Age hardening characteristics and mechanical properties of Mg-3.5Dy-4.0Gd-3.1Nd-0.4Zr alloy
LI De-hui(李德辉)1, DONG Jie(董 杰)1, ZENG Xiao-qin(曾小勤)1, LU Chen(卢 晨)1, DING Wen-jiang(丁文江)1
National Engineering Research Center for Light Alloy Net Forming, Shanghai Jiao Tong University,
Shanghai 200030, China
Received 28 July 2006; accepted 15 September 2006
Abstract:
Age hardening characteristic and tensile property of Mg-3.5Dy-4.0Gd-3.1Nd-0.4Zr alloy were investigated. The alloy exhibits a considerable age hardening effect up to 250 ℃. Increasing the aging temperature leads to a shorter aging time to reach the peak hardness and a lower peak hardness. The tensile results show that the peak-aged specimens have higher tensile strength at the temperature below 200℃. However, with the increase of temperature further, the tensile strength decreases dramatically and elongation increases drastically. The loss in tensile strength and increase in elongation at high temperature are possibly associated with the instability of secondary precipitates. The fracture mechanism of alloy transfers from intergranular to transgranular with the increase of experimental temperature.
Key words:
Mg-Dy-Gd-Nd alloy; heavy rare-earth; aging characteristics; high temperature tensile; microstructure;
1 Introduction
The industry, as well as the scientist, focused their attention on the development of the promising magnesium alloys, particularly those based on the Mg-Y-Nd system, to offer attractive properties for light mass materials in aerospace and automobile applications. Magnesium and magnesium alloys present a number of advantages, such as: high specific strength, good castability, excellent machinability, good weldability under controlled atmosphere.
Recent work in Japan and Russia has suggested that mechanical properties can be further improved using the alloys based on Mg-Gd and Mg-Dy systems, and Mg-20Gd and Mg-20Dy alloys are superior to the conventional heat-resistant alloy WE54A and the aluminum alloy 332.0 in mechanical properties at room and high temperatures[2-8]. However, a large amount of heavy rare earth elements are added to magnesium, as a result, the alloy is more expensive, its density is higher and the ductility at room temperature deteriorates[2]. It was reported that it could not only increase the mechanical properties, but also lower the cost, when part of Dy and Gd elements was replaced by Nd or Y[9-12], However, till to now, the precipitation processes and their relationship with mechanical properties in more complex magnesium-rare earth alloys are not fully understood. The present work aims at developing a heat-resistant magnesium alloy which is relatively cheap and has higher elevated temperature strength than commercial heat-resistant magnesium alloys. Therefore, part of Dy and Gd elements in Mg-Dy-Gd-Nd-Zr alloy was replaced by Nd. In addition, Zr was added to the alloy as a grain refining agent.
2 Experimental
The master alloy was melted in a crucible electric resistance furnace protected with a mixed gas of CO2 and SF6 in the volume ratio of 100∶1. Mg-25Dy, Mg-25Gd and Mg-25Nd interalloys were added at 760 ℃, then the melt was held at 800 ℃ for 20 min. Zr in the form of an Mg-30Zr master alloy was added to the melt at 830 ℃. After that the melt was refined by JDMJ flux (mainly chlorate) and was held at 800 ℃ for 30 min before being adjusted to the required temperature and cast. Pouring temperature and steel mold temperature were about 760 ℃ and 200 ℃ respectively. Chemical composition of the alloy ingots obtained is Mg-3.5Dy- 4.0Gd-3.1Nd- 0.4Zr.
Solution heat treatment of the alloy was performed at 525 ℃ for 8 h, followed by water quenching. Aging treatment was carried out in an oil-bath furnace at 200, 225 and 250 ℃ in order to investigate the age hardening characteristics.
The microstructures of the as-cast and solution- treated specimens were observed by optical microscope. The fracture surface and microstructure near the surface area were observed by an optical microscope and a scanning election microscope PHLIIPS SEM515 (SEM).
Hardness of aged specimens was determined by Vickers hardness tester under load of 49 N and holding time of 30 s to evaluate the aging response of the alloy. Tensile test of peak-aged specimens was conducted from room temperature to 300 ℃ using Shimadzu AG-100KNA tensile tester at a constant crosshead speed of l mm/min.
3 Results and discussion
The microstructures of as-cast and solution-treated specimens are shown in Fig.1. The grains of as-cast specimen are fine because of Zr addition, and most of the eutectic compounds precipitate along grain boundaries. The average grain size of alloy is about 25 μm. After solution-treatment, the eutectic compounds are dissolved, the alloy consists of simple magnesium solid solution and the grain grows up distinctly.
The hardness evolution, as a function of aging time, is shown in Fig. 2 for isothermal aging at 200, 225 and 250 ℃, respectively. As shown in Fig.2, the alloy shows a remarkable increase in hardness at all examined aging temperatures, and the peak Vickers hardness can reach HV 99 after being aged at 200 ℃ for 64 h. The increase of peak hardness and the occurrence of age hardening may be caused by the increase in the amount of precipitates. With the increase of aging temperature, the aging time to reach the peak hardness is shortened, and the peak hardness decreases.
Fig.1 Microstructures of as-cast (a) and solution treated (b) specimens
Fig.2 Hardness curves of solid solution treated alloy aged at different temperatures
Fig.3 shows the tensile properties of the peak-aged specimens as a function of temperature. Tensile strength and elongation of alloy are 298.25 MPa and 3.31% at room temperature, 285.23 MPa and 11.6% at 200 ℃, and 187.95 MPa and 18.13% at 300 ℃, respectively. It can also be seen that the tensile strength and elongation of the alloy have only little change below 200 ℃, however, the tensile strength and elongation change dramatically as the experimental temperature is over 250 ℃.
Fig.3 Tensile properties of peak-aged specimens as function of temperature
Figs.4 and 5 show the fracture surface and microstructures near the surface, respectively. At room temperature, the fracture surface presents an intergranular cleavage characterized as a brittle fracture. At 150 ℃, the overall fracture surface appears in a mixed intergranular/transgranular mode. In contrast, the fracture surface is fully filled with transgranular fracture and reveals the dimple pattern characterized as a ductile fracture at the temperature above 250 ℃, especially at 300 ℃.
Fig.4 Fracture surfaces of peak-aged specimens at different temperatures: (a) 20 ℃; (b) 150 ℃; (c) 250 ℃; (d) 300 ℃
Fig.5 Microstructures near fracture surfaces at different temperatures: (a) 20 ℃; (b) 150 ℃; (c) 250 ℃; (d) 300 ℃
The tensile strength of the alloy significantly decreases and the elongation increases as fracture morphologies change from intergranular to transgranular. The loss in tensile strength at high temperature may result from the disappearance of the metastable phase. The metastable phase may effectively prohibit the slip on the basal plane at the temperature below 200 ℃, leading to a higher strength and a lower elongation at low temperature.
4 Conclusions
1) With the increase of the aging temperature, the aging time of Mg-3.5Dy-4.0Gd-3.1Nd-0.4Zr alloy to reach the peak hardness is shortened, and the peak hardness decreases.
2) The peak-aged specimens have higher tensile strength at the temperature below 200 ℃, however, the tensile strength decreases significantly and the elongation increases distinctly when the experimental temperature is higher than 250 ℃.
3) The fracture mechanism of alloy transfers from intergranular to transgranular with the increase of experimental temperature.
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(Edited by CHEN Can-hua)
Corresponding author: LI De-hui; Tel: +86-21-62932549; E-mail: lidehuilee@sjtu.edu.cn
