Effect of erbium on microstructure and thermal compressing flow behavior of Mg-6Zn-0.5Zr alloy

WANG Zhong-jun(王忠军)^{1, 2}, ZHANG Cai-bei(张彩碚)¹, SHAO Xiao-hong(邵晓宏)¹,

CUI Jian-zhong(崔建忠)², LE Qi-chi(乐启炽)²

1. College of Science, Northeastern University, Shenyang 110004, China

2.The Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China

Received 28 July 2006; accepted 15 September 2006

Abstract: A series of thermal compressing tests of Mg-6Zn-0.5Zr and Mg-6Zn-0.5Zr-1Er alloys were performed on a Gleeble-1500D thermal simulator. The microstructures of thermal compressed Mg-6Zn-0.5Zr and Mg-6Zn-0.5Zr-1Er alloys were determined by optical microscopy, transmission electron microscopy and X-ray diffractometry. The results show that Mg-6Zn-0.5Zr alloy mainly consists of α-Mg and MgZn₂ phase, while Mg-6Zn-0.5Zr-1Er alloy comprises α-Mg phase, coarse Mg₃Zn₄Er₂ eutectic, rod-liked Mg₃Zn₄Er₂ precipitated phase, fine I phase particle (Mg₃Zn₆Er, icosahedral quasicrystal structure). The peak flow stress becomes larger with increasing strain rate and erbium addition at the same temperature, and gets smaller with increasing deformation temperature at the same strain rate. The deformation activation energy increases with increasing temperature, strain rate and erbium addition. In addition, it is observed that the growth of dynamic recrystallization (DRX) grains of Mg-6Zn-0.5Zr-1Er alloy was markedly suppressed due to the pinning effect of fine I phase and Mg₃Zn₄Er₂ phase during thermal compression.

Key words: Mg-6Zn-0.5Zr alloy; erbium; flow behavior; I phase; Mg₃Zn₄Er₂ phase

1 Introduction

Significant interest exits in the developing high strength magnesium alloys[1-3]. Some high strength magnesium alloys have been developed for applications at elevated temperature, for example, Mg-Zn-Y, Mg-Zn-Y-Zr, Mg-Zn-Zr-Nd and Mg-Ce-Zn-Zr system alloys[4-10]. Usually two kinds of phases containing rare earth (RE) were observed in these alloys after deformation. They are both Mg-Zn-RE ternary phase[11-15]. One of them is face-centered cubic (FCC) structure, another is icosahedral quasicrystal structure. These ternary phases can suppress the growth of dynamic recrystallization (DRX) grains formed during deformation of these alloys, which can result in refined microstructure. In addition, The disadvantage of the low eutectic temperature of a Mg-Zn and Mg-Zn-Zr system alloys (about 340 ℃) was also surmounted since a small amount of RE could increase the eutectic temperature significantly [16]. Recently, Mg-Gd-Y-Zr system alloys containing heavy rare earth (HRE) Gd was developed, which presented superior strength and creep resistance due to the strengthening effect of Mg-Gd-Y phase particles on Mg matrix[17-18]. However, other HRE elements, for example, erbium (Er) and holmium (Ho), etc, were seldom used in magnesium alloys[19]. And few researches for the deformation behavior of magnesium alloys containing HRE have been found so far. Here the HRE element of Er was selected as the additive element in the Mg-Zn-Zr-HRE system alloy, and the effect of Er on the microstructure and thermal compressing flow behavior of Mg-6Zn-0.5Zr magnesium alloy was investigated.

2 Experimental

The materials used in this study are Mg-6Zn-0.5Zr and Mg-6Zn-0.5Zr-1Er alloy. The alloys were melted in a mild steel crucible with the protection of a mixed gas of 1% SF₆ and 99% CO₂ in volume fraction. Cylindrical specimens of 15 mm in height and 8 mm in diameter were machined from the ingots. The samples were homogenized at 420 ℃ for 6 h and cooled in air, then thermally compressed by the Gleeble-1500D materials simulator with the strain rates of 1.0×10^-4, 1.0×10^-2 and 1.0 s^-1, respectively, and over a temperature range from 150 ℃ to 450 ℃.

Metallographic specimens were polished and etched with an etchant of 1 mL oxalic acid, 1 mL nitric acid and 98 mL water. Phase structure analyses were performed with an X-ray diffractometer(XRD, D/max 2400). Microstructures were observed with an optical microscope (OM, LEICA DMR) and a transmission electron microscope (TEM, TECNAI F30). For TEM observations, samples were sliced by a low speed diamond saw, thinned mechanically, and twin-jet electropolished in a methyl alcohol solution containing 15% nitric acid (in volume fraction), at -40 ℃ and 0.1 A, and finally ion milled to clear away the oxides on the surface of the specimen.

