Investigation on the Microstructure and Mechanical Properties of Mg

Materials Transactions, Vol. 52, No. 6 (2011) pp. 1088 to 1095 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, V #2011 The Japan Institute of Metals

Investigation on the Microstructure and Mechanical Properties of Mg-Al-Yb Alloys Su Mi Jo1 , Kyung Chul Park1 , Byeong Ho Kim1 , Hisamichi Kimura2 , Sung Kyun Park3; * and Yong Ho Park1; * 1

Department of Material Science and Engineering, Pusan National University, Busan 609-735, Korea Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 3 Department of Physics, Pusan National University, Busan 609-735, Korea 2

The effects of Yb addition on the microstructure and mechanical properties of Mg-5Al alloy are investigated. The results indicate that the addition of Yb to the Mg-5Al alloy facilitates the formation of a thermally stable Al2 Yb phase, the refinement of the microstructure and the suppression of the volume fraction of Mg17 Al12 phase in Mg-5Al alloy. Yb addition has little effect on the mechanical properties of the experimental alloys tested at room temperature. At elevated temperatures, however, the ultimate tensile strength (UTS) is significantly increased by Yb addition and Mg-5Al-1Yb has the highest UTS value than other experimental alloys. On the other hand, the yield strength (YS) increases at all tested temperatures due to the grain refinement and dispersion strengthening of the secondary phase. Meanwhile, the elongation (") of the experimental alloys decreases at all tested temperatures. Tensile fractographic analysis indicates that cleavage fracture is the dominant mechanism of the Mg-5Al and Mg-5Al-xYb alloys at room temperature. At elevated temperatures, however, the fracture mechanism of experimental alloys mainly changes from cleavage to quasi-cleavage fracture. [doi:10.2320/matertrans.MC201011] (Received December 1, 2010; Accepted April 7, 2011; Published May 25, 2011) Keywords: magnesium alloys, magnesium-alminum-rare earth, ytterbium addition, mechanical property

1.

Introduction

As one of the lightest structural materials, magnesium and its alloys have great potential for applications in automotive, aerospace and other industries due to their low density, high specific strength and good damping capacity.1–3) Mg-Al series alloys, such as AZ91 and AM60, are widely used due to their high castability and wide range of room temperature mechanical properties.4) However, the applications of these alloys are limited at elevated temperatures due to their poor creep resistance, which is typically improved by forming thermally stable precipitates to prevent grain boundary sliding during creep deformation. The most effective alloying elements for such purposes are rare earth metals (RE) that significantly improve the creep-resistance.5) Although some creep-resistant Mg-Al alloys with RE have already been developed, not many investigations have been reported in the literature concerning the effect of RE addition on the mechanical properties of Mg alloys. RE are normally added to Mg alloys such as misch metal (MM) because MM has a similar behavior with that of RE, but the cost is lower than RE. In spite of its lower price, some studies reported that the mechanical properties of single RE-added Mg alloys are higher than those of the alloys with added MM. Earlier studies investigated some single RE such as La, Ce, Nd and Sm.6–10) However, no research about the effects of ytterbium (Yb) addition on the microstructure and mechanical properties of Mg alloys has been reported. In this investigation, the effects of Yb addition on the microstructure and mechanical properties of Mg-5Al alloy are studied in order to provide a reference for the development of new RE-containing Mg alloys. *Corresponding

author, E-mail: [email protected], [email protected]

Table 1 Nominal compositions of as-cast Mg-5Al alloys with and without Yb addition (mass%). Alloy

Al

Mg-5Al

2.

Yb

Mg

—

Bal.

Mg-5Al-0.5Yb

5

0.5

Bal.

Mg-5Al-1Yb

5

1

Bal.

Mg-5Al-2Yb

5

2

Bal.

Experimental Procedure

The Mg-5Al-xYb (x ¼ 0, 0.5, 1 and 2 mass%) alloys with nominal compositions listed in Table 1 were prepared using high purity magnesium (99.9%), aluminum (99.99%) and Mg-40 mass%Yb master alloy. The alloys were melted at 750 C in a magnesia crucible under a CO2 + SF6 gas atmosphere and poured into a preheated permanent mould at 200 C. Microstructural analysis was carried out using an optical microscope (OM) and a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). A solution of an acetic picral (10 mL acetic acid + 4.2 g picric acid + 10 mL distilled water + 70 mL ethanol (95%)) was used to etch the samples. The samples selected at the corresponding position were homogenized at 420 C for 4 h in order to delineate the grain. The phases in the as-cast alloys were analyzed by X-ray diffraction (XRD) using monochromatic CuK radiation. The volume fractions of the precipitates were measured using an image analyzer. The dimensions of the tensile test specimens were 2 mm 2 mm 13:2 mm. The tensile tests were carried out at an initial engineering strain rate of 3:33 102 s1 at room and elevated temperatures (150 C, 200 C). An offset method is used to determine the yield strength of the material

Investigation on the Microstructure and Mechanical Properties of Mg-Al-Yb Alloys

tested. The values of 0:2 achieved by the strain gage. SEM was used for the fractographic observations to clarify the fracture process. 3.

