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

摘要

Strengthening precipitate phases in a Mg-4Sm-0.4Zr (wt%) alloy, aged isothermally at 473 K, were characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. The beta '' (D0(19)) precipitates appear in continuous and discontinuous D0(19) structures, which are designated as beta '' (I) and beta '' (II) respectively and the beta '' (II) is the dominant phase in the peak-aged state. A small number of beta' (cbco) precipitates were observed in Mg-Sm-Zr alloy, and these phases have two types of lattice parameters where one is a = 0.64 nm, b = 1.14 nm and c = 0.52 nm, and the other is a = 0.64 nm, b = 2.28 nm and c = 0.52 nm, designated as beta'(L)and beta'(s)respectively. The precipitation sequence in Mg-Sm-Zr alloys can be presented as super-saturated solid solutions (S.S.S.S)->beta ''->beta'->beta(1)(fcc)-> beta(bct), indicating almost Mg-RE (RE = Nd, Sm, Gd, Y, Dy) alloys have the same ageing precipitation sequence. A phase transformation model between beta ''/beta' was proposed based on the experimental results. (C) 2016 Elsevier Ltd. All rights reserved.