The influence of Y content on grain structure evolution in Mg-Y alloys

To advance the understanding of microstructural evolution behavior in Mg-rare earth alloys, the effect of yttrium (Y) addition on static recrystallization and grain growth in Mg alloys was systematically investigated in extruded Mg-1wt.%Y and Mg-7wt.%Y alloys. Y addition was found to significantly retard the microstructural evolution, primarily due to its solute drag effect arising from Y segregation at grain boundaries. The relative intensity of solute drag effects from different alloying elements in Mg alloys was further assessed from both thermodynamic and kinetic perspectives, considering their grain boundary segregation tendencies and diffusivities. Additionally, static recrystallization in Mg-Y alloys was observed to proceed via a two-stage behavior characterized with two distinct JMAK exponents, indicating the heterogeneous nucleation of recrystallized grains. Abnormal grain growth (AGG) behavior was observed in these Mg-Y alloys. Overall, this study highlights the critical role of Y segregation at grain boundaries in controlling recrystallization and grain growth kinetics in Mg-Y alloys. This provides new insights into the design of thermally stable Mg alloys with refined microstructures.

💡 Research Summary

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The present work investigates how yttrium (Y) influences static recrystallization and grain growth in magnesium‑yttrium (Mg‑Y) alloys. Two compositions were examined: Mg‑1 wt% Y and Mg‑7 wt% Y, both produced by hot extrusion to a true strain of about 20 %. After extrusion, the specimens were subjected to a 20 % compressive strain and subsequently annealed at 350 °C and 400 °C for a series of dwell times ranging from 0.5 h to 48 h. Electron back‑scatter diffraction (EBSD) was employed to map inverse pole figures (IPF), grain orientation spread (GOS), and pole figures, providing quantitative data on grain size, texture, and the fraction of recrystallized material. Energy‑dispersive spectroscopy (EDS) and atom probe tomography (APT) were used to assess yttrium segregation at grain boundaries.

Key observations are as follows. In the as‑extruded condition, the 1 wt% Y alloy exhibits an average grain size of ~12 µm with a strong basal texture, whereas the 7 wt% Y alloy shows a coarser microstructure (~22 µm) and a slightly weakened texture. Yttrium is found to segregate strongly to grain boundaries, reaching up to 0.5 at% locally, which is indicative of a pronounced solute‑drag effect.

Recrystallization kinetics were analyzed using the Johnson‑Mehl‑Avrami‑Kolmogorov (JMAK) model. Neither alloy follows a single‑exponent behavior; instead, a two‑stage model is required. For Mg‑1 wt% Y, the early stage yields an Avrami exponent n₁≈0.4 (indicating limited nucleation) and a rate constant k₁≈1.2×10⁻⁴ s⁻¹, while the later stage shows n₂≈1.2 (growth‑controlled) and k₂≈3.5×10⁻³ s⁻¹. The 7 wt% Y alloy displays an even smaller n₁ (≈0.2) and a reduced n₂ (≈1.0), reflecting a delayed onset of recrystallization and a slower overall progress. This two‑step behavior is interpreted as a consequence of heterogeneous nucleation: Y‑rich precipitates (e.g., Mg₁₄Y₂) and segregated Y at boundaries create localized sites where nuclei can form, while the surrounding matrix remains largely untransformed.

Grain growth experiments reveal contrasting behavior. At 350 °C, the 1 wt% Y alloy follows normal grain growth with an exponent close to 0.33, reaching an average size of ~45 µm after 8 h. In stark contrast, the 7 wt% Y alloy exhibits abnormal grain growth (AGG) under the same conditions, with average grain sizes exceeding 80 µm and occasional ultra‑large grains (>200 µm). The grain‑size distribution becomes bimodal, indicating that a subset of grains experiences rapid, uncontrolled growth after the pinning effect of Y‑segregated boundaries is locally lost. Similar AGG is observed at higher temperatures (≥450 °C) for prolonged anneals, confirming that high Y content promotes a pinning‑unpinning instability.

Thermodynamic and kinetic analyses were performed using CALPHAD and DICTRA simulations. The segregation free energy of Y to a Σ3 grain boundary is calculated to be ΔG_seg ≈ ‑0.45 eV at 350 °C, substantially more negative than that of common alloying elements such as Al (‑0.12 eV). The diffusivity of Y in Mg at 400 °C is D_Y ≈ 1.5×10⁻¹⁴ m² s⁻¹, roughly an order of magnitude lower than Mg‑Al or Mg‑Zn systems. Consequently, Y acts as a “slow” solute that remains at the boundary for extended periods, exerting a strong drag on boundary migration.

When compared with other rare‑earth additions (e.g., Nd, Gd, Ce), Y provides the most potent combination of high segregation tendency and low diffusivity, resulting in the greatest retardation of both recrystallization and grain growth. This insight enables alloy designers to tailor Y content to achieve specific microstructural stability targets. For instance, a moderate Y level (3–5 wt%) combined with a 350 °C, 2–4 h anneal yields sufficient recrystallization for ductility while preserving a fine grain structure for strength. In contrast, a high Y level (≥7 wt%) is advantageous for applications requiring exposure to 400 °C+ where abnormal grain growth must be avoided; short‑duration anneals (≤4 h) suffice to retain a refined microstructure while providing high-temperature strength.

In summary, the study demonstrates that yttrium’s strong grain‑boundary segregation produces a dominant solute‑drag effect that delays nucleation, creates a two‑stage recrystallization kinetic signature, and, at high concentrations, triggers abnormal grain growth through a pinning‑unpinning mechanism. The combined thermodynamic‑kinetic framework presented here offers a quantitative basis for designing thermally stable Mg‑Y alloys for automotive, aerospace, and other lightweight‑structural applications.