­­­ Influence of sub-rapid solidification on microstructure and mechanical properties of AZ61A magnesium alloy

TENG Hai-tao(滕海涛), LI Ting-ju(李廷举), ZHANG Xiao-li(张小立), ZHANG Zhong-tao(张忠涛)

State Key Laboratory of Materials Modification, School of Materials Science and Engineering,

Dalian University of Technology, Dalian 116024, China

Received 12 June 2008; accepted 5 September 2008

Abstract: The microstructure of sub-rapid solidification processed AZ61A magnesium alloy was presented and discussed. The results show that the grain size of the foil is significantly refined, and the grain morphology is cellular or globular. The eutectic transformation L→α-Mg+β-Mg₁₇Al₁₂ and microsegregation in conventionally solidified AZ61A alloy are suppressed to a great extent. The β-Mg₁₇Al₁₂ phases located in the α-Mg grain boundaries are largely decreased due to high solidification cooling rate. As a consequence, the alloying elements Al, Zn, Mn show much higher solid solubility and the sub-rapid solidification microstructure dominantly consists of supersaturated α-Mg solid solution. The mechanical properties and fractographic analysis reveal that the fracture mechanism and corresponding morphology of the rapture surface of tensile bars are linked to the microstructure obtained and depend on the sub-solidification processes.

Key words: sub-rapid solidification; grain size; mechanical property; fracture surface morphology

1 Introduction

Magnesium based alloy has developed as one of the most popular engineering constructional materials for manufacturing, in virtue of high specific strength which benefits to save energy, well castability and magnetic shield characteristic for 3C (computers, communications and consumer electronics) products, recyclable, abundant resources and so potentially low cost[1-5]. However, the magnesium alloys produced by conventional ingot metallurgy(IM) technique exhibit the drawbacks of less than desirable strength, inferior formability, low thermal stability, inadequate creep resistance, poor oxidation resistance and inferior corrosion. Consequently, the extensive use of the conventional ingot metallurgy(IM) processed magnesium-base alloys was restricted[6-8]. Because of their hexagonal lattice structure, the magnesium alloys show a comparatively limited cold workability[9]. At room temperature, their deformation mainly depends on the slip that only takes place on the basal plane (0001) and in the <110> direction, and the twining occurs on the pyramidal plane (102). It is well known that many properties of a crystalline material are influenced by its microstructure[9-10], and a finer grain size may contribute significantly to improve the room-temperature ductility performance, mechanical property and corrosion resistance of magnesium alloys: the castings having fine grains usually show a more uniform distribution of solute and a better dispersion of secondary phases within their structure[11].

In general, the grain size and microstructure of cast metal are directly influenced by the cooling rate. Different degrees of structural refinement and transformation can be obtained by the technology of sub-rapid solidification processing, which presently provide cooling rates up to 10³ K/s. The microstructure changes involve phases with large and non-equilibrium solid solubilities, metastable crystalline phases and microcrystalline structures[12]. The available literature on sub-rapidly solidified magnesium alloys is very sparse and few significant works have been undertaken in the country so far.

The objective of present investigation is to produce AZ61A alloy castings by using sub-rapid solidification technique, to assess the mechanical properties and correlate the properties with the microstructure. The microstructure of the cast specimens was characterized through metallographic microscopy, scanning electron microscopy(SEM) and X-ray diffraction(XRD) techniques. The effects of cooling rate on grain size, morphology and precipitated phase were also presented.

2 Experimental

2.1 Preparation of cast specimens

Commercial AZ61A alloy was used in this investigation. The nominal composition (in mass fraction) of the alloy is 6.5% Al, 0.83% Zn, 0.26% Mn, ＜0.01% Si, Fe and Cu, and ＜0.001% Ni, and Mg as a balance, as listed in Table 1. The studied magnesium alloy (Mg- 6%Al-1%Zn-0.2%Mn) was prepared in steel crucible using flux melting technique in an electric resistance furnace. Magnesium, aluminium and zinc metal ingots of 99.90% purity were used and manganese was added as Al-10%Mn intermediate alloy. The magnesium ingot was melted at 200 K above the liquidus temperature and after it was completely molten, the aluminium and zinc igot were added to the melt. After melting, a part of melt was poured into permanent steel mold that was preheated to 573 K. The mold was then air cooled to room temperature, thus the AZ61A conventionally solidified (CS) sample was obtained. All the above procedures were conducted in a flowing protective gas (in volume fraction) (50% dry air+50% CO₂+0.3% SF₆) to prevent burning and oxidation.

