International Journal of Modern Physics and Application 2018; 5(3): 21-26 http://www.aascit.org/journal/ijmpa ISSN: 2375-3870 Refined Microstructure and Enhanced Mechanical Properties of Mg-6Al-Ca-Mn Alloy by Sn Addition Fang Da-qing, Luo Xiao-ping * College of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan, China Email address * Corresponding author Citation Fang Da-qing, Luo Xiao-ping. Refined Microstructure and Enhanced Mechanical Properties of Mg-6Al-Ca-Mn Alloy by Sn Addition. International Journal of Modern Physics and Application. Vol. 5, No. 3, 2018, pp. 21-26. Received: February 1, 2018; Accepted: May 9, 2018; Published: May 11, 2018 Abstract: The effects of tin on microstructure and mechanical properties of as-cast Mg-6Al-Ca-Mn alloy are investigated. The addition of 3.0 wt% tin leads to a significant grain refinement of Mg-6Al-Ca-Mn alloys owing to the formation of dispersed distribution of Mg 2 Sn phases that inhibited the formation of β-mg 17 Al 12 phase. Few Al 2 Ca and Al 8 Mn 5 phases are found. Furthermore, the Mg-6Al-Ca-Mn-3Sn alloy exhibits the best combined mechanical properties at ambient temperature, and the ultimate tensile strength, yield strength, and elongation are 222MPa, 145 MPa, and 14.31%, respectively. These results are attributed to the grain refinement and precipitation strengthening of the Mg 2 Sn phases. Keywords: Magnesium Alloy, Tin, Microstructure, Mechanical Properties 1. Introduction A Mg-Al alloy is currently the most widely used casting magnesium alloy due to its superior casting and mechanical properties [1-4]. However, these alloys are unsuitable for transmission and engine parts because of their net-like β-mg 17 Al 12 precipitation at temperatures over 125 C, which limits the mechanical properties at high temperatures and ultimately the application scope [5-8]. Therefore, the design of alloys with improved thermal performance should be based on the reinforcement of the thermal stability of the strengthening phase and reduction of β-mg 17 Al 12 precipitation. The addition of calcium can refine the alloy microstructure by forming Al-Ca compounds, which are thermally stable and improve the high-temperature properties [9]. Investigations of AM60-xCa alloy characteristics showed that when calcium addition was greater than 1.2%, it can inhibit the formation of β-mg 17 Al 12 phase and promote the generation of beneficial, thermally stable Al 2 Ca precipitate [10]. In addition, 0.2% calcium addition added to the Mg-3Al alloy acted as a heterogeneous nucleation agent could increase the nucleation rate and refine alloy grains [11]. Recently, the role of tin in magnesium alloy has been studied [12, 13]. Tin has high solid solubility in Mg (5.8%, mass fraction) and when added to Mg, forms Mg 2 Sn (face-centered cubic (fcc), a β =0.676 nm) precipitates, which reinforces the grain/dendrite boundaries. The Mg 2 Sn phase has a high melting temperature of approximately 770 C, which could provide the barriers needed for preventing dislocation slipping and climbing at high temperature, thus improving the thermal properties of the alloy. Mg 2 Sn precipitates generated after adding tin to an AZ91 alloycould improve the mechanical properties and creep resistance at high temperatures [14]. Tin content higher than 1.72% in the AZ80 alloy resulted in Mg 2 Sn precipitation and preferentially suppressed the formation of DP-Mg 17 Al 12 by hindering their growth [15]. In AZ82 alloy, tin content less than 4% increases tensile strength and compressive yield strength, and lowers the elongation [16]. In this work, relatively low calcium content and different tin contents were used to refine the grains, form high melting point intermetallics, and reduce the formation of Mg 17 Al 12. Finally, the improved microstructure and mechanical properties of as-cast Mg-6Al-Ca-Mn-xSn alloy were investigated and quantified.
22 Fang Da-qing and Luo Xiao-ping: Refined Microstructure and Enhanced Mechanical Properties of Mg-6Al-Ca-Mn Alloy by Sn Addition 2. Experimental The Mg-6Al-Ca-Mn-xSn alloys were prepared from pure magnesium, aluminum, tin (>99.9 wt.%), Mg 30Ca master alloy, and anhydrous. The experimental alloys were melted in a crucible resistance furnace and protected by a flux addition of sulfur hexafluoride and carbon dioxide. After being homogenized and mixed completely by mechanical stirring at 750 C, the melt was held at 750 Cfor 30 min and poured into a permanent mold with Ф125 mm Ф280 mm dimensions, which was preheated to 150 C to obtain experimental alloys. Alloys were obtained by treating the solid solutions at 400 C for 10 h. According to the different tin contents, alloys were defined as T1, T2, T3, and T4, as summarized in Table 1, which shows the nominal compositions (wt. /%) of the alloys. Al Mn Ca Sn 0 1.0 3.0 5.0 3. Results and Discussion 3.1. Microstructure Table 1. Nominal compositions of the alloys (wt./%). Material T1 T2 T3 T4 alloy was then examined using an Olympus optical microscope and S-4800 type scanning electron microscope (SEM) equipped with an Oxford energy dispersive spectrometer (EDS) which operated at 20 kv. The grain size was measured by the standard linear intercept method using an Olympus stereomicroscope. The phases in the as-cast experimental alloys were analyzed by D/Max-1200X type X-ray diffraction (XRD) operated at 40 kv and 40 ma. Tensile tests were performed using a CMT5105 universal electronic testing machine at a rate of 0.2 mm/min at room temperature. The ultimate tensile strength (UTS), 0.2% yield strength (YS) and elongation were obtained based on the average of the three tests. Mg The as-cast and solution samples of the experimental alloys were polished and sequentially etched in a 5% nitric acid solution in distilled water and a solution of 1.2 g picric, 25 ml ethanol, 5 ml acetic acid and 5 ml distilled water. The resulting In order to understand the role of Sn additions on the Mg-6Al-Ca-Mn-based alloy, the optical microstructure of cast Mg-6Al-Ca-Mn alloys with different Sn contents was analyzed, as shown in Figure 1. The Figure 1 reveals that all the alloys are composed of primarily dendritic microstructures surrounded by interdendritic eutectic second phase particles. The remarkable changes in the microstructure observed with the addition of Sn are morphological and are a significant volume fraction of the precipitated phase. Figure 1. The optical microstructures of the experimental alloys containing different amounts of Sn.
