Study on the strengthening mechanism of rare earth yttrium on magnesium alloys

The thermodynamic stability of intermetallic compounds formed by most conventional alloying elements in Mg alloys is not high, resulting in the poor heat resistance of Mg alloys. Although rare earth (RE) compounds have superior thermal stability, the melting point of Al-RE compounds is generally higher than that of Mg-RE compounds. Therefore, in-depth exploration of the influence mechanism of RE compounds on the heat resistance of Al-containing Mg alloys is necessary. In this work, the thermodynamic stability of the might exist precipitates in Mg–Al–Zn–Mn–Y alloys was analyzed using first-principles calculations and verified via scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), and other experimental methods. Based on their results, the precipitation sequence of the key RE precipitates was deduced. Furthermore, it was investigated whether the preferentially precipitated precipitates could serve as the nuclei of primary α-Mg based on the mismatch theory. Consequently, the refinement mechanism of Y on Mg alloys was elucidated. Besides, the complex alloying reactions in the multi-elemental Mg alloy system were simplified using the electronegativity theory and the Al–Y, Mg–Y, Mg–Al binary phase diagrams, and the strengthening mechanism of Y on Mg alloys was elucidated eventually. The results indicated that the preferentially precipitated granular Al8Mn4Y and Al2Y prevented Mg17Al12 from forming a network structure, and a reinforced structure was formed in which the modified Mg17Al12 particles were mixed with the newly formed granular Al8Mn4Y and Al2Y. Tensile tests and fracture analyses indicated that the reinforced structure improved the mechanical properties of the alloy both at room and elevated temperatures.