Effect of neodymium on the as-extruded ZK20 magnesium alloy

JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010, p. 631

Effect of neodymium on the as-extruded ZK20 magnesium alloy ZHAO Yazhong (䍉Ѯᖴ)1,2, PAN Fusheng (┬໡⫳)1,3, PENG Jian (ᕁ ᓎ)1,3, WANG Weiqing (⥟㓈䴦)1, LUO Suqin (㔫㋴⨈)1 (1. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China; 2. Department of Mechanic and Electronic Engineering, Nanyang Institute of Technology, Henan 473004, China; 3. National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China) Received 8 June 2009; revised 15 March 2010

Abstract: The effect of Nd addition on the microstructure and mechanical properties of ZK20 magnesium alloy was investigated by room tensile test, optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to develop a magnesium alloy with higher ductility. Results showed that the crystal grains of as-extruded ZK20+0.5%Nd magnesium alloy were effectively refined, and the alloy exhibited higher strength and ductility, with the UTS of 237 MPa and the elongation of 32.8%, increasing by 5% and 50% for those of ZK20 alloy respectively. Some fine granular Mg-Zn-Nd ternary phases (IJ1, IJ4) appeared in the as-extruded ZK20+ 0.5% Nd magnesium alloy. It was found that ZK20 showed brittle fracture surface, while the fracture surface of ZK20+0.5%Nd showed a mixed characteristics of brittleness and ductility. Keywords: ZK20 magnesium alloy; neodymium; extrusion; microstructure; mechanical properties; rare earths

Magnesium alloys are attractive materials due to their low density, high specific strength, good machineability and recyclability, etc. They are applied mainly for weight savings, such as on automobiles, mobile phones, aerospace components, and computers[1,2]. However, magnesium alloys have low ductility compared with aluminum alloys because of their hexagonal close packed crystal structure which has limited slip systems[3]. They have been fabricated into mechanical parts mainly by die casting even in the case of suitability for deformation shaping where wrought magnesium alloys show better mechanical properties than cast alloys. Therefore, it is very important to develop new magnesium alloy with perfect ductility to meet the needs of their widespread applications in industries. The rare earth elements have been applied in magnesium alloys for many years. The solidification behavior, microstructure, mechanical properties of Mg alloys with rare earth addition were studied mainly for increasing the strength and improving the high temperature stability[4,5]. But the effects of Nd addition in magnesium alloys was studied less, and the purpose of increasing alloy’s ductility and plastic deformation ability with Nd addition was not mentioned[6–9]. The influence of Nd addition on microstructure and mechanical properties of ZK20 alloy was investigated in the present work .

1 Experimental The chemical compositions of the sample alloys are listed in Table 1. The alloys were melted in a 60 kW well type

Table 1 Chemical compositions of studied alloys (wt.%) Alloys

Zn

Zr

Nd

Si

Fe

Mn

Mg

ZK20

2.11

0.021

0.00

0.0063

0.0046

0.011

Bal.

ZK20+0.1Nd

2.05

0.023

0.09

0.0062

0.0038

0.017

Bal.

ZK20+0.3Nd

2.07

0.021

0.28

0.0061

0.0020

0.026

Bal.

ZK20+0.5Nd

2.06

0.062

0.45

0.0065

0.0018

0.033

Bal.

ZK20+0.7Nd

1.98

0.045

0.66

0.0058

0.0016

0.031

Bal.

electric resistance furnace under the protection of 5# flux and the mixed gas of 0.5%SF6+99.5%CO2. Metallic Mg (99.98%), metallic Zn (99.7%), master alloy Mg-20%Nd and master alloy Mg-31%Zr were used as raw materials. Zn and Mg-20%Nd were put in the stove at 730 ºC, while Mg31%Zr at 760 ºC. The alloy melts were cast into ĭ92 ingots at 720 ºC at semi-continuous cast equipment. The ingots were homogenized with a heat treatment of 420 ºC×12 h, and then were hot extruded into rods of 16 mm in diameter with a reduction ratio of 28:1. The extrusion speed was 58–92 mm/s, and the temperature of the extrusion container and die was 390 ºC. The microstructure of as-cast and as-extruded alloys were examined with an optical microscope and scanning electron microscope (XL30-TMP) equipped with an energy dispersive X-ray spectrometer. The tensile samples were 40 mm in gauge and 8.0 mm in gauge diameter. All the tests were carried out at an initial strain rate of 4 mm/min.

