Lamellar Structure Stability of a Two-Phase α-Mg/C14–Mg₂Ca Alloy

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)_α 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 = k_T t, where λ₀ is the α/C14 lamellar spacing for the as-cast specimen, and k_T 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.