Results and Discussion

3.1 Microstructure Figure 1 shows the XRD results of the as-cast experimental alloys. The XRD patterns revealed the main phases of the Mg-5Al alloy to be -Mg and Mg17 Al12 . With increasing Yb content, the peak of the Al2 Yb phase was newly appeared. The optical microstructures of the four studied alloys are shown in Fig. 2. Some phases were precipitated in the

Fig. 1 X-ray diffraction patterns of AZ51 alloys with and without Yb addition.

Fig. 2

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Mg-5Al alloy (Fig. 2(a)), and were revealed by the XRD results to be Mg17 Al12 phase. With Yb addition, other phases appeared at the grain boundaries in the Mg-5AlxYb alloys. (Fig. 2(b)–(d)) Mapping analysis was performed to determine the elemental distribution of Mg, Al and Yb in the microstructure shown in Fig. 3. Al and Yb elements appeared around the grain boundaries, which indicated that Mg17 Al12 and Al2 Yb phases had precipitated at the grain boundaries. Figure 4 shows the SEM images and EDS-spectra of the experimental alloys. According to the XRD and EDS results, the only secondary phase precipitated at the grain boundaries was Mg17 Al12 in the Mg-5Al alloy (Fig. 4(a)) and Al2 Yb phase was newly formed at the grain boundaries in the Mg5Al-xYb alloys (Fig. 4(b)–(d)). The morphology of Al2 Yb phase was changed to a more clearly lamellar structure and became finely dispersive at the grain boundary with increasing Yb content. However, at a Yb content of 2 mass%, Al2 Yb phase was coarsened (Fig. 4(d)). The volume fractions of the Mg17 Al12 and Al2 Yb phases were decreased and increased, respectively, with increasing Yb content. According to L. Ke-jie et al.,9) the electronegativity difference between two elements and the solidification kinetics of a metallic melt can be used to predict the possibility of intermetallic formation. The larger the electronegativity differences between the two elements, the stronger the bond and the higher the possibility of intermetallic formation. As the electronegativity difference between Al and Yb is higher than that between Al and Mg, the formation of Al-Yb intermetallics is favored over that of Mg-Al intermetallics in the Mg-Al-Yb system, and Mg17 Al12 phases are suppressed by Yb addition.10,11)

Optical micrographs of the experimental alloys: (a) Mg-5Al, (b) Mg-5Al-0.5Yb, (c) Mg-5Al-1Yb and (d) Mg-5Al-2Yb alloy.

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Fig. 3 Mapping analysis of alloying elements on the surface of Mg-5Al-0.5Yb alloy.

Figure 5 shows the microstructures of the experimental alloys after homogenized treatment. Homogenized treatment at 420 C for 4 h ensured the complete dissolution of the Mg17 Al12 phase, which is evident from the microstructure of the homogenized Mg-5Al alloy shown in Fig. 5(a). Figure 5(b)–(d) shows the microstructures of homogenized Mg-5Al-0.5, 1 and 2Yb alloys, respectively. As already shown in Fig. 5(a), there was no Mg17 Al12 phase, and only Al2 Yb phase still appeared at the grain boundaries even after 4 h of homogenized treatment, which indicated that the Al2 Yb phase was thermally more stable than the Mg17 Al12 phase.12) In addition, the grain size was gradually decreased with increasing Yb content in the Mg5Al. J. Zhang et al.13) have reported that based on the principles of solidification and the binary phase diagram of the Mg-RE system, the distribution coefficient of solute RE is less than 1, so that during the solidification process, solute atoms of RE, as well as Al, are enriched in the liquid ahead of the solid–liquid interface. This may lead to constitutional undercooling and a reduced diffusion rate of atoms, thereby increasing the number of nuclei and restricting the grain growth. On the other hand, the enrichment of solute atoms leads to the formation of AlRE phases, which are mainly distributed in the grain boundary, which further inhibits the grain growth. As an RE, therefore, Yb addition seems to induce a certain effect on grain refinement.

