To evaluate the stability of α-Mg/C14–Mg2Ca lamellar microstructure, aging treatment was carried out for a Mg–14.8 mass% Ca near-eutectic alloy at 573–723 K for 1–150 hours. The spacing of the lamellar microstructure obtained by the eutectic transformation L → α-Mg + C14–Mg2Ca during solidification was approximately 250 nm. High-resolution transmission electron microscopy observations show that the α/C14 interface is composed of terraces and steps, with terraces parallel to the (1101) pyramidal plane of the α-Mg lamellae. The α/C14 lamellar microstructure is stable in morphology at temperatures below 573 K. In contrast, the lamellar spacing (λ) continuously increases with increasing aging time (t) above 573 K, and the increase in λ can be described as λ2 − λ02 = kTt, where λ0 is the α/C14 lamellar spacing for the as-cast specimen, and kT is a constant depending on aging temperature. The activation energy for the coarsening of α/C14 lamellar microstructure was evaluated as 112 kJ/mol, which is close to the activation energy for the inter-diffusion of Ca in Mg. The hardness of the α/C14 lamellar region decreases with increasing λ, indicating that the α/C14 interface acts as an obstacle to the basal slip of dislocations in α-Mg lamellae.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 399–405. The abstract of this paper, together with the caption (Table 1 and Figs. 1–10), are slightly changed through the English polishing service by native speakers, compared to those of the paper published in J. Japan Inst. Met. Mater.
Fig. 3 FE-SEM image of α-Mg/C14–Mg
2Ca lamellar microstructure observed in the Mg–14.8 mass% Ca alloy. The bright phase is α-Mg, while the dark phase is C14–Mg
2Ca.
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