Three-Dimensional Morphology of C15–Al2Ca Precipitates in a Mg–Al–Ca Alloy

Sakiko Kashiwase*, Mariko Unekawa*, Hiromu Hisazawa** and Yoshihiro Terada*
*Department of Materials Science and Engineering, School of Materials and Chemical Technology,
Tokyo Institute of Technology, Yokohama 226-8502, Japan
**Graduate School of Technology, Industrial and Social Sciences, Tokushima University, Tokushima 770-8506, Japan

The three-dimensional morphology and thickness of the C15–Al2Ca Laves phase, which precipitated within the primary α-Mg grains, were investigated for the Mg–5Al–1.5Ca alloy over-aged at 523 K for 100 h using the high-resolution transmission electron microscopy (HRTEM). The C15–Al2Ca phase precipitated with a hexagonal plate-like morphology along the (0001)α basal plane of the α-Mg matrix phase, where the sides of the hexagonal plates were parallel to the second columnar plane {11-20}α of the α matrix. The typical coffee bean contrast was clearly visible around the precipitates, indicative of coherent precipitation of the C15–Al2Ca phase. The thickness of the C15–Al2Ca precipitates, which corresponds to the six layers of (111)C15 plane composed of Ca atoms, was evaluated to be approximately 1.5 nm.

Figure 1 shows the HRTEM image of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [11-20]α (a). The SADP and its illustration with indices are shown in (b) and (c). Figure 2 shows the HRTEM image of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [01-11]α (a). The SADP and its illustration with indices are shown in (b) and (c). Figure 3 shows the HRTEM image of the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [11-20]α (a). The edge portion of the C15–Al2Ca precipitate, surrounded by the square in (a), is magnified in (b). The inverse fast-Fourier transformation (IFFT) image of (b) is shown in (c). Figure 4 shows a reduced-sphere unit cell (a) and [-110] projection (b) of the C15–Al2Ca.

[Published in Journal of Japan Institute of Metals and Materials, 83 (2019), pp. 193-197]

Fig. 1 HRTEM image of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [11-20]α (a). The SADP and its illustration with indices are shown in (b) and (c).

Fig. 2 HRTEM image of the C15–Al2Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [01-11]α (a). The SADP and its illustration with indices are shown in (b) and (c).
Fig. 3 HRTEM image of the Mg–5Al–1.5Ca alloy aged at 523 K for 100 h, taken with B = [11-20]α (a). The edge portion of the C15–Al2Ca precipitate, surrounded by the square in (a), is magnified in (b). The inverse fast-Fourier transformation (IFFT) image of (b) is shown in (c).
Fig. 4 A reduced-sphere unit cell (a) and [-110] projection (b) of the C15–Al2Ca.