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Review of grain refi nement methods for as-cast microstructure of magnesium alloy

Male, born in 1959, is now the Assistant President of Shanghai University and Director of the Center for Advanced Solidifi cation Technology of Shanghai University. He obtained his bachelor degree from the Northeastern University in 1982 and Ph.D degree from the University of Science and Technology Beijing in 1991. His research interests mainly focus on metal solidifi cation and microstructure refi nement. His academic research has led to the publication of more than 170 papers and 2 books. So far he holds 18 invention patents in China, of which 11 have been put into production.E-mail: qjzhai@shu.edu.cnReceived: 2009-03-09; Accepted: 2009-04-20

*Zhai Qijie

Song Changjiang 1, Han Qingyou 2 and *Zhai Qijie 1

(1. Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, China; 2. Mechanical

Engineering Technology Department, Purdue University, West Lafayette, IN 47907, USA)

Abstract: As the lightest structural metal, Mg and Mg-based alloys have great potential applications in the aerospace, automotive and nuclear industries. However, such applications have been limited by low ductility and strength. Theoretically, small grain sized structure can synchronously improve its ductility and strength. Yet, universally reliable grain refi nement techniques for the magnesium alloys are still under investigation and some are in strong debating. This paper presents a brief review of development of grain refi nement methods for magnesium alloys, which would contribute to a better understanding of the factors controlling grain refi nement and provide an outlook of future research in this fi eld.

Key words: magnesium alloy; grain refi nement; microstructureCLC number: TG146.1+3 Document code: A Article ID: 1672-6421(2009)02-093-11

Magnesium is the lightest structural metal with a density of only 1.738 g/cm3 at 20℃. For engineering applications,

magnesium is usually strengthened by alloying mechanism; it can be alloyed with other alloying elements such as aluminum, zinc, manganese, zirconium and rare earth [1-3]. Besides the low density, Mg and Mg-based alloys also have other attractive properties, such as an excellent damping capacity and good electromagnetic shielding. Moreover they have good castability and machinability, and potential for availability because of its wide distribution in the Earth’s crust and sea[1-4]. Due to the characteristics mentioned above, Mg and Mg-based alloys are increasingly used for aerospace, automotive and nuclear applications [1-3]. However, due to the hexagonal close packed crystal structure (a=0.32092 nm and c=0.52105 nm at room temperature), it has low (only two basal) independent slip systems [1, 4]. At the room temperature, the low ductility and high degree of anisotropy in mechanical properties signifi cantly limit their formability.

Although Mg-based alloys have outstanding strength/weight

ratio, the important disadvantages of Mg alloys are low strength and low ductility compared with the other competitive structural materials such as Al and steel [5, 6]. It is well known that a finer grain size may contribute synchronously to the strength and ductility. The grain size dependence of the yield stress (and tensile strength) can be expressed through the classical Hall-Petch relationship:

(1)

Where d is the average grain diameter, v0 is a constant and KY is the stress intensity factor for plastic yielding. For Mg and Mg-based alloys, the value of KY is about 210 MPa·µm1/2

and depends on temperature, structure, composition and preparation [1]. Therefore, grain refinement is an effective method for improving the mechanical properties of Mg-based alloy and can signifi cantly extend the applications of Mg-based alloys [5]. As a result, the grain refinement of Mg-Al based alloys has been an active research topic over several decades.

Up till now, there have been various grain-refi ning methods developed for Mg-based alloy, mainly applied to the wrought and cast products. Fine grain microstructure favors uniform deformation and improves isotropic mechanical properties of the materials with hexagonal close-packed (hcp) structure [7]. It is also well-known that, the microstructure prior to forging or extrusion, i.e. the solidifi ed structures of an ingot, has a signifi cant impact on the subsequent forging properties [7]. Therefore, this paper reviews the grain refinement methods for as-cast Mg-based alloys with an aim of promoting its development.

