Investigation on the microstructural refinement of an Mg 6 wt.% Zn alloy

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1 Materials Science and Engineering A 419 (2006) Investigation on the microstructural refinement of an Mg 6 wt.% Zn alloy A. Das, G. Liu, Z. Fan Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge UB8 3PH, UK Received 11 July 2005; received in revised form 9 January 2006; accepted 12 January 2006 Abstract The possibility to refine the microstructure of an Mg 6 wt.% Zn alloy utilising a physical grain refinement mechanism employing melt shearing under turbulence and casting near the liquidus temperature has been explored and compared against chemical grain refinement by Zr master alloy. The extremely coarse dendritic solidification microstructure of the base alloy can be somewhat refined through near liquidus-temperature casting. Employing melt shearing under turbulence in conjunction with liquidus-temperature casting promotes dramatic grain refinement and uniform polycrystalline microstructure throughout the cross section. The extent of microstructural refinement and uniformity achieved is better than chemical grain refinement using Zr containing master alloy in this investigation possibly due to inefficient Zr dissolution in the latter. The combination of chemical and physical grain refining promotes about a further 10% reduction in grain size over sheared and liquidus cast microstructures Elsevier B.V. All rights reserved. Keywords: Mg-alloy; Grain refinement; Solidification; Microstructure; Forced convection 1. Introduction Due to its high strength to weight ratio and being the lightest of all structural metals, Mg and Mg-based alloys are increasingly used for structural, aerospace and automotive applications among others. The increasing demand for reduced emission levels is expected to continue the growth of Mg in the automotive industry in the coming years [1]. The key potential of Mg usage in the automotive industry lies in the successful evolution of Mg wrought products that now constitute a meagre 1 2% of total Mg usage [2]. However, the main deterrent to the development of Mg wrought alloys is poor formability at room temperature. The critical resolved shear stress for basal slip at room temperature is 1/100th of those of prismatic or pyramidal slip, and thereby, slip in coarse-grained Mg occurs predominantly by basal slip constituting only two independent slip systems, far lower than the five necessary for homogeneous deformation [3]. Interestingly, recent research activities have revealed that grain refinement can substantially improve the room temperature ductility of Mg alloys by introducing cross slip to non-basal planes and increased Corresponding author. Tel.: ; fax: address: Amitabha.Das@brunel.ac.uk (A. Das). activity of the grain boundaries such that elongation of more than 46% is achievable in fine-grained alloys [3,4]. In view of this, grain-refinement in Mg-alloys has recently raised considerable interest from the research community. Considerable research activities have concentrated on equal-channel-angular-extrusion (ECAE) of Mg-alloys to achieve significant grain refinement and enhanced ductility [3 6]. However, recent observations seem to suggest that ECAE produces a strong deformation texture that persists after recrystallisation resulting in anisotropic tensile properties that are favourable only along the extrusion axis due to such texture development [7]. A more conventional grain refinement approach to Mg-alloys is through grain-refiner addition during solidification. However, the vast majority of Al-containing Mg-alloys still do not have a suitable grain refiner. On the other hand, Zr is the most powerful grain refiner for Mg alloys free from alloying elements such as Al, Si, Sn, Ni, Fe, Co, Mn and Sb [8]. As such Zn (and rare earth) containing alloys are the major Mg alloy group grain refined by Zr at present. Zr is almost exclusively added to the melt as Mg Zr master alloy, the most notable one is the 33.3 wt.% Zr containing Zirmax alloy developed and marketed by Magnesium Elektron Limited. Although the maximum solubility of Zr in molten Mg is 0.6% (all the compositions are expressed in wt.% unless otherwise stated), in reality a much /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.msea

