Interphase boundary segregation induced phase transformation in a high-pressure die casting Mg-Al-La-Ca-Mn alloy

The well-known Al11La3 structure in a die casting Mg-4Al-2La-2Ca-0.3Mn alloy was decomposed when heat treated at 400 °C. Transmission electron microscopy characterizations reveal the decomposition process as: Ca firstly segregated at Mg-Al11La3 interphase, then Al and Ca simultaneously segregated at the interphase boundaries and the radial Al2Ca phase formed, finally the Al11La3 phase gradually decomposed into Al2(Ca,La) structure. First-principles calculations indicate that Ca atoms substituting for La atoms would decrease the stability of Al11La3 and result in that Al11La3 become metastable even at 0 K. Therefore, the decomposition of Al11La3 is attributed to that the interphase boundary segregation of Ca and the diffusion of Ca into Al11La3 change its thermodynamic stability.