­­­ Microstructures and properties of rapidly solidified Mg-Zn-Ca alloys

ZHOU Tao(周 涛), CHEN Ding(陈 鼎), CHEN Zhen-hua(陈振华)

School of Materials Science and Engineering, Hunan University, Changsha 410082, China

Received 12 June 2008; accepted 5 September 2008

Abstract: Ternary alloys based on the Mg-Zn-Ca system were produced by twin-roll rapid solidification. The alloys were characterized by OM, SEM, HRTEM, XRD, EDS and Micro-hardness. The results show that the rapidly solidified flakes are of fine dendritic cell structures with the cell size ranging from 1 to 5 μm. The Mg-6Zn-5Ca alloy in RS and annealing (200 ℃ for 1 h) states are mainly composed of α-Mg, Mg₂Ca, Ca₂Mg₆Zn₃ and a small quantity of Mg₅₁Zn₂₀, MgZn₂ and Mg₂Zn₃. Micro-hardness increases with the increment of Ca content and age hardening occurs after aging at 200 ℃ in the flakes probably due to the precipitation strengthening of the fine precipitates Mg₂Ca and Ca₂Mg₆Zn₃. Some phases at the grain boundary in Mg-6Zn-5Ca alloy are identified by means of HRTEM, which may be beneficial to the improvement in thermal stability of the alloy.

Key words: Mg-Zn-Ca alloy; rapid solidification; microstructure; micro-hardness

1 Introduction

Magnesium alloys are considered as potential candidates for many structural applications due to their low density, high specific strength and excellent machinability. However, the widespread use of magnesium alloy has been critically limited by its low strength at ambient and elevated temperatures, poor corrosion and oxidation resistance and inferior formability[1-2]. A common approach to design of high strength, creep resistant alloys involves production of alloy microstructures containing uniform fine-scale dispersions of thermal stable intermetallic precipitates and such microstructures can be developed for magnesium alloys by non-equilibrium processing, such as rapid solidification[3]. Thus, rapid solidification can play an important role due to its advantages, such as homogeneity of refined microstructure, extended solid solubility and formation of non-equilibrium phases[4]. On the other hand, it has been reported that Ca addition to magnesium alloy improves oxidation resistance due to the formation of a protective oxide layer and increases hardness and creep resistance[5-7] due to the formation of an intermetallic compound, Mg₂Ca, with a high melting point[8]. Moreover, Zn and Ca together with Mg may form a stable intermetallic compound Ca₂Mg₆Zn₃ (a=0.97 nm, c=1.0 nm)[9-10], which is responsible for the age hardening response of the alloy[11]. MIYAZAKI et al[12] suggested that both tensile strength and ductility were increased by ternary addition of Zn to Mg-Ca alloys and the highest tensile strength of 483 MPa was obtained in Mg-5Ca-5Zn (mass fraction, %) alloy with 2% tensile elongation. NIE and MUDDLE[3] found that the ternary addition of a small content of Zn (1%) to the Mg-Ca binary alloy leads to a substantial increase in peak hardness and an accelerated rate of ageing. However, it is difficult to find a systematic research on the both rapid solidification and the mechanical behavior in the Mg-Zn-Ca alloy flakes although a few were reported on RS Mg-Zn-Ca alloys[10-13].

The Mg-Zn-xCa alloys (x=3, 5, 7.5, 10, mass fraction, %) were prepared by twin-roll rapid solidification in this work. The objective of the present work is to investigate the microstructure characterization, thermal stability and precipitation hardening behavior of the rapidly solidified Mg-Zn-Ca alloys.

2 Experimental

Rapidly solidified (RS) Mg-6Zn-xCa (x=3, 5, 7.5, 10) alloys were prepared using 99.9% magnesium, 99.9% zinc and 99.9% calcium. The RS flakes were produced by melting the alloys under the protection of Aratmosphere in a steel crucible, atomizing the alloy melt and subsequently splat-quenching of the atomized drops on the water-cooled copper twin-rollers. The process parameters used for the atomization were as follows. The diameter of the nozzle was 1.5 mm; pouring rate was 1.5 kg/min; and the pressure of Ar gas used for atomization was 0.25 MPa. For the splat-quenching process, the diameters of the wheels were both 400 mm, the width of the roll gap between the rollers was 0.1 mm and wheel velocity was 1 200 r/min (25.12 m/s). The RS flakes were annealed in the temperature range of 100-400 ℃ for 1 h.