2 350 A. Das et al. / Materials Science and Engineering A 419 (2006) larger Zr addition to the melt is practiced (as high as 2%) to result in the necessary grain refinement effect. This originates from the fact that higher specific gravity of Zr makes the Zr particles settle to the bottom and a large Zr concentration is needed at the bottom of the melt to maintain a necessary concentration gradient in the melt [8]. Not only does this make grain refinement through Mg Zr master alloys an expensive process but the effect of grain refinement is reported to fade on subsequent melting, necessitating further additions of the master alloy to retain the grain refinement effect [8]. It has also been reported that at Zn concentrations higher than 4%, the soluble Zr (and grain refinement effect) is reduced probably due to the formation of Zn Zr intermetallics [9]. Although it has been suggested that the efficiency of grain refiner utilisation can be improved by alloying at lower temperature and coating steel crucibles with boron nitride to prevent C and Fe pickup [10], a more effective utilisation is needed for cost reduction. Moreover, Zr grain refinement being limited to specific alloying additions, a more universal approach to grain refinement of wrought Mg-alloys via solidification will be beneficial for future development and usage of such alloys. In the present investigation, a universal physical grain refinement approach is explored through near-liquidus temperature casting in conjunction with intensive melt shearing. The aim is to create an ideal condition for copious nucleation and then ensuring high survival rate of such nuclei to achieve microstructural refinement through solidification. The physical refining approach is compared against the conventional chemical grain refinement approach through Zr containing master alloy (Zirmax ) addition for the Mg 6% Zn alloy. Finally, a combination of both physical and chemical refining is also investigated. 2. Experimental The Mg 6% Zn alloy was prepared by melting pure Mg (99.9% purity, composition of base ingot given in Table 1) and Zn (99.95% purity) in a mild steel crucible under a protective cover gas (SF 6 +N 2 ) and casting into small billets. Composition analysis suggested that the alloy has a nominal composition of 5.6% Zn. For the experiments the billets were melted under similar conditions at 700 C in an electrical resistance furnace. Zr was added to some of the alloy melts at 2% level through preheated Zirmax addition. The melt was puddled with a mild steel rod consisting of a perforated plate at the end (coated with barium nitride) for about 50 strokes, then held for 5 min without disturbing followed by puddling, held for another 2 min followed by puddling and immediately either cast into a steel mould of 32 mm diameter and 120 mm height or sheared in a twin-screw device [11] followed by casting into the steel mould. The steel mould was maintained at ambient temperature. Fig. 1 schemati- Table 1 Composition (wt.%) of the pure magnesium ingots used for alloying Impurity content (wt.%) Zn Al Si Cu Mn Fe Ni Mg (wt.%) Balance Fig. 1. Schematic illustration of the geometry and the dimensions of the steel mould including the mould cavity (sample geometry). Microstructural investigation was conducted on the edge and the centre of the shaded section as indicated by the black circles. cally illustrates the geometry of the cast sample and the section selected for microstructural observations. The twin-screw device was operated at 632 C at 300 rpm for 35 s prior to casting of the melt. Alloy melts without Zr addition were sheared under the same conditions inside the twin-screw device and cast similarly. Representative samples were cut as circular disks 35 mm from the bottom of the cast billet (Fig. 1), ground and polished and etched with a solution containing 2 g oxalic acid in 100 ml distilled water. Microscopic observations were conducted in a ZEISS Axioscop2 MAT optical microscope equipped with an automated Zeiss AxioVision image analyser and a JEOL JXA- 840A Scanning Electron Microscope (SEM) equipped with an EDX system operating at 20 kv. To compare against the sheared specimen, direct casting from 700 and 638 C into the steel mould and from 700 C in a wedge shaped Cu mould was also performed. All the solidification experiments were conducted at least three times to ensure reproducibility of the results. 3. Results 3.1. Effect of melt superheat and shearing on cast microstructure Fig. 2a and b present microstructures from the alloy specimen solidified from 700 C in the steel mould from the edge and the centre of the section, respectively (as indicated in Fig. 1). It is evident from Fig. 2 that the solidification morphology consists

3 A. Das et al. / Materials Science and Engineering A 419 (2006) Fig. 2. Optical micrographs from (a) the edge and (b) the centre of the specimen cast into the steel mould directly from 700 C. Note the huge directional dendrites formed at the mould wall and coarse dendrites formed at the centre of the mould. of huge dendritic grains (few millimetres long) growing in a directional manner near the mould wall and in a more equiaxed growth pattern near the centre of the specimen as expected out of the heat transfer direction during solidification. What is noteworthy are the extremely long dimension and the well-developed dendritic nature of the columnar grain structure near the mould wall. Casting the alloy into a wedge shaped Cu mould also indicated the formation of well-developed dendrites (instead of chill zone) even at the tip of the mould where the cooling rate is in excess of 1000 C/min. Significant refinement of the