Effect of si and p on microstructure and mechanical properties of mg 7al 1zn 3 597cu xsi alloys

Research of Materials Science June 2013, Volume 2, Issue 2, PP.23-27

Effect of Si and P on Microstructure and Mechanical Properties of Mg-7Al-1Zn-3.597CuxSi Alloys Keqiang Qiu#, Xiaocheng Wang, Junhua You School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China Corresponding author: kqqiu@yahoo.com.cn

Abstract In order to improve the heat resistant properties of magnesium alloys, the effects of P modification on phase constituent, microstructure and mechanical properties of Mg-7Al-1Zn-3.597Cu-xSi (x=1, 2, 3) alloys were investigated by means of x-ray diffractometer (XRD), scanning electron microscopy (SEM), and universal electronic testing machine. The results indicate that the Mg2Si phase presented in the form of Chinese script shape was observed in the microstructure of the alloy without P element.

The addition of P element can refine the microstructure by making Mg2Si phase change from Chinese script shape into block shape. The optimum properties with tensile strength, yield strength and elongation of 192.91 MPa, 115.42 MPa and 4.6% respectively are obtained for the alloy by adding 0.5% P and 1% Si elements. The mechanical properties of the alloys decrease with further increasing the Si content.

Keywords: Magnesium Alloy; Microstructure; Mechanical Strength; P addition

1 INTRODUCTION Magnesium alloys have been attracting attention as an important lightweight material and are being widely utilized in the automobile and aerospace industries [1-2].. It is well known that Mg-Al-Si based alloys are potential high temperature lightweight materials[3-4] due to the existence of Mg2Si phase in the microstructure. This is because that Mg2Si presents a high melting point, a high hardness, a low density, a high elastic modulus and a low thermal expansion coefficient. Furthermore, Mg2Si phase is very stable and can impede grain boundary sliding at high temperatures[5-7]. Therefore, the modification and refinement of Mg2Si phase is one of the means to improve the mechanical properties of Mg-Al-Si based alloys[8-10]. It has been reported that the Chinese script shaped Mg2Si phases in Mg-Al-Si based alloys might be modified and refined by Sb[11], Ca and P additions[12-13], However, Sb is not an effective modifier of the Mg2Si phase, and Ca could result in cast defects such as a hot-crack. Therefore in order to control the Mg2Si morphologies, a small amount of P was used to control the the Chinese script shaped Mg2Si phases.

2 EXPERIMENTAL TABLE 1 THE NOMINAL COMPOSITION OF THE ALLOYS Alloys 1 2 3 4

Al 7 7 7 7

Zn 1 1 1 1

Si 1 1 2 3

P 0 0.5 0.5 0.5

Cu 3.597 3.597 3.597 3.597

Mg Bal Bal Bal Bal

Nominal chemical composition of the alloys used in this study are listed in Table 1. High purity Mg (99.99%), high purity Zn (99.999%), and high purity Al (99.99%), as well as Mg-30%Si and Cu3P intermediate alloys were used for the investigated alloys. The experimental alloys were melted in a crucible under the protection of the nitrogen gas at - 23 http://www.ivypub.org/rms

700℃. The melt was homogenized by mechanical stirring and held at 700℃ for 30min, then it was cast in a metal mold preheated at a temperatire of 200℃. The slices were cut from the ingots by an electrodes charge wire-cut machine for tensile tests and structural characterization. The microstructure and the fracture surfaces of the specimens were studied by scanning electron microscopy (SEM). The X-ray diffraction (XRD) and energydispersive X-ray (EDS) analysis were carried out on selected samples to identify the phases and compositions.

3 RESULTS AND DISCUSSION 3.1

Microstructure analysis

▼ ●

●◆

Fig. 1 shows the XRD patterns of the as-cast alloys. It can be seen that four studied alloys are mainly composed of αMg, AlCu3, Mg32(Al Zn)49, Mg2Si and MgZn2 phases. We have not found the P-containing compounds due to it is too low in quantity. Fig. 2 shows the SEM image of the as-cast alloys. As show in Fig. 2a, Chinese script shaped Mg2Si phase formed in the microstructure of the alloy without P element. The addition of 0.5wt% P element and 12wt% Si can make their morphologies change from Chinese script shape to block shape. As show Fig. 2b and Fig. 2c.

