Effects of Ce addition on microstructure, mechanical properties and corrosion resistance of as-cast AZ80 magnesium alloy

Transcription

1 Research & Development CHINA FOUNDRY Effects of Ce addition on microstructure, mechanical properties and corrosion resistance of as-cast AZ80 magnesium alloy Wang Wei, *Xu Chunxiang, Zhang Jinshan, Cheng Weili, and Niu Xiaofeng College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, , Shanxi China Abstract: In this study, Ce was introduced into the AZ80 alloy and the effects of Ce addition on the microstructure, mechanical properties and corrosion resistance of the as-cast AZ80 magnesium alloy were investigated. The results show that the addition of Ce into the AZ80 alloy can not only refine the microstructure, but also result in the formation of the needle-like Al 4 Ce phase. These tiny Al 4 Ce phases are homogeneously distributed at grain boundaries and within grains. An appropriate Ce addition can also change the β-mg 17 Al 12 phase at the grain boundaries from continuous network to small island-like. At the same time, with the increase of Ce content from 0 to 2.0wt.%, the macro-hardness of the as-cast alloy is enhanced linearly, while impact toughness, tensile strength and elongation all firstly increase and then decrease. The AZ80 alloy containing 1.0wt.% Ce exhibits the optimal properties. Its macro-hardness, impact toughness, tensile strength and elongation are HB, J cm -2, MPa and 3.35%, increase by 9.95%, 63%, 13.3% and 36.7%, respectively compared with the base alloy. In addition, Ce can enhance the corrosion resistance of the AZ80 magnesium alloy. Key words: Ce; AZ80 magnesium alloy; microstructure; mechanical properties; corrosion resistance CLC numbers: TG Document code: A Article ID: (2014) * Xu Chunxiang Magnesium alloys have attracted much attention in the automobile and aerospace industries due to their advantages of high specific strength, specific stiffness, good damping capacity and machinability [1]. In recent years, the study on strengthening of AZ80 magnesium alloy has been a hot point of scientific research due to the increasing application of the alloy as a structural material in the automobile industry. The AZ80 magnesium alloy can be strengthened by thermal processing or by extruding deformation [2-4]. Although they give good results, these methods have a long production cycle and a high production cost compared with the casting process. The alloying technique is an effective way to improve the strength and ductility of magnesium alloys [5]. It has been found that addition of Ce is effective in refining grains and enhancing properties of several typical alloys such as AZ31, AZ61 and AZ91 [6-7]. To explore Female, born in 1964, Ph.D, Professor. Research interests: forming and strengthening of magnesium alloys. So far she has published about 50 papers. xuchunxiang2012@126.com. Received: ; Accepted: the strengthening effect of Ce on AZ80 magnesium alloy, the present study mainly investigates the form of existence of Ce in the AZ80 magnesium alloy and the effect of Ce addition on the microstructure, mechanical properties and corrosion resistance of the as-cast AZ80 magnesium alloy. The strengthening mechanism is also discussed. 1 Experimental procedure The AZ80 alloy was melted in a crucible furnace under RJ-6 protection. When the AZ80 magnesium alloy was melted completely at 740 C, Ce was added into the melt. After being stirred for 2 min, the melt was held at 760 C for 20 min to ensure the alloying elements were completely dissolved and diffused. Then the molten alloy was poured at 690 C into a cylindrical steel mold preheated to 220 C. The mold has a diameter of 20 mm and a length of 100 mm. The nominal chemical compositions of the alloys with different Ce addition are shown in Table 1. The samples, with dimensions of Ф20 mm 10 mm, for optical microscopy (OM) observation were polished and then etched in 3vol.% HNO 3 ethanol 157