microstructure including elimination of the huge columnar dendrites can be obtained through casting near the liquidus temperature as opposed to the higher superheat casting. Fig. 3a and b represent microstructures from the alloy cast into the steel mould from 638 C. The liquidus temperature of the alloy was determined to be around 632 C through CALPHAD calculations. Melt handling became progressively more difficult below 638 C for the normal casting process due to the uneven cooling of the melt and initiation of solidification at the surface of the melt. Fig. 3 shows considerable refinement of microstruc- Fig. 3. Optical micrographs from (a) the edge and (b) the centre of the specimen cast into the steel mould from 638 C, close to the liquidus temperature of the alloy (632 C). The tendency to columnar dendrite formation has been eliminated. ture compared to the one cast with high superheat (Fig. 2). The columnar dendrites have been replaced by a smaller equiaxed grain structure that appears to originate from coarsening of small dendrites, the collapsed dendritic arms left liquid pocket inside many grains. The microstructure still appears slightly nonuniform comprising larger and smaller grains throughout the specimen cross section. Remarkable grain refinement is observed when the melt was poured at 700 C into the twin-screw device, which was maintained at 632 C (barrel temperature) and sheared for 35 s at 300 rpm followed by casting into the steel mould. The microstructure from the edge and the centre of the billet is presented in Fig. 4a and b, respectively. The grain structure throughout the specimen is polygonal and considerably finer than that in the normal low superheat cast specimen (Fig. 3). The grain structure also appears to be very uniform throughout the specimen. Fig. 5 presents a backscattered electron image from the specimen. The grain boundaries appear thicker due to Zn enrichment and precipitation is evident in the form of bright white phase. EDX spectra from the precipitates confirm them

4 352 A. Das et al. / Materials Science and Engineering A 419 (2006) Fig. 4. Optical micrographs from (a) the edge and (b) the centre of the specimen cast into the steel mould from 632 C following shearing inside the twin-screw device at 300 rpm for 35 s. Note the uniform grain structure and dramatic refinement achieved by shearing in conjunction with liquidus temperature casting. to be Mg Zn intermetallics. The uniform and refined polycrystalline grain structure is clearly evident in the micrograph and further corroborates the microstructural observation from Fig Microstructural observation in Zr grain-refined specimen Conventional chemical grain refining of the Mg 6Zn alloy was performed by Zr-master alloy (Zirmax ) addition and casting into the steel mould for a direct comparison with refining observed in the sheared and liquidus-cast specimen. Fig. 6a and b presents the microstructure from the edge and the centre of the grain refined sample cast into the steel mould directly from 700 C. The excellent grain refining effect of Zr is clearly evident compared to the non grain-refined specimen under similar casting conditions (Fig. 2). Large dendrites have been completely replaced by equiaxed and dramatically refined grain structure. The variation in the grain size from the edge towards the centre of the billet is larger compared to that in the sheared and liquidus- Fig. 5. (a) Backscattered electron image from the specimen cast in the steel mould from 632 C following shearing inside the twin-screw device at 300 rpm for 35 s. Grain boundaries appear thick due to Zn enrichment. (b) EDX spectra from the grain boundary phase (marked by arrow in (a)) confirm them to be Mg Zn intermetallic. cast specimen (Fig. 4). It should be noted that the chemical grain refinement effect is not optimised in the present castings where the primary objective is to illustrate the refining effect under comparable conditions rather than optimising the effect of grain refinement under various conditions. Under the present conditions, both the sheared and liquidus-cast, and the chemically grain refined specimens show a similar grain size at the edge of the casting. However, towards the centre of the billet, where the cooling rate is low during solidification, the former shows a finer grain structure compared to that of the latter. It should also be noted that in all the casting experiments around 20 ppm Be was added in the form of Al 5%Be master alloy to prevent oxidation. It has earlier been shown that Be addition to Mg promotes grain coarsening [12]. Although the effect of such coarsening is noticeable beyond 10 ppm addition, the effect becomes prominent beyond 50 ppm addition level. Moreover, Al forms a stable compound with Zr and therefore Al Be addition is not practiced in alloys grain refined with Zr. However, at such low addition level the effect is probably small as evidenced from the reasonably efficient grain refining observed in the samples.

