Effect of Li on formation of long period stacking ordered phases and mechanical properties of Mg-Gd-Zn alloy

Abstract

M g-Re-Zn alloys have attracted increasing attention due to their superior mechanical properties, attributing to the formation of lamellar long period stacking ordered (LPSO) phases within an α-Mg matrix [1][2][3] . Nevertheless, as a kind of second phase, the coarse block-like LPSO phase usually precipitates at grain boundaries during traditional solidification or subsequent solid solution treatment [4][5] , which was reported to cause a decrease in strength [6][7] . Recently, various processes, such as extrusion [8][9] , friction stir processing (FSP) [10][11] and severe plastic deformation [12][13] , have been applied to achieve refined microstructure and enhanced mechanical properties. However, the coarse LPSO phase with excellent plasticity is hard to be broken. What's worse, all these methods will raise the production cost. Hence, there is a great importance to develop an effective and simple technology to decrease the detrimental influence of the coarse second phase.Alloying treatment is a simple and economical method to improve the properties of alloys. To further improve the Abstract: Alloys with composition of Mg 96-x Gd 3 Zn 1 Li x (at.%) (x=0, 2, 4, and 6) were prepared by conventional casting. The microstructures of these alloys under as-cast and solid-solution conditions have been observed, and the mechanical properties were investigated. The results showed that Li is an effective element to refine the grains and break the eutectic networks in as-cast MgGd 3 Zn 1 alloy. During solid solution treatment, these broken eutectic networks are spheroidized and highly dispersed. In addition, plentiful lamellar long period stacking ordered (LPSO) phases are precipitated in an α-Mg matrix when the Li addition is not more than 4%. Solid-solution treated Mg 92 Gd 3 Zn 1 Li 4 alloy exhibits an optimal ultimate tensile strength (UTS) of 226 MPa and elongation of 5.8%. The strength of MgGd 3 Zn 1 alloy is improved significantly, meanwhile, the toughness is apparently increased. properties, heat treatment is usually adopted following the alloying treatment, which gives another simple and effective property improving method. Therefore, a proper combination of alloying and heat treatment is needed to obtain optimal mechanical properties. It has been demonstrated that Mn, Sr and Li are effective elements to modify the size and morphology of the second phases in cast aluminum alloys [14][15][16] . After heat treatment, the modified A380 aluminum alloy shows a significant improvement in tensile properties [17] . The method and principle of alloying and heat treatment for magnesium and aluminum alloys are similar. So, is it possible that the addition of these elements could modify the eutectic networks of as-cast Mg-Re-Zn alloy or turn the coarse second phase into less harmful forms during the following heat treatment? Furthermore, the addition of Li can decrease the lattice constant ratios (c/a) through the substitution of Mg atoms by Li atoms, which will improve the ductility [18][19][20] . The reduction of the atomic...