Effects of composition and growth rate on the microstructure transformation of β-rods/lamellae/α-rods in directionally solidified Mg-Li alloy

Mg-Li alloy is a unique system with a narrow dual-phase region and superstructure transformation. These characteristics will have significant influence on the solidification behavior. Using Bridgman-type vacuum furnace with liquid metal cooling method, the directional solidification experiments were performed on alloys with different Li contents (6.3 wt.%, 6.6 wt.%, 6.9 wt.%, 72 wt.%, 7.5 wt.%, 7.8 wt.%) at varied growth rates (3 mu m/s, 5 mu m/s, 10 mu m/s, 15 mu m/s, maximum to 21 mu m/s). As the Li content increases, the microstructures transform from divorced eutectic structure to lamellar structure, then to lamellae/alpha-Mg rod mixed structure, and finally to alpha-Mg rod-like structure. Fully eutectic structure is produced at a wide range of off-eutectic compositions. The effect of growth rate on the alpha-Mg rod-like structure is mainly to diminish the inter-rod spacing as growth rate increases, while additionally to affect the morphology of lamellar structure. High growth rate is found to be beneficial for producing straight and uniform lamellar structure. The eutectic spacing of Mg-Li system is one or two orders of magnitude lager than most other alloy systems. Moreover, a controllable three-stage transformation of beta-rods/lamellae/a-rods is achieved in one single alloy system. A structure selection map is also given to define the microstructure transformation of the Mg-Li alloys. (C) 2017 Elsevier Ltd. All rights reserved.