The microhardness measurements were conducted for all specimens using a 0.5 N load and a dwell time of 10 s with Vickers indenter. The thermal stability of the flakes was analyzed by differential scanning calorimetry (DSC/DTA 404PC) at heating rate of 10 ℃/min and under the protection of Ar atmosphere.

The microstructure of the flakes was characterized by X-ray diffractometry (XRD, D8 Advance) using monochromatic Cu K_α radiation, optical microscopy (OM) and scanning electron microscopy (SEM, JSM-6700F). To understand the role of precipitation particles in the flakes, high-resolution transmission electron microscopy (HRTEM) with EDS was also conducted. Thin foil specimens for HRTEM observation were twin-jet electropolished in a solution of 5.3 g lithium chloride, 11.6 g magnesium perchlorate, 500 mL methanol and 100 mL 2-butoxy-ethanol, at -55 ℃ and 85 V, and then by an ion beam milling and were observed in JEM-3010 at 300 kV. The “free” side of the flakes was adopted for the examinations mentioned above.

3 Results and discussion

3.1 Phase constitution

The as-prepared flakes are 100-150 μm in thickness and the alloy composition has no obvious influence on the thickness. The phase compositions of Mg-6Zn-5Ca alloy in different states are shown in Fig.1. The main peaks for RS Mg-6Zn-5Ca alloy correspond to Mg, Mg₂Ca, Ca₂Mg₆Zn₃, whereas a small quantity of Mg₅₁Zn₂₀, MgZn₂ and Mg₂Zn₃ are also detected in as-quenched specimens (Fig.1(a)), which is similar to the results of PARK et al[13] except for Ca₂Mg₆Zn₃ phase. After annealing at 200 ℃ for 1 h (Fig.1(b)), more Ca₂Mg₆Zn₃, Mg₂Ca, Mg₅₁Zn₂₀ and MgZn₂ are detected. LEVI et al[11] suggested that Ca₂Mg₆Zn₃ is more stable than Mg₂Ca at lower temperatures (＜350 ℃), and the age hardening response of the Mg-Ca-Zn alloys is concerned with its precipitation. However, PARK et al [13] revealed that the peak hardness in melt-spun Mg- Zn-Ca alloy is caused by the formation of Mg₂Ca precipitates (about 10 nm) finely dispersed in the grain interior. MIYAZAKI et al[12] suggested that age hardening of the Mg-Ca-Zn ternary alloy is caused by precipitates of the Mg-Zn binary system. Thus, the hard phase in the alloy may be Ca₂Mg₆Zn₃, Mg₂Ca or Mg-Zn binary phase.

Fig.1 X-ray diffraction patterns of Mg-6Zn-5Ca alloy: (a) RS; (b) Annealed at 200 ℃ for 1 h

As seen from Fig.2, the phase composition of Mg-6Zn-5Ca alloy annealed at 200 ℃ for 72 h is similar to that in RS and 200 ℃, 1 h annealing states. This means that the phase transformation is not visible in the alloy after annealing at 200 ℃ for 72 h.

Fig.2 X-ray diffraction pattern of Mg-6Zn-5Ca alloy annealed at 200 ℃ for 72 h

3.2 Microstructure characterization

The microstructures of the Mg-6Zn-5Ca alloy are shown in Fig.3. The Mg-6Zn-5Ca alloy both in RS state and after annealing at 200 ℃ for 1 h (Figs.3(a) and (b)) exhibits finely dendritic structures with the grain size of 1-5 μm. After exposure at 200 ℃ for 72 h, some grains of the alloy grow up to 10 μm. As seen from Fig.4(a), the Mg-6Zn-5Ca alloy annealed at 200 ℃ for 1 h exhibits very fine grains with the size of 1-5 μm and a large number of dispersive precipitates (as marked by arrows in Fig.4(b)) distribute in the grain inners for the alloy

Fig.3 Optical micrographs of Mg-6Zn-5Ca alloy: (a) RS;    (b) Annealed at 200 ℃ for 1 h; (c) Annealed at 200 ℃ for 72 h

annealed at 200 ℃ for 1 h. These precipitates vary in size (Fig.4(b)). The coarser and the finer ones are 300-500 nm and 50-100 nm in size, respectively, and most of the precipitates are in spherical shape.

