Fatigue Characterization of Ultrahigh Strength Mg-Gd-Y-Zn-Zr Alloy with Hierarchical Anisotropic Nanostructure

2022 Fiscal Year Annual Research Report

• Project/Area Number: 22F22720
• Allocation Type: Single-year Grants
• Research Institution: Kyushu University
• Principal Investigator: 陳 強 九州大学, 工学研究院, 教授 (30264451)
• Co-Investigator(Kenkyū-buntansha): CHEN YAO 九州大学, 工学(系)研究科(研究院), 外国人特別研究員
• Project Period (FY): 2022-11-16 – 2025-03-31
• Keywords: Mg-RE alloy / Very high cycle fatigue / Crack initiation / Oxidation / Oxygen embrittlement

Outline of Annual Research Achievements

We investigated the very high cycle fatigue behavior of a Mg alloy with long-period stacking ordered (LPSO) phase. Fatigue fracture and damage observation revealed that microcracks nucleate from the thick alpha-Mg layers, away from the dense LPSO lamellae. Notably, microcrack nucleation was observed inside the localized oxides. This investigation highlights the critical role of highly localized oxide thickening, orders of magnitude thicker than native oxide, in the process of microcrack nucleation. The proposed model reaffirms the importance of the extrusion/intrusion process, but more importantly highlights a mechanism in which fatigue-induced oxidation results in the embrittlement of the extrusion/intrusion structure, which in turn affects the nucleation of microcracks.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

As planned, macroscopic fatigue strength and damage evolution investigation have been conducted on the Mg/LPSO alloy after solution hot treatment. The results obtained have been submitted to the journal of Scripta Materialia. Further research will be conducted to investigate the plastic localization according to the hierarchical anisotropic nanostructures during fatigue crack nucleation, as well as the microstructural mechanisms involved in the early propagation of microcracks. On the whole, we have achieved the research goals set up for FY2022, and the progress of the project was partially advanced.

Strategy for Future Research Activity

T-EBSD and AC-STEM will be utilized to investigate the impact of the hierarchical anisotropic nanostructures and the aging heat treatment on the mechanism of microcrack nucleation, especially fatigue-induced oxidation process.
By identifying critical microstructure configurations, we will establish a correlation between the configurations and the mechanism of microcrack nucleation. This, in turn, will provide novel insights into the design of microstructures that can enhance the fatigue damage resistance of Mg-RE alloys.