1 Superheating methodThe superheating process was originally described in a British

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patent granted in 1931 [8, 9]. It refers to heating the magnesium melt to an elevated temperature (generally between 150℃ and 260℃ above the equilibrium liquidus temperature), followed by a rapid cooling to the pouring temperature and holding for a short period of time before casting [3]. The required holding time at the superheating temperature decreases with the increasing of aluminum content, the amount of scrap in the charge, the crucible size and the heating rate [3]. The grain refi nement by superheating is characterized as follows:

(1) A significant grain refinement can only occur in the aluminum-containing magnesium alloys. High aluminum content magnesium alloys (>8wt.%Al) are more readily grain refi ned than low aluminum content alloys. Grain refi nement by superheating is not practicable for Mg alloys with smaller than 1wt% Al [2,10].

(2) Existence of Fe and Mn promotes grain refinement. Compared with the alloys with high Fe and Mn contents, high-purity Mg-Al alloys with low Fe and Mn contents are less susceptible to superheating treatment. But excess (>1wt.%) Mn would inhibit grain refi nement of superheating treatment [2, 8, 10]. In addition, Zr, Be, Ti and some other elements also suppress the grain refi nement effect of superheating treatment [2].

(3) Successful grain refinement requires adequate superheating temperature range and holding time. For example, the optimum overheating temperature for Mg-9Al-2Zn alloy is within the range between 850℃ and 900℃ [9]. Although lower superheating temperature effect can be compensated by a longer holding time and the alloys with low aluminum contents require more superheating [9], grain coarsening can occur at higher superheating temperature and with longer holding time [2].

(4) Rapidly cooling from superheating temperature to pouring temperature and pouring immediately are crucial for successful gain refinement. Holding the melt below 820℃, especially below about 750℃, will cause grain coarsening [2].

Although superheating process has been extensively studied from various perspectives, it is still under debating with regard to underlying mechanism. Early investigators proposed a temperature-solubility theory, in which it is supposed that the existing coarse particles with low density at normal pouring

temperature will dissolve upon superheating and then re-precipitate as a large number of fi ne nucleation sites during the rapid cooling [2, 8, 10].

The second hypothesis is to attribute the grain refinement to the formation of Al4C3 nuclei since carbon may present by decarburizes the walls of steel crucibles at high temperature [2]. Peng Cao et al recently report that native grain refinement was found in high-purity Mg-Al alloys (as evidenced in Figs.1-3), but not in the high-purity Mg-Zn and Mg-Ca alloys [11]. Such grain refi nement in Mg-Al alloys only appears with superheating, so it is believed that the native Al4C3 nucleant particles are poisoned by the impurities (Fe and/or Mn) in commercial Mg-Al type alloys [8]. However, at high superheating temperature (850-900℃), the impurity coating on the nucleant particle surface is removed, and subsequent rapid cooling prevents the forming of impurities coating. Thus they can serve as nucleation sites and gives rise to grain refi nement (as shown in Fig.4).

Another hypothesis is that one or more Al-Fe or Al-Mn-Fe intermetallics (e.g. Al8(Mn, Fe)5, as reported by Young Min Kim et al. [12]) may precipitate from the melt at high temperature and act as nuclei in the subsequent cooling. However, crystallographic analysis by using edge-to-edge matching model shows that Al8(Mn, Fe)5 particle has

Fig.1 Effect of source magnesium purity on the grain size of Mg-Al alloys [11]

Fig.2 Grain structures in Mg-9%Al alloys made of commercial purity magnesium metal, average grain size AGS=200 μm (a), and high purity magnesium metal, AGS=140 μm (b)[11]

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Fig.3 Grain structures in Mg-0.5%Al alloys made of commercial purity magnesium metal, AGS=2,500 μm (a), and high purity magnesium metal, AGS=1,500 μm (b); Sample diameter: 25 mm [11]

Fig.4 Schematic of typical temperature profi les (a) and evolution of nucleation particles (b) during superheating [8]

low potency as a nucleation site [5]. It is recently suggested that the metastable x-AlMn phase possesses significantly better crystallographic matching to Mg grain than other Al-Mn intermetallic phases and can act as nucleation sites [9]. Moreover, it can only be produced from the high temperature f-AlMn phase through a massive transformation. Therefore, it is postulated that at the superheating temperature, the high temperature f-AlMn phase is formed and then f-AlMn phase transforms into the metastable x-AlMn phase during the subsequent rapid cooling to the pouring temperature [9].