Grain refinement is one of the important ways to improve the mechanical properties of magnesium alloys. It is an essential and fundamental approach since grain size significantly influences the mechanical properties of the Mg alloys. It is well known that the nucleation depends on the number and size of clusters in the melt [14]. The number of grains per unit area is estimated according to

                              (1)

where    is the nucleation ratio and G is the growth rate. Thus it can be seen that effective nucleation ratio or

Fig.4 SEM image of Mg-6Zn-5Ca alloy at 200 ℃ for 1 h:    (a) In lower magnification; (b) In higher magnification

restraint of growth rate will result in grain refinement. Generally in the supercooling range during common metal solidification, the ratio of  increases with the increase of super cooling rate, which can refine microstructure [14].

During the rapid solidification processing, the solute atoms will be frozen, which is called “solute trap” due to its very high solidification velocity, so the refined and homogeneous microstructure can be obtained. But in the course of the solidification process, when the solidification velocity falls below the absolute stability velocity, segregation starts and the planar interface is destabilized[10]. In this work, the interfacial instability in the flakes can also be verified by the shape of certain grains marked by square in Fig.4(b), similar to that in Ref.[10].

As seen from Fig.5(a), some spherical precipitates about 200 nm in size are observed at the grain boundary. In order to understand the distributions of alloying elements, EDS was carried out in Mg matrix and on the precipitates respectively, as shown in Figs.5(b) and (c). Only some contents of Zn atoms (2.84%, mole fraction) but no Ca atoms are detected in the Mg matrix (marked as M in Fig.5(b)) probably because the solid solubility of the Ca in Mg matrix is very little at room temperature. As seen in Fig.5(c), the mole fractions of Mg, Zn and Ca atoms in the precipitates are 52.34%, 35.92% and 11.74%, respectively. Obviously, the contents of the latter

Fig.5 HRTEM results of Mg-6Zn-5Ca alloy ribbon after annealing at 200 ℃ for 1 h: (a) Image of matrix and precipitate; (b, c) EDS of matrix and precipitate

two atoms in the precipitates are much higher than those in Mg matrix and a Ca/Zn molar ratio in the precipitates is blew unity. According to the XRD patterns (Fig.1) and HRTEM with EDS, it can be inferred that the precipitates may be Ca₂Mg₆Zn₃, which is so stable that it is very effective for pinning the grain boundary. A detailed study about precipitates within the grain and at the boundary is under way by means of TEM and selected area diffraction patterns.

3.3 Thermal stability

The phase stablility of the Mg-Zn-xCa (x=5, 7.5,  10) alloys was investigated by DSC. The results obtained for heating from 100 to 550 ℃ with a rate of 10 ℃/min are shown in Fig.6. The Mg-6Zn-5Ca, Mg-6Zn-7.5Ca and Mg-6Zn-10Ca alloys exhibit a large endothermic peak at 411.3, 415.6 and 409.8 ℃ respectively during heating, while the onset peak is 409.4, 412.5 and 407.5 ℃, respectively. Furthermore, another flat endothermic peak at about 480 ℃ occurs in the alloy with 7.5%Ca addition. According to the binary alloy phase diagrams and the XRD patterns (Fig.1), the first endothermic peak at about 411 ℃ in the alloys is related to the melting onset of some metastable phases, such as Mg₂Zn₃ with a lower melting point. Meanwhile, the second endothermic peak in the Mg-6Zn-7.5Ca alloy may be related to the dissolution of some stable precipitates such as MgZn₂ or Ca₂Mg₆Zn₃.

