Evolution of precipitates and precipitation-free zone in Mg-7Gd-2Nd-0.5Zr alloy under different ageing conditions: mechanical properties and strengthening mechanisms

Abstract

Ageing treatment is a simple yet effective method for enhancing the mechanical performance of magnesium (Mg) alloys. This study investigates the precipitation behaviour, precipitation-free zone (PFZ) evolution, and strengthening mechanisms of Mg–7Gd–2Nd–0.5Zr (wt.%) alloy under different ageing temperatures and times. The as-cast microstructure was found to consist mainly of α-Mg matrix, Mg₅ (Gd, Nd) phase, and a small amount of cubic GdH₂ phase. After solution treatment, the eutectic Mg₅ (Gd, Nd) phase was dissolved into the matrix, while cubic GdH₂ remained. Ageing treatment promoted the formation of fine β′ precipitates, which served as the primary strengthening agent. The key finding is that lower-temperature ageing (200 °C, 96 h) maximised the retention of β′ phases, leading to optimal strength with a yield strength (YS) of 193.3 MPa, ultimate tensile strength (UTS) of 347.8 MPa, and elongation (EL) of 2.7%. In contrast, higher-temperature ageing (250 °C, 3 h) accelerated the β′ → β₁ transformation, reduced the density of effective precipitates, and resulted in a lower YS of 185 MPa and UTS of 313.3 MPa. However, this condition concurrently produced the narrowest PFZ (110 nm), leading to an enhanced EL of 5.5%. These findings provide practical guidance for optimising ageing treatments to enhance the high-performance potential of Mg alloys in lightweight structural applications.

Keywords

Mg alloy / ageing treatment / precipitation behaviour / precipitation-free zone

References

[1] Yang Y, Xiong X-m, Chen J, et al. . Research advances in magnesium and magnesium alloys worldwide in 2020 [J]. Journal of Magnesium and Alloys, 2021, 9(3): 705-747

[2] Song J-f, Chen J, Xiong X-m, et al. . Research advances of magnesium and magnesium alloys worldwide in 2021 [J]. Journal of Magnesium and Alloys, 2022, 10(4): 863-898

[3] Wang S, Pan H-c, Xie D-s, et al. . Grain refinement and strength enhancement in Mg wrought alloys: A review [J]. Journal of Magnesium and Alloys, 2023, 11(11): 4128-4145

[4] Wei K, Xiao L-r, Gao B, et al. . Effect of aging temperature on the hardening behavior and precipitation evolution of Mg-10Gd alloy [J]. Materials Characterization, 2023, 196: 112580

[5] Li Q-q, Jiang W-m, Xu Y-c, et al. . Development of prominent bonding strength in Al/Mg bimetal composites prepared by ultrasonic vibration-assisted compound casting: Effects of ultrasonic powers [J]. Journal of Materials Science & Technology, 2024, 197: 78-93

[6] Xu Y-c, Jiang W-m, Li Q-q, et al. . Superior bonding of Al/Mg interface by introducing nanocrystals via high-entropy alloy coating and ultrasonic vibration [J]. Materials Research Letters, 2025, 13(2): 131-139

[7] Ali N A, Ismail M. Advanced hydrogen storage of the Mg–Na–Al system: A review [J]. Journal of Magnesium and Alloys, 2021, 9(4): 1111-1122

[8] Javaid A, Czerwinski F. Progress in twin roll casting of magnesium alloys: A review [J]. Journal of Magnesium and Alloys, 2021, 9(2): 362-391

[9] Li Q-q, Jiang W-m, Xu Y-c, et al. . New insights into the influencing mechanism of ultrasonic vibration on interface of Al/Mg bimetal composites by compound casting using simulation calculation and experimental verification [J]. Composites Part B: Engineering, 2024, 284: 111726

[10] Malik A, Wang Y-w, Cheng H-w, et al. . Post deformation analysis of the ballistic impacted magnesium alloys, a short-review [J]. Journal of Magnesium and Alloys, 2021, 9(5): 1505-1520

[11] Nie K B, Wang X J, Deng K K, et al. . Magnesium matrix composite reinforced by nanoparticles - A review [J]. Journal of Magnesium and Alloys, 2021, 9(1): 57-77

[12] Sekar P, Narendranath S, Desai V. Recent progress in in vivo studies and clinical applications of magnesium based biodegradable implants – A review [J]. Journal of Magnesium and Alloys, 2021, 9(4): 1147-1163

[13] Shi Z-z, Chen H-t, Zhang K, et al. . Crystallography of precipitates in Mg alloys [J]. Journal of Magnesium and Alloys, 2021, 9(2): 416-431

[14] Wu G-h, Wang C-l, Sun M, et al. . Recent developments and applications on high-performance cast magnesium rare-earth alloys [J]. Journal of Magnesium and Alloys, 2021, 9(1): 1-20