2 Carbon inoculationDeveloped at the end of World War II, carbon inoculation is another major grain-refi ning process for Mg-Al based alloys. This method is featured with low operating temperature, less fading, short processing time and crucible wear, and therefore favors practical applications [2-3]. It is noted that carbon inoculation grain refinement is only applicable to aluminum-containing magnesium alloys (normally with > 2% Al) [10, 13]. Moreover, the presence of Be, Zr, Ti and rare earth (RE) elements would interfere with grain refi nement of carbon inoculation [2]. As an effective grain refiner, carbon can be added in various forms, such as granular graphite, calcium carbide (CaC2) in flux, organic compounds (e.g. hexachloroethane (C2Cl6), hexachlorbenzene (C6Cl6)), paraffi n

wax, lamp black, carbonaceous gases (e.g. CO, CO2, CH4), and even lumps of magnesite (MgCO3)

[2, 3, 10]. Among those, the hexachloroethane treatment used to be the most popular, however degassing from such treatment can cause severe environmental pollution [2, 10]. Accordingly, some researchers put forward that the high-purity carbon powder or the magnesite particles should be added to replace harmful hexachloroethane in the carbon inoculation treatment [14-15].

In the carbon inoculation treatment, both aluminum and carbon are required to successfully refi ne the solidifi ed grain structure, therefore, it was widely accepted that formed Al4C3 particles are effective nucleants for carbon inoculation treatment [2, 3, 10]. However, recent investigations indicate that in the grain refi nement of carbon inoculation, the nuclei are composed of Al, C and O, and considered Al2CO particles as effective nucleants for grain refi nement of carbon inoculation [16-17]. Theoretical investigations made by Zhang M X also supported this hypothesis, which assumed that Al2CO particles are more effective than the Al4C3 particles as nucleants of a-Mg phase from the crystallography viewpoint [5]. Experimental results by Young Min Kim et al [12] show that carbon inoculation through addition of 0.6wt.% C2Cl6 lead to considerable grain refinement of commercial AZ91 with 0.25wt.% Mn (shown in Fig.5) and Mg-9Al-0.3Mn alloys, while adverse effect was observed for Mn-free Mg-9Al alloys. This might indicate that Mn is necessary for considerable grain refinement of Mg-Al

(a) (b)

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alloys in the light of carbon inoculation. Young Min Kim et al proposed a duplex nucleation theory, i.e. (1) The formation of Al4C3 particles after carbon addition; (2) Al8Mn5 layer formed on the surfaces of Al4C3 particles; and (3) The a-Mg phases nucleated on the surfaces of Al8Mn5 particles [12]. In the mean time, Jin Qinglin et al proposed carbon segregation mechanism based on their observation of carbon segregation in AZ31 castings after treated with carbon (C2Cl6)

[18-19]. Carbon segregation or clustering causes constitutional undercooling ahead of advancing solid/liquid interface, this will induce remelting and therefore restricts the grain growth to achieve the ultimate effect of grain refinement [18]. Jin’s proposal received objection from Ma Qian et al [13] and Kim et al. [12] The focus of such argument is that the solubility of carbon in a molten magnesium alloys is limited (around 22 ppm). Moreover, the commercial purity magnesium generally contains about 20 ppm of carbon, and no grain refi nement has ever been documented [13].

3 The Elfi nal process (FeCl3)The Elfi nal process was fi rst developed in Germany at the end of World War II. This method involves plunging anhydrous FeCl3 into melts at the temperatures between 740 and 780℃ [2, 10], and it

Fig.5 The microstructure changes of AZ91 alloys without (a) and with (b) an addition of 0.6wt.% C2Cl6; average grain sizes are 460 μm and 97 μm, respectively [12]

Fig.6 Grain refi nement of Mg-3%Al alloys by FeCl3 at 750℃ [20]

Fig.7 Grain refi nement of Mg-9%Al alloys by FeCl3 at 750℃ [20]

is effective for grain refi nement of both aluminum-containing Mg-Al based alloys and aluminum-free Mg-Zn based alloys [2]. For aluminum containing Mg-Al based alloys, Zr, Be elements inhibit the refining effect; and Al, Si and Th are inhibiting elements for grain refi nement of aluminum-free Mg-Zn based alloys [2].

