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) Ll(2)-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 L1(2) -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 L1(2)-type TM6RE8 clusters play an important role in governing LPSO phase formation. (C) 2021 Chongqing University. Publishing services provided by Elsevier B.V. on behalf of KeAi Communications Co. Ltd.

Automated summary

The microstructures of an as-cast Mg97.49Ho1.99Cu0.43Zr0.09 alloy were investigated using atomic resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The study revealed the presence of ordered long period stacking/order (LPSO) phases and short-range order (SRO) clusters embedded in stacking fault layers. The results support the importance of L1(2)-type TM6RE8 clusters in the formation of LPSO phases.