2 & Development CHINA FOUNDRY Research Table 1: Nominal chemical compositions of experimental alloys (wt.%) Alloy Al Zn Mn Be Ce Mg A0 (AZ80) - A A A A solution for 30 s. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to study the morphological and micro-chemical characterization of the second phase. The phase constitutions were confirmed by X-ray diffraction with Cu Kα radiation at a rate of ( ) s-1. Samples with dimensions of Ф20 mm 10 mm were also used for HB hardness test on a brinell hardness tester (HB3000B) with a loading force of 625 N and a loading time of 30 s. Each measurement was the average value of at least 7 individual measurements. Square bars with dimensions of 10 mm 10 mm 60 mm were used for impact toughness testing on the impact testing machine (JBN-300) at room temperature. Tensile samples with Ф6 mm and 30 mm gauge length were fabricated according to GB/T (a) The tensile tests were carried out using a DNS100 universal electronic testing machine with a tensile speed of 0.5 mm min-1 at room temperature. Polarization curves of the specimens (with an exposed area of 1 cm2) were measured using a Land CS350 electrochemical system in NaCl solution (3.5 wt.%). 2 Results 2.1 Microstructure Figure 1 shows the optical microstructures of the AZ80 magnesium alloy with different Ce addition. Figure 1(a) shows that in the AZ80 alloy without Ce addition most of β-mg17al12 phase distributes along grain boundaries in the form of a continuous network or island. With the addition of Ce to the alloy, the coarse eutectic β-mg 17Al 12 phase is refined and becomes discontinuous, and a new tiny acicular-shaped phase (arrows in Fig. 1) occurs within the matrix and along the grain boundaries, as shown in Figs. 1(b) to (e). As the Ce content increases, the amount of the fine acicular-shaped phases clearly increases, the grain size decreases gradually and reaches a minimum when the Ce content is 1.0wt.%. With further increase of Ce, the acicular-shaped phases gather and coarsen (b) Acicular phase (c) 158 (d)

3 Research & Development CHINA FOUNDRY (e) Fig. 1: Optical microstructures of AZ80 magnesium alloy with different Ce additions: AZ80 alloy (a); AZ80+0.5wt.%Ce alloy (b); AZ80+1.0wt.%Ce alloy (c); AZ80+1.5wt.%Ce alloy (d); AZ80+2.0wt.%Ce alloy (e) gradually [marked with ellipses in Fig. 1(d)]. The morphology of the acicular-shaped phases turns to short rhabditis form or rod-like, even long rhabditiform through the grain [arrows in Figs. 1(d) and 1(e)]. SEM, EDS and XRD were used to identify the different phases. Figures 2(a) and 2(b) show the SEM images of A0 alloy and A2 alloy, respectively. Table 2 lists the EDS results of phases A to G in Fig. 2. The phase constitutions of the samples containing 0wt.%, 0.5wt.% and 2.0wt.% Ce (A0, A1 and A4), were analyzed using X-ray diffraction, as shown in Fig. 3. According to the EDS and XRD results (Table 2 and Fig. 3), the black areas in Fig. 2 are α-mg matrix (marked as A and D in Fig. 2), and the gray compound with network, ellipsoidal and islet-like morphology is β-mg17al12 phase (B and F in Fig. 2). The white granular compound is identified as Al8Mn5 (C in Fig. 2), and the acicular-shaped phase is the compound Al4Ce (E in Fig. 2). In addition, small white lump-like compound consisting of Al, Mn and Ce elements is observed (G in Fig. 2). It may be a mixed phase of Al8Mn5 and Al4Ce, which need to be fully identified in further study. Figure 3 shows that the A0 alloy is mainly composed of (a) α-mg matrix, β-mg 17Al 12 and Al 8Mn 5 phases, while some new peaks of Al4Ce phases appear with the addition of Ce, which is consistent with Fig. 1. It is known that the higher the difference in electronegative values between two elements is, the more easily the inter-metallic forms. The electronegative values of Mg, Al, Zn, Mn and Ce elements are 1.31, 1.61, 1.65, 1.55 and 1.12, respectively [6, 8], which means Ce should combine with Zn easily, but most of the Ce combines with the Al of the alloy to form the acicular-shaped Al4Ce compounds. This may be due to the content of Zn being too low. There is no Zn compound, so Zn exists in the α-mg solid solution [5]. 2.2 Mechanical properties Figure 4 shows the changes in hardness and impact toughness of the alloy with different Ce additions. It can be seen that with an increase in Ce content the hardness increases gradually. The impact toughness increases at first, reaches its peak value of J cm-2 when the Ce addition is 1.0wt.%, and then decreases with a further increase in the Ce content. With the addition of 1.0wt.% Ce, the experimental alloy exhibits the highest impact toughness and a higher hardness of J cm-2 (b) A (α-mg) D (α-mg) E (Al4Ce) B (β-mg17al12) G (Al8Mn5+Al4Ce) F (β-mg17al12) C (Al8Mn5) Fig. 2: SEM images of investigated alloys: A0 without Ce (a); A2 with 1.0wt.% Ce (b) 159

