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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.

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.

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 Mg5(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.

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 Mg3RE phase variants, which are bulk Mg3RE phase with a size range of 10–30 µm, spot Mg3RE phase with a size range of 1–2 µm and spherical Mg3RE phase with a size range of 0.3–1 µm, respectively. The spherical Mg3RE phase has a strong dispersion strengthening effect, while the bulk Mg3RE 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 Mg3RE phase variants, and β 1 R phase.