5 A. Das et al. / Materials Science and Engineering A 419 (2006) Fig. 6. Optical micrographs from (a) the edge and (b) the centre of the specimen cast into the steel mould from 700 C following Zirmax addition at 2% Zr level. Grain refinement and conversion of dendritic structure to an equiaxed grain structure is evident. Dark regions represent clusters of excess Zr particles. Although up to 2% Zr addition level is normal in large-scale castings in order to maintain the necessary Zr concentration level in the melt, such high addition level for the small castings in the present work resulted in excess insoluble Zr particles in the microstructure. Dark regions in the micrographs in Fig. 6a and b represent clusters of insoluble Zr particles. The combined grain refining efficiency through Zr addition and liquidus temperature casting under melt shearing was also investigated in the present work. Fig. 7a and b present the microstructures from the edge and centre of the steel mould cast sample containing Zr following shearing in the twin-screw device at 300 rpm for 35 s at 632 C. The melt was introduced in the twin-screw device at 700 C where it is immediately cooled under intense turbulence to 632 C. The microstructures show a further improvement in the refiner efficiency compared to the grain-refined specimen cast without shearing (Fig. 6). Moreover, the large clusters of undissolved Zr particles frequently observed in the grain-refined sample (Fig. 6) are completely absent following shearing in Fig. 7. This was further confirmed by microstructural examination of the specimen under SEM as illustrated in Fig. 8 indicating Fig. 7. Optical micrographs from (a) the edge and (b) the centre of the specimen (containing Zirmax addition at 2% Zr level) cast into the steel mould from 632 C following shearing inside the twin-screw device at 300 rpm for 35 s. Note the more uniform grain size, reduced grain size, and the absence of Zr clusters compared to the specimen cast without shearing (Fig. 6). that shearing has helped in refining and dispersing the undissolved Zr particle clusters throughout the microstructure. The average grain size calculated from the edge and the centre of the observed sections in the different specimens are presented in Table 2. Each average was calculated from 15 to 25 individual linear intercept measurements each consisting of at least 50 grains. The measured average grain size appears to be consistent with the microstructural observation. The sheared and liquidus cast specimens show a dramatic grain refinement and grain size uniformity over the entire sample section compared to nearliquidus cast specimens without melt shearing. The chemically grain refined specimens show a larger grain size towards the centre of the casting, although still below 42 m, and the maximum variation in grain size across the cross-section among the different grain refined specimens. The refinement is maximum in the specimens combining chemical grain refinement and melt shearing and liquidus casting, although the variation in grain size with the sheared and liquidus cast specimen is relatively small.

6 354 A. Das et al. / Materials Science and Engineering A 419 (2006) Discussion The present investigation clearly demonstrates that the extremely coarse dendritic solidification morphology in the Mg 6%Zn alloy can be dramatically refined into an equiaxed fine-grain structure through three grain refining approaches; chemical grain refinement, physical grain refinement or a combination of these two approaches. The mechanisms and relative merits of these approaches are discussed below Physical grain refinement by increased effective nucleation Fig. 8. Backscattered electron image from Zirmax added specimen: (a) cast into the steel mould from 700 C. Arrows at the bottom right corner indicate the clusters of Zr particles, (b) cast in the steel mould from 632 C following shearing inside the twin-screw device at 300 rpm for 35 s. Dispersion of Zr particles throughout the microstructure is evident in the form of tiny white dots. Network phase at the grain boundary are Mg Zn intermetallics. Table 2 Average grain size at the edge and the centre of the observed section (Fig. 1) from specimens cast under different conditions Processing condition Cast into a steel mould from 638 C Sheared at 300 rpm for 35 s, cast into a steel mould from 632 C Zirmax addition at 2% Zr level, no shearing, cast into a steel mould from 700 C Zirmax at 2% Zr addition level, sheared at 300 rpm for 35 s, cast into a steel mould from 632 C Average grain size ( m) Edge Error tolerance is reported at 95% confidence level. Centre ± ± ± ± ± ± ± ± The creation of an ideal condition for nucleation