Mg–5Bi–3Al 마그네슘 고속 압출재의 미세조직과 고주기피로 특성

초록

In this study, the microstructural characteristics of a high-speed-extruded Mg–5Bi–3Al (BA53) alloy and its tensile, compressive, and high-cycle fatigue properties are investigated. The BA53 alloy is successfully extruded at a die-exit speed of 16.6 m/min without any hot cracking using a large-scale extruder for mass production. The homogenized BA53 billet has a large grain size of ~900 μm and it contains fine and coarse Mg3Bi2 particles. The extruded BA53 alloy has a fully recrystallized microstructure with an average grain size of 33.8 μm owing to the occurrence of complete dynamic recrystallization during high-speed extrusion. In addition, the extruded BA53 alloy contains numerous fine lath-type Mg3Bi2 particles, which are formed through static precipitation during air cooling after exiting the extrusion die. The extruded BA53 alloy has a high tensile yield strength of 175.1 MPa and ultimate tensile strength of 244.4 MPa, which are mainly attributed to the relative fine grain size and numerous fine particles. The compressive yield strength (93.4 MPa) of the extruded BA53 alloy is lower than its tensile yield strength, resulting in a tension-compression yield asymmetry of 0.53. High-cycle fatigue test results reveal that the extruded BA53 alloy has a fatigue strength of 110 MPa and fatigue cracks initiate at the surface of fatigue test specimens, indicating that the Mg3Bi2 particles do not act as fatigue crack initiation sites. Furthermore, the extruded BA53 alloy exhibits a higher fatigue ratio of 0.45 than other commercial extruded Mg–Al–Zn-based alloys.

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