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Research of Materials Science  December 2012, Volume 1, Issue 1, PP.19‐24 

Effect of Heat Treatment on Mechanical Properties and High Temperature Properties of Extruded MgSnSiSr Alloy Keqiang Qiu 1#, Qiang Gou 1, Junhua You 1 1.School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China #Email: kqqiu@yahoo.com.cn

Abstract In order to give a suitable heat treatment technique for as-extruded Mg-5Sn-2Si-2Sr alloy, the microstructure and mechanical properties of the as-extruded alloy after T4, T5, T6 heat treatment were investigated by means of hardness tester, the X-ray diffraction, the tensile tester and the optical microscopy. The results show that the T5 treatment is appropriate for the as-extruded Mg-5Sn-2Si-2Sr alloy because a large amount of precipitated phases are observed on the grain boundary. The yield and tensile strengths for the alloy treated by T5 technique are 210.9 MPa and 257.0 MPa, respectively, which is higher than the alloy treated by other technique. The mechanical property is decreased after T4 treatment because the effect of softening annealing is much stronger than solid solution strengthening. The mechanical properties can not be improved after T6 treatment due to the increase of the second phase size and the grain growth. Keywords:Magnesium alloys; hot extrusion; solid solution; aging; tensile properties; high temperature properties

I.

INTRODUCTION

Magnesium alloy is the lightest metallic materials can be applied to the current industry. It will be widely used in the field of automotive, electronics, aerospace and national military because of its high specific strength and stiffness, good damping capacity and noise reduction, recyclable and no polluted to environment[1-2]. However, conventional magnesium alloy is limited to use in heat-resistant parts by its low temperature resistance, not ideal corrosion resistance and bad plastic forming. It is shown that Si, Sn, and Sr is useful to improve the heat resistance of magnesium alloy. Si and Mg generate intermetallic compound Mg2Si phase, it has high melting point (1085 ℃), high hardness(460HV), low density (1.93g/cm3), low thermal expansion coefficient (7.5×10-6/K)[3]. Sn and Mg generate Mg2Sn phase, it also has high melting point (771.5℃) and high hardness (119HV)[4]. Element Sr[5] and Ca[6] can refine Mg2Si phase. Compared with rare earth, alkaline earth is cheaper. A large amount of research has been done abroad on Mg-Si heat-resistant magnesium alloys. South Korea's Hyundai Motor Company and Japan's Ube Industrial Corporation has developed patent alloys of Mg-Al-Zn-Si-Ca and Mg-Al-Zn-Si-Sr, respectively[7]. Compared with casting magnesium alloy products, the comprehensive properties increase obviously because extruded magnesium alloy products (Extrusion, forging, rolling) eliminate casting defects and refine the grain in the process of plastic deformation[8-11]. In recent years, widely concern is attended about Mg-Sn heat-resistant magnesium alloys. This article design and discuss the properties and structure variation of alloy after solid solution (T4), aging (T5) and solid solution + aging (T6) heat treatment. It can provide a reference to the alloy application and heat treatment process. II.

EXPERIMENTAL

The as-extruded alloy with the nominal composition of Mg-5Sn-2Si-2Sr was prepared by purity of 99.9% melting Mg, Sn and Mg-30%Si, Mg-30% Sr master alloy. The alloys were melted in a crucible under the protection of N2 and SF6 gas mixture. Heat Mg ingot to 700℃ until completely melted and put in Sn, Mg-30% Si and Mg-30% Sr master alloy insulating 20min at 680℃, then stir and pour into mold after 10min. Analyzed by spectrum, the chemical composition of the ingot is shown in Table 1. TABLE 1 CHEMICAL COMPOSITION OF MG-5SN-2SI-2SR ALLOY (WT%) Sn

Si

Sr

Mg

5.01

1.98

1.97

Bal.

