Dilute long period stacking/order (LPSO)-variant phases along the composition gradient in a Mg-Ho-Cu alloy

We have systematically investigated the microstructures of as-cast Mg97.49Ho1.99Cu0.43Zr0.09 alloy by atomic resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), revealing the coexistence of 18R, 14H and 24R long period stacking/order (LPSO) phases with fully coherent interfaces along step-like composition gradient in a blocky intermetallic compound distributed at grain boundary. The short-range order (SRO) L12-type Cu6Ho8 clusters embedded across AB'C’A-stacking fault layers are directly revealed at atomic scale. Importantly, the order degree of SRO clusters in the present dilute alloy is significant lower than previous 6M and 7M in-plane order reported in ternary Mg-TM (transition metal)-RE (rare earth) alloys, which can be well matched by 9M in-plane order. This directly demonstrates that SRO in-plane L12-type clusters can be expanded into more dilute composition regions bounded along the definite TM/RE ratio of 3/4. In addition, the estimated chemical compositions of solute enriched stacking fault (SESF) in all LPSO variants are almost identical with the ideal SESF composition of 9M in-plane order, regardless of the type of LPSO phases. The results further support the viewpoint that robust L12-type TM6RE8 clusters play an important role in governing LPSO phase formation.