It is suggested that the hydrolysis of FeCl3 in the magnesium melt give rise to copious hydrogen chloride (HCl) fumes, which would attack steel crucibles to liberate some free carbon from the surface into the melt; yet another hypothesis assumes that magnesium grains nucleate on Fe or Fe-Al-Mn compound particles [2,10]. A recent examination by Cao P et al indicates that the addition of anhydrous FeCl3 can lead to grain refinement of high-purity Mg-3Al (shown in Fig.6) and Mg-9Al (as shown in Fig.7) alloys melted in the carbon-free aluminum titanite (Al2TiO5) crucibles, thus the authors assumed that grain refinement by Elfinal process has little to do with the Al4C3 particles [20]. In addition, Fe and Al-rich intermetallic particles were usually found in the magnesium grains, which were considered as the nucleants for magnesium grains in Elfi nal process [20]. But further experiment shows the addition of Fe powder in the form of an ALTAB (trademark) Fe75 powder compact (75%Fe, 15%Al and 10% Na-free fl ux) did not give rise to grain refinement [21]. On the other hand, Y Tamura et al show that small addition of Fe would cause the grain coarsening of high-purity Mg-Al alloy ingots [22-23]. Moreover, the decrease of Fe content in B2O3-containing fl ux can significantly refine the grain of AZ91 alloys (shown in Fig.8) [24]. Further and systematic researches in this respect are necessary to rule out the confusions.

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Fig.8 Dependency of grain size on Fe concentration [24]

Although the Elfinal process can significantly reduce the grain size of Mg alloys, the addition of Fe has detrimental effect on corrosion resistance of Mg alloys. In addition, the release of Cl or HCl raises great concerns in environmental impact [3, 10, 24]. Thus, the Elfi nal process has limited industrial application.

4 Zr additionZirconium has an exceptional grain refining ability for magnesium alloys that contain little (impurity level) or no Al, Mn, Si, Fe, Ni, Co, Sn and Sb (because zirconium readily forms stable compounds with these elements) [10, 25]. For example, the addition of zirconium can signifi cantly decrease the grain size of pure Mg [26] (shown in Fig.9), Mg-Zn [27] and Mg-RE [28] type alloys. On the contrary, the addition of Zr leads to grain coarsening for natural fi ne-grained high-purity Mg-9Al alloys due to presence of Al [29]. Although zirconium can be introduced into magnesium through many forms such as zirconium metal, metallic hardener alloys, various zirconium halides and zirconium oxide, it generally was added to the magnesium alloys as Mg-Zr master alloys that contain one third their weight of zirconium [2, 25].

The peritectic mechanism was usually accepted for grain refi nement by Zr additions, i.e., Zr particles fi rstly precipitate as Zr-rich magnesium, which promotes nucleation of primary magnesium grains through peritectic reaction [10]. In addition, it had been assumed that only the dissolved Zr at the time of pouring can cause grain refinement, and the insoluble Zr is irrelevant to grain refinement of Mg alloys [10]. Based on this hypothesis, the zirconium content must be greater than the peritectic composition (0.443%Zr) to refine the grain of magnesium alloys. But recent work, by Ma Qian et al [27, 30] and Y Tamura. et al [31], indicates that an addition of zirconium lower than the peritectic composition can also lead to grain refinement of Mg alloys. Moreover, a-Zr also has hexagonal crystal structure, and its lattice parameters (a=0.323 nm, c=0.514 nm [2]) are nearly the same with those of the magnesium [2, 32]. So they suggested that the undissolved zirconium particles also are effective nucleation sites for magnesium alloy. Statistical study of zirconium particles in the