After sawing and cutting, it turns into a smooth cylindrical ingot, the size is d127 mm× 200 mm. The ingots extrude at 380℃ after annealing 24h at 415℃. Extrusion temperature is 400℃, extrusion ratio is 40: 1, extrusion speed is 1.4m/min. The extruded alloy bar use solid solution (T4), aging (T5) and solid solution + aging (T6) heat treatment. Heat treatment

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system refer to magnesium alloy phase diagram[12] which listed in Tab.2 TABLE 2 HEAT TREATMENT TECHNIQUE Technique

Solution temperature/ ℃

Solution time/ h

Aging temperature/ ℃

Aging time/ h

T4 T5 T6

415 415

24 24

200 200

12 12

The HB-3000B Digital Brinell hardness tester was used to measure the hardness of the alloy specimens in the various states. The load is 62.5N loading 30s. Four hardness test values were prepared for each sample and then averaged. The XRD-7000 X-ray diffractometer was used to analyze phase. Oxalic acid and nitric acid were used to corrode metallographic specimen and then observe microstructure by using Neophot-21 microscope. Tensile test was conducted by using WDW-100 electronic testing machine. At least three samples were prepared for each component and then averaged. The S-3400N scanning electron microscope was used to analyze the fracture. III.RESULTS AND DISCUSSION  A. Microstructure and phase analysis of extruded Mg-5Sn-2Si-2Sr alloy Fig.1 shows the microstructure of extruded Mg-5Sn-2Si-2Sr alloy under different states. It can be seen that dynamic recrystallization occurs in test alloys after hot extrusion. That is because extrusion deformation occurs in high temperature and it is easy to produce dynamic recrystallization (DRX) in hot deformation during high temperature deformation of magnesium alloy due to the low stacking fault energy of magnesium. The dynamic recrystallization grains are mixed between the grains which deformed seriously. Generally, their size is small. Extrusion deformation can refine its grain because the microstructure occurs dynamic recrystallization[13]. As it is shown in Fig.1a and Fig.1b, the gain size increases obviously, Mg2Si phase and Mg2Sn phase are solid dissolved to the matrix gradually after T4 treatment. After T5 treatment, the grain size decreases, a large amount of the Mg2Si phases precipitated at the grain boundaries as it is shown in Fig.1c. Compared with T5 state, the gain size increased a little after T6 treatment, there are also some Mg2Si phases precipitated at the grain boundaries as it is shown in Fig.1d. The grain growth reduces the strength of the alloy, which is the main reason for the low strength of T4 and T6 state alloy. The cleavage plane size increases due to the growth of the grain size which decreases ductility of the alloy. b

a

20μm

20μm

c

d

20μm

20μm

FIG. 1 MICROSTRUCTURES OF AS‐EXTRUDED MG‐5SN‐2SI‐2SR ALLOY AS WELL AS UNDER DIFFERENT HEAT TREATMENT CONDITIONS  (A) AS‐EXTRUDED; (B) T4 TREATMENT; (C) T5 TREATMENT; (D) T6 TREATMENT

Fig.2 shows the XRD pattern of extruded Mg-5Sn-2Si-2Sr alloy. The graph shows that extruded Mg-5Sn-2Si-2Sr alloy is mainly composed of α-Mg matrix phase, Mg2Si phase, Mg2Sn phase and MgSnSr phase. B. Mechanical properties of extruded Mg-5Sn-2Si-2Sr alloy Brinell hardness of Mg-5Sn-2Si-2Sr alloy in different conditions is shown in Fig.3. It can be seen from the graph that the hardness of cast alloy is low and the hardness increase obviously after hot extrusion deformation. Compared with asextruded, the hardness of the alloy decline after T4 treatment. Two reasons make the hardness values decline. First, the alloy insulate in high temperature during the solution treatment, element Si and Sn are solid dissolved to the Mg matrix. It will not only have a solid solution strengthening effect, but also the same as annealing in high temperature. Second, the 20 http://www.ivypub.org/rms/


crystal structure of magnesium is close-packed hexagonal. Elements diffuse slowly in magnesium matrix cause annealing softening effect becoming much stronger than solid solution strengthening. Compared with as-extruded, the hardness values increase after T5 treatment.    

60

C --- As-cast E --- Extrusion

Brinell Hardness

50

   

40 30 20 10

0

C

E

T4

T5

T6

Conditions

FIG. 2 XRD PATTERN OF EXTRUDED MG‐5SN‐2SI‐2SR ALLOY 

This is because the directly aging of extruded alloy can not only keep the extrusion effect, but also produce dislocations, sub-boundary during the extrusion process and promote the second phase precipitated. Both Mg2Sn phase (119HV) and Mg2Si phase (460HV) have high hardness and it can improve the hardness of the alloy[14]. The hardness of the alloy reach the highest after T6 treatment. The higher temperature of T6 treatment makes the second phase fully dissolved into the matrix, precipitating more strengthening phase in the aging process and enhancing the effect of age hardening. As a result, the hardness of the alloy reach the highest after T6 treatment.   ◆

Mg-5Sn-2Si-2Sr Alloy As-extruded

□ ◆

 

Intensity (a.u.)

