Short communicationEffects of Ti addition on the formation of LPSO phase and yield asymmetry of Mg-Zn-Y-Mn alloy
Introduction
Magnesium alloys are lightweight metallic structural materials with high specific strength, specific stiffness, and good recyclability. Hence, they are increasingly employed in automation, transportation, and electronics. In recent years, Mg-Zn-Y alloys with long period stacking ordered (LPSO) structure have attracted increasing attention due to their unique microstructures and excellent mechanical properties [1], [2], [3], [4]. Mg97Y2Zn1 (at%) alloy processed by rapidly solidified power metallurgy (RS P/M) was reported to exhibited a high yield strength of 610 MPa and a relatively high tensile ductility of 5% at room temperature [5]. But the mechanical properties of Mg-Zn-Y alloys severely decrease when prepared by conventional casting technology. To improve the mechanical properties of Mg-Zn-Y alloys containing LPSO phase, a modification in composition is necessary by adding other alloying elements. Our previous studies have showed that Mn, Ca and B can effectively improve the mechanical properties of the Mg94Zn2.5Y2.5Mn1 alloy by refining grains and promoting the formation of LPSO phase [2], [3], [4]. However, LPSO phase showed tensile-compressive behavior opposite to that of magnesium. The yield strength in compression test was larger than that in the tension test for LPSO phase. The asymmetry of Mg-Zn-Y alloys containing LPSO phase should depend on the volume fraction of magnesium matrix and LPSO phase [6], [7]. It is, therefore, necessary to study the relationship between volume fraction of LPSO phase and asymmetry to obtain desired mechanical properties. Ti is an effective grain refiner for cast magnesium alloy [8], [9]. In addition, the addition of Ti can reduce the axial ratio (c/a) of magnesium alloy, which is beneficial to deformation. [10]. However, up to now, there is no detail report about the effect of Ti on the formation of LPSO. In this work, the microstructure and mechanical properties of Mg94-xZn2.5Y2.5Mn1Tix (at%) (x = 0, 0.1, 0.3. 0.5, and 0.7) alloys were systematically investigated. In particular, the effects of Ti addition on the formation of LPSO and yield asymmetry were discussed, which have great significance to reasonably design the alloy.
Section snippets
Experimental procedures
Ingots of Mg94-xZn2.5Y2.5Mn1Tix (at%) (x = 0, 0.1, 0.3. 0.5, and 0.7) alloys were prepared by melting mixtures of industrial high-purity Mg, Zn, Y, Mn, and Ti in an electric resistance furnace under protective Argon atmosphere at 1023 K. The mixtures were then cast into a preheated die at 993 K. The microstructures and compositions of the alloys were observed by optical microscopy (OM), scanning electron microscopy (SEM, TESCAN-MIRA3) equipped with energy dispersive spectroscopy (EDS), and
Microstructure analysis
SEM image of Mg94Zn2.5Y2.5Mn1 alloy is shown in Fig. 1a. The alloy was composed of α-Mg matrix, block second-phase, and eutectic phase. After adding 0.5 at% Ti into Mg94Zn2.5Y2.5Mn1 alloy, the amount of block second-phase increased while that of eutectic phase decreased. The block second-phase was identified as 18R-LPSO (Mg12ZnY) and the eutectic phase was W-phase (Mg3Zn3Y2), as verified by EDS (Fig. 1c and d), bright field (BF) TEM, as well as the corresponding selected area electron
Conclusions
- (1)
The as-cast Mg-Zn-Y-Mn-Ti alloys were composed of α-Mg matrix, 18R-LPSO phase (Mg12ZnY), and W-phase (Mg3Zn3Y2).
- (2)
Adding moderate Ti element to Mg94Zn2.5Y2.5Mn1 can promote the formation of LPSO phase and reduce yield asymmetry.
- (3)
The yield strength, ultimate strength, and elongation first increased and then decreased with increasing Ti contents regardless of tension and compression. The Mg93.7Zn2.5Y2.5Mn1Ti0.3 exhibited the best strength. However, the addition of 0.5 at%Ti led to the lowest yield
Acknowledgements
The authors acknowledge the financial support from National Natural Science Foundation of China (Nos. 51474153 and 51574175) and Shanxi Key Laboratory of Advanced Magnesium-based Materials, Taiyuan University of Technology (No. 2016-06).
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