Jonghyun Kim

This study investigates the effects of Y, Yb, and their combined addition on the oxide film structure and ignition behavior of Mg–4Al–1Ca (AX41) alloy. Y/Yb co-addition significantly enhances ignition resistance, with the Y15Yb15 alloy exhibiting the highest ignition temperature of 1097 °C, representing an increase of ~370 °C over the base alloy. FIB–SEM observations show that oxide films in the base and single-element-added alloys contain pores and interfacial irregularities, whereas the Y15Yb15 alloy forms a compact and uniform oxide structure. Quantitative analysis indicates that ignition resistance is governed by oxide structural integrity rather than thickness alone, as evidenced by a stronger correlation between ignition temperature and the coefficient of variation (COV) of oxide thickness than with thickness itself. Although thermodynamic analysis indicates that CaO, Y₂O₃, and Yb₂O₃ are more stable than MgO, ignition behavior cannot be fully explained by thermodynamic parameters alone. TEM and EDS analyses suggest the formation of endogenous CaO beneath the oxide layer, attributed to inward oxygen diffusion and its subsequent reaction with Ca²⁺ near the Al₂Ca/matrix interface. In the Y15Yb15 alloy, this endogenous CaO is uniformly distributed, contributing to oxide densification by reducing ionic diffusion pathways. A mechanism is proposed in which the balance between inward oxygen diffusion and outward diffusion of Mg²⁺ and Ca²⁺ is considered to govern oxide structural integrity, and Y/Yb co-addition optimizes this balance through the controlled formation of endogenous CaO, providing a strategy for designing flame-resistant magnesium alloys.

This study investigates the effects of Y and Yb additions on the microstructure and mechanical properties of a low-Ca AX41 magnesium alloy. The addition of rare-earth elements transforms the Al₂Ca phase into Al₂(Ca,RE) and Al₂RE phases, leading to grain refinement, increased dislocation density, and a higher volume fraction of second phases. Texture evolution is also observed, although its contribution to mechanical behavior is secondary compared to microstructural heterogeneity. As a result, the yield strength increases while elongation decreases, exhibiting a typical strength–ductility trade-off. For example, the Y15Yb15 alloy shows a yield strength of 264 MPa (~43% increase compared to AX41), while elongation decreases from 13.5% to 9.6%. Quantitative analysis indicates that grain-boundary strengthening is the primary contributor to strength increase, while dislocation strengthening provides a secondary contribution. Furthermore, descriptor-based analysis reveals that δ and Δχ are correlated with strength, whereas the second-phase fraction shows a stronger correlation with elongation reduction. These results indicate that descriptor-correlated microstructural evolution plays a key role in governing the strength–ductility trade-off in the present alloy system.

This study investigates the effects of Y, Yb, and their combined addition on the oxide film structure and ignition behavior of Mg–4Al–1Ca (AX41) alloy. Y/Yb co-addition significantly enhances ignition resistance, with the Y15Yb15 alloy exhibiting the highest ignition temperature of 1097 °C, representing an increase of ~370 °C over the base alloy. FIB–SEM observations show that oxide films in the base and single-element-added alloys contain pores and interfacial irregularities, whereas the Y15Yb15 alloy forms a compact and uniform oxide structure. Quantitative analysis indicates that ignition resistance is governed by oxide structural integrity rather than thickness alone, as evidenced by a stronger correlation between ignition temperature and the coefficient of variation (COV) of oxide thickness than with thickness itself. Although thermodynamic analysis indicates that CaO, Y₂O₃, and Yb₂O₃ are more stable than MgO, ignition behavior cannot be fully explained by thermodynamic parameters alone. TEM and EDS analyses suggest the formation of endogenous CaO beneath the oxide layer, attributed to inward oxygen diffusion and its subsequent reaction with Ca²⁺ near the Al₂Ca/matrix interface. In the Y15Yb15 alloy, this endogenous CaO is uniformly distributed, contributing to oxide densification by reducing ionic diffusion pathways. A mechanism is proposed in which the balance between inward oxygen diffusion and outward diffusion of Mg²⁺ and Ca²⁺ is considered to govern oxide structural integrity, and Y/Yb co-addition optimizes this balance through the controlled formation of endogenous CaO, providing a strategy for designing flame-resistant magnesium alloys.