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

Abstract

To evaluate the stability of the α-Mg/C14–Mg₂Ca lamellar microstructure, the aging treatment was carried out for the Mg–14.8 mass％Ca near-eutectic alloy at 573-723 K for 1-150 h. The spacing of the lamellar structure obtained by the eutectic reaction L → α-Mg＋C14–Mg₂Ca after casting is approximately 250 nm. It was found by the HRTEM observation that the α/C14 interface is composed of terraces and steps, and the terrace is parallel to the (1101)α pyramidal plane of the α-Mg lamellae. The lamellar microstructure is stable below 573 K. By contrast, the lamellar spacing (λ) continuously increases with the increasing aging time (t) above 573 K, and the increase of λ can be described as λ²−λ₀² = k_T t, where λ₀ is the lamellar spacing for the as cast specimen and k_T is a constant depending on aging temperature. The activation energy for the coarsening of the 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 lamellar region decreases with the increasing λ, which is indicative that the α/C14 interface acts as an obstacle to the basal slip of dislocations in the α-Mg lamellae.

Fig. 3 FE-SEM image of the α-Mg/C14–Mg₂Ca lamellar microstructure observed in the Mg–14.8 mass％Ca alloy, where the bright phase is α-Mg and the dark one is C14–Mg₂Ca, respectively.