While for the alloy by adding 3wt% Si and 0.5wt% P, two kinds of morphologies, i.e., Chinese script shape and block shape, for Mg2Si phase appaear in the microstructure. The average size of the primary Mg2Si phases in both the modified alloy and the unmodified one are 23 μm and 85 μm respectively. Therefore, the addition of P can also increase the nucleation rate of primary Mg2Si phase.

1 Mg-7Al-1Zn-1Si-3.597Cu

AlCu3 Mg2Si MgZn2

▼ ▼

★■

●▼◆ ★ ■ ●

●

●

●▼ ◆■ ●

I（a.u.）

◆★

4 Mg-7Al-1Zn-3Si-0.5P-3.597Cu

α-Mg

Mg32（AlZn）49

■ ★

3 Mg-7Al-1Zn-2Si-0.5P-3.597Cu

◆

2 Mg-7Al-1Zn-1Si-0.5P-3.597Cu

4 3 2 1 0

20

40

60

80

100

2（°）

FIG.1 XRD PATTERNS OF AS-CAST EXPERIMENTAL ALLOYS

a

b

c

d

FIG.2 MICROSTRUCTURES OF THE ALLOYS (A) ALLOY 1; (B) ALLOY 2; (C) ALLOY3; (D) ALLOY 4 - 24 http://www.ivypub.org/rms

3.2

EDS analysis mapping of Mg-7Al-1Zn-1Si-0.5P-3.597Cu alloy

We choose a typical alloy to show the elements distribution both in the phases and the in the matrix. Fig. 3a shows the

SEM image of Mg2Si phase in the matrix of Mg-7Al-1Zn-1Si-0.5P-3.597Cu alloy. While Figs.3b-e show element mapping on the Mg2Si phase. The results of the map scan on the Mg2Si phase of the alloy show that P atoms dissolve in Mg2Si matrix, Al atoms which congregate in the core of Mg2Si phase. It suggests that Al-P compound exist in the core of the Mg2Si phase. It suggests that Al-P compound exist in the core of the Mg2Si phase.

(a)

(b)

(c)

(d)

(e)

FIG.3 SEM IMAGES OF AS-CAST MG-7AL-1ZN-1SI-0.5P-3.597CU ALLOY WITH WHITE NUCLEUS (A) AS WELL AS MAGNESIUM (B), SILICON (C), ALUMINUM (D), AND PHOSPHORUS (E) DISTRIBUTION IN MG2SI PHASE

3.3

Mechanical properties

Tensile properties including ultimate tensile strength (UTS), yield strength (YS) and longation (Îľ). The alloys are tested and the results are listed in Table 2. The alloy with addition of P 0.5 wt% and 1wt% Si shows the best properties, the UTS, YS and Îľ are 192.91 MPa, 115.42 and 4.6 %, resppectively. This mainly because P addition causes Mg2Si morphologies change from Chinese script shape to block shape. With the increasing of the Si content, the ultimate tensile strength yield strength and elongation decrease. When the alloy containing 3wt% Si, it show the poorest properties due to different type of Mg2Si formation. The fracture surfaces shown in Fig. 4 indicate that only the fractograph for alloys2 (Fig.4b) present deeper caves than others, which is coincident with mechanical peoperties of the alloy. While fractograph for alloy1, a number of cleavage planes and steps indicates that the tensile fracture surface of the experimental alloys exhibit mixed characteristics of cleavage and quasi-cleavage fractures. With the Si addition up to 1wt%, the degree of deep for the caves decrease, therefore the mechanical properties is also decreased.

- 25 http://www.ivypub.org/rms

a

b

c

d

FIG. 4 TENSILE FRACTURE MORPHOLOGY OF ALLOY (A) ALLOY 1; (B) ALLOY 2; (C) ALLOY3; (D) ALLOY 4

4 CONCLUSIONS (1) The Mg-7Al-1Zn-1Si-0.5P-3.597Cu alloy with 0.5wt% P addition, the Mg2Si phase morphologies change from Chinese script shape to block shapes, compareing with the Mg-7Al-1Zn-1Si-3.597Cu alloy without P. (2) Al-P Compound distribution in the core of the blok shapes Mg2Si phase as the nucleation of the primary Mg2Si, it indicates that the Al-P compounds has excellent modifying and refining effect on the Mg2Si phase.