International Journal of Modern Physics and Application 2018; 5(3): 21-26 Figure 2. The X-ray diffractionanalyses of the as-cast alloys. The results of X-ray diffraction analysis of these alloys are illustrated in Figure 2. The α-mg and β-mg17al12 phases are observed only in the T1 and T2 alloys. Few Al2Ca and Al8Mn5 phases are found and the diffraction peaks of Mg2Sn are also not observed due to tiny addition dissolved in both 23 the matrix and β-mg17al12 phase particles, as reported by Li et al. [17]. However, additional peaks corresponding to the Mg2Sn phase appear in T3 and T4 alloy. With increasing Sn content, the intensity of the β-mg17al12 peaks weakens while the intensity of the Mg2Sn peaks increases. Together, it is obvious that Sn content plays a key role for the inhibition of β-mg17al12 phase and the promotion of Mg2Sn phase. Next, the microstructures of the solutionized alloys were investigated. As seen in Figure 3, dendritic crystals are converted into equiaxed grain and most of the eutectic phases are dissolved into the matrix after solid solution treatment. However, after solid solution treatment, there are small amounts of residual intermetallic compound in alloys with Sn addition, which mostly occur in the grains but part of the distribution occurs in the grain boundaries. Because Mg2Sn phase has a higher melting point than β-mg17al12, the residual compounds are Mg2Sn in the T3 alloy. Thus, Mg2Sn phase is more stable than β-mg17al12, which is beneficial for the improved thermal properties of the resulting alloy. Figure 3. The microstructure of the solutionized alloys. The magnified SEM images characterizing the secondary phases are shown in Figure 4. It is found that two types of precipitates having different morphologies are present in the solutionizedmicrostructures of the experimental alloys. One has a continuous and quasi-continuous network, and the other has an isolated shape. Furthermore, the volume fraction of Mg2Sn phase increases while the β-mg17al12 phase decreases with increasing Sn content. Together, the XRD and EDS results show that the precipitates in the T1 and T2 alloy are β-mg17al12 phase, while the precipitates in the T3 and T4
24 Fang Da-qing and Luo Xiao-ping: Refined Microstructure and Enhanced Mechanical Properties of Mg-6Al-Ca-Mn Alloy by Sn Addition alloy are the mixed phases of β-mg17al12 and Mg2Sn. Figure 4. The SEM images and EDS analyses of the solutionizedalloys. 3.2. Mechanical Properties Figure 5. Mechanical properties of the solutionized alloys.
International Journal of Modern Physics and Application 2018; 5(3): 21-26 The mechanical properties of the solutionizedalloys are shown in Figure 5. All of the Mg-6Al-Ca-Mn-xSn alloys exhibit greater hardness and strength than the Mg-6Al-Ca-Mn alloy, and the hardness and tensile strength increase with increasing Sn. The hardness and strength of the Mg-6Al-Ca-Mn-3Sn alloy are increased by 23% and 35%, respectively, compared with that of the Mg-6Al-Ca-Mn alloy, while excessive Sn addition results in decreased strength and elongation. The Mg-6Al-Ca-Mn-3Sn alloy exhibits the optimal combination of mechanical properties at room temperature 25 (RT), with UTS, YS, and elongation of 222MPa, 145 MPa, and 14.31%, respectively. The following aspects are considered to be related to the improved strength and plasticity at RT. The first most important aspect is the formation of large amount of Mg2Sn intermetallics, which provide considerable reinforcement of dendrite boundaries. Second, grain refinement can be achieved through the addition of Sn, resulting in the enhanced strength and elongation. The smaller grain size results in larger deformation resistance and more uniform stress distribution, thus improving the strength of the alloy. Figure 6. Tensile fracture micrographs of the solutionizedalloys. Figure 6 shows the SEM images of a fractured surface tested at RT, which can provide partial explanation for alloy strengthening. Many river patterns and obvious cleavage-type facets are observed in the Sn-free alloy (Figure 6a), where dimples appear in the T2 alloy (Figure 6b). In the T3 alloy, even more dimples are observed, which are deeper and contain some second phase particles, and the cleavage-type facets are relatively few (Figure 6c). This suggests that the alloy suffers relatively large plastic deformation prior to tensile fracture. For the T4 alloy, the dimples are low and the cleavage-type facets increase relatively, and the micro-cracks originate at the interface between the Mg matrix and the second phase (Figure 6d). It is known that initial cracks originate from the bulky second phase, especially at the grain boundary. The more bulky the compounds are, the easier the crack is to produce and spread, thus the strength and elongation of alloys decreases. 4. Conclusions The microstructure of as-cast Mg-6Al-Ca-Mn alloy with 1.0 wt.%sn mainly consists of α-mg and β-mg17al12. Few Al2Ca and Al8Mn5 phases are found. Higher than 1.0 wt.%sn addition leads to the increased formation of Mg2Sn phases. The microstructure is significantly refined and the volume fraction of β-mg17al12 phase is decreased when 3.0
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