Synergistic enhancement of strength and ductility in Mg-Gd-Y-Zn-Zr alloys through LPSO phases with multiple morphologies

doi:10.1016/j.metadv.2026.02.027

Abstract

Abstract:

As a crucial strengthening phase in rare-earth magnesium alloys, the content and morphology of the long period stacking ordered (LPSO) phase significantly influence the mechanical properties of the alloys, yet the specific regulatory mechanisms and synergistic effects remain unclear. In this study, Mg-Gd-Y-Zn-Zr alloy sheets with different LPSO phase contents and morphologies were fabricated by adjusting alloy composition and employing a multi-pass rolling process. The microstructure, mechanical properties, and strain distribution of the alloys were systematically analyzed using scanning electron microscopy, electron backscatter diffraction, and digital image correlation techniques. The results demonstrate that a multi-morphology LPSO structure—comprising bulk (B-LPSO), intragranular lamellar (I-LPSO), and dispersed lamellar (L-LPSO) phases—was controllably achieved via a rolling-induced “W→LPSO” phase transformation. When the LPSO volume fraction reached 13%, all three morphologies coexisted in the alloy, which exhibited optimal tensile properties: ultimate tensile strength of 334 ± 14 MPa, tensile yield strength of 310 ± 5 MPa, and elongation of 13.8% ± 1.2%. These properties are markedly superior to those of the alloy containing only a single LPSO morphology with a higher content (18% LPSO). Mechanistic studies revealed that B-LPSO originates from the deformation-induced transformation of grain-boundary W-phase, I-LPSO forms through segregation at stacking faults within the supersaturated matrix, and L-LPSO results from the fragmentation, dissolution, and subsequent reprecipitation of I-LPSO. During deformation, B-LPSO and I-LPSO act as stress-bearing “hard zone” while the Mg matrix containing L-LPSO serves as a strain-accommodating “soft zone”. Their synergistic interaction leads to the simultaneous enhancement of both strength and ductility. Furthermore, the multi-morphology LPSO phases improve deformability and fracture toughness by promoting non-basal slip and inducing crack deflection. This study introduces a novel approach for regulating LPSO phase morphology through W-LPSO phase transformation, offering valuable insights for designing high-performance rare-earth magnesium alloys.

Key words: Mg-Gd-Y-Zn-Zr alloys, Long period stacking ordered (LPSO) phase with multiple morphologies, Rolling, Phase transformations, Deformation behavior

Zeyu Zhao, Zhibin Chen, Hailong Shi, Jinhua Cao, Xuejian Li, Xiaoshi Hu, Xiaojun Wang, Weimin Gan, Xinghong Zhang. Synergistic enhancement of strength and ductility in Mg-Gd-Y-Zn-Zr alloys through LPSO phases with multiple morphologies[J]. Metals Advances, 2026, 46: 34-49.

Figures/Tables 21

Fig. 1. Schematic diagram of the homogenization and rolling process.

Fig. 2. Schematic diagrams of the tensile specimens: (a) mechanical property testing specimen, (b) in situ tensile testing specimen.

Fig. 3. Calculated phase diagrams under thermodynamic equilibrium conditions for the three alloy compositions: (a) Mg-3.1Gd-0.9Y-1.4Zn-0.3Zr, (b) Mg-5.1Gd-1.5Y-2.3Zn-0.3Zr, and (c) Mg-7.1Gd-2.1Y-3.2Zn-0.3Zr (wt%).

Fig. 4. Distribution of secondary phases in the initial microstructures of the three alloys: (a-d) 7%-LPSO alloy in as-cast and as-homogenized conditions; (e-h) 13%-LPSO alloy in as-cast and as-homogenized conditions; (i-l) 18%-LPSO alloy in as-cast and as-homogenized conditions.

Fig. 5. EDS elemental distribution of the 13%-LPSO alloy in as-cast and solution-treated states.

Fig. 6. Morphologies of secondary phases in the as-rolled alloys: (a) 7%-LPSO alloy, (b) 13%-LPSO alloy, (c) 18%-LPSO alloy, (d) W-phase present in the 7%-LPSO alloy, (e) bulk LPSO phase (B-LPSO) present in the 13%-LPSO and 18%-LPSO alloys, (f) intragranular LPSO phase (I-LPSO) and lamellar LPSO phase (L-LPSO) present in the 13%-LPSO alloy.

Fig. 7. Elemental composition in characteristic regions of the three as-rolled alloys: (a) 7%-LPSO alloy, (b) 13%-LPSO alloy, (c) 18%-LPSO alloy.