3.2 Mechanical properties 3.2.1 Tensile behavior The tensile properties of ultimate tensile strength (UTS), yield strength (YS) and elongation (") were tested from room temperature to 200 C and the results are shown in Fig. 6. Figure 6(a) shows the tensile properties of the alloys tested at room temperature. Compared to Mg-5Al alloy, although the difference was very slight, Mg-5Al-1Yb had the highest UTS. Meanwhile, the YS of the alloys gradually increased and " slightly decreased with increasing Yb content. The elevated temperature tensile properties are shown in Fig. 6(b) and (c). Yb addition improved UTS and YS, but decreased " at elevated temperatures in Fig. 6(b) and (c). Mg-5Al-1Yb alloy had the highest UTS than other experimental alloys at both elevated temperatures and room temperature. The Mg5Al-1Yb alloy was the optimized composition among the experimental alloys at room and elevated temperatures. At room temperature, the slight UTS variation among the four alloys was attributed to the similar room temperature property of the Mg17 Al12 and Al2 Yb phases. At elevated temperatures, however, the increased tensile strength with Yb addition was attributed to the thermal stability of the Al2 Yb phase. The main strengthening phase of the Mg-5Al alloy is Mg17 Al12 , which has a low melting point (approximately 437 C) and poor thermal stability, and hence can readily be softened at temperatures exceeding 120–130 C. However, upon Yb addition to the Mg-5Al alloy, Al2 Yb phase formed


Fig. 4

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SEM images and EDS spectra of Mg-5Al alloys: (a) Mg-5Al, (b) Mg-5Al-0.5Yb, (c) Mg-5Al-1Yb and (d) Mg-5Al-2Yb alloy.

at the grain boundary. Al2 Yb, which has a high melting point of approximately 980 C, has great thermal stability, and therefore acted to increase the UTS in the 1 mass% Yb added alloy. However, when Yb addition exceeded 2 mass%, the Al2 Yb phase was coarsened. The presence of massive secondary phase causes stress concentration, and intergranular cracks are readily formed when the alloy is loaded.14) Therefore, the UTS of the Mg-5Al-2Yb alloy was slightly decreased at elevated temperatures. Meanwhile, YS increased in all tensile tests at different temperatures. In general, YS is related to grain size. According to the well-known Hall– Petch relationship, grain refinement can obviously increase the YS.10) As shown in Fig. 5, grain size was efficiently refined with increasing Yb content. The " was decreased with increasing Yb content at room and elevated temperatures. This was attributed to the brittleness of Al2 Yb, which has a higher thermal stability but is rather brittle. 3.2.2 Fracture behavior The SEM images of the tensile fracture surfaces of the experimental alloys at room temperature and 200 C are shown in Figs. 7 and 8. In general, cleavage fracture, quasicleavage fracture and intergranular fracture are known to be

the main fracture modes of magnesium alloys. Figure 7(a) shows fracture surface of the Mg-5Al alloys tested at room temperature. The fracture surface reveals a cleavage plane with some secondary cracks. In the Mg-5Al alloy, Mg17 Al12 phase was the only secondary phase and the Mg17 Al12 phases were discontinuously precipitated and very susceptible to fracture because of their brittleness.15) The presence of many cleavage planes with cleavage steps of different sizes and river patterns indicated the action of brittle failure. With Yb addition, more and larger cleavage surfaces were observed in Fig. 7(b)–(d), which was attributed to the decrease in ductility with increasing Yb content. At elevated temperatures, the fracture mode was changed from cleavage to quasi-cleavage, as evidence in the fracture surface by the presence of numerous plastic zones spread over the entire fracture surface, as seen in Fig. 8.16) However, some cleavage planes with secondary cracks remained along with the deformation zone, which indicated a brittle fracture that differed from cleavage. This is a rather complicated fracture pattern according to Y. Lu et al.17) The planes on the fracture surface were incoherent with the Mg matrix but were formed through a combination of locally formed microcracks and

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Fig. 5 Microstructures of the alloys homogenized treatment at 420 C for 4 h: (a) Mg-5Al, (b) Mg-5Al-0.5Yb, (c) Mg-5Al-1Yb and (d) Mg-5Al-2Yb alloy.

Fig. 6 Tensile properties of the experimental alloys at (a) 25 C, (b) 150 C and (c) 200 C.


Fig. 7 Tensile fractograph of the experimental alloys at room temperature: (a) Mg-5Al, (b) Mg-5Al-0.5Yb, (c) Mg-5Al-1Yb and (d) Mg5Al-2Yb alloy.

Fig. 8 Tensile fractograph of the experimental alloys at 200 C: (a) Mg-5Al, (b) Mg-5Al-0.5Yb, (c) Mg-5Al-1Yb and (d) Mg-5Al-2Yb alloy.