[15] Xie J-s, Zhang J-h, You Z-h, et al. . Towards developing Mg alloys with simultaneously improved strength and corrosion resistance via RE alloying [J]. Journal of Magnesium and Alloys, 2021, 9(1): 41-56

[16] Zhao Z-j, Wang K, Li M, et al. . Simultaneous enhancement of strength and plasticity in extruded Mg-4Gd-0.5Zr alloys: Tailoring grain size and precipitated phases via Nd and Y addition [J]. Journal of Materials Research and Technology, 2025, 39: 2502-2517

[17] Liu Y-f, Long N-n, Zhang D-dong. Effects of hot rolling reduction and intermediate annealing on microstructure and mechanical properties of Mg-Gd-Y-Zr plates [J]. Journal of Alloys and Compounds, 2025, 1039: 183177

[18] Deng D-y, Cheng R-j, Jiang B, et al. . Investigation on the microstructure evolution of high strength and ductility as-cast Mg-9.5Gd-2.3Y-1Zn-0.5Zr alloy via double peak-aging [J]. Nano Materials Science, 2025, 7(5): 686-696

[19] Dang C, Dou X-x, Wang J-f, et al. . Investigations on microstructure and mechanical properties of cast Mg-Gd-Y-Zn-Zr-Nd alloy [J]. Journal of Materials Research and Technology, 2023, 24: 4852-4862

[20] Maghzi M, Mirzadeh H, Mahmudi R. Temperature-dependent mechanical properties of as-cast Mg-Gd-Zr alloys evaluated by shear punch testing [J]. Journal of Materials Research and Technology, 2025, 36: 417-423

[21] Zheng L-w, Zhuang Y-p, Li J-j, et al. . Mechanical properties of Mg-Gd-Zr alloy by Nd addition combined with hot extrusion [J]. Transactions of Nonferrous Metals Society of China, 2022, 32(6): 1866-1880

[22] Nie J F. Effects of precipitate shape and orientation on dispersion strengthening in magnesium alloys [J]. Scripta Materialia, 2003, 48(8): 1009-1015

[23] Wang F-l, Bhattacharyya J J, Agnew S R. Effect of precipitate shape and orientation on Orowan strengthening of non-basal slip modes in hexagonal crystals, application to magnesium alloys [J]. Materials Science and Engineering: A, 2016, 666: 114-122

[24] Cai H-s, Zhao Z, Wang Q-d, et al. . Study on solution and aging heat treatment of a super high strength cast Mg-7.8Gd-2.7Y-2.0Ag-0.4Zr alloy [J]. Materials Science and Engineering: A, 2022, 849: 143523

[25] Wu Q-y, Wu Y-j, Deng Q-c, et al. . Effect of Gd content on microstructure and mechanical properties of Mg-xGd-Zr alloys via semicontinuous casting [J]. Journal of Magnesium and Alloys, 2025, 13(11): 5500-5510

[26] Yamada K, Hoshikawa H, Maki S, et al. . Enhanced age-hardening and formation of plate precipitates in Mg – Gd – Ag alloys [J]. Scripta Materialia, 2009, 61(6): 636-639

[27] Zhao Y-x, Wang X, Liu Y, et al. . Microstructure evolution and mechanical properties of bimodal microstructure Mg-Gd-Y-Zn-Zr alloy under different heat treatment processes [J]. Journal of Materials Research and Technology, 2025, 38: 3851-3865

[28] Wei X-j, Liu Q-z, Han S-ping. Effects of heat treatment on microstructure and mechanical properties of Mg-8Gd-2.5Nd-0.5Zr alloy [J]. The Chinese Journal of Nonferrous Metals, 2017, 27(1): 82-88 (in Chinese)

[29] Li J-c, He Z-l, Fu P-h, et al. . Heat treatment and mechanical properties of a high-strength cast Mg – Gd – Zn alloy [J]. Materials Science and Engineering: A, 2016, 651: 745-752

[30] Meng M, Zhang H-l, Gao Z, et al. . Effect of aging treatment on the precipitation transformation and age hardening of Mg-Gd-Y-Zn-Zr alloy [J]. Journal of Magnesium and Alloys, 2023, 11(12): 4628-4643

[31] Du Z-w, Peng Y-g, Teng H, et al. . Formation and growth of precipitates in a Mg-7Gd-5Y-1Nd-2Zn-0.5Zr alloy aged at 200 ° C [J]. Journal of Magnesium and Alloys, 2023, 11(7): 2326-2339

[32] Xie Z-z, Xu B-s, Sun W. Study on precipitating phases in Mg-Gd-Nd alloy aged at limiting high temperature [J]. Hot Working Technology, 2011, 40(2): 170-172 (in Chinese)

