Investigation on β′ phase inversion of peak-aging hot extruded GW84 alloy after short high temperature secondary aging
Introduction
Mg-Gd-Y alloys have typical aging strengthening effect which have been widely concerned by researchers due to their excellent mechanical properties at room and high temperatures [1], [2], [3], [4], [5]. And the precipitation order of strengthening phase in Mg-Gd-Y alloys after aging treatment and aging sequence is as follows [6], [7], [8], [9]: SSSS → β″ → β′ → β1 → β.
Among these precipitate phases, β′ phase play an important role in improving mechanical properties and could markedly increase precipitation strengthening effectiveness [10], [11], [12]. Most studies believe that Mg-Gd-Y alloys have good heat resistance, because aging precipitation phases have the characteristic of high melting point [13], [14], [15]. But there are few researches on heat resistance mechanism stretching at high temperature and whether β′ phase of Mg-Gd-Y alloys changing during high temperature tensile process has not been clearly and emphatically studied. Therefore, the microstructure of hot extruded Mg-8Gd-4Y-1Sm-0.5Zr alloy at peak-aging condition after high temperature secondary aging for7 min was analyzed in this paper, and the transformation of β′ phase was revealed, which could provide important theoretical guidance for heat resistance mechanism of Mg-Gd-Y alloys at high temperature.
Section snippets
Experimental
The preparation process of as-cast GW84 (Mg-8Gd-4Y-1Sm-0.5Zr) alloy is as follows: pure Mg (99.95%), Mg-Gd (30%), Mg-Y (30%) and Mg-Zr (30%) (wt%) were heated in a crucible, introducing CO2 and SF6 as protective gas with a volume ratio of 99:1. Then these materials were heated to 760 °C for 10 min, the molten alloy was cast into a steel mold with Φ 100 mm diameter and cooled to form GW84 alloy ingot. The Φ 50 × 45 mm samples, cut from ingot, were heated at 525 °C for 8 h and then quenching in
TEM images of hot extruded GW84 alloy after first aging at 200 °C for 96 h
As shown in Fig. 1a, the striped precipitates at grain boundary extend along [0 0 0 1]α orientation, viewed from . According SAED pattern in Fig. 1b, the weak diffraction spots distribute in 1/4, 2/4 and 3/4 positions and by calculating plane spacing and crystal plane angle between diffraction spots, we could confirm that these striped precipitates are composed of β′ phase (a ≈ 0.65 nm, b ≈ 2.27 nm, c ≈ 0.52 nm), cbco structure, which is completely coherent with α-Mg, and the
Conclusion
In this work, the precipitated phase of hot extruded GW84 magnesium alloy is β′ phase at first aging for 200 °C × 96 h. After secondary aging at 250 °C for 7 min, part of metastable phase transforms into stable phase (β′ → β), β precipitates about (21–32) nm × (51–86) nm forming at adjacent grain boundary, and β and β′ phase coexists in grain. After secondary aging at 300 °C for 7 min, the β precipitates size increases obviously, about 80 nm × 362 nm at adjacent grain boundary, and triangular β
CRediT authorship contribution statement
Quanan Li: . Lipeng Yan: Data curation, Writing - original draft. Limin Zhu: Writing - review & editing. Xiaoya Chen: .
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by National Natural Science Foundation of China through Projects No. 51571084 and No. 51171059.
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