3 Results and discussion

3.1 Microstructure of alloys

Micrographs of Mg-6Zn-0.5Zr and Mg-6Zn-0.5Zr- 1Er two alloys after homogenization are shown in Fig.1. Small amounts of undissolved intermetallic compounds are distributed along dendrite boundaries in Mg-6Zn- 0.5Zr alloy (Fig.1(a)). However, lots of eutectic exist in Mg-6Zn-0.5Zr-1Er alloy, and the dendrites are refined to some degree due to Er addition (Fig.1(b)).

Fig.1 Micrographs of casting Mg-6Zn-0.5Zr alloy (a) and Mg-6Zn-0.5Zr-1.0Er alloy (b) after homogenized at 420 ℃ for 6 h and cooled in air

The TEM image of phase and its corresponding SAED pattern of thermally compressed Mg-6Zn-0.5Zr alloy (Fig.2) suggest that the intermetallic compound should be MgZn₂ (HCP structure) phase. The XRD results (Fig.3) show that the structure is not changed after deformation even at 450 ℃.

Fig.2 TEM image of intermetallic compound and its SAED pattern (area zone [010]) of Mg-6Zn-0.5Zr alloy compressed at 350 ℃ with strain rate of 1×10^-4 s^-1

Fig.3 XRD patterns of alloy under different conditions: (a) Before compression; (b) After compression at 450 ℃ with  strain rate of 1×10^-4 s^-1

However, as shown in Fig.4, some new phases were formed in thermally compressed Mg-6Zn- 0.5Zr-1Er alloy. Firstly, coarse and not fully destroyed eutectic distributed along original dendrite boundaries was observed. It has an FCC structure (Fig.4(a)), and the lattice constant is 0.7061 nm by calculation. Secondly, fine I phase (Mg₃Zn₆Er, icosahedral quasicrystal structure) particle was found near destroyed Mg₃Zn₄Er₂ phase, which has a representative quintic symmetry structure characterized by its SAED pattern (Fig.4(b)). Lastly, the fine and rod-liked Mg₃Zn₄Er₂ precipitated observed, which is parallel to each other or has an angle of 120? with each other due to its orientation relationship with matrix, which is [114]_Mg3Zn4Er2//[0 2 1]_α-Mg,  _Mg3Zn4Er2//(12)_α-Mg, as shown in Fig.4(c). In addition, we also can see the peaks of both I phase and Mg₃Zn₄Er₂ phase of the alloy containing Er before and after thermally compressed deformation below 450 ℃ in X-ray pattern (Fig.5, patterns (a) and (b)). But only the peak of Mg₃Zn₄Er₂ phase was seen when deformation temperature increased to 450 ℃(Fig.5 pattern (c). From the TEM observation, we can find that the fine and rod-liked Mg₃Zn₄Er₂ phase is precipitated during thermal compression at 150 ℃. And the Mg₃Zn₄Er₂ phase is not dissolved during thermal compression even at 450 ℃, but meanwhile I phase is disappeared.

Fig.4 TEM images and corresponding SAED patterns of phases in thermal compressed Mg-6Zn-0.5Zr-1Er alloy: (a) Not fully destroyed Mg₃Zn₄Er₂ phase; (b) I phase (Mg₃Zn₆Er), with representative quintic symmetry structure; (c) Precipitated Mg₃Zn₄Er₂ phase, with orientation relationship with matrix

As shown in Fig.6, the dynamic recrystallization (DRX) grains were observed in both alloys after thermal compression. The DRX grains of Mg-6Zn-0.5Zr alloy are found along twins or original dendrite boundaries (Figs.6(a) and (c)), while those of Mg-6Zn-0.5Zr-1Er alloy are near the intermetallic compounds (Fig.6(b) and (d)). Although the DRX grain size of both alloys increases with increasing temperature, the DRX grain size of Mg-6Zn-0.5Zr-1Er alloy is much smaller than those of Mg-6Zn-0.5Zr alloy at both 350 ℃ and 450 ℃. Compared with casting alloy, the coarse net-like Er-containing eutectics in thermally compressed Mg-6Zn-0.5Zr-1Er alloy were broken during deformation. And the cause of refined DRX grains is as followed factors. Firstly, thermally stable Er-containing eutectics with a relatively high melting point pins the grain boundaries and impedes grain growth during thermal compression. Secondly, it is considered that the addition of Er changes the valence electron structure of the alloy and the bond-energy between atoms is increased, hence the structure stability of the alloys is improved. Thirdly, both fine I phase particles and precipitated Mg₃Zn₄Er₂ phase can induce DRX, and suppress the growth of DRX grains, and finally result in the refined microstructure. All of above factors can confine the formation and movement of large angle grain boundaries. As a result, the growth of recrystallized grains in Mg-6Zn-0.5Zr-1Er alloy is restricted.