4 CHINA FOUNDRY Research & Development Table 2: EDS results for experimental alloys (wt.%) Element A0 Fig. 4: Effect of Ce addition on hardness and impact toughness of AZ80 magnesium alloy and HB, respectively. Compared with the values for the AZ80 magnesium alloy, their values increase by 63% and 9.95%, respectively. The changes in tensile strength and elongation of the experimental alloys with different content of Ce are shown in Fig. 5. It can be seen that the values of tensile strength and elongation increase at first and then decrease with the increase of Ce content. When the Ce content reaches 1.0wt.%, the experimental alloy exhibits the highest ultimate tensile strength and elongation of MPa and 3.35%, respectively. Compared with A0 base alloy, the ultimate tensile strength and elongation are enhanced by 13.3% and 36.7%, respectively. A2 A B C D E F G Ce Mn Zn Al Mg Total cps Hardness ( HB ) Fig. 3: XRD patterns of AZ80 magnesium alloy with different Ce additions α - Mg β - Mg Al Al8Mn5 Al Ce θ( ) Hardness Impact toughness Ce ( wt.%) 4 A4 A1 A Impact toughness (J cm ) -2 Tensile strength ( MPa ) Ce ( wt.%) Fig. 5: Effect of Ce addition on tensile strength and elongation of AZ80 magnesium alloy 2.3 Corrosion resistance Tensile strength Elongation Figure 6 shows the electrochemical polarization curves of the experimental alloys with different additions of Ce in 3.5wt.% NaCl solution. The polarization curves of all the experimental alloys are similar, and cathode and anode branches of the polarization curves are not symmetrical. The polarization curves of the Ce-containing alloys shift slightly in the positive potential direction. The free corrosion potentials of the alloys increase at first and then decrease with the increase of Ce content. The values for the alloys with Ce addition are higher than that of the A0 alloy, and the free corrosion potential of the alloy with 1.0wt.% Ce is the highest. When coupling with the same cathode, the higher the free corrosion potential value of the alloy is, the lower the corrosion rate, i. e. the better the corrosion resistance of the alloy. Thus the addition of Ce element can improve the corrosion resistance of AZ80 alloy, and the 1.0wt.% Ce-containing alloy exhibits the best corrosion resistance. E ( V ) A0 A1 A2 A3 A4 1E-6 1E-5 1E-4 1E Fig. 6: Electrochemical polarization curves of experimental alloys in 3.5wt.% NaCl solution 3 Discussions 3.1 Microstructure When adding Ce to the AZ80 magnesium alloy, on the one hand, the Ce could consume some of the Al of AZ80 Elongation (%) 160