● □ ◆

□ ■

MgSnSr Mg2Sn

● □

 

a-Mg Mg2Si

■ ■

0

20

● ● ●

40

● ●

60

80

100

2deg

FIG.3 BRINELL HARDNESS OF MG‐5SN‐2SI‐2SR ALLOY IN DIFFERENT CONDITIONS 

The mechanical properties will be varied because of different heat treatment processes. As it is listed in Tab.3 and Tab.4, the alloy take T4 treatment for 24h at 415℃, element Si and Sn are solid dissolved to the Mg matrix, producing a solid solution strengthening effect and the same as annealing in high temperature, so mechanical properties of the alloy will decline. The mechanical properties reach the highest point after T5 treatment, the yield strength and tensile strength are 210.9MPa and 257.0MPa, respectively. Compared with T5 state alloy, the mechanical properties of T6 state decline and the ductility is equivalent to T5 state. The mechanical properties of T5 state is higher than T6 state related to the more intensive and smaller strengthening phase precipitating in the alloy after T5 treatment. The variation of elevated tensile properties in different conditions under 150℃ is the same as it is under the room temperature, but the mechanical properties decrease obviously and the ductility increase significantly. As a result, the Mg-5Sn-2Si-2Sr alloy after hot extrusion taking T5 treatment not only simplify heat treatment process, but also obtain good mechanical properties. It is an effective heat treatment system. C. Tensile fracture surface observation and analysis The crystal structure of magnesium is close-packed hexagonal, it has few slip systems and only occurs some very limited macroscopic deformation. Cleavage fracture is the most common fracture mode of magnesium alloy.

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TAB.3 ROOM TENSILE PROPERTIES OF EXTRUDED MG‐5SN‐2SI‐2SR ALLOY IN DIFFERENT CONDITIONS  Conditions 

σ0.2/MPa

σb/MPa

δ/%

Extrusion

202.3

240.6

9.0

T4

195.6

225.7

8.2

T5

210.9

257.0

12.5

T6

206.1

248.1

12.4

TAB.4 HOT TENSILE PROPERTIES OF EXTRUDED MG‐5SN‐2SI‐2SR ALLOY IN DIFFERENT CONDITIONS(150℃)  Conditions 

σ0.2/MPa

σb/MPa

δ/%

Extrusion

103.7

139.0

12.5

T4

101.0

133.6

9.9

T5

107.7

149.2

15.9

T6

105.1

144.0

16.7

Fig.4 shows the tensile fracture morphology of extruded Mg-5Sn-2Si-2Sr alloy in different conditions. It can be seen that the tensile fracture have mixed feature of cleavage and quasi cleavage. The fracture surface has some cleavage planes, cleavage steps and river patterns, there are also a few tearing ridges and few dimples exist. It can be seen from Fig.4a, Fig.4b and Fig.4c, the fracture morphology of as-extruded, T4 state and T6 state is similar. The fracture surface has a large number of tiny quasi-cleavage planes, a few tearing ridges and few dimples. However, fracture morphology of T5 state is quite different from others, it has many bright tearing ridges and obvious river patterns. The number of tiny quasi-cleavage plane decrease which indicates that T5 state alloy has good tensile strength.

a

b

c

d

                  FIG. 4 TENSILE FRACTURE MORPHOLOGY OF EXTRUDED MG‐5SN‐2SI‐2SR ALLOY AT ROOM TEMPERATURE  (A) AS‐EXTRUDED; (B) T4 TREATMENT; (C) T5 TREATMENT; (D) T6 TREATMENT 

