On the strengthening precipitate phases and phase transformation of β″/β′ in a Mg-Sm-Zr alloy

Publication date: 15 February 2017
Source:Materials & Design, Volume 116
Author(s): Bingbing Li, Jie Dong, Zhenyan Zhang, Jian-Feng Nie, Laure Bourgeois, Liming Peng
Strengthening precipitate phases in a Mg-4Sm-0.4Zr (wt%) alloy, aged isothermally at 473K, were characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. The β″ (D019) precipitates appear in continuous and discontinuous D019 structures, which are designated as β″ (I) and β″ (II) respectively and the β″ (II) is the dominant phase in the peak-aged state. A small number of β' (cbco) precipitates were observed in Mg-Sm-Zr alloy, and these phases have two types of lattice parameters where one is a=0.64nm, b=1.14nm and c=0.52nm, and the other is a=0.64nm, b=2.28nm and c=0.52nm, designated as βL′ and βS′ respectively. The precipitation sequence in Mg-Sm-Zr alloys can be presented as super-saturated solid solutions (S.S.S.S)→β″→β'→β1 (fcc)→β (bct), indicating almost Mg-RE (RE=Nd, Sm, Gd, Y, Dy) alloys have the same ageing precipitation sequence. A phase transformation model between β″/β′ was proposed based on the experimental results.

Graphical abstract

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