Fig. 8. EBSD results of the as-rolled 7%-LPSO, 13%-LPSO, and 18%-LPSO alloys on the ND×RD plane: (a–c) morphologies of secondary phases in the EBSD observation areas; (d–f) IPF maps of the three as-rolled alloys; (g–i) grain size distribution graphs of the three as-rolled alloys; (j–l) (0001) and ($10 \overline{1} 0$) pole figures of the three as-rolled alloys.

Fig. 9. (a-c) Grain boundary distribution maps and (d-f) twin boundary distribution maps of as-rolled 7%-, 13%-, and 18%-LPSO alloys.

Fig. 10. Tensile stress-strain curves of the three as-rolled alloys.

Fig. 11. Fracture surface morphologies after tensile testing of the as-rolled alloys: (a, d) 7%-LPSO, (b, e) 13%-LPSO, (c, f) 18%-LPSO.

Fig. 12. (a-h) Microstructures and (i-l) elemental distribution of the 13%-LPSO alloy sheets under different reductions during rolling: (a, e, i) as-rolled with 25% reduction; (b, f, j) annealed with 25% reduction; (c, g, k) as-rolled with 50% reduction; (d, h, l) annealed with 50% reduction.

Fig. 13. TEM images of a typical W-phase particle embedded within the α-Mg matrix after rolling: (a) dark-field TEM image; (b) high-resolution TEM image; (c) selected area electron diffraction figure of area A; (d) fast Fourier transform figure of area B; (e-i) energy-dispersive X-ray spectroscopy elemental mapping.

Fig. 14. TEM images of the residual W-phase and the newly formed LPSO phase along with their interface: (a) dark-field TEM image; (b) high-resolution TEM image; (c, d) EDS line scan profile across the interface.

Fig. 15. TEM images of the lamellar LPSO phases: (a) dark-field TEM image; (b-g) energy-dispersive X-ray spectroscopy elemental mapping.

Fig. 16. Schematic diagrams of the secondary phase transformation process during (a) the early, (b) the middle, (c) the late stages of rolling.

Table 1. Features of the LPSO phases with different morphology.

Item	Bulk LPSO phase (B-LPSO)	Intragranular lamellar LPSO phase (I-LPSO)	Lamellar LPSO phase (L-LPSO)
BSE morphology	High contrast, dense bulk, elongated along the RD	Low contrast, coarse lamellar	Relatively low contrast, dispersed elliptical lamellae
EDS composition	Alloying element content significantly higher than the Mg matrix; RE: Zn ≈ 2:1 (at.%)	Alloying element content slightly higher than the Mg matrix; RE:Zn ≈ 2:1 (at.%)	Alloying element content close to the Mg matrix due to the small size of the phases (statistical average)
Distribution	At grain boundaries, near undissolved particulate W-phase	Inside coarse deformed grains, interleaved with the Mg matrix	Dispersed at the grain boundaries of recrystallized grains
Formation process	Deformation-induced dissolution of the W-phase enriches alloying elements, leading to precipitation near grain boundaries; subsequent rolling elongates the phase along the RD.	Stacking faults generated during deformation act as sites for elemental segregation from the supersaturated solid solution, followed by coarsening.	Fragmentation and partial dissolution of coarse lamellar LPSO phases during deformation and recrystallization, followed by reprecipitation as fine lamellar phases.

Fig. 17. EBSD analysis of the as-rolled 7%-, 13%-, and 18%-LPSO alloys on the ND×RD plane: (a-c) KAM maps; (d-f) Schmid factor distributions for basal slip {0001} <11̅2̅0> along the RD direction; (g-i) Schmid factor distributions for prismatic slip {10̅1̅0} <11̅2̅0> along the RD direction.

Fig. 18. Initial microstructure and in-situ DIC-based local strain distribution of the as-rolled 13%-LPSO alloy under different tensile strains: (a) SEM image before tensile testing; (b) IPF map before tensile testing; (c) normal strain (εxx) distribution in the early deformation stage; (d) overall average strain and the local average strain values within the tested regions; (e, f) normal strain (εxx) distribution in the middle and late deformation stages.

Fig. 19. Surface morphologies of the sample under different deformation levels during in-situ tensile testing: (a) IPF map of the observed area; (b-f) surface morphologies at different deformation stages.

Fig. 20. Surface morphologies near the fracture area of the sample after in-situ tensile testing: (a) macroscopic morphology near the fracture zone; (b, c) local views of the fracture surface; (d-f) local regions containing LPSO phases with multiple morphologies near the fracture zone.

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