and ensuring high nuclei survival has been employed as a physical grain refinement strategy in the present investigation. Consequently, physical grain refinement increases effective nucleation by tailoring the solidification conditions without necessitating the addition of inoculants. Casting near the liquidus temperature has been known to promote fine equiaxed microstructure [13 15]. There has been significant controversy in explaining the columnar to equiaxed transition (CET) in castings without grain refiner addition. In a comprehensive overview, Hutt and StJohn have discussed the five major available theories and critically assessed the applicability of the proposed mechanisms [16]. It has been concluded by the authors that all proposed mechanisms or a combination of them may be operative depending on the alloy composition, casting conditions or the types of nucleating substrates present. A similar comprehensive analysis of CET and the plausible mechanisms have been discussed by Flood and Hunt [17]. Both of these reviews suggest that in the absence of grain refiner (where constitutional supercooling driven nucleation is important), big bang (also known as free chill crystal or wall mechanism) and dendrite detachment mechanisms are the primary contributors to the creation of equiaxed grains [16,17]. During low superheat casting the convection associated with the mould filling remains strong as solidification commences. Although it is argued that deformation or melting of the dendrite arms is promoted by the fluid flow [18], the big bang mechanism becomes progressively important as the melt superheat is reduced. Low superheat casting indeed promoted refinement of microstructure as evidenced in Fig. 3 compared with Fig. 2 for high superheat casting. The refinement is still not comparable to the extent of chemical grain refinement (Fig. 6) and the microstructure is non-uniform. However, the columnar growth of dendrites near the mould wall has completely been eliminated. The non-uniformity of microstructure is indicative of temperature non-uniformity and the coarsening of small dendrites is evident in the form of liquid entrapped grains probably during the release of latent heat of fusion. In comparison, the extent of refinement achieved due to melt shearing in conjunction with liquidus casting (Fig. 4) is more remarkable than refinement achieved by low-superheat casting alone (Fig. 3). In the present investigation, the extent of such physical grain refinement is superior to chemical grain refinement as shown in Table 2. The twin-screw device consists of a

7 A. Das et al. / Materials Science and Engineering A 419 (2006) pair of closely matched screws that are capable of delivering very high intensity shear and a high level of turbulence in the melt even at low rotational speed. Moreover, inside the twin screw the entire melt is broken up into very small packets that are in intimate contact with the screw surfaces and the barrel resulting in very efficient heat transfer. The combined effect of such fast heat transfer and intense turbulence results in very uniform cooling of the melt and complete temperature and compositional homogenisation of the entire melt close to its melting point. In addition, foreign particles present that can act as heterogeneous nuclei are uniformly dispersed throughout the melt by the intense turbulence inside the twin-screw device. As a result of the processing conditions, the melt of uniform temperature close to its melting point with uniformly dispersed natural nucleant particles is poured out of the twin-screw device into the steel mould held at ambient temperature. Consequently, the whole melt can undercool uniformly (and quickly) and the latent heat is quickly dissipated preventing local heating. Such a condition appears to favour the big bang nucleation over dendrite fragmentation. Nucleation can occur very efficiently throughout the melt and most of the chill crystals formed and detached from the mould wall due to the convection (and distributed in the bulk of the melt) can survive and contribute to the uniform equiaxed microstructure and significant grain refinement. Moreover, the globular non-dendritic evolution of the microstructure also suggests an enhanced nucleation event and early impingement due to particle crowding, as equiaxed growth in the undercooled melt is still likely to occur in a dendritic fashion otherwise. It is not known if the intense turbulence in the melt is still remnant and plays any role during the fast solidification of the sheared melt in the steel mould. However, establishing an uniform temperature and dispersion of natural nucleants in the liquid at its solidification temperature