Structural changes of precipitates by aging of an Mg–4at%Dy solid solution studied by atomic-scaled transmission electron microscopy

DOI Web Site 6 References
  • Saito Kaichi
    Department of Materials Science and Engineering, Graduate School of Engineering and Resource Science, Akita University
  • Yasuhara Akira
    EM Application Group, EM Business Unit, JEOL Ltd.
  • Nishijima Masahiko
    Institute for Materials Research, Tohoku University
  • Hiraga Kenji
    Institute for Materials Research, Tohoku University Ministry of Education, Culture, Sports, Science and Technology in Japan (MEXT)

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  • 原子識別透過電子顕微鏡法で評価したMg–4at%Dy合金の時効析出物の構造変化
  • ゲンシ シキベツ トウカ デンシ ケンビキョウホウ デ ヒョウカ シタ Mg 4at Dy ゴウキン ノ ジコウ セキシュツブツ ノ コウゾウ ヘンカ

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Abstract

Phase transformation of solid solution decomposition occurring in a 96 at%Mg–4 at%Dy alloy, which was solution-treated at 540°C and subsequently aged at 250°C for various lengths of time, has been investigated by conventional transmission electron microscopy (TEM)in combination with high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). The atomic-scaled observations based on both techniques provide the evidence that the first appreciable change in microstructure caused by aging is the occurrence of a short-range ordered state in Dy-segregated regions and that the short-range ordered state allows full of the nuclei of β′ phase associated with an Mg7Dy-type structure to occur in the domains, just as in cases of Mg–Gd and Mg–Y systems. With an increase of age-hardening effect, the β′ precipitates become larger and increasingly anisotropic in morphology, accompanying three orientation variants in coherent with the Mg-matrix. When reaching at the stage of hardness maximum (as-aged at 250°C for 100 h), the β′ precipitates, which have an orthorhombic structure with lattice parameters of a=0.659 nm, b=2.231 nm, c=0.523 nm, take the form of a thin disk-shape with a thickness of 20~100 nm and a diameter of 200~400 nm. With an advance of over-aging effect, the β′ precipitates are gradually reduced in volumes and replaced by β precipitates of cubic structure.

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