(3) The alloy with addition of 0.5 wt% P and 1wt% Si shows the best Mechanical properties contrasted with the alloy without P addition. While further increase Si content from 1wt%, the ultimate tensile strength, yield strength and elongation decrease.

ACKNOWLEDGMENTS This work was financially supported by the Technology Tackling Project of Shenyang City (F10-063-2-00), and Technology Tackling Project of Liaoning Province (2010221005).

REFERENCES [1]

Dahle A. K., Lee Y. C., Nave M. D., Schaffer P. L., St John D. H. Development of the as-cast microstructure in magnesium– aluminum alloys [J]. Light Metals 2001; 1: 61-72

[2]

Kim J. M., Shin K., Kim K. T., Jung W. J. Microstructure and mechanical properties of a thixocast Mg Cu–Y alloy [J]. Scr Mater 2003; 49: 687-91

[3]

LUO A., PEKGULERYUZ M. Z. Review: cast magnesium alloys for elevated temperature applications [J]. Mater Sci, 1994, 29: 5259-5271

[4]

DARGUSCH M. S., BOWLES A. L., PETTERSEN K., BAKKE P., DUNLOP G. L. The effect of silicon content on the microstructure and creep behavior in die-cast magnesium AS alloys [J]. Metal Mater Trans A, 2004, 35: 1905-1909

[5]

YOO M. S., SHIN K. S., KIM N. J. Effect of Mg2Si particles on the elevated temperature tensile properties of squeeze-cast Mg-Al alloys [J]. Metal Mater Trans A, 2004, 35: 1629-1632

[6]

LU Y. Z., WANG Q. D., ZENG X. Q., DING W. J., ZHU Y. P properties, and fracture behaviour of Mg-6Al alloy [J]. Mater Sci Technol, 2001, 17: 207-214

[7]

YANG Ming-bo, PAN Fu-sheng, BAI Liang, TANG Li-wen. Sb modification on the heat-treated microstructure and mechanical - 26 http://www.ivypub.org/rms

properties of Mg-6A1-1Zn-0.7Si magnesium alloy [J]. The Chinese Journal of Nonferrous Metals, 2007, 17(12): 2010-2016 [8]

JIANG Q. C., WANG H. Y., WANG Y., MA B. X., WANG J. G. Modification of Mg2Si in Mg-Si alloys with yttrium [J]. Mater Sci Eng A, 2005, 392: 130-135

[9]

LU Y. Z., WANG Q. D., ZENG X. Q., ZHU Y. P., DING W. J. Behavior of Mg-6Al-xSi alloys during solution heat treatment at 420 [J]. Mater Sci Eng A, 2001, 301: 255-258

[10] BARBAGALLO S. Microstructural evolution of AS21X HPDC alloy during thermal treatment [J]. Inter J Cast Metals Research, 2004, 17(6): 370-375 [11] SRINIVASAN A., PILLAI U T. S., PAI B. C. Microstructure and mechanical properties of Si and Sb added AZ91 magnesium alloy [J]. Metall Mater Trans A, 2005, 36: 2235-2243 [12] QUIMBY P. D., LU S. Z., PLICHTA R., VISSER D. K., JACOBE K. P. Effects of minor addition and cooling rate on the microstructure of cast Mg-Si alloys[C] gnesium technology. San Antonio, Texas, USA: TMS, 2006: 535-538 [13] KIM J. J., KIM D. H., SHIN K. S., KIM N. J. Modification of Mg2Si morphology in squeeze cast Mg-Al-Zn-Si alloys by Ca or P addition [J]. Scripta Mater, 1999, 41: 333-340

AUTHORS Keqiang Qiu (1962-), male, born in Jinzhou, Liaoning Province.

Xiaocheng Wang (1987-), male, born in Yingkou, Liaoning

Professor, Ph.D. supervisor, doing research on amorphous alloys

Province. Email: wangxiaocheng8@126.com

and casting magnesium alloys.

Email: kqqiu@yahoo.com.cn

Junhua You (1980-), male, born in Jinzhou, Liaoning Province, PHD. Email: youjunhua168@163.com

- 27 http://www.ivypub.org/rms