Microstructure Evolution and Mechanical Properties Improvement of Al-11Mg₂Si Hypoeutectic Alloy with Different La Content

Abstract

The microstructure, tensile properties, and fracture behavior of Al-11% Mg₂Si hypoeutectic alloys with rare earth La (0, 0.1wt%, 0.2wt%, 0.4wt%, 0.6wt%, and 0.8wt%) were systematically studied. The experimental results show that the addition of La remarkably refines the primary α-Al dendrites and eutectic Mg₂Si particles of the alloy and decreases the secondary dendrite arm spacing of the alloy. In addition, the introduction of La transforms the shape of the eutectic Mg₂Si phase into a fine granular or fibrous structure from a coarse plate-like structure. The constitutional undercooling formed by the enrichment of La atoms and La containing particles at the primary Al grain boundary and eutectic Mg₂Si/α-Al phase interface is responsible for α-Al refinement and Mg₂Si modification. The addition of La significantly improves the tensile properties of the alloy and at an optimum addition of 0.4wt% of La; meanwhile, the ultimate tensile strength, yield strength and elongation of the alloy increase from 231 MPa, 136 MPa and 2.1% to 322 MPa, 164 MPa and 4.8%, respectively, which are 39.4%, 20.6% and 128.6% higher than those without the addition of La. The improvement of the tensile properties of La-containing alloys is mainly due to the fact that the reduction in the size and the change in morphology of eutectic Mg₂Si crystals caused by the addition of La enhance the anti-cracking stress and reduce stress concentration at the Mg₂Si/Al phase interface.

Keywords

hypoeutectic Al-Mg-Si alloy / lanthanum / grain refinement / modification / mechanical properties

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