Study on the precipitates in various aging stages and composite strengthening effect of precipitates and long-period sta
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Study on the precipitates in various aging stages and composite strengthening effect of precipitates and longperiod stacking ordered structure of Mg–Gd–Y–Ni alloy Lei Zhou1
, Jingxu Zheng1, Xiaodong Wang1, Xiaoqin Zeng1, Bin Chen1,a)
1
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Address all correspondence to this author. e-mail: [email protected]
a)
Received: 17 September 2019; accepted: 18 December 2019
In this paper, the atomic resolution high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) was used as the main research method. Using HAADF-STEM, two types of long-period stacking ordered structure (LPSO)—14H and 18R-LPSO—were observed in Mg96Gd2Y1Ni1 alloy, and the precipitates at various stages of aging were observed. Moreover, a type of rectangular b precipitates were found, and the atomic models of b precipitates along the [0001]Mg and 〈1120]Mg directions were identified. At the aging peak stage, a three-dimensional network structure composed of LPSO/c9 precipitates and b9 precipitates and b precipitates was observed. The hardness of the unaged homogenized Mg96Gd2Y1Ni1 alloy was only 87 HV and the hardness value of aging peak was 128.4 HV. Compared with the unaged alloy, the hardness of the peakaged alloy increased by 47.59%. The composite strengthening of the three types of precipitates induced a significant strengthening to the alloy.
Introduction As is known, magnesium is the lightest structural material. Magnesium alloy exhibits excellent properties such as low density, high specific strength, and good damping properties, and it has good recyclability too. Nowadays, there is more and more attention to environmental protection; simultaneously, applying magnesium to automotive, aerospace, and other fields can achieve significant weight loss and reduce carbon dioxide emissions. Thus, magnesium alloys have great development potential in the automotive, aerospace, and other fields [1]. However, the disadvantages of magnesium such as poor corrosion resistance and insufficient strength, especially insufficient high-temperature strength, limit its application. But these problems are being solved. It has been found that the addition of heavy rare earth (RE) elements such as Gd and Y to the magnesium alloy leads to a significant age hardening response, and the long-period stacking ordered structure (LPSO) can be formed by adding a transition metal to the RE magnesium alloy. The formation of LPSO brings an important contribution to the improvement of the mechanical properties of magnesium alloys [2]. Different types of LPSO such as 10H, 12H, 14H, 15R, 18R,
ª Materials Research Society 2020
21R, and 24R have been discovered [3], of which 14H and 18RLPSO are the most common. Mg–Gd–Y alloys have attracted much attention due to their high-temperature strength and good creep resistance [4]. Due to the promotion of Ni to the formation of the LPSO [5, 6, 7, 8], a Mg–Gd–Y–Ni alloy was designed by adding Ni to the Mg– Gd–Y alloy. So far,
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