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S. M. Jo et al. Table 3 The comparison of yield strength (0:2 ) between the calculated values using Hall–Petch relationship and measured ones. 0:2

Alloy Calculated

Measured

Mg-5Al

52.9

83

Mg-5Al-0.5Yb

60.2

88

Mg-5Al-1Yb

66

89

Mg-5Al-2Yb

83.5

90

Table 4 Quantitative volume fractions of secondary phases of the experimental alloys. Alloy

Mg-5Al Mg-5Al-0.5Yb Mg-5Al-1Yb Mg-5Al-2Yb

Fig. 9 High-magnification of Mg-5Al-1Yb at 200 C.

Volume fraction of Mg17 Al12 (%)

3.25

2.32

1.46

0.27

Table 2 Quantitative grain size of the experimental alloys.

Volume fraction of Al2 Yb (%)

—

1.08

2.12

4.43

Alloy Grain size (mm)

Mg-5Al Mg-5Al-0.5Yb Mg-5Al-1Yb Mg-5Al-2Yb 98.5

63.8

47.9

24.3

simultaneously formed tearing ridges. The bottoms of the pits were not strict cleavage planes but rather consisted of several somewhat sunken planes with secondary cracks. Figure 9 shows a locally high magnification image of Fig. 8(c) that presents the tearing ridges and secondary crack. This indicated that Yb addition led to quasi-cleavage-type failure of the alloys at elevated temperatures. In summary, the improved mechanical properties of Mg5Al-xYb alloys at elevated temperatures were based on the following two main strengthening effects. First is the grain refinement effect. As shown in Fig. 5, with increasing Yb content, the grain size of the Mg-5Al alloys was decreased. The larger surface volume of the grain boundary effectively acted as a dislocation barrier, which increased the deformation resistance.8) According to the Hall–Petch relationship grain refinement can obviously increase the YS of Mg alloys due to the increased value of the coefficient k value. pffiffiffi 0:2 ¼ 0 þ k= d ð1Þ where 0:2 is the YS, 0 a constant, d the average grain pffiffiffiffiffiffiffi diameter, and for Mg k ¼ 280{320 MPa mm,18) the value pffiffiffiffiffiffiffi of k was chosen as 300 MPa mm. And the values for 0 (35 MPa) and d (590 mm) of pure Mg by permanent mold casting were used in this calculation.19) The quantitative grain size of the experimental alloys needed for calculating YS (0:2 ) shows in Table 2. The YS (0:2 ) could be estimated using the Hall–Petch relationship, as seen in Table 3. As shown in Table 3, the measured 0:2 was higher than the calculated 0:2 . This indicated the action of another effect in improving the YS of the alloys, in addition to the grain refinement effect. This second effect is the dispersion strengthening. The heat resistance of the Mg alloys was determined by the softening resistance of the -Mg, the morphology, and the size and distribution of the second phases in the alloy. With increasing Yb content, Mg17 Al12 phase and thermally stable Al2 Yb phase were decreased and increased, respectively (Table 4),

and Al2 Yb phase was finely dispersive at the grain boundary (Fig. 4). Therefore, the Mg-5Al-1Yb alloys, which had a finer grain size and more uniform distribution of grain boundary phases, had the highest mechanical properties than the other alloys. For the Mg-5Al-2Yb alloy, the decrease of the gap between the measured YS and the YS calculated by the Hall–Petch relationship can be understand in the same context (Table 3). In conclusion, Yb addition can further improve the tensile properties of Mg-Al-RE alloys for the two aforementioned reasons. 4.

Conclusions

The microstructure and mechanical properties of Mg-5AlxYb (x ¼ 0, 0.5, 1 and 2 mass%) alloys were investigated. The main results were as follows: (1) The Mg-5Al alloy consisted of -Mg and Mg17 Al12 phases. Upon Yb addition, Al2 Yb phase was observed with a gradually reduced Mg17 Al12 phase. The grain size was also refined with increasing Yb content in the Mg-5Al alloy. (2) Among the experimental alloys, the UTS values of the Mg-5Al-1Yb alloy were the greatest than other alloys at both room and elevated temperatures due to the grain refinement and dispersion strengthening. (3) At Yb addition less than 2 mass%, both grain refinement and dispersion strengthening affected the UTS results, which improved with increasing Yb content. at a Yb content of 2 mass%, although the grain was more refined, secondary phases were coarsened at the grain boundary and the UTS of Mg-5Al-2Yb was decreased at room and elevated temperatures. Meanwhile, the elongation (") of the experimental alloys decreased at all tested temperatures. (4) Cleavage fracture was dominant mechanism of Mg-5Al and Mg-5Al-xYb alloys tested at room temperature. Elevated temperatures and Yb addition both induced a change in the failure mechanism of the alloys to quasicleavage fracture.


Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials, from the Ministry of Knowledge Economy, Republic of Korea and a grant-in-aid for support from the Tohoku University Global COE program.

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