along with the fast heat extraction during solidification creates the ideal conditions for copious nucleation and ensures the survival of most of the nuclei formed thereby increasing the number of effective nucleation events. The advantage of melt shearing over normal low-superheat casting originates from the ability of intense turbulent mixing in completely homogenising the entire melt with respect to composition and temperature; especially while handling a large quantity of melt. In addition, shearing in the twin-screw device is able to cool and maintain the liquid much closer to its liquidus temperature without encountering melt handling problems as observed without shearing Chemical grain refining by Zr master alloy and combined chemical and physical refinement Zr has long been identified as a notable grain refiner for Mg [8] but the mechanism of grain refinement by Zr is not yet completely understood. It has been proposed that the refinement effect is caused by peritectic reaction involving soluble Zr in the melt [8]. Although the maximum solubility of Zr in molten Mg is approximately 0.6% [19], in reality a lower dissolved Zr content is often obtained due to the low alloying efficiency using the Zirmax master alloy [10,20]. The characteristic Zrrich cores observed in the grain interiors of grain-refined Mg and thought to be representative of the peritectic solidification [21] were absent in the present specimens suggesting that dissolved Zr in the melt was considerably lower than the peritectic limit. In Mg Zn alloys, the dissolved Zr content was observed to decrease beyond 4% Zn and precipitation of Zn Zr intermetallic phases was suggested to be responsible for the decrease in soluble Zr [9]. In the present investigation, Mg Zn Zr precipitates were identified from EDX analysis in the grain-refined specimen indicative of a reduction of dissolved Zr content in the melt. However, the low dissolved Zr content of the melt cannot be accounted for solely due to the high Zn content and minor Al Be addition. It is likely that the high Fe pickup from the steel crucible and stirrer used in the experiments have prevented Zr from going into solution. Consequently, the chemical grain refinement effect observed in the specimen is not ideal and is presumably contributed by the large quantity of undissolved Zr particles rather than the peritectic solidification involving soluble Zr. This, however, illustrates that good chemical grain refinement necessitates very careful melting practice and cannot be guaranteed by use of sufficient grain refiner addition alone. The comparatively large difference in grain size from the edge to the centre of the specimen is a direct consequence of different undercooling achieved in those parts. The superior grain refining obtained through combined effect of melt shearing at the liquidus temperature and Zr addition can be viewed from the dissolved and undissolved Zr content on the refining efficiency. Qian et al. have earlier observed no noticeable change in the total dissolved Zr content on prolonging the stirring time in a Mg melt refined using Zirmax [10]. It, therefore, appears that melt shearing will not improve grain-refining efficiency due to dissolution of Zr assisted by the intense shearing. In addition, there is a further possibility of Fe contamination from the twin-screw device reducing the dissolved Zr content, although the processing time inside the device is short. Intense shearing may, however, increase the refining efficiency by deagglomerating Zr clusters, reducing the rate of settling of the Zr particles, and dispersing them uniformly throughout the melt as evident in Figs In practical casting undissolved Zr clusters may remain in the microstructure unless the melt is allowed to settle for sufficient time prior to pouring [22]. Shearing can reduce the detrimental effect of such undesirable inclusions by breaking the clusters and dispersing the particles evenly throughout the microstructure. The increased refining efficiency is a direct consequence of better utilisation of the numerous nucleating Zr particles dispersed throughout the melt under the ideal condition for nucleation and nuclei survival created through the physical grain refinement mechanism resulting in the finest grain structure in the present study. Chemical grain refinement can be very effective in refining selected Mg-alloys through carefully designed master alloy addition, careful selection of melting environment to prevent Fe and C pickup, and controlled solidification of the grain refined melt. Chemical grain refinement through Zr addition is also effective within a wide temperature range (between 680 and 780 C as reported) [20]. However, the implication of the

