Effects of various Mg-Sr master alloys on microstructural refinement of ZK60 magnesium alloy
CHENG Ren-ju(程仁菊)1, PAN Fu-sheng(潘复生)1, 2, YANG Ming-bo(杨明波)3, TANG Ai-tao(汤爱涛)1
1. National Engineering Research Center for Magnesium Alloys, Chongqing University,
Chongqing 400044, China;
2. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China;
3. College of Materials Science and Engineering, Chongqing Institute of Technology, Chongqing 400050, China
Received 12 June 2008; accepted 5 September 2008
Abstract: The effects of various Mg-Sr master alloys (conventional as-cast, rapidly-solidified, rolled and solutionized) on microstructural refinement of ZK60 magnesium alloy were investigated. The results indicate that the refinement efficiency of various Mg-Sr master alloys in ZK60 alloy is different. The rolled Mg-Sr master alloy is found to have relatively higher refinement efficiency than the conventional as-cast, solutionized and rapidly-solidified Mg-Sr master alloys. After being treated with the rolled Mg-Sr master alloy, the ZK60 alloy obtains the minimum average grain size of 33 μm. The difference of various Mg-Sr master alloys in refinement efficiency might be related to the initial microstructure change of various Mg-Sr master alloys.
Key words: magnesium alloy; Mg-Sr master alloy; microstructural refinement; strontium
1 Introduction
Although wrought magnesium alloys have been used as structural materials since 1990s, the volume of wrought magnesium products is considerably less than that of casting products. This is because their mechanical properties and processing performances could not yet meet the needs of currently industrial application[1]. However, wrought magnesium alloys are thought to have more development potential. Therefore, a lot of ways are investigated in order to improve the mechanical properties and processing performances of wrought magnesium alloys. It is known that the grain refinement is an important method to improve properties and formability of magnesium alloys. Recent investigation indicated that Sr additions, which have been widely used in industrial practice especially for the modification of Al-Si alloys, could effectively refine the grains of wrought magnesium alloys[2-7]. However, the previous investigations focus on the AZ31 magnesium alloy. The investigation about wrought magnesium alloys, such as ZK60 magnesium alloy, which is thought to have the higher strength at room and elevated temperatures, is very scarce. In the present work, the effects of various Mg-Sr master alloys (conventional as-cast, rapidly- solidified, rolled and solutionized) on microstructural refinement of ZK60 magnesium alloy are investigated.
2 Experimental
The conventional as-cast and rapidly-solidified Mg-Sr master alloys were made by using pure Sr and pure Mg. The Sr content is 9.41% (mass fraction) in the conventional as-cast master alloy and 9.17% (mass fraction) in the rapidly-solidified master alloy. The rolled and solutionized Mg-Sr master alloys were prepared from the conventional as-cast Mg-Sr master alloy by the following method. The rolled Mg-Sr alloy was prepared by CMT-5105 tensile testing machine under the condition of 1 mm/min compression rate and 20% defor- mation degree at 340 ℃, and the solutionized Mg-Sr master alloy was prepared at 350 ℃ for 24 h.
The ZK60 magnesium alloy was melted in an electrical resistance furnace using a graphite crucible and protected by a flux addition. When the melt temperaturereached approximately 740 ℃, various Mg-Sr alloys, whose mass was calculated according to the Sr adding amount of 0.1% (mass fraction), were added to the melt respectively. After holding at 740 ℃ for 80 min, the melt was poured into a permanent coated mould. In order to examine the crystal grains of the alloys clearly, the solid solution treatment was carried out at 400 ℃ for 5 h and followed by air cooling. Optical microscope samples of the as-cast and heat-treated experimental ZK60 magnesium alloys were etched with 8% nitric acid and picric acid (a solution of 1.5 g picric, 25 mL ethanol, 5 mL acetic acid and 10 mL distilled water), respectively. The microstructure was examined by using optical microscope, scanning electron microscope and X-ray diffractometer.
3 Results
3.1 Microstructures of various Mg-Sr master alloys
Fig.1 shows the XRD pattern of conventional as-cast Mg-Sr master alloy. It is found that the microstructures of Mg-Sr master alloy are composed of α-Mg and Mg17Sr2 phases. Fig.2 shows the microstructures of the various Mg-Sr master alloys. Although the Mg17Sr2 phases in the various Mg-Sr master alloys are found to distribute on the grain boundaries, their morphology, amount and size are relatively different. Mg17Sr2 phases in the conventional as-cast and rapidly-solidified Mg-Sr master alloy mainly exhibit particle shape, while in the solutionized and rolled Mg-Sr master alloys, they exhibit block shape. Further- more, it is found from Fig.2 that Mg17Sr2 phase has different size in various Mg-Sr master alloys.
