Solidification behavior, microstructure and tensile properties of ZK60-Er magnesium alloys

JOURNAL OF RARE EARTHS, Vol. 29, No. 6, Jun. 2011, P. 558

Solidification behavior, microstructure and tensile properties of ZK60-Er magnesium alloys WANG Zhongjun (⥟ᖴ‫)ݯ‬, XU Yang (ᕤ䰇), WANG Zhaojing (⥟䆣䴭), CHENG Jia (⿟Շ), KANG Baohua (ᒋᅱढ), ZHU Jing (ᴅ᱊) (School of Materials and Metallurgy, University of Science and Technology Liaoning, Liaoning 114051, China) Received 1 November 2010; revised 31 January 2011

Abstract: ZK60-Er (erbium) alloys were made by melting ZK60 and Mg-Er magnesium alloys (20 wt.% Er) in an electric resistance furnace. The contents of Er were 0, 0.5, 1, 2, 3 wt.%, respectively. The influence of Er on solidification behavior, microstructure, corrosion resistant and mechanical properties of ZK60 magnesium alloy was studied. The results showed that long rod-like Ȗ phase (ErZn5) formed during solidification increased with increasing Er content in the range investigated, which resulted in the decrease of the amount of galvanic couplings between phase particles and alloy matrix and the marked improvement of corrosion resistant. It was also found that elongation of the alloys decreased with increasing Er content, but tensile strength of the alloys were improved by the addition of Er due to the strengthening effect of Ȗ phases distributing along grain boundaries. Keywords: ZK60; erbium; corrosion resistance; microstructure; mechanical properties; rare earths

As the lightest metallic structure materials, magnesium alloys offer many advantageous properties including low density (<2 g/cm3), high strength/weight ratio, high specific stiffness, superior damping and magnetic shielding capacities. Therefore, magnesium alloys are attracting great interest from material researchers[1,2]. ZK60 is a typically high strength magnesium alloy. But it is easy to form shrinkages and micro-segregation during casting due to its large solidification range[3,4]. Micro-segregation phases are nubble-like and discontinuously distribute along interdendritic boundaries, which increases the amount of galvanic couplings between interdendritic phase and alloy matrix, deteriorates corrosion resistant and severely limits the application of the alloy[3,5]. Some researchers have found that the addition of Y or the addition of a mixture of Y and other rare earth (RE) in ZK60 magnesium alloy can improve casting characteristics and increase higher temperature strength by elevating the eutectic temperature, markedly decrease micro-shrinkages, avoid hot cracking and delay the onset of over-ageing[4,6]. Unluckily, ZK60-Y and ZK60-Y-RE alloys are extremely difficult to smelt, and the cost of production is much higher than that of other magnesium alloys. Recently, Rosalbino et al.[7] have found that Mg-Al-Er system alloys exhibit good corrosion resistance, which was ascribed to the incorporation of Er atom solute in the hexagonal Mg(OH)2 lattice on the corrosion surface of magnesium alloys via substitution of the magnesium cations, to increase its volume ratio with respect

to the underlying bulk alloy. But the effect of heavy rare earth element Er on microstructure and properties of ZK60 magnesium alloy has not been found, so far. With the increasing application of Er for advanced materials and light metals such as aluminium alloys, etc., the cost of Er has markedly decreased[8,9]. Thus the research about influence of Er addition on microstructure, corrosion resistant and mechanical properties of magnesium alloys are of great commercial and scientific interests.

1 Experimental In the present experiments, ZK60 was chosen as the master alloy. Er additions were made in the form of Mg-20Er alloy (containing 20 wt.%Er). The ZK60-Er alloys were made by melting ZK60 alloy in an electric resistance furnace being flushed continuously with SF6 and CO2 gas at about 720 °C, and then Er was added. The melt was held at that temperature for about 15 min to ensure that the Er addition was completely dissolved. After that, the melt was poured into metal mould and the ZK60-Er alloy bars were made. The chemical compositions of ZK60-Er alloys were studied with an inductively coupled plasma spectrum machine (ICP, IRIS Advantage 1000) produced by the Thermo Jarrell Ash Company. Chemical compositions of the studied alloys are shown in Table 1.