grain-fi ned magnesium alloys shows that the majority of active particles are in the range between 1 µm and 5 µm in size when solidifi ed in a mild steel cone mold, as shown in Fig.10 [10, 32]. Moreover, Y C Lee et al [33] proposed that the signifi cant grain refi nement of Zr addition for pure magnesium is mainly caused by its high growth restriction effect during solidifi cation. But the fact is that at least one Zr-rich core was found in almost each grain of alloy with high Zr addition (0.40% and 0.51%) (shown in Fig. 11) and no such cores were found in the alloy with a low Zr addition (0.21%) in Mg-9Gd-4Y alloy [34]. Therefore, it is concluded that the restricting grain growth is main refining mechanism for low Zr-addition alloy, and heterogeneous nucleation is responsible for grain refi nement

Fig. 10 Size distribution of active particles observed on polished sections [32]

(a)

Fig.9 Grain sizes of samples solidifi ed under the same cooling conditions from 730℃: pure magnesium (750 μm) (a) and Mg-0.56Zr alloy (50 μm) (b) [26]

(b)

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for high Zr-addition alloy [34]. Without doubt, Zr is an effective grain refi ner for aluminum-

free magnesium alloys. However the high cost is a major drawback for commercial practice.

5 Other element additionsThe addition of solute element usually causes constitutional undercooling in a diffusion layer ahead of the solid/liquid interface, which would restrict grain growth by forming a dendrite-like network in the front melt [33]. The effect of solute element on the grain size can be expressed as the growth restriction factor (GRF) [3]. The GRF is defi ned by

Where mi is the slope of the liquidus line, C0,i the initial composition, and ki is the equilibrium partition coeffi cient for element i [3]. A larger GRF indicates stronger forming tendency of constitutional undercooling ahead of advancing solid/liquid interface, and therefore larger grain growth restriction. Thus, larger GRF is likely to give rise to a small grain size when potential nucleant particles are present [3]. In magnesium alloys, Zr element has relatively larger GRF value compared with other elements, so it possesses stronger grain refining ability as mentioned previously. Similar to Zr, Ca, Sr and Sb can be effective additions for refi ning grain size of magnesium alloys.

5.1 Ca addition Many studies have reported that Ca addition can signifi cantly reduce the gain size of pure Mg [33, 35], AZ31 [35], Mg-5Al-1Zn-1Si [36], AZ63 [37], AZ91 [38-39], AZ91D [40-41] alloys. For instance, the addition of 0.4wt.% and 1.0wt.% Ca make the grain size of AZ91 and AZ91D alloy decrease from 65 µm to 20 µm and from 166 µm to 118 µm, respectively [38, 41]. Ca with GRF of 11.94 possesses strong segregation ability and can be an effective grain refiner through growth restriction mechanism [33]. Other than this, Yuan G Y et al also suggested that the formed CaSi2 particles in Mg-5Al-1Zn-1Si alloy with 0.2wt.%Ca addition promote heterogeneous nucleation

of Mg2Si particles, which restrict the grain growth of a-Mg during the solidifi cation [36].

5.2 Sr additionsIt is well-known that a small amount addition of Sr can effectively refine the pure Mg, AZ31, AZ91 and low Al-containing (1%-3%) magnesium alloy [33, 42-43]. A comparable experiment by Y C Lee et al also shows that Sr addition has a signifi cant grain refi ning effect in low-Al containing alloy but a negligible effect on grain size of Mg-9Al alloy [33]. This suggests that an increase of Al content maybe weaken the refi nement effect of Sr element. Because the solid solubility of Sr in magnesium is relatively small, rapid enrichment of Sr in the liquid ahead of the growing interface would easily occur [33]. Consequently, Y C Lee et al thought that the grain refi nement is mainly caused by its growth restriction effects [33]. Gruzleski et al believed that Sr may poison the grain surface or poison the fast-growing direction of the grains by preferential adsorption of Sr at these sites [33, 42]. Y C Lee et al [33] and Zeng Xiaoqin et al [42] both found that with increasing Sr content, the grain size fi rst decreases, then increases, and fi nally decreases again. Therefore, Zeng Xiaoqin et al [42] proposed two principles: (1) for Sr content below the solid solubility limit, Sr fi lm on the advancing interface of solid/liquid reduces the diffusion rates of Al and Zn solutes and lead to constitutional undercooling resulting in grain refi nement; (2) for Sr content above the solid solubility, Mg16(Al, Zn)2Sr particles are formed and distribute on the advancing interface of solid/liquid to hinder grain growth.