Fig.6 DSC curves for Mg-Zn-xCa alloys

3.4 Age hardening response

The microhardness changes of the as-quenched flakes after isochronal aging for 1 h at various temperatures are shown in Fig.7. The Mg-6Zn-3Ca alloy exhibits relatively low hardness(HV) of 59.3 and 85.5 in the as-quenched and the as-annealed state at 200 ℃ for  1 h, respectively. The hardness of the alloys increases with increasing content of Ca. This may be resulted from the fact that Mg₂Ca and Ca₂Mg₆Zn₃ phases were found to be the hardening phases with a high microhardness of 125-128 kg/mm²[9]. The Mg-6Zn-3Ca and Mg-6Zn-5Ca alloys exhibit a strong age hardening behaviour and the peak value of hardness(Hv) of the Mg-6Zn-5Ca alloy is 120.8. Mg-6Zn-7.5Ca alloy has the highest hardness(HV) of about 158.1 experienced ageing at 200 ℃ for 1 h. According to the XRD analysis (Fig.1) and the SEM images (Fig.4), the increase in hardness observed in Mg-

Fig.7 Changes in microhardness after isochronal aging for 1 h at various temperatures

6Zn-5Ca alloy after aging at 200 ℃ for 1 h should be attributed to the grain refinement and homogeneous distribution of precipitates.

Changes in microhardness of Mg-6Zn-5Ca alloy after isochronal aging at 200 ℃ for various time are shown in Fig.8. The highest hardness of the alloy is obtained when exposure at 200 ℃ for 1 h, and then the hardness slightly decreases with the increase of aging time. However, there are no distinct changes on the hardness of the alloy until 62 h. When the alloy experiences the process at 200 ℃ up to 72 h, the hardness decreases again. This means that the alloy is of a relatively high hardness even after aging at 200 ℃ for  62 h.

Fig.8 Changes in microhardness of Mg-6Zn-5Ca alloy after isochronal aging at 200 ℃ for various times

According to the previous investigations[15-16], discontinuous precipitation and decomposition of the intermetallic compounds (such as Mg₁₇Al₁₂) are considered the main factor leading to poor creep properties of Mg-Al based alloys at elevated temperatures due to a low melting point (437 ℃) of Mg₁₇Al₁₂. Hence, the thermal stability of the phases in the alloy is very important for heat resistant Mg alloys. FINKEL et al[17] suggested that the as-cast structure of Mg-5%Ca-6%Zn alloy is composed mainly of α-Mg solid solution, while in the grain boundaries large (2 μm in diameter) elliptical precipitates of Mg₂Ca and a eutectic structure of Mg and Ca₂Mg₆Zn₃ are found, and the structural stability of the as-cast alloy at 160 ℃ is good due to the high-melting point precipitates (Mg₂Ca and Ca₂Mg₆Zn₃) at the grain boundaries. In the present study, Ca₂Mg₆Zn₃ phases are also observed at the grain boundary and the relatively high hardness obtained in the alloy after aging at 200 ℃ for 62 h may be associated with the formation of this phase. The microhardness of the alloy relates to a number of factors, e.g. the average grain size and the composition of the phases. The compositions of the Mg-6Zn-5Ca alloy after aging at 200 ℃ for 72 h are similar to those of the alloy RS and annealed at 200 ℃ for 1h, so the effect of the compositions of the phases on hardness can be neglected. On the other hand, some grains of the alloy grow up to 10 μm after exposing at 200 ℃ for 72 h, as seen in Fig.3. Thus, the decrease of hardness in the Mg-6Zn-5Ca alloy after aging at 200 ℃ for 72 h may be interpreted as a slight coarsening of the grain size.

4 Conclusions

1) The RS Mg-6Zn-5Ca alloy consists of Mg, Mg₂Ca, Ca₂Mg₆Zn₃ and a few Mg₅₁Zn₂₀, Mg₂Zn₃ and MgZn₂. The Mg-6Zn-5Ca alloy both in RS state and after annealing at 200 ℃ for 1 h exhibits finely dendritic structures with the grain size of 1-5 μm and a large number of dispersive precipitates distribute in the grain inners for the alloy annealed at 200 ℃ for 1 h, which may be beneficial to enhancing hardening peak.

2) The relatively high stability obtained in Mg-6Zn-5Ca alloy after aging at 200 ℃ for 62 h may be associated with the formation of Ca₂Mg₆Zn₃ phases that are observed at the grain boundary.

3) Mg-6Zn-5Ca alloy is expected to be a promising alloy composition with high strength, good creep resistance and moderate ductility in the Mg-Zn-Ca alloy system.