Foundation item: Projected supported by the University Science Study Project (2008329) of Liaoning Province Education Ministry, Science & Technology Project (2008SF54) of Anshan City, the College Student Science Study Project of University of Science and Technology Liaoning (No.6), and also partly supported by the Open-end Funds of Key Lab of Material Forming and Microstructure & Properties Control of Liaoning Province of China (200909) Corresponding author: WANG Zhongjun (E-mail: [email protected]; Tel.: +86-412-5929535) DOI: 10.1016/S1002-0721(10)60497-8

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Table 1 Chemical compositions of the studied alloys Nominal alloy

Compositions/wt.% Mg

Zn

Zr

Er

ZK60

Bulk

5.82

0.27

–

ZK60-0.5Er

Bulk

5.86

0.26

0.39

ZK60-1.0Er

Bulk

5.81

0.29

0.88

ZK60-2.0Er

Bulk

5.80

0.31

1.76

ZK60-3.0Er

Bulk

5.82

0.30

2.80

Differential thermal analysis (DTA, SHIMADZU, DT-30B) was carried out on the alloys while solidifying at a cooling rate of 5.0 °C/s from 680 to 200 °C. Corrosion test specimens were machined into plates of 20 mm in diameter and 2 mm in thickness. Then they were abraded successively with 2000# emery paper. The specimens were washed in ethanol and dried by flowing air. Then the total surface area (SA in cm2) and the weight (W0 in mg) of the specimens were measured. Following that, the specimens were immersed in a 5 wt.% NaCl aqueous solution for 2 d. All tests were conducted at room temperature. At the end of the test, the corroded specimens were brushed and then cleaned by acetone and ethanol. Subsequently, the specimens were dried by warm air. The measured weight of the corroded specimen (WC in mg) was used to calculate the weight loss WL, WL=W0–WC. So, corrosion rate can be characterized by WL/ (SA·day). Tensile specimens were machined into a gauge length of 25 mm, and 5 mm in diameter. Tensile testing was performed in a SHIMADZU AG-100KNA materials testing machine. As for each alloy at least five bars were tested and the average value was calculated. The morphologies of phases were examined by a scanning electron microscope (SEM, SHIMADZU SSX-550). Metallographs were observed by an optical microscopy (OM, LEICA-DMR). Composition of phase was analyzed using an energy dispersive spectroscope (EDS) attached to the SEM. Phases of the alloys were analyzed by X-ray diffraction (XRD, D/max 2400).

2 Results and discussion Examination of the DTA curves, two examples of which are shown in Fig. 1, shows that formation of Į-Mg primary dendrites began at 622, 621, 620, 619 and 618 °C in alloys ZK60, ZK60-0.5Er, ZK60-1.0Er, ZK60-2.0Er and ZK603.0Er, respectively. The formation of Į-Mg primary dendrites was ahead with increasing the Er content in the alloys,

Fig. 1 DTA curves of ZK60 and ZK60-3Er alloys Table 2 Location of peak for different alloys by the results of DTA tests Alloy

Peak 1 (0C)

Peak 2 (0C)

Peak 3 (0C)

Peak 4 (0C)

ZK60

618

416

326

–

ZK60-0.5Er

619

510

416

326

ZK60-1.0Er

620

510

416

326

ZK60-2.0Er

622

510

–

ZK60-3.0Er

622

510

–

which resulted in microstructure refined. According to Mg-Zn binary phase diagram[10], the peak at 416 °C (Fig. 1) should be corresponding to the eutectic reaction or formation of Mg51Zn20 phase, which was the interdendritic compound. When the temperature was decreased to 326 °C (Fig. 1) during solidification, Mg51Zn20 decomposed, and MgZn2 phase formed[11]. As shown in Table 2, a peak occurred at 510 °C due to Er addition, and it can be assumed that this peak corresponded to interdendritic eutectic solidification. When the Er content was more than 1.0 wt.%, the peaks of Mg51Zn20 and MgZn2 both disappeared. Fig. 1 also indicates that Er additions markedly increased the latent heat of solidification, decreased temperature gradient of interface between metal liquid and primary Į-Mg, and decreased micro-segregation. What’s more, the addition of Er increased the volume of eutectic of the alloys, which is known to effectively improve castability[3]. The solidification microstructures of the cast alloys are shown in Fig. 2. Here it can be seen that there was only one kind of morphology of interdendritic compounds, MgZn2 phase, in ZK60 magnesium alloy (Fig. 2(a)), while two morphologies were present in ZK60-Er alloys, and the phase

Fig. 2 Microstructures of ZK60-Er alloys in as-cast condition (a) ZK60; (b) ZK60-0.5Er; (c) ZK60-1.0Er; (d) ZK60-3.0Er