5.3 Sb additionIt was found that a small amount of Sb addit ion can significantly refine the Mg-5Al-1Zn-1Si and AZ91 alloys, and make their grain size decrease from 134 µm to 68 µm and from 115 µm to 80 µm, respectively [36, 44-45]. Experiments made by N Balasubramani et al indicate that the addition of 1.0wt.%Sb makes the grain size of ZA84 reduce from 62 µm to 35 µm [46]. Sb has a small GRF of 0.53, compared with Zr (38.29), Ca (11.94) and Sr (3.51) [10]. Thus, people usually attribute the grain refi nement of Sb addition to other factors. In ZA84 alloys, N Balasubramani et al found that Sb additions give rise to the rod-like Mg3Sb2 particles distributed at the grain boundaries. It is therefore believed that the presence of Mg3Sb2 particles act as effective obstacles restricting grain growth during solidification [46]. Controversially, Yuan G Y et al suggested that Mg3Sb2 particles act as the ultrafine heterogenerous nuclei for generating fi ner Mg2Si particles, and the Mg2Si particles restrict grain growth [36].

Moreover, Nd [47-48], Y [47-48], Ce [49], Ti [50], Si [33], Sc [51] and B [52] etc. all can refine cast Mg alloys. And some elements have other benefi cial effects besides refi nement, for example, Si can improve the creep strength of the alloys by forming Mg2Si particles at the grain boundary [3], and Ca can improve the rolling behavior [2]. It is also recommended that the combination of two or more elements additives may achieve better refi nement of Mg alloys, as evidenced in Fig.12 [3, 43, 53].

Fig.11 Average grain size of Mg-9Gd-4Y alloys as function of zirconium content [34]

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Fig.14 Dependence of grain size of the AZ31 on addition of the master alloy [55]

increase of Al content [56-58]. For example, in the refinement experiments by Liu Yanhui et al, the average grain size of AZ31 and AZ63 alloys decreases from 1,300 µm to 225 µm, and from 300 µm to 200 µm, respectively, but little effect on AZ91 alloy [56]. Mark A Easton et al also reported a similar fi nding [57]. According to theoretical analysis of R Günther et al, the optimum mean diameter of the SiC particles should be about 1-5 µm for grain refi nement of Mg alloys [58]. The larger diameter results in a low number of available heterogeneous sites, and the small diameter particle needs greater undercooling [58]. On the other hand, the following reaction possibly occurs from the thermodynamic viewpoint, 3SiC (s) + 4Al (l) = Al4C3 (s) + 3Si (l), SiC particles would transform to Al4C3 particles, or Al4C3 layer be formed on the surface of SiC particles [56-57, 59]. It is therefore believed that the chemically formed Al4C3 particles might enhance grain refi nement in Mg alloys when SiC is added [56-57, 59]. Liu Yanhui et al found the existence of Al4C3 particles in the prepared SiC master alloy [56]; and the break down of SiC has been suggested with the appearance of Mg2Si phase reported in the Mg-Al alloys with SiC additions [57].

TiB2 has a hexagonal structure with a = 0.3028 nm and c = 0.3228 nm, and the smallest disregistry is 5.6% in the crystallographic orientation relationship of (0001)TiB2

//(0001)

Mg between TiB2 and a-Mg. TiB2 particles also can be added as the heterogeneous nuclei for Mg alloys [55]. Wang Yingxin et al reported that the addition of 0.3wt.% Al-4Ti-5B master alloy made the average grain size of AZ31 magnesium alloy dramatically decrease from 1,100 µm to about 80 µm, as shown in Fig.14 [55].