Fig.1 XRD pattern of conventional as-cast Mg-Sr master alloy
3.2 Microstructure of ZK60 magnesium alloys
Fig.3 shows the as-cast and heat-treated micro- structures of ZK60 alloy without Sr addition. It is found that ZK60 alloy without Sr addition mainly exhibits coarse dendrite structure, and the dendrite arm spacing and grain size of the alloy are relatively larger. However, after treatment with 0.1%Sr by using various Mg-Sr master alloys, ZK60 alloys exhibits fine dendrite structure and approximately equiaxed structure, and their grain size drastically decreases (Figs.4-6). The above results indicate that adding various Mg-Sr master alloys (conventional as-cast, rolled, solutionized and rapidly-solidified) to ZK60 alloy has obvious refining effect on the microstructure of ZK60 alloy. By comparing Figs.4-6, it is found that the various Mg-Sr master alloys have different refinement efficiency. The rolled Mg-Sr master alloy has higher refinement efficiency than the conventional as-cast, solutionized and rapidly-solidified Mg-Sr master alloys. After being treated with the rolled Mg-Sr master alloy, the ZK60 magnesium alloy can obtain the minimum average grain size of 33 μm.
Fig.2 SEM images of various Mg-Sr master alloys: (a) Conventional as-cast; (b) Rapidly-solidified; (c) Solutionized; (d) Rolled
Fig.3 Microstructures of ZK60 magnesium alloys without Sr modification: (a) As-cast; (b) Heat-treated
Fig.4 As-cast microstructures of ZK60 magnesium alloy treated with various Mg-Sr master alloys: (a) Conventional as-cast; (b) Rapidly-solidified; (c) Solutionized; (d) Rolled
4 Discussion
Generally, the grain refinement in industrial applications usually involves adding nuclei and/or solute elements into a melt before casting, and the effect of a solute element may be explained in terms of the growth restriction factor(GRF)[8-9]. In addition, the grain refinement is also related to the heterogeneous nucleation during solidification. One criterion of heterogeneous nucleation is that the disregistry of unclean planes is less than 6%[10]. According to the information from XRD results, the various Mg-Sr master alloys are composed of α-Mg and Mg17Sr2 phases. It is well known that the Mg17Sr2 phase has a hexagonal close-packed crystal structure with a=b=10.469 nm and c=10.3 nm[11]. The α-Mg phase has a hexagonal close-packed crystal structure with a=0.320 nm and c=0.52 nm[12]. Obviously, the lattice disregistry between Mg17Sr2 and α-Mg phases is larger than 6%, indicating that the Mg17Sr2 phase could not act as heterogenerous nucleus for the α-Mg phase.
Fig.5 Solutionized microstructures of ZK60 magnesium alloy treated with various Mg-Sr master alloys: (a) Conventional as-cast; (b) Rapidly-solidified; (c) Solutionized; (d) Rolled
Fig.6 Average grain size of ZK60 magnesium alloys treated with various Mg-Sr alloys
Fig.7 and Table 1 show the SEM image and EDS result of the ZK60 magnesium alloy treated with conventional as-cast Mg-Sr master alloy. It is found from Fig.7 and Table 1 that the element Sr mainly distributes in the white second phases. According to the XRD result of the as-cast ZK60 magnesium alloy treated with the Mg-Sr master alloys, the Sr-containing phases are not detected, indicating that Mg17Sr2 phases have been completely dissolved into the melt. Since the solid solubility of Sr in magnesium is relatively limited, Mg17Sr2 phases in the various Mg-Sr master alloys may not directly influence the grain refinement unless the free Sr is obtained by the dissolution of Mg17Sr2 phases[6, 13]. Thus, after Mg17Sr2 phases are dissolved into the melt of ZK60 magnesium alloy, the free Sr will rapidly enrich in the liquid ahead of growing interface and then restrict the grain growth during solidification[14]. In the present work, Mg17Sr2 phases in the rolled Mg-Sr master alloy are larger but less, so, more Sr dissolves in the α-Mg matrix, and during rolling, the Mg-Sr master alloy can storage energy, which could promote the dissolution rate of Mg17Sr2 phases. As a result, the GRF would be larger. Therefore, the refinement efficiency of the rolled Mg-Sr master alloy is better than that of other three Mg-Sr master alloys. However, the refining mechanism of various Mg-Sr master alloys, especially the difference of various Mg-Sr master alloys in refining efficiency, is not completely clear.
Fig.7 SEM image of ZK60 magnesium alloy treated with conventional as-cast Mg-Sr master alloy
Table 1 EDS result of ZK60 magnesium alloy treated with conventional as-cast Mg-Sr master alloy (mass fraction, %)
5 Conclusions
1) The preparing methods of Mg-Sr master alloys, conventional casting, rapid solidification, rolling and solution treatment, are found to have obvious effects on the microstructure of Mg-Sr master alloys.
2) The various Mg-Sr master alloys all refine the grains of ZK60 magnesium alloy. However, the refinement efficiency of various Mg-Sr master alloys to ZK60 magnesium alloy is different. The rolled Mg-Sr master alloy is found to have relatively better refinement efficiency than the as-cast, solutionized and rapidly-solidified Mg-Sr master alloys. After being treated with the rolled Mg-Sr master alloy, ZK60 magnesium alloy with 0.1%Sr can obtain the minimum average grain size of 33 μm.
3) The difference of various Mg-Sr master alloys in refinement efficiency may be related to the initial microstructures of various Mg-Sr master alloys, but the mechanism remains unclear.
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(Edited by YANG Bing)
Foundation item: Project(50725413) supported by the National Natural Science Foundation of China for Distinguished Young Scholar; Project (2007CB613704) supported by the National Basic Research Program of China
Corresponding author: FAN Fu-sheng; Tel: +86-23-65112635; E-mail: fspan@cqu.edu.cn