5 Research & Development CHINA FOUNDRY magnesium alloy through the formation of Al 4 Ce phase during the crystallization process, which would decrease the grain size and the relative proportion of β-mg 17 Al 12 phase. On the other hand, according to Chen [9] and Liu et al. [10], the Al 4 Ce phase might act as a heterogeneous nucleating agent and promote nucleation during solidification. The Al 4 Ce phase is a kind of thermally stable phase, and its crystallization temperature is higher than that of the β-mg 17 Al 12 phase. Therefore the Al 4 Ce phase might be precipitated preferentially during the crystallization process, which could restrain the growth of α-mg at high temperatures; this would result in grain refinement. However, the excess Ce might cause segregation in front of the solid/liquid interface, which would result in the aggregation of the Al 4 Ce phase. In this way, the inhibition of the Al 4 Ce phase on the growth of α-mg grain would reduce, therefore the α-mg grain of the experimental alloy would tend to grow. 3.2 Mechanical properties The addition of Ce can improve the mechanical properties of the AZ80 magnesium alloy, mainly through the comprehensive action of the fine grain strengthening and dispersion strengthening, which can be analyzed by means of the Hall- Petch correlation [11] : where s is the yield stress, d is the grain size, s 0 is the friction stress of mobile dislocations, and k defines the characteristic constant that depends on the content of impurities and alloying elements [11]. The fine grain strengthening can be analyzed as follows. The k value of the magnesium alloy is about 4 times that of the aluminum alloy (k values of the Mg alloy and the Al alloy are about 280 MPa m -1/2 and 68 MPa m -1/2, respectively [12] ). According to the Hall-Petch relationship, the grain size of the magnesium alloy has a significant effect on its strength; the refinement of grain size would improve the yield strength of the alloy. In this experimental study, the appropriate addition of Ce apparently refines the grain size of AZ80 magnesium alloy, which leads to an evident increase in the value of k d -1/2. Therefore the fine grain strengthening caused by the Ce effectively improves the strength of AZ80 magnesium alloy. The enhancement of ductility of AZ80 magnesium alloy also benefits from the fine grain strengthening. When the grains of the alloy are refined, the grain number could be increased, which might enhance the cooperation deformation probability that occurs in the interior and surrounding of any grain, thereby improving the ductility of AZ80 magnesium alloy. The dispersion strengthening of Ce accords with the Orowan mechanism [13]. According to the Orowan mechanism, when the alloy deforms under the stress, the pinning action of dispersive phases can inhibit the dislocation movement, thereby leading to the dispersion strengthening. As for the AZ80 magnesium alloy, the β-mg 17 Al 12 phase is the main dispersive strengthening phase. After adding the appropriate amount of Ce, the β-mg 17 Al 12 phase is effectively refined and distributes (1) homogenously along the grain boundary. Meanwhile, the Ce could combine with the Al from the alloy to form many small dispersive phases of Al 4 Ce, which homogenously distribute at grain boundaries and within grains. So the dispersion strengthening by the Al 4 Ce phase with high melting point and the decrease of discontinuous β-mg 17 Al 12 phase can improve the strength of AZ80 magnesium alloy. 3.3 Corrosion resistance The effect of Ce on the corrosion resistance of AZ80 magnesium alloy was analyzed as follows. Firstly the Ce makes the corrosion products compact because of being incorporated into the corrosion products in the form of CeO 2 in the course of corrosion, which improves the protective effectiveness of corrosion products. Secondly, the Ce addition decreased the volume fraction and the size of the β-mg 17 Al 12 phase. In this way, the effect of a galvanic battery, produced by the free corrosion potential difference between the α-mg matrix (corrosion potential of 1.6 V) and the β-mg 17 Al 12 phase (corrosion potential of 1.3 V), might be reduced following the decrease in the area ratio of the cathode and the anode through decreasing the size of the β grains [15], which could also improve the corrosion resistance of AZ80 magnesium alloy. Also, the formation of the Al 4 Ce phase restrains the corrosive effect of the galvanic difference between the β-mg 17 Al 12 phase and the α-mg matrix to some extent, because the corrosion rate of the galvanic difference between the Al 4 Ce phase and the α-mg matrix is lower than that of the galvanic difference between the β-mg 17 Al 12 phase and the α-mg matrix [16]. However, an excess addition of Ce could result in coarsening of β-mg 17 Al 12 phase and forming a greater amount of Al 4 Ce phases, in which case the corrosion caused by the galvanic difference would be strengthened, which consequentially would worsen the corrosion resistance of the experimental alloy. 4 Conclusions (1) A new phase of Al 4 Ce is observed in the AZ80 alloy following the addition of Ce. The amount of this new phase is small and it distributes homogeneously in the matrix. (2) An appropriate Ce addition can not only refine the matrix microstructure, but can also change the β-mg 17 Al 12 phase distributed at the grain boundaries from continuous network to small island-like. (3) With an increase in the Ce amount, the macro-hardness of the as-cast experimental alloys is enhanced linearly, while the impact toughness, tensile strength and elongation firstly increase and then decrease. With the addition of 1.0wt.% Ce, the experimental alloy exhibits the optimal mechanical properties, with values of HB, J cm -2, MPa and 3.35%, respectively. Compared with those of the AZ80 magnesium alloy, their values are increased by 9.95%, 63%, 13.3% and 36.7%, respectively. (4) The addition of Ce can improve the corrosion resistance of the AZ80 magnesium alloy and the optimal content of Ce is 1.0wt.%. 161

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