In order to explore the heat resistance of magnesium alloys in this series, this article has also studied the elevated tensile properties of extruded Mg-5Sn-2Si-2Sr alloy. Fig.5 shows the tensile fracture morphology of extruded Mg-5Sn-2Si-2Sr alloy at elevated temperature in different conditions. It can be seen that the elevated fracture mode of extruded Mg-5Sn2Si-2Sr alloy at 150℃ is mixed fracture of brittle and ductile. The fracture surface has some cleavage planes, cleavage steps and river patterns. Compared with the fracture surface at room temperature, the fracture surface at elevated temperature has more dimples indicate that the ductility increase obviously at high temperature. It can be seen from Fig.5a and Fig.5b, the fracture morphology of as-extruded, T4 state is similar. The fracture surface has quasi-cleavage planes, tearing ridges and dimples. Fig.5c shows that the fracture surface of T5 state has many bright tearing ridges and the cleavage plane become smaller which indicate that T5 state alloy has good tensile strength. Fig.5d shows that the number of the dimple on the T6 state fracture surface increase obviously and there are also many bright tearing ridges exist which indicate that T6 state alloy has good ductility. 22 http://www.ivypub.org/rms/


a

b

c

d

FIG. 5 TENSILE FRACTURE MORPHOLOGY OF EXTRUDED MG‐5SN‐2SI‐2SR ALLOY AT HIGH TEMPERATURE(150℃)  (A) AS-EXTRUDED; (B) T4 TREATMENT; (C) T5 TREATMENT; (D) T6 TREATMENT 

IV.CONCLUSIONS (1) Extrusion deformation can refine the gain of Mg-5Sn-2Si-2Sr alloy grain because the microstructure occurs dynamic recrystallization. The increasing gain size of the T4 state alloy lead to a decline of the mechanical properties. The gain size of the T5 state alloy is small, it is also precipitated a large amount of strengthening Mg2Si phases and it has good mechanical properties. (2) The hardness of Mg-5Sn-2Si-2Sr alloy increases obviously after hot extrusion deformation. Compared with extruded state, the hardness of the alloy decreases after T4 treatment while the T5, T6 state hardness increases obviously. The room yield and tensile strengths for the alloy treated by T5 technique are 210.9MPa and 257.0MPa, respectively, which is higher the alloy treated by other technique. The variation of high tensile properties in different conditions under 150℃ is the same as it is under the room temperature, but the mechanical properties decrease obviously and the ductility increase significantly. (3) The better mechanical properties of Mg-5Sn-2Si-2Sr alloy are obtained for T5 treatment.

ACKNOWLEDGMENT 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).

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[8] YU Kun, LI Wen-xiang, WANG Ri-chu. Research, development and application of wrought magnesium alloys [J]. The Chinese Journal of Nonferrous Metals, 2003, 13(2): 277−288. [9] LU Zhi-wen, WANG Lin-yun, PAN Fu-sheng, CHEN Lin. Wrought Magnesium Alloys and Their Forming Processes [J]. Materials Review, 2004, 18(9): 39−42. [10] CHEN Zhen-hua. Wrought Magnesium Alloys [M]. Beijing: Chemical Industry Press, 2005. [11] ZHANG Ya, ZENG Xiao-qin, LIU Liu-fa, LU Chen, ZHOU Han-tao, LI Qiang, ZHU Yan-ping. Effects of yttrium on microstructure and mechanical properties of hot-extruded Mg-Zn-Y-Zr alloys[J]. Mater Sci Eng A, 2004, 373: 320−327. [12] LIU Chu-ming, ZHU Xiu-rong, ZHOU Hai-tao. Magnesium alloy phase diagram [M]. Changsha: Central South University Press, 2006. [13] ZENG Xiang-liang, LIU Chu-ming, JIANG Hui. Research on Microstructure and Mechanical Property of Mg-Cd-Nd-Zn-Zr Alloy[J]. Hot Working Technology, 2009, 38(16): 1-7. [14] YU Kun, LI Wen-xiang, WANG Ri-chu. Effects of heat treatment on microstructures and mechanical properties of ZK60 magnesium alloy [J].The Chinese Journal of Nonferrous Metals, 2007, 17(2): 188-192.

AUTHORS QIU Ke-qiang (1962-), male, born in Jinzhou, Liaoning Province. Professor, Ph.D. supervisor, doing research on amorphous alloys and casting magnesium alloys. E-mail: kqqiu@yahoo.com.cn

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Effect of Heat Treatment on Mechanical Properties and High Temperature Properties ……  

In order to give a suitable heat treatment technique for as-ext ruded Mg-5Sn-2Si-2Sr alloy, the microstructure and mechanical properties of...

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