highest peak-aging hardness, which was ascribed to the precipitate chains comprising of the dominate β' , β′ F phase tail-like "hexagonal ring" layers, individual "zigzag" chain and individual "hexagonal ring" metastable phases. These precipitate chains could segregate the matrix into lots of nano-scale units forming honeycomb structures, which could improve the strength significantly without an obvious reduction of EL together with the bimodal microstructure. Thus, the GE61K-300 °C alloy sheet showed UTS of 367 ± 3 MPa, YTS of 349 ± 4 MPa and EL of 9.1 ± 1.5%, respectively. [Display omitted] In this study, we investigated the microstructures and mechanical properties of Mg-6Gd-1Er-0.5Zr (GE61K) sheets. The results show that the rolling temperature has a remarkable influence on microstructures, age-hardening responses and mechanical properties of GE61K sheets. The microstructures varies with the increase in rolling temperatures, i.e. microstructures were dominated by the mixture of dynamic recrystallized (DRXed) grains and un-DRXed grains at 300 °C, by DRXed grains at 350 °C, and by un-DRXed grains at 400 °C and 450 °C. Rolling at 300 °C leads to a formation of the bimodal microstructure which contains equilibrium Mg 5 (Gd, Er) precipitates and a large number of dislocations. These dislocations leads to a formation of novel chain-like precipitates which comprise dominant β ', β′ F (or β′ T), tail-like "hexagonal ring" layers, individual "zig-zag" chain and individual "hexagonal ring", eventually improving age-hardening responses. The chain-like precipitates are beneficial for achieving a good strength-ductility balance by dividing the matrix into nano-scale units and forming subsequent honeycomb structures. As a result, the R-300 °C sheet shows good mechanical properties, i.e. ultimate tensile strength (UTS) of 367 ± 3 MPa, yield strength (YS) of 349 ± 4 MPa and elongation (EL) of 9.1 ± 1.5%, respectively. However, such strength-ductility balance is destroyed sharply as soon as the rolling temperature increases to 400 °C because of the microstructure variations. [ABSTRACT FROM AUTHOR]