Fig.12 Relationship between the grain size and the amount of added Sr/B to AZ91 [43]

6 Particle additionIn principle, heterogeneous nucleation for grain refinements can be enhanced in two methods: (1) in situ precipitation from the parent melt through special processing technique (for example carbon inoculation and Elfinal process); and (2) external particle addition, i.e. the particles or particles containing master alloy are directly added to the melt. It was reported that the particles of Al4C3, AlN, SiC, TiB2, and TiC all can be used as the heterogeneous site for primary Mg grains and effectively refi ne grains of Mg alloys [54-58].

It was found that SiC master alloys can significantly refine the grain size of Mg alloys containing small Al contents (shown in Fig.13), but refining efficiency decreases rapidly with the

The Al-1wt.%C master alloy with Al4C3 particles can reduce the mean grain size of AZ63B alloy to approximately 48 µm, which is about 17 % of that of the unrefi ned alloy [60]. Another investigation also shows that the Mg-Al-C master alloy has a remarkable refi ning effect on the grain size of AZ91 alloy, and the corresponding refi nement mechanism is attributed to the formation of large amount of heterogeneous nucleus of Al4C3 or Al-C-O compounds [61].

7 Rapid solidifi cation It is well known that rapid solidification processing (RSP) Fig.13 Microstructure of a conventional AZ31 alloy

refi ned by SiC inoculation [58]

(a) AZ31 (145 μm)

(b) AZ31+0.4wt.% SiC (49 μm)

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is an important grain refinement method. There are two basic techniques for rapidly solidifying melts: substrate quenching and atomization [62]. Substrate quenching refers to the solidification of the melt against one or two surfaces at a lower temperatures (e.g. room temperature, or near liquid nitrogen) [62]. Substrate quenching includes thermal spray methods, melt-spinning technique, planar fl ow casting, copper mold casting, twin rolling etc [62-66]. Atomization is a process of breaking up a molten stream of liquid into small spheres by using gas jet [62]. Gas atomization includes high pressure and centrifugal gas atomization etc [62, 67]. In substrate quenching, rapid solidifi cation is achieved by increasing the rate of heat extraction and in atomization by increasing the amount of undercooling before nucleation [62]. An average grain size of 0.2-3 µm can be achieved in the rapid solidification of Mg alloys, and the rapidly solidifi ed Mg-Al-Zn system presented an outstanding ultimate tensile strength of about 500 MPa [63, 67]. Besides microstructure refi nement, RSP can effectively extend solid solubility in magnesium, for example 1.5 times for Mg-Ag and about 1,000 times for Mg-Ba alloys [68].

The combination of grain refinement and solid solution hardening effect makes RSP a suitable technique for enhancing the mechanical properties and corrosion resistance of Mg alloys. To fabricate structural components, subsequent thermal mechanical processing (e.g. extrusion, forging or rolling and consolidation) is necessary. Depending on the working temperature and processing rate, such hot working signifi cantly impacts the structure of the as-solidifi ed Mg alloys [67, 69-70]. It should be pointed out that RSP of magnesium alloys poses critical challenges due to the high chemical reactivity of magnesium [63].

8 Physical grain refi ning methodsPhysical grain refi ning methods involve promoting nucleation, dispersion and multiplications of solidified crystals under mechanical force or external physical fi eld without any further chemical additions. Physical grain refinement generally targets creating a favorable condition for nucleation and nuclei survival or breaking the solidifi ed crystal structures [4].

Grain refi nement by mechanical force can be achieved by melt shearing near liquidus temperature before the casting or mold vibrations during solidification [4, 71]. Shearing melt near the liquidus temperature before casting using twin-screw device leads to remarkable grain refi nement in Mg-6wt.%Zn alloy, as reported by A Das et al [4]. The authors attributed grain refinement to uniform temperature and dispersion of natural nucleates in the liquid at its solidifi cation temperature, resulting from the fast heat transfer and intense turbulence by melt shearing in the twin-screw device [4]. In the high-purity magnesium and a commercial AZ91 magnesium alloys, the grains were signifi cantly refi ned by mold vibrations at a frequency of 160 Hz and an amplitude of 0.12 mm [71]. The reasons for grain refi nement are the detachment and separation of dendrites from the mold caused by the vibration [71].