Fig.5 XRD patterns before and after thermal compressed deformation of Mg-6Zn-0.5Zr-1Er alloy: (a) Un-deformation; (b) Thermally compressed deformation at 350 ℃ with a strain rate of 1×10^-4 s^-1; (c) Thermally compressed deformation at 450 ℃ with a strain rate of 1×10^-4 s^-1

3.2 Thermal compressing flow behavior

Fig.7 illustrates the stress—strain curves of the two alloys. The flow curves have a typical shape of a peak true stress followed by a softening process. There are serrated flow behaviors of the two alloys with a low strain rate of 1.0×10^-4 s^-1, which may be mainly due to the interaction between dislocation and solute atom [20-21]. The main difference of thermal compressing flow behaviors between Mg-6Zn-0.5Zr alloy and Mg-6Zn-0.5Zr-1Er alloy is the higher true stress of the latter due to Er addition.

Fig.6 Microstructures of compressed Mg-6Zn-0.5Zr alloy (a, c) and compressed Mg-6Zn-0.5Zr-1Er alloy (b, d): (a), (b) Compression at 350 ℃ and strain rate of 1.0 s^-1; (c), (d) Compression at 450 ℃ and strain rate of 1.0×10^-4 s^-1

The true stress—true strain curves are satisfactorily fitted by the following equation by the constitutive analysis for the compression tests curves[22]:

                      (1)

where  A(s^-1) and α(MPa^-1) are materials constants, R is the gas constant, and  is the strain rate (s^-1) with the temperature T(K) through an Arrhenius function with activation energy Q(kJ/mol). Eqn.(2) can be derived from Eqn.(1):

Fig. 7 True stress—true strain curves of two alloys: — Mg- 6Zn-0.5Zrally; … Mg-6Zn-0.5Zr-1Er alloy; (a) 1.0×10^-4 s^-1; (b) 1.0×10^-2 s^-1; (c) 1.0 s^-1

Q=2.3Rns                                   (2)

where  n and s are the slopes of lg versus lg[sinh(ασ)] and lg[sinh(ασ)] versus T ^-1 with the other invariable parameters, respectively, obtained by linear-regression analysis. The values of Q can be then calculated at ε=0.45.

As shown in Fig.8, Q value has a small increase with increasing temperature between 150 ℃ and 250 ℃because of the dominating dynamic recovery mechanism. However, there is a marked increase of Q value with increasing temperature from 250 ℃ to 450 ℃ ascribed to the dominating DRX mechanism. There is a little increase of Q value when increasing strain rate. In addition, Er addition can result in the increase of Q value. The reason for this is that the fine I phase particles or precipitated Mg₃Zn₄Er₂ phases suppress the migration of DRX grain boundaries, which can increase the Q value.

Fig.8 Relations between Q values and temperature for two alloys at different strain rates

4 Conclusions

1) The thermally compressed Mg-6Zn-0.5Zr alloy mainly consists of α-Mg and MgZn₂ phases, while the Mg-6Zn-0.5Zr-1Er alloy consists of α-Mg phase, coarse Mg₃Zn₄Er₂ eutectic, rod-liked Mg₃Zn₄Er₂ precipitated phase, and fine I phase particle (Mg₃Zn₆Er, icosahedral quasicrystal structure).

2) There are serrated flow behaviors of the two alloys at a low strain rate of 1.0×10^-4 s^-1. The peak flow stress becomes larger with increasing strain rate and with erbium addition at the same temperature, and gets smaller with increasing deformation temperature at the same strain rate. The deformation activation energy Q increases with increasing temperature, strain rate and with erbium addition.

3) Fine I phase particles and precipitated Mg₃Zn₄Er₂ phase can suppress the growth of DRX grains, which can increase the Q value and refine DRX grains.

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(Edited by HE Xue-feng)

Foundation item: Project(2003AA331110) supported by the High-Tech Research and Development Program of China

Corresponding author: ZHANG Cai-bei; Tel: +86-24-83678479; E-mail: cbzhang4616@yahoo.com.cn