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

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