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containing Er (tentatively named after Ȗ phase) substantially increased with increasing Er content (Fig. 2(b,c)). When the content of Er was 3.0 wt.%, the microstructure was markedly refined, and few of MgZn2 phase was found (Fig. 2(d)). The morphology of MgZn2 shown in Fig. 3(a) suggests that it is the result of a eutectic reaction, and there were a lot of holes in it. The composition of MgZn2 phase by the result of EDS was as follows: Mg 67.4 at.%, Zn 58.5 at.%, Zr 0.1 at.%, which also suggests that the areas of holes in it should be the matrix. Thus this kind of morphology had a lot of galvanic couplings between phase particles and magnesium matrix due to the extremely strong electro-negativity of magnesium (standard electric potential of magnesium is –2.363 V)[12], which can decrease the corrosion resistant. The Ȗ phase is shown in Fig. 3(b), which is assumed rod-like, and the composition of it by the result of EDS is as followes (at.%): Mg 0.2%, Zn 82.8%, Er 16.9%, Zr 0.1%. The atom ratio of Zn/ Er was 4.899. Fig. 4 shows the result of XRD of ZK60-3Er alloy, which suggests that the Ȗ phase should be ErZn5. The peak of MgZn2 phase does not appear for the alloy, and the reason for which may be the quantity of MgZn2 phase is too little to be checked by the method of XRD. Fig. 5 shows the corrosion rates of ZK60-Er alloys immersed in NaCl aqueous solution versus different Er contents. It can be seen that the corrosion rate markedly decreases as the content of Er increases. The corrosion rate of ZK60 alloy is about 8 times higher than that of ZK60-3Er alloy. The microstructure of ZK60 alloy consisted of Į grains with the grain boundaries decorated by discontinuous MgZn2

Fig. 3 SEM image of MgZn2 phase in ZK60 alloy (a) and ErZn5 phase in ZK60-1Er (b)

JOURNAL OF RARE EARTHS, Vol. 29, No. 6, Jun. 2011

Fig. 4 XRD result of ZK60-3Er alloy

Fig. 5 Dependency of corrosion rate on Er content

phase particles (Fig. 2(a)), and these phase particles with magnesium matrix formed a lot of galvanic couplings, which is the reason for the bad corrosion resistant of this alloy. However, MgZn2 phase particles markedly decreased and rod-like Ȗ phases increased and some of it connected with each other (Fig. 2(d)) with increasing Er content, which substantially impeded the corrosion behavior of magnesium matrix. The influence of Er addition on tensile properties of ZK60 is shown in Fig. 6. The results show that the ultimate and yield strength of ZK60-Er alloys were improved with increasing Er content in the range investigated, but the elongation markedly decreased due to Er addition at both room and elevated temperature. At room temperature, the ultimate and yield strength of ZK60-Er alloys can be improved from 236 and 108 MPa to 268 and 170 MPa, respectively. That is, the ultimate and yield strength can be increased by 13.6% and 57.4%, respectively. At 120 °C, the ultimate and yield strength of ZK60-Er alloys can be improved from 120 and 60 MPa to 192 and 126 MPa, respectively, that is, ultimate and yield strength can be increased by 60% and 110%, respectively. However, the elongation at room temperature and 120 °C decreased from 16.3% and 21.8% to 2.8% and 10%, respectively, that is, the elongation at room temperature and 120 °C decreased by 82.8% and 54%, respectively. Thus Er addition can lead the tensile properties of ZK60-Er alloys at elevated temperature to be more markedly improved than that at room temperature. As mentioned in Figs. 1 and 3, Ȗ phase

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loys at elevated temperature more markedly improved than that at room temperature. It was also found that the improvement of tensile properties of ZK60-Er alloys was primarily due to grain boundaries strengthening effect of Ȗ phase on alloy matrix. The elongation of the alloys decreased with increasing Er content due to the increased amount of Ȗ phase along grain boundaries.

References:

Fig. 6 Tensile properties of ZK60-3Er alloys (a) At room temperature; (b) at 150 °C

formed during solidification increased the eutectic temperature, subsequently enhanced the stability of phase particles distributed along primary Į grain boundaries, which is of the strengthening effect on magnesium alloy matrix, namely grain boundaries strengthening.

3 Conclusions The solidification behavior, microstructure, corrosion resistant and tensile properties of ZK60-Er magnesium alloys was studied. It was demonstrated that Er addition increased the latent heat of solidification and markedly improved castability of the alloys. The long rod-like Ȗ phase (ErZn5) formed during solidification increased with increasing Er content in the range investigated, which resulted in the substantial improvement of corrosion resistant. Er addition could result in reduced grain size of the ZK60 alloy. The ultimate and yield strength increased at both room and elevated temperature with Er content in the range investigated. Er addition could lead the tensile properties of ZK60-Er al-

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