4.7 Article

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

Journal

JOURNAL OF MAGNESIUM AND ALLOYS
Volume 10, Issue 6, Pages 1573-1580

Publisher

KEAI PUBLISHING LTD
DOI: 10.1016/j.jma.2021.11.007

Keywords

Magnesium alloys; Long period stacking/order (LPSO) phases; Short-range order (SRO) clusters; High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM)

Funding

  1. JSPS KAKENHI [JP18H05475, JP18H05479]
  2. MEXT, Japan
  3. JSPS [JP19F19775]
  4. Open Funds of the State Key Laboratory of Rare Earth Resource Utilization [RERU2020012]

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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.
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.

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