Effects of Eutectic Compounds Formed along Grain Boundaries in Thixomolded^® Alloy Mg₉₆Zn₂Y₂ on Creep Deformation

Abstract

During thixomolding^® of Mg alloys, fine grains (size of ∼10 μm) and continuous eutectic compounds are formed along grain boundaries. These compounds are partly responsible for the superior creep strength of Mg alloys. In this study, we investigate the effects of eutectic compounds formed along grain boundaries in thixomolded^® Mg₉₆Zn₂Y₂ on creep deformation at 573 K. By performing microstructure analyses, we find that two types of continuous eutectic compounds—Mg₃Y₂Zn₃ and Mg₁₂YZn—are formed in the alloy. The strain rate of Mg₉₆Zn₂Y₂ is about two orders of magnitude lower than that of a heat-resistant alloy Mg₉₃Al₅Ca₂ (Mg-6 mass%Al-3 mass%Ca) at 573 K. The stress exponent n at 573 K is approximately 7, and it is considered that creep deformation is controlled by dislocation motion. All scribe lines produced on a specimen of Mg₉₆Zn₂Y₂ are curved. Furthermore, in the case of a specimen with a creep strain of 7%, no abrupt steps are observed at grain boundaries and interfaces between the α-Mg matrix and eutectic compounds. Our results indicate that eutectic compounds formed along grain boundaries are effective in suppressing grain boundary sliding.