References

[1] WEI L Y, DUNLOP G L, WESTENGEN H. Precipitation hardening of Mg-Zn and Mg-Zn-RE alloys [J]. Metallurgical and Materials Transactions A, 1995, 26(7): 1705-1716.

[2] DECKER R F. The renaissance in magnesium [J]. Advanced Materials and Processes, 1998, 154(3): 31-33.

[3] NIE J F, MUDDLE B C. Precipitation hardening of Mg-Ca(-Zn) alloys [J]. Scripta Materialia, 1997, 37(10): 1457-1481.

[4] MORDIKE B L, RIEHEMANN W. Mechanical properties and thermal stability of rapidly solidified magnesium alloys [J]. Key Engineering Materials, 1994, 97/98: 13-28.

[5] BAI Jing, SUN Yang-shan, XUE Feng, XUE Shan, QIANG Jing, TAO Wei-jian, LIU Hai-feng. Microstructure and creep properties of Mg-6Al-(Sr, Ca) alloys [J]. Acta Metallurgica Sinica, 2006, 42(12): 1267-1273. (in Chinese)

[6] LUO A A, BALOGH M P, POWELL B R. Creep and microstructure of magnesium-aluminum-calcium based alloys [J]. Metallurgical and Materials Transactions A, 2002, 33(3): 567-574.

[7] VOGEL M, KRAFT O, ARZT E. Effect of calcium additions on the creep behavior of magnesium die-cast alloy ZA85 [J]. Metallurgical and Materials Transactions A, 2005, 36(7): 1713-1719.

[8] OH J C, OHKUBO T, MUKAI T, HONO K. TEM and 3DAP characterization of an age-hardened Mg-Ca-Zn alloy [J]. Scripta Materialia, 2005, 53(6): 675-679.

[9] LARIONOVA T V, PARK W W, YOU B S. A ternary phase observed in rapidly solidified Mg-Ca-Zn alloys [J]. Scripta Materialia, 2001, 45(1): 7-12.

[10] JARDIM P M, SOLORZANO G, VANDER SANDE J B. Second phase formation in melt-spun Mg-Ca-Zn alloys [J]. Mater Sc Eng A, 2004, A381(1/2): 196-205.

[11] LEVI G, AVARAHAM S, ZILBEROV A, BAMBERGER M. Solidification, solution treatment and age hardening of a Mg-1.6wt.% Ca-3.2wt.% Zn alloy [J]. Acta Materialia, 2006, 54(2): 523-530.

[12] MIYAZAKI T, KANEKO J, SUGAMATA M. Structures and properties of rapidly solidified Mg-Ca based alloys [J]. Mater Sci Eng A, 1994, A181/182: 1410-1414.

[13] PARK W W, YOU B S, MOON B G, KIM W C. Microstructural change and precipitation hardening in melt-spun Mg-X-Ca alloys [J]. Science and Technology of Advanced Materials, 2001, 2(1): 73-78.

[14] CAI J, MA G C, LIU Z, ZHANG H F, HU Z Q. Influence of rapid solidification on the microstructure of AZ91HP alloy [J]. Journal of Alloys and Compounds, 2006, 422(1/2): 92-96.

[15] DU Wen-wen, SUN Yang-shan, MIN Xue-gang, XUE Feng, ZHU Min, WU Deng-yun. Microstructure and mechanical properties of Mg-Al based alloy with calcium and rare earth additions [J]. Mater Sci Eng A, 2003, A356(1/2): 1-7.

[16] ZHANG P. Creep behavior of the die-cast Mg-Al alloy AS21 [J]. Scripta Materialia, 2005, 52(4): 277-282.

[17] FINKEL A, SHEPELEVA L, BAMBERGER M, RABKIN E. The effect of exposure to elevated temperatures on the microstructure and hardness of Mg-Ca-Zn-Si compared with Mg-Ca-Zn alloy [C]// KAPLAN H I ed. Magnesium Technology 2003. San Diego: TMS, 2003: 189-194.

(Edited by CHEN Wei-ping)

Foundation item: Project(04GK1008-1) supported by the Major Fund of Science and Technology of Hunan Province, China

Corresponding author: CHEN Ding; Tel: +86-731-8821648; E-mail: ztlw8118@yahoo.com.cn