2 Results and analysis

Foundation item: Project supported by the Major State Basic Reasearch and Development Program of China (973) (2007CB613700), National Natural Science Foundation of China (50725413) and National Key Technologies R&D Program of China (2007BAQ00134-04) Corresponding author: ZHAO Yazhong (E-mail: [email protected]; Tel.: +86-23-65112291) DOI: 10.1016/S1002-0721(09)60169-1

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2.1 Microstructure of the as-extruded alloys

2.2 Mechanical properties of the as-extruded alloys

Table 2 shows the optical micrographs and SEM images of the as-extruded alloys. With the increase of Nd addition from 0 to 0.5 wt.%, the average grain size of as-extruded alloys gradually decreased. The average grain size is smaller when 0.5%Nd is added to ZK20 alloy, and then the grain size becomes larger when 0.7%Nd is added. As-extruded ZK20 alloy is mainly composed of Į-Mg. The element Zn dissolved into the Į-Mg matrix. With the increase of Nd addition, part of Zn, Nd and Mg form second phase particles dispersed in the as-extruded alloys, and the particles become more. The amount of particles becomes larger in ZK20+0.5Nd alloy and ZK20+0.7Nd alloy.

The tensile properties of as-extruded alloys at room temperature are shown in Fig. 1. With the increase of Nd addition from 0 to 0.5 wt.%, the YS (yield strength) and UTS (ultimate tensile strength) of alloys increase, while they decrease slightly with 0.7% Nd addition. The higher values of the YS (152.0 MPa) and the UTS(237.4 MPa) were obtained in ZK20+0.5Nd alloy, which increased by 5% and 10%, respectively than those of ZK20. According to the Hall-Petch equation ıy=ı0+Ky×d(–1/2)[10], the increase of strength were mainly caused by the grain refining. The elongation of the alloy with Nd addition increases obviously compared with ZK20, as shown in Fig. 2. The elongation of alloy increases with the increase of Nd addition from 0.1 wt.% to 0.5 wt.%, but it decreases slightly with 0.7% Nd addition. The maximum value of elongation is 32.8% of the ZK20+0.5Nd alloy, increasing by 50% compared with the alloy without Nd addition.

Table 2 Optical and SEM microstructures of the as-extruded alloys Alloys

Optical micrographs

SEM images

2.3 Fracture surface of tensile specimens ZK20

ZK20 +0.1Nd

The fracture surface of as-extruded alloys is composed of tear ridges, smaller cleavage planes and some dimples, as shown in Fig. 3. With the increase of Nd addition, the fracture structure becomes much finer, while the size of cleavage planes becomes smaller and the dimple increases in number[11]. The alloys with Nd addition exhibit mixed characteristics of brittleness and ductility. As shown in Fig. 3(d), there are large numbers of dimples and a few tear ridges, so the ZK20+0.5Nd alloy has better ductility than the others.

ZK20 +0.3Nd

Fig. 1 Effects of different Nd additions on mechanical properties of as-extruded alloys ZK20 +0.5Nd

ZK20 +0.7Nd

Fig. 2 Stress-strain curves of as-extruded alloys

ZHAO Yazhong et al., Effect of neodymium on the as-extruded ZK20 magnesium alloy

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Fig. 3 Fracture surface of tensile specimens (a) ZK20; (b) ZK20+0.1Nd; (c) ZK20+0.3Nd; (d) ZK20+0.5Nd; (e) ZK20+0.7Nd

The necking becomes clearer at failed tensile specimens with Nd addition. A relatively significant yield steps come into being on the stress-strain curve with Nd addition. All those show the characteristics of ductile fracture.

3 Discussion 3.1 The alloy phases The composition of studied alloys in isothermal section of Mg-Nd-Zn ternary phase diagram[12] at 300 ºC is shown at location A in Fig. 4.

Fig. 4 Mg-Nd-Zn phase-diagram at 300 ºC

According to the ternary phase diagram, the microstructure of ZK20, ZK20+0.1Nd, ZK20+0.3Nd alloys is composed of Į-Mg without any other alloy phases containing Nd under the equilibrium solidification condition. But IJ4 (Nd3Mg6Zn11) appears in ZK20+0.5Nd and ZK20+0.7Nd alloys. Under semi-continuous casting condition, non-equilibrium solidification microstructure is likely to appear owing to the higher cooling speed. Therefore, other second phase still can be found in as-cast alloys. The XRD spectrum and EDX results of as-cast alloys are used for analysis the ternary phases. Fig. 5 shows the XRD spectrum of as-cast alloy. The as-cast ZK20 alloy is composed of D-Mg and granular MgZn phase. But the as-extruded ZK20 alloy does not show any alloy phase due to Mg-Zn phase dissolved into the matrix. The as-cast ZK20+0.5Nd alloy is composed of D-Mg and broken network Mg-Zn-Nd (IJ1, IJ4) ternary phase. Network Mg-Zn-Nd (IJ4) ternary phase appears in as-cast ZK20+