8 356 A. Das et al. / Materials Science and Engineering A 419 (2006) physical grain refinement approach (without necessitating inoculation) is significant. This offers a cost effective solution to grain refinement over expensive chemical refinement using Zr containing master alloys. In addition, large Zr and Zr containing intermetallic inclusions, detrimental to the mechanical performance of the casting, are avoided. The most significant implication of physical grain refinement is its universal applicability for wrought Mg alloys without an efficient grain refiner, such as Al containing alloys. 5. Conclusions In this work the microstructural refinement of a Mg 6 wt.% Zn alloy has been investigated through different means including Zr-inoculation, liquidus casting and melt shearing, and combination of these methods. The following conclusions can be drawn under the conditions of this study: (1) Solidification of the base alloy with high superheat produces an extremely coarse microstructure consisting of huge and well-developed dendritic grains. The microstructure can be greatly refined through casting near the liquidus temperature but still suffers from an uneven microstructure and presence of coarsened rosette type grains. (2) Dramatic grain refining and extremely uniform grain size over the entire sample section has been obtained in the Mg 6 wt.% Zn alloy by employing intensive melt shearing combined with liquidus temperature casting. The temperature and compositional homogenisation of the melt near its liquidus under turbulence creates an ideal condition for physical grain refinement involving undercooling of the entire melt, copious nucleation and, nuclei survival improving the nucleation efficiency. (3) Chemical grain refinement through Zr containing master alloy addition produces refined polycrystalline grain structure in samples cast from high temperature but the grain size variation from the edge to the centre of the samples is still considerable as compared to physical grain refinement in the present investigation, where the relatively poor refinement is contributed to by the low dissolved Zr despite the addition of sufficient amount of master alloy. (4) Combining the physical and chemical grain refinement by shearing of Zr refined melt under turbulence and casting near the liquidus temperature produces the finest grain structure although the extent of refinement is marginal over physically grain refined samples. Melt shearing also appears to deagglomerate and disperse the clustered zirconium particles uniformly in the microstructure. Acknowledgements Financial support from EPSRC (UK) and material support from Magnesium Elektron Ltd. (UK) are gratefully acknowledged. The authors would like to acknowledge Dr. Ma Qian of BCAST for helpful discussions. References [1] T. Kaneko, M. Suzuki, in: Y. Kojima, T. Aizawa, K. Higashi, S. Kamado (Eds.), Magnesium Alloys 2003, Trans Tech, Zurich, 2003, pp [2] M.M. Avedesian, H. Baker, ASM Speciality Handbook Magnesium and Magnesium Alloys, ASM, 1999, p. 66. [3] J. Koike, T. Kobayashi, T. Mukai, H. Watanabe, M. Suzuki, K. Maruyama, K. Higashi, Acta Mater. 51 (2003) [4] T. Mukai, M. Yamanoi, H. Watanabe, K. Higashi, Scripta Mater. 45 (2001) [5] W.J. Kim, C.W. An, Y.S. Kim, S.I. Hong, Scripta Mater. 47 (2002) [6] A. Yamashita, Z. Horita, T.G. Langdon, Mater. Sci. Eng. A 300 (2001) [7] S.R. Agnew, J.A. Horton, T.M. Lillo, D.W. Brown, Scripta Mater. 50 (2004) [8] E.F. Emley, Principles of Magnesium Technology, Pergamon, Oxford, 1966, pp [9] Z. Hildebrand, M. Qian, D. StJohn, M. Frost, in: A.A. Luo (Ed.), Magnesium Tech., TMS, 2004, pp [10] M. Qian, D. Graham, L. Zheng, D.H. StJohn, M.T. Frost, Mater. Sci. Tech. 19 (2003) [11] S. Ji, Z. Fan, Mater. Sci. Eng. A 299 (2001) [12] P. Cao, M. Qian, D.H. StJohn, Scripta Mater. 51 (2004) [13] K. Xia, G. Tausig, Mater. Sci. Eng. A 246 (1998) [14] H. Wang, D.H. StJohn, C.J. Davidson, M.J. Couper, Aluminium Trans. 2 (2000) [15] H. Wang, C.J. Davidson, D.H. StJohn, in: G.L. Chiarmetta, M. Rosseo (Eds.), Proceedings of Sixth International Conference on Semisolid Processing of Alloys and Composites, Edimet Spa, Brescia, Italy, 2000, pp [16] J. Hutt, D. StJohn, Int. J. Cast Met. Res. 11 (1998) [17] S.C. Flood, J.D. Hunt, ASM Handbook, vol. 15, ASM, Materials Park, 1998, p [18] W. Kurz, D.J. Fisher, Fundamental of Solidification, Trans Tech, Zurich, 1998, p. 89. [19] H. Okamoto, J. Phase Equilib. 23 (2003) [20] M. Qian, D.H. StJohn, M.T. Frost, in: H. Kaplan (Ed.), Magnesium Tech. 2003, TMS, 2003, pp [21] M. Qian, D.H. StJohn, M.T. Frost, Scripta Mater. 46 (2002) [22] M. Qian, L. Zheng, D. Graham, M.T. Frost, D.H. StJohn, J. Light Met. 1 (2001)