Paper
Effect of the Mean Size of Fine Intermetallic Compounds on the Strength Property of Sintered Magnesium Alloy by Gas Atomization
2016 Volume 63 Issue 7 Pages 657-662
Details
2016 Volume 63 Issue 7 Pages 657-662
Magnesium is promising as a light weight material because its density is lower than that of aluminum. In order to enhance this advantage, various alloying elements have been studied. However, embrittlement caused by the simple addition of alloying elements is problematic. In this study, an atomized powder obtained by adding an element to improve the properties of the magnesium was prepared, then sintered by pulse current pressure. The strength property of magnesium sintered compacts was assessed by a transverse rupture test. Based on the results of the transverse rupture test and microstructure observations, the effect of the mean size of the intermetallic compounds on the strength and ductility was discussed.
Magnesium alloys are used for mobile device housings, sound equipment and automotive parts, etc. because these are the lightest metallic materials for practical use and have good damping properties1). Recently, die casting and plastic forming materials using magnesium have begun to be actively used in the automotive and aircraft industries. Thus, various magnesium alloys with a high strength and heat resistance are being studied2–4). Powder metallurgy can be used to alleviate formability problems through near net shape processing, and also offers the possibility of creating new alloys with highly specialized material properties. Therefore, powder metallurgy may lead to the creation of new magnesium alloys that are industrially useful.
In our past study, the sintering between elemental powders5–7), for example, magnesium and tin powder mixtures, was investigated7). It was evident from the magnesium-tin phase diagram8) that the mixtures would be sintered due to a low melting liquid phase and the intermetallic compound, Mg2Sn, would be formed. Tin additions were within the solid solubility limit of magnesium. As a result, the strength and ductility of the Mg-Sn sintered material was higher than that of other sintered materials (Mg-Al, Mg-Zn, Mg-Bi and Mg-Sb powder mixtures). Both properties reached their peak at 1 vol.% tin. However, these results were predicted to change depending on the processing condition.
Therefore, in this study, we focused on the influence of the size of the intermetallic compound on the strength properties of the sintered magnesium alloy made from atomized powder with different tin contents.
Three kinds of ingots for obtaining the atomized powder were prepared by dissolving the raw materials of pure magnesium, tin and zinc. A schematic of the gas atomization device supplied by Makabe R&D Co., Ltd., is shown in Fig. 1. These ingots, charged in a graphitic crucible, were dissolved by high frequency induction heating. These were heated at 973 K for 300 s at the heating rate of about 1.1 K/s. The molten metals were passed through a nozzle and became the three different kinds of alloy powders by blowing argon gas. The nozzle diameter was 15 mm, and the atomizing pressure was between 8 and 9 MPa. The surface and sectional microstructures of these powders were observed and analyzed by SEM (ERA-8900FE, Elionix, Inc.) equipped with EDS (Genesis, Ametek Co., Ltd.).
Schematic illustration of gas atomization device.
The sintering of these powders was performed by a pulsed current and pressure (SPS-625, Fuji Electronic Industrial Co., Ltd.) method. The graphitic mold for packing the powders was designed to form the transverse test specimens. The mold core in order to avoid stress concentration was divided into four sections. The energization and pressing area in the cavity was 35 × 10 mm2 because the size of the specimens was 35 × 10 × 6 mm2. The powders within the mold were heated at 723, 773 and 823 K at the heating rate of about 3.4 K/s, and simultaneously pressed at 40 MPa, then the temperature and pressure were held for 180 s. The sintered materials were cooled together in the mold at the rate of about 1.2 K/s. The degassing by using the open pores was also carried out during the processing and then the pores of the sintered materials almost disappeared at the pressure of 40 MPa.
The transverse rupture strength and bending strain were examined using a universal test machine (UH-500kNX, Shimadzu Corporation) and performed at the cross-head velocity of 1.7 × 10−5 m/s until the specimen’s destruction. The specimen size was 35 × 10 × 6 mm3 (Fulcrum distance: 30 mm). For microstructural examination, the sintered materials were sectioned and mounted in an epoxy resin. The polishing of the specimens was then performed on a buffing machine using up to 1/4 micron diamond. The polished section was etched in a 0.5 % hydrofluoric acid aqueous solution for 5 to 10 s until the microstructure was revealed. The microstructure and fractured surface of the sintered materials were observed by an optical microscope (EPIPHOT TME300, Nikon Corporation) and SEM-EDS. The Vickers hardness was measured by a hardness testing machine (MVK-H2, Akashi Corporation). The load was 98 mN in order to avoid as much as possible the grain boundary and the compound. The average hardness values were obtained from 5 measurements of the materials.
