Effect of Gd–Ca combined additions on the microstructure and creep properties of Mg–7Al–1Si alloys

Abstract

The microstructure and creep properties of Mg–7Al–1Si alloys with combined additions of alkaline earth element Ca and rare earth element (RE) Gd were investigated using scanning electron microscope (SEM), optical microscope, energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and compressive creep tests. It was found that the combined additions of Ca and Gd contributed to grain refinement, modification of the morphology of coarse Chinese script Mg2Si phase, and reduction of β-Mg17Al12 content. Mg–7Al–1Si alloys containing 1wt.% compound modifier (0.5wt.% Gd+0.5wt.% Ca) exhibited the minimal steady-state creep rate, and were even superior to Mg–7Al–1Si alloys containing single Gd addition. The great improvement of creep properties is mainly attributed to the reduction in the amount and continuity of eutectic β-Mg17Al12 phase, morphology modification of Mg2Si phase, solution strengthening of Gd, as well as the formation of thermally stable intermetallic Al2Gd, which acted as an effective barrier against grain boundary sliding and dislocation movement.