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Modification of Mg2Si morphology in Mg–9%Al–0.7%Si alloy by the SIMA process
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0–3 6 6
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Modification of Mg2Si morphology in Mg–9%Al–0.7%Si alloy by the SIMA process G.R. Ma, X.L. Li⁎, L. Li, X. Wang, Q.F. Li Center for Biomedical Materials and Engineering, College of Materials Science and Chemical Engineering, Harbin Engineering University, No. 145 Nantong Street, Harbin 150001, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
The effect of predeformation on the semisolid microstructure of Mg–9%Al–0.7%Si alloy
Received 22 July 2010
produced by the strain induced melt activation (SIMA) process was studied. Experimental
Received in revised form
results indicated that a non-dendritic microstructure could be obtained by the SIMA process
9 September 2010
in the alloy. With the reduction ratio varying from 0% to 30%, the volume fraction of liquid
Accepted 7 January 2011
gradually enlarged from 37.26 vol.% to 48.28 vol.%, and the morphology of α-Mg grains transformed from dendritic shape to spherical shape, with a size of 52.16 μm. Also, the liquid
Keywords:
islands in the grains become less with the increase of the reduction ratio. In addition, the
Predeformation
eutectic Mg2Si phase transformed from Chinese script shape to globular shape during the
Semisolid
isothermal heat treatment. A possible reason of the modification of Mg2Si morphology is the
SIMA
shifting of the eutectic composition of the liquid towards lower silicon contents with
Mg2Si phase
increasing Al content.
Modification
1.
Introduction
Magnesium alloys containing Mg2Si particles have received a wide attention since the intermetallic compound Mg2Si exhibits high melting temperature, low density, high hardness, low thermal expansion coefficient and reasonably high elastic modulus [1–3]. However, Mg2Si phases in the Mg–Al–Si alloys are prone to forming undesirable, coarse Chinese script shape under a low solidification rate resulting from the eutectic reaction, which will deteriorate the mechanical properties of the magnesium alloys [4]. In order to modify the Chinese script shaped Mg2Si phases in the Mg–Al–Si alloys, many methods have been studied, such as hot extrusion [5,6], rapid solidification [7], mechanical alloying [8,9] and micro-alloying (Sb, Ca, and P) [10–15]. But some investigations indicated that these methods may lead to the increase of the production cost and other problems [11,15,16].
© 2011 Elsevier Inc. All rights reserved.
Semisolid metal (SSM) processing has been developed rapidly since the 1970s [17]. SSM processing is regarded as an advanced forming technology for magnesium alloy. The methods to obtain a semisolid structure are mainly electromagnetism stirring, mechanical stirring, spray stirring and semisolid isothermal heat treatment [18–21]. Comparing with these methods, SIMA has several advantages. It omits the procedure of molten metal treatment, and is applicable for both low and high melting alloys [22]. Recent results indicate that the Chinese script shaped Mg2Si phases in Mg–6Al–1Zn– 0.7Si alloy can be modified to granule and/or polygon shapes by semisolid isothermal heat treatment [23]. Our previous research shows [24] that the eutectic Mg2Si phase changes from the initial Chinese script shape to granule and/or polygon shape in Mg–9Al–1Si alloy during the isothermal heat treatment. But less work has been done on the Mg–Al–Si alloy by the SIMA process.
⁎ Corresponding author. Tel.: +86 451 8251 8173; fax: +86 451 8251 8644. E-mail address: [email protected] (X.L. Li). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.01.006
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0 –3 6 6
In the present work, the microstructure evolution of Mg–9% Al–0.7%Si alloy during the SIMA process is studied, and the effects of the reduction ratio on the semisolid microstructure of Mg–9%Al–0.7%Si alloy are discussed.
2.
Experimental Procedure
Commercial pure Mg (> 99.9 wt.%, purity), Al (> 99.8 wt.%, purity) and Si (> 99.4 wt.%, purity) were used to prepare the experimental alloy. The actual compositions of the experimental alloy were measured by the ARL4460 Metals Analyzer, and given as following (wt.%): 89.92 Mg, 9.32 Al, and 0.76 Si; thus it can be termed Mg–9%Al–0.7%Si alloy. The alloy was melted in a graphite crucible electric resistance furnace and then poured into a preheated steel mold to form ingots. Subsequently, the ingots were machined into the samples with sizes of 60 mm × 12 mm × 12 mm, and then were predeformed with different reduction ratios from 0% to 30% by rolling at 350 °C. The eutectic temperature of (α-Mg + Mg2Si)E in the Mg–9%Al–0.7%Si alloy was 485.4 °C, determined by differential scanning calorimeter (DSC) (Pekin-Elmer, USA) with a heating rate of 5 °C/min (Fig. 1). Based on the DSC result, the semisolid holding temperature was selected at 565 °C, ranging between the eutectic temperature and the liquidus temperature of the alloys (596.6 °C). After deformation, the samples were holding at a temperature of 565 °C for 20 min in the electric resistance furnace with the protective atmosphere of CO2/SF6, and then were quenched in cold water. Microstructure analyses were carried out at the mid section of the sample by means of optical microscopy (OM) and X-ray diffraction (XRD). The samples were carefully ground and polished, and then were etched by 3% HNO3 in alcohol. In this paper, quantitative metallography analyses including the volume fraction and size of α-Mg grains were conducted using the quantitative analysis system (Omnimet Imaging Systems-Buehler, USA) to study the evolution of the microstructures during the SIMA process.
Fig. 1 – DSC curve of Mg–9%Al–0.7%Si alloy.
3.
361
Results and Discussion
Fig. 2 shows the XRD pattern and microstructure of the as-cast Mg–9%Al–0.7%Si alloy. The XRD result reveals that the components of the alloy consist of α-Mg, Mg2Si and Mg17Al12. As seen from Fig. 2(b), the as-cast microstructure of Mg–9%Al– 0.7%Si alloy contains three constituents: white primary α-Mg dendrites, Chinese script shaped eutectic Mg2Si and gray Mg