Table 1 shows the chemical compositions of some spots and areas of the Mg-Sn-Zn alloy powder cross section. The grain boundaries and compounds were not included in the spot, but were included in the area of a 2000 times power field. The tin and zinc contents in the magnesium grains were lower than the mean compositions of the atomized powder. Fig. 2 shows the observation images of the section of the atomized 7Sn alloy powder. It was evident from the Mg-Sn phase diagram8) and previous reports7,9) that the microstructure consisted of α-Mg and Mg2Sn, but it was unclear whether the magnesium-zinc compound was formed. Not all the tin and zinc were uniformly distributed in the atomized powder although the solidification rate of the atomization would be high. However, the matrix hardness of the pure Mg, 4Sn, 7Sn and 10Sn powders were about 43, 52, 56 and 67 HV, respectively. That is, not only the formation of Mg2Sn but also the solid solution strengthening was caused by the tin addition to the powder. The solid solution strengthening was also confirmed by the next result. The microstructure change and the matrix hardening with the increasing sintering temperature of the elemental powder mixtures are shown in Fig. 3. The magnesium-zinc eutectic was dissolved sooner than the magnesium-tin eutectic. It is now necessary to investigate the influence of zinc on the microstructure of the atomized magnesium powder. Fig. 4 shows an SEM image and the analysis results of the sintering boundary between the atomized powders. The spectral intensity of the oxygen and tin in the phase on the sintering boundary were higher than that of matrix. The Mg2Sn would be formed in the sintering neck. The oxygen peak would be due to the oxide film10) formed on the powder surface. The fine oxides about 0.2 μm or less were observed. These fine oxides would hardly affect the static strength properties although a coarse oxide is generally regarded as a notch defect.
| Symbol | Composition, C/mass% | |||||
|---|---|---|---|---|---|---|
| spot in Mg grain | 2000 times power field | |||||
| Mg | Sn | Zn | Mg | Sn | Zn | |
| 4Sn | 96.0 | 2.7 | 1.3 | 94.3 | 3.8 | 1.8 |
| 7Sn | 94.9 | 3.8 | 1.3 | 92.1 | 6.3 | 1.6 |
| 10Sn | 92.0 | 6.6 | 1.5 | 89.8 | 8.8 | 1.4 |
Observation images of the cross section of the atomized 7Sn powder: (a) optical microscope image and (b) SEM image.
Hardness measurement and optical microscopy results of the Mg-Zn and Mg-Sn powder mixtures sintered between 613 and 823 K for 300 s.
Analysis results by EDS standard-less method in the vicinity of the sintering boundary between atomized powders: (a) the sintering boundary and (b) the matrix of atomized powder.
Fig. 5 shows the various microstructures of the 4Sn and 10Sn powders sintered at 723 and 823 K for 180 s. In the case of a high tin content and a high sintering temperature, a large size Mg2Sn particle was observed, few pores were observed, and the magnesium grains were coarsened. Therefore, tin contained in the magnesium affected the sinterability. The relation between the tin content in the matrix and the sintering temperature is shown in Fig. 6. The tin content increased with the increasing sintering temperature in comparison to the atomized powders. That is, the grain boundary and Mg2Sn compound decreased, and the solid solution strengthening would occurred11,12). Fig. 7 shows the mean grain size dependence of the transverse rupture strength5) and the SEM images of a fractured surface of the 10Sn sintered materials prepared at (a) 773 and (b) 723 K for 180 s. The sintering, the bonding between powders, was confirmed by the fracture of Fig. 7 (a), but was not confirmed by the fracture of Fig. 7 (b). The two inclined lines are based on the Hall-Petch law. The lower line is pure Mg, while the upper line is T4-treated AZ91 magnesium alloy (T4: solution heat treatment and natural aging). The heating for the AZ91 was performed at 693 K for 36 ks. The strength of the materials in the vicinity of the magnesium alloy line was good. The sintered materials below the pure Mg line in Fig. 7 were not improved by the addition of alloying elements. Thus, in the materials near its line, the pure Mg should be used instead of (b). Fig. 8 shows the dependence of the mean size of the IMC on the strength properties of the well-sintered Mg-Sn-Zn alloy powders. The transverse rupture strength and bending strain of the sintered material reached their peak at about 1 μm. Therefore, the control of the IMC size is important in order to maintain the solid solution strengthening of the sintered atomized powders. The atomized powders were not extruded or heat treated, only sintered. It is expected that the selection of the magnesium alloy process will be increased.
SEM images of the 4Sn and 10Sn sintered materials processed at 723 and 823 K for 180 s.
Relation between the tin content in matrix and the sintering temperature. The atomized powders data are plotted for comparison.
Mean grain size dependence of the transverse rupture strength5) and SEM images of a fractured surface of the 10Sn sintered materials prepared at (a) 773 and (b) 723 K for 180 s.
Mean IMC size dependence of the strength properties: (a) the transverse rupture strength and (b) the bending strain of the sintered Mg-Sn-Zn alloy powder.
In this study, the dependence of the mean size of the intermetallic compound (IMC) on the strength properties of the sintered atomized powders was investigated. The results were as follows:
A part of this study was supported by a grant (No. 26820330) from the Japan Society for the Promotion of Science (JSPS), and the atomized powders were prepared thanks to the Faculty of Engineering at Iwate University.