[33] Shi H, Huang Y-d, Yang L-x, et al. . Compressive creep behavior and microstructural evolution of sand-cast and peak-aged Mg–12Gd–0.4Zr alloy at 250 °C [J]. Materials Science and Engineering: A, 2023, 882: 145422

[34] Jiang A X, You Z Y, Jin S S, et al. . Study on the precipitation strengthening mechanism of Mg–10Zn–5Al–0.2Sc alloy [J]. International Journal of Metalcasting, 2025, 19(3): 1579-1589

[35] Zheng J-x, Chen B. Interactions between long-period stacking ordered phase and β′ precipitate in Mg – Gd – Y – Zn – Zr alloy: Atomic-scale insights from HAADF-STEM [J]. Materials Letters, 2016, 176: 223-227

[36] Zhang Q, Wang F-l, Zeng J, et al. . Nd microalloying significantly enhances precipitation strengthening of Mg-Gd-Y-Zn-Zr alloy [J]. Journal of Alloys and Compounds, 2025, 1025: 180324

[37] Zhang H-q, Zhang C-j, Liang X-j, et al. . Plastic slip mechanisms in high-temperature titanium alloys: Insights into silicide and Ti₃Al precipitates on ductility [J]. Scripta Materialia, 2025, 256: 116412

[38] Liu Z-y, Chen B, Zhao P-y, et al. . Atomic-scale characterization of the precipitates in a Mg-Gd-Y-Zn-Mn alloy using scanning transmission electron microscopy [J]. Vacuum, 2023, 207: 111668

[39] Shao X, Tan Z-d, Xia L, et al. . Theoretical investigation on aging precipitation mechanisms and precipitation strengthening effect of Mg-Gd alloy [J]. Journal of Solid State Chemistry, 2025, 346: 125285

[40] Ye L-y, Liu X-d, Tang J-g, et al. . Ostwald ripening and stability of precipitates during two successive overaging in an Al-Mg-Li alloy [J]. Materials Letters, 2021, 291: 129616

[41] Shi H, Huang Y-d, Yang L-x, et al. . Microstructural evolution of Mg–14Gd–0.4Zr alloy during compressive creep [J]. Journal of Magnesium and Alloys, 2023, 11(9): 3161-3173

[42] Chen M-y, Liu S-d, He K-z, et al. . The effect of precipitate-free zone on mechanical properties in Al-Zn-Mg-Cu aluminum alloy: Strain-induced back stress strengthening [J]. Journal of Alloys and Compounds, 2023, 969: 172426

[43] Wang Y, Li H, Wang Z-x, et al. . The effects of artificial aging following cyclic strain on the microstructure and mechanical properties of Al-Cu-Mg alloys [J]. Materials Science and Engineering: A, 2025, 943: 148832

[44] Li L-z, Liu W-t, Xiao N-m, et al. . Effect of Cu content on microstructural evolution and mechanical behavior of Al – Cu – Mg – Ag alloys [J]. Journal of Materials Research and Technology, 2025, 36: 5222-5236

[45] Shen S-m, Zhuang Y-p, Luo P-l, et al. . Influence of creep aging on structural and mechanical properties of Mg-9Gd-2Nd-0.5Zr alloys [J]. Journal of Materials Research and Technology, 2023, 22: 2383-2397

[46] Shen S-m, Zhuang Y-p, Li M, et al. . Effects of stress loading mode on microstructures and properties of Mg-9Gd-2Nd-0.5Zr alloy treated by creep aging [J]. Journal of Magnesium and Alloys, 2023, 11(11): 4263-4273

[47] Dos Santos D C, Magnabosco R. Kinetic study to predict sigma phase formation in duplex stainless steels [J]. Metallurgical and Materials Transactions A, 2016, 47(4): 1554-1565

[48] Yang Z, Li J P, Guo Y C, et al. . Precipitation process and effect on mechanical properties of Mg–9Gd–3Y–0.6Zn – 0.5Zr alloy [J]. Materials Science and Engineering: A, 2007, 454–455: 274-280

[49] Li Z-h, Xie S-k, Wang G-d, et al. . Dependence of recrystallization behavior and magnetic properties on grain size prior to cold rolling in high silicon non-oriented electrical steel [J]. Journal of Alloys and Compounds, 2021, 888: 161576

[50] Zhang Z-r, Huo Q-h, Zhang Y-x, et al. . Unraveling the individual effect of yttrium and neodymium on the precipitate-free zone development and the dislocation creep mechanism of Mg-Y-Nd alloy [J]. Materials Characterization, 2023, 195: 112537

[51] Xu X Y, Hua Z-m, Hu C, et al. . Impact of aging-induced precipitate-free zones on yielding mechanisms in a low-alloyed Mg-Zn-Ca-Mn-Y alloy [J]. Scripta Materialia, 2026, 272: 117067

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Central South University