Grain refi nement of Mg alloys by electromagnetic stirring

has been extensively studied. Electromagnetic stirring can be generated by several methods, such as running an alternating or pulsed current and rotating a static magnetic field [7, 72-75]. AZ31, AZ91D and pure magnesium alloys were signifi cantly refined by imposing of a magnetic field with an alternating current, as shown in Fig.15 [7, 73, 75]. Depending on the alloy, it was found that there was an optimal frequency for effective grain refi nement. The optimal frequencies are 500-2,000 Hz, 200 Hz and 500 Hz for AZ31, AZ91D and pure magnesium alloys, respectively [7, 73, 75]. Yoshiki Mizutani et al proposed that the major reason for the grain refi nement is the collapse of dendrite arms due to cavitation phenomenon caused by the electromagnetic stirring [73, 75]. Li Mingjun et al thought that low frequency can not produce cavitation phenomenon,

Fig.15 Effect of vibration caused by a magnetic fi eld with an alternating current on grain size

(C) AZ91D (B=1.6T, I=42A) [73]

(a) Pure magnesium (B=10T, I=60A) [75]

(b) AZ31 (B=10T, I=60A)[7]

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and proposed that relative velocity and relative displacement, resulting from the signifi cant difference in electronic resistivity between the solid and the liquid, lead to grain refi nement of Mg alloys [7].

Moreover, AZ31 and AZ80 direct-chill (DC) casting billets could be effectively refi ned by low-frequency (30 Hz) electromagnetic stirring, as shown in Fig.16 [50, 76]. It was thought that the low-frequency electromagnetic stirring makes the initial solidifi ed crystals close to mold wall dissociate into the melt and breaks the dendritic arms, then these crystals are uniformly dispersed in the entire liquid metal by the forced convection. In addition, the forced convection can also promote heat transfer and reduce the temperature gradient of the melt. So the low-frequency electromagnetic stirring can signifi cantly refi ne the grain of DC casting billets of Mg alloys [50, 76].

(a) AZ80 [76]

(C) Schematic diagram of experimental apparatus in the AZ80 alloy (upper coil with direct current; lower coil with alternating current) [76]

(b) AZ31 [50]

In a recent experiment by Yang Yuansheng et al, AD91D Mg alloy was refi ned by using the pulsed electric current [74,77]. Even, the application of a static magnet fi eld also can be used to refi ne ZK60 magnesium alloy [78].

Another physical grain refi ning method of magnesium alloy is ultrasonic vibrations [79-80]. The AZ31, AM60B, AZ91 and pure magnesium all were significantly refined by ultrasonic vibrations [79-80]. The grain refi nement mechanism results from the cavitation phenomenon caused by ultrasonic vibrations.

9 Summary Fine grain size can result in structural uniformity and enhance the mechanical properties, hence improving the service performance of the products. During the solidification, the grain refi nement can be achieved by three major mechanisms: promoting the heterogeneous nucleation, restricting the grain growth and breaking the solidifi ed crystals (multiplication of the crystals). For Mg alloys, many grain refi nement methods have been developed, but their refining mechanism are still unclear. For example, as the effective grain refinement method, there still is debate in the heterogeneous nuclei for superheating and carbon inoculation of aluminum-containing magnesium alloys.

A significant amount of research has been focused on the grain refiner for the magnesium alloys, yet there is not a universally reliable commercial grain refiner, especially for aluminum-containing magnesium alloys. Although aluminum-free magnesium alloy can be readily refi ned by Zr, its cost is high. Moreover, the grain refiner is likely to lead to a detrimental impact on the properties of Mg alloys and make it difficult to recycle the alloys. Maybe, the external physical fi eld or the grain refi ner combined with the external physical fi eld will be a better alternative for gain refi nement of Mg alloys. Further investigations are needed for a more comprehensive understanding of the grain refi ning mechanism, and to develop reliable commercial grain refiners or novel grain refi nement processes.

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This work was supported by National Natural Science Foundation of China (No.50701030) and China Postdoctoral Science Foundation (No.: 20070410716).