Formation of 14H-type long period stacking ordered structure in the as-cast and solid solution treated Mg-Gd-Zn-Zr alloy
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National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China; and School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China (Received 14 November 2008; accepted 6 February 2009)
The coherent fine lamellae consisting of the 2H-Mg and the 14H-type long period stacking ordered (LPSO) structure within a0 -Mg matrix have been first observed in an ascast Mg96.32Gd2.5Zn1Zr0.18 alloy. During subsequent solid solution heat treatment at 698– 813 K, in addition to the lamellae within matrix, a novel lamellar X phase (Mg– 8.371.0Zn–11.321.0Gd, at.%) with the 14H-type LPSO structure was transformed from the dendritical b phase, and a corresponding time–temperature–transformation (TTT) diagram was established. The 14H-type LPSO structure existing in Mg–Gd–Zn–Zr alloys derives from two variant means: the formation of LPSO structure within a0 -Mg matrix and the transformation of the dendritical b phase to a lamellar X phase with the LPSO structure. The alloy solid solution treated at 773 K for 35 h exhibits higher tensile strength and better elongation than the nonheated alloy because of the lamellar X phase with the 14H-type LPSO structure and the 14H-type LPSO structure within matrix.
I. INTRODUCTION
Magnesium (Mg) alloys have attracted much attention as structural materials widely used in automobile and electronics as well as military and aerospace industries because of their light weight, high specific strength and rigidity, good damping, and casting capability. Magnesium alloys containing rare earth (RE) elements (especially heavy RE) have widely been developed and systematically investigated as promising materials with high strength and high creep resistance.1–6 Moreover, the addition of Zn/Cu can further improve the strengthening response of Mg–RE alloys.1–3,7–9 Because of a combined strengthening by solid solution precipitation,1–6 novel long period stacking ordered (LPSO) structures7–30 as well as grain refinement,13,26 Mg–RE–X (RE = Y, Dy, Ho, Er, Gd, Tb, Tm; X = Zn/Cu) alloys have attracted much attention. At present, there are various types of LPSO structures in these alloys such as 6H, 10H, 14H, 18R, 24R.7–30 The equilibrium solid solubility of Gd in Mg at 821 2 K is relatively higher (4.53 at.% or 23.49 wt%) than the solubility of Y (3.75 at.% or 12.47 wt%) at 839 1 K1–3 and decreases exponentially to 0.61 at.% (3.82 wt%) at 473 K with a temperature decrease,1–3 which forms a a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0215
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J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 24 Mar 2015
more significant system for precipitation hardening. The atomic size of Mg (0.160 nm) is larger than Zn (0.133 nm) and smaller than Gd (0.178 nm).7 Furthermore, Gd, Zn, and Mg have hexagonal closed packed (hcp) structures, and the mixing enthalpies of Mg–Zn, Mg–Gd, and Gd–Zn are 4, 6, and 31 kJ/mo
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