 Fig. 5 XRD patterns of ZK20 (a) and ZK20+0.5Nd (b)

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0.7Nd alloy, but the ternary phase becomes particles with the size of 1–3 Pm in the as-extruded alloy. The results of EDX show that the phase is Mg-Zn-Nd ternary phase in ZK20+0.5Nd and ZK20+0.7Nd alloys, as shown in Fig. 6. According to the atom ratio of Zn to Nd and the Mg-Zn-Nd ternary phase diagram, the as-cast ZK20+0.7Nd alloy contains phase IJ4 (Nd3Mg6Zn11), while the as-cast ZK20+0.5Nd alloy could contain phase IJ4 (Nd3Mg6Zn11) and IJ1 (Nd6Mg7Zn12). 1.85% Zn dissolved into D-Mg matrix of as-cast ZK20 alloy, but 1.21% Zn in ZK20+0.5Nd alloy, 1.02% Zn in ZK20+0.7Nd alloy. The Zn amount in D-Mg solid solutions decreased with the increase of Nd addition because of the formation of Mg-Zn-Nd ternary phase. 3.2 Mechanical properties The average grain size of as-extruded ZK20 alloy is 24 ȝm,

while that of ZK20+0.5Nd alloy is 4.6 ȝm, as shown in Fig. 7. The elongation and UTS of the alloys increase with the refinement of the grains and the best values were obtained when 0.5% Nd was added. A large part of Nd is dissolved into Į-Mg matrix after homogenized. With the increase of Nd addition, the nucleation rate of the alloy increased during the dynamic recrystallization, which could result in the grain refinement. Further more, 1 at.% Nd in the a-Mg matrix can cause an axial ratio c/a decrease of 0.34%[13], which is helpful to the activity of non-basal slip systems during deformation. In polycrystalline Mg, grain refinement can cause more grain boundary slipping (GBS)[14] and nonbasal slipping near grain boundary. According to Koike’s study[15], basal slip causes strain incompatibility at grain boundaries. This incompatibility stress can give rise to the activation of nonbasal dislocations as well as twins. So grain refinement has remarkable effect on increasing ductility of alloys with Nd addition. The grain size of the as-extruded alloys gradually decreased with the increase of Nd addition, which is helpful to increase alloy strength. Mg-Zn-Nd phases in as-cast ZK20+0.7Nd alloy did not completely dissolve into the D-Mg matrix at homogenized treatment because of the restriction of holding time or solid solubility, as shown in Fig.8. During extrusion deformation process, network phases caused non-uniformity of stress. An inhomogeneous dynamic recrystallization generated by the excessive non-uniformity stress, more chance is thus given of the growing of refine grain formed in earlier recrystallization. Therefore, coarser grain was generated for as-extruded

Fig. 7 Relationship between grain size and alloy mechanical properties

Fig. 6 EDX analysis of as-cast alloys (a) ZK20; (b) ZK20+0.5Nd; (c) ZK20+0.7Nd

Fig. 8 Ternary phases after homogenized treatment (a) ZK20+0.5Nd; (b) ZK20+0.7Nd

ZHAO Yazhong et al., Effect of neodymium on the as-extruded ZK20 magnesium alloy

ZK20+0.7Nd alloy than that for the ZK20 +0.5Nd alloy.

4 Conclusions (1) Nd addition was effective on refining crystal grains of as-extruded ZK20 alloy, and the 0.5%Nd addition had a predominant effect. (2) Better UTS and elongation were obtained in the as-extruded ZK20+0.5Nd alloy. The UTS was up to 237 MPa, while the elongation was up to 32.7%, increasing by 5% and 50% respectively on those of ZK20. Nd addition imposed an indistinctive effect on increasing strength, but a remarkable effect on increasing ductility. (3) More characteristics of ductility fracture were shown in the fracture surface of as-extruded ZK20+0.5Nd alloy with a large number of dimples and a few tear ridges, embodying that the alloy has better ductility. (4) The Mg-Zn-Nd (IJ4) ternary phase network appeared in the as-cast ZK20+0.7Nd alloy, which could not be dissolved into the D-Mg matrix completely during homogenized treatment. It could be the main reason that the grain of as-extruded ZK20+0.7Nd alloy was coarser than the ZK20+ 0.5Nd alloy.

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