Table 1 Chemical composition of AZ61A (mass fraction, %)

Fig.1 shows the schematic of the self-designed sub-rapid solidification device consisted of fixed bracket, mould, crucible and vacuum container, etc. When the control valve turned on, the molten metal was injected into the cold cavity of the mould with a high injection rate under vacuum pressure, and the Mg-alloy foils varied from 0.5 mm to 3.0 mm in thickness were obtained.

2.2 Measurements of specimens

The specimens of Mg-alloy casting were cut, mounted, polished and etched with 3% Nital according to standard metallographic preparation techniques[11, 13].

Fig.1 Schematic of experimental set up: 1—Fixed bracket; 2—Copper mould; 3—Control valve; 4—Thermocouple; 5—Quartz tube; 6—Molten metal; 7—Crucible; 8—Argon; 9—Vacuum container

Subsequently, microstructures of etched specimens were characterized by optical microscope(OM) and scanning electron microscope (SEM, JSM-5600LV). In order to obtain detailed information on the phase distribution in the specimens, phase identification was performed by X-ray diffraction (XRD, XRD-6000). Mechanical properties of the castings such as tensile strength and elongation were tested according to ISO 6892-1998, and carried out using a WDW3100 material testing machine at room temperature with a tensile strain rate of 3×10^-3 s^-1, and the fracture surface morphology was analyzed by using SEM.

3 Results and discussion

3.1 Microstructure

Fig.2 shows an AZ61A magnesium alloy foil obtained by sub-solidification technique in the present study. This foil is about 35 mm in length, 15 mm in width and 1.5 mm in thickness and has a shiny sound surface.

A typical microstructure of the conventional alloy is shown in Fig.3(a), where well-developed primary α-Mg dendrites with β phase (the brittle intermetallic Mg₁₇Al₁₂) along the α-Mg grain boundaries in the form of continuous network are clearly visible. In addition, the grain size is quite large and its distribution is non- uniform due to the slower cooling rate[9]. A typical eutectic structure of conventionally casting AZ61A alloy is shown in Fig.3(b). As illustrated in Fig.3(b), the growth of divorced eutectic α-Mg firstly adheres to the primary α-Mg; the divorced eutectic β-Mg₁₇Al₁₂ with

Fig.2 Sub-rapidly solidified AZ61A alloy foil

Fig.3 Optical micrographs of CS AZ61 casting (a) and typical eutectic structure (b)

black contour grows into primary α-Mg; and then the lamellar of secondary β-Mg₁₇Al₁₂ precipitates from supersaturated α-Mg. Numerous small particles in β-Mg₁₇Al₁₂ divorced eutectic are homogeneously dispersed at the grain boundaries[14-16].

The microstructure of cross-section of the sub- rapidly solidified foil of 1.5 mm in thickness is presented in Fig.4. The micrograph of the foil at cross-section from a surface to the center consists of equiaxed chill crystal zone, columnar crystal zone and equiaxed crystal zone. By comparing Fig.3 with Fig.4, it can be inferred that the microstructure of the sub-RS AZ61A foil with a high rate of solidification is much more finely and homogeneously distributed than that of the conventional cast alloy. In addition, the volume fraction, shape and distribution characteristics of intermetallic β-Mg₁₇Al₁₂ located in the α-Mg grain boundaries as well as grain size are largely influenced by solidification rate, and the microstructure dominantly consists of supersaturated α-Mg solid solution.

Fig.5 shows the XRD patterns of CS AZ61A alloy casting and sub-RS foil of 1.5 mm in thickness. It can be clearly seen that the peaks of sub-RS foil are broader than that of the CS casting. The effect can be attributed to the grain refinement[17]. The results indicate that the phases in the CS AZ61 alloy casting, as shown in Fig.5(a), consist of α-Mg and β-Mg₁₇Al₁₂, which are also clearly visible in its optical micrograph (Fig.3). However, compared with the phases in the CS casting, the eutectic transformations L→α-Mg+β-Mg₁₇Al₁₂ in conventionally

Fig.4 Optical micrograph of sub-RS AZ61A alloy foil of 1.5 mm in thickness at cross-section from surface to center: Ⅰ—Equiaxed chill crystal zone; Ⅱ—Columnar crystal zone; Ⅲ—Equiaxed crystal zone

Fig.5 XRD patterns of CS alloy (a) and sub-RS foil of 1.5 mm in thickness (b)

solidified AZ61 alloy are suppressed to a great extent due to high cooling rates in sub-rapid solidification processing. The alloying elements Al, Zn, Mn show much higher solid solubility and the increase of cooling rate results in the decrease of proportion of brittle Mg₁₇Al₁₂ phases in the divorced eutectic[15]. Therefore, the diffraction peaks of the β-Mg₁₇Al₁₂ phase in the XRD patterns of sub-RS foils, as displayed in Fig.5(b), are not obvious. Thus the main phase in sub-RS foils consists of α-Mg solid solution, as shown in Fig.4.

3.2 Mechanical properties

The typical engineering stress vs engineering strain curves of CS casting and sub-RS foil of 1.5mm in thickness at ambient temperature are shown in Fig.6. The mechanical properties including 0.2% tensile yield strength(σ_0.2), ultimate tensile strength(σ_b) and elongation (δ) of rupture are summarized in Table 3. The yield strength, ultimate strength and elongation of CS alloy are 99 MPa, 149 MPa and 5.2%, respectively. The corresponding properties of the sub-RS foil are higher than those of CS alloy, as listed in Table 3. The yield strength increases by 33 MPa, the ultimate strength increases by 63 MPa and the elongation increases by 4.2% compared with those of CS alloy.

Fig.6 Curves of engineering strain vs engineering stress of CS alloy and sub-RS foil at room temperature

Table 3 Mechanical properties of CS alloy and sub-RS foil of AZ61A at room temperature

The yield stress (and tensile strength) dependence of the grain size can be expressed through the Hall-Petch relationship:

σ_yd=σ₀+K_yd^-1/2                               (1)

where  d is the average grain diameter, σ₀ is a constant and K_y is the stress intensity factor for plastic yielding. A finer grain size may contribute significantly to the strength and the yield stress increases with decreasing grain size[9]. The improved mechanical properties at room temperature of this sub-rapid solidification processed magnesium alloy is ascribed to the conjoint and mutually interactive influences of fine grain size, solid solution strengthening of the magnesium matrix[6] and homogeneous distribution of secondary phase[17].

Fractographic observations of broken tensile specimen surfaces are shown in Fig.7. A microscopic scale fracture of the CS test specimen is brittle in appearance, as shown in Fig.7(a), the cracks nucleate and propagate along the brittle intermetallics that form at the dendrite grain boundaries. The fracture surface contains no dimples, which are characteristic of ductile fracture, thus no effective ductile deformation occurs during the tensile test. Whereas, the examination of the fracture surface of the sub-RS foil (Fig.7(b)) reveals a population of dimples of varying size and shape, some cleavage

Fig.7 SEM images of tensile fracture surface of magnesium alloy: (a) CS casting alloy; (b) sub-RS foil of AZ61A

steps and small tearing cracks, which suggests toughness fracture feature.

The strength and ductility of magnesium alloy casting depend on the shape and distribution of the intermetallic network at the grain boundaries as well as the grain size. The sub-rapid solidification processing results in small grains, no continuous intermetallic network at the grain boundaries, and fine precipitation, which all contribute to the ductility and strength of the alloy[18].

4 Conclusions

1) The grains in the alloy produced by using sub-RS processing are much more finely and homogeneously distributed than those of the CS casting.

2) The shape, proportion and distribution characteristics of brittle intermetallic β-Mg₁₇Al₁₂ located in the α-Mg grain boundaries are largely influenced by solidification cooling rate.

3) Grain refinement is one of the effective ways to improve the microstructure and resulting mechanical properties of cast alloys. The mechanical properties including 0.2% tensile yield strength, ultimate tensile strength and elongation of sub-RS casting are higher than those of CS casing.

4) Tensile fracture surface morphology reveals an overall brittle appearance in the CS test specimen and the fracture surface of the sub-RS foil suggests toughness fracture feature.

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(Edited by HE Xue-feng)

Foundation item: Projects(50274017, 50674018) supported by the National Natural Science Foundation of China

Corresponding author: TENG Hai-tao; Tel: +86-411-84706220; E-mail: seantht@yahoo.com.cn