Investigation of the crystallographic structure and orientations of the Al2Ca phase in a Mg-Al-Ca-Mn alloy

Because of the second phases with high melting point, Mg-Al-Ca system alloys have better high temperature strength and creep resistance and attract much attention on the second phases. Crystallographic structure and orientations of the Al2Ca(C15) phase had been investigated in the aged Mg-5.0 wt%Al-3.0 wt%Ca-0.5wt%Mn alloy. Several typical characteristics of Al2Ca particles had been repeatedly observed. Firstly, Al2Ca particles containing 180° rotation twin [101]T had been observed and analyzed by simulating the electron diffraction patterns. Then, Al2Ca particle exhibiting different orientations in its different areas, with a transition area containing stacking faults, was observed. Atomic structure was simulated and analyzed to better understand this transition. By transmission electron microscope analyses and calculations based on crystallographic knowledge, a new crystallographic orientation relationship between Al2Ca phase and the magnesium matrix was identified as: [1-210]α//[110]C15, (0001)α//(001)C15, (10-10)α//(-110)C15. Stereographic projections of this orientation relationship, together with the reported one ([-1100]α//[-110]C15, (0001)α//(111)C15), were calculated and analyzed. Along with the orientation relationships in FCC/HCP, discussions had been carried out about their relationships.