Substantial undercooling brings about microstructural change for ternary eutectic alloy

The Department of Applied Physics, Northwestern Polytechnical University (NPU) in Xi'an, China-Research, has shown that the substantial undercooling of liquid state brings about novel microstructural transition for Al-Cu-Si ternary eutectic alloy. The study is reported in Issue 54 (January, 2009) of Chinese Science Bulletin because of its significant research value. Al–Cu–Si alloy is widely applied in industry mainly as a light construction material. It is also a promising material for applications such as engine block and cylinder heads. It is important for designers to have an intimate knowledge of how Al–Cu-Si alloy solidifies because its mechanical performance is usually controlled by solidification mechanism and microstructural characteristics. However, so far there has been little research on its rapid solidification mechanism under the extremely nonequilibrium condition.

In this work, Al80.4Cu13.6Si6 eutectic alloy was undercooled up to 147 K (0.18TE). Generally speaking, under the conventional solidification condition, a liquid aluminium alloy can be hardly undercooled because of oxidation. Dr. Ruan and Prof. Wei made it realized by choosing dehydrated B2O3 agent as a denucleating agent and providing a high-vacuum environment and special melting processing. In addition, the experimental parameters, such as cooling rate, superheating, and holding time, were controlled in order to get a wider undercooling range for comparison.

The undercooling level of alloy melt has a strong effect on its crystallization kinetics, structure morphology, and final physical and chemical properties. This study presents the microstructural characteristics of highly undercooled Al80.4Cu13.6Si6 ternary eutectic alloys. So far, most investigations on eutectic growth focus on binary alloy systems. The rapid solidification of undercooled ternary alloys involves the competitive nucleation and coupled growth of three eutectic phases from one liquid phase, which makes it more complicated than the case of binary alloys. The authors showed that the phase selection of Al80.4Cu13.6Si6 ternary eutectic alloy is influenced by undercooling, which makes the competitive nucleation and cooperative growth among (Al), (Si) and (CuAl2) phases become more drastic. Once undercooling exceeds 73 K, the primary phase will transform from (Al) dendrite to faceted (Si) block.

"This paper emphasizes the phase selection and microstructure formation of substantially undercooled ternary eutectic alloys. The result is of academic and practical significance." said one journal reviewer. A series of papers about rapid solidification of ternary eutectic alloys written by Dr. Ruan and Prof. Wei have been published in Chin. Sci. Bull., Sci. Chin. G, etc. "It enriched and expanded the research results on nonequilibrium solidification. It offered us new references to develop solidification and nonequilibrium phase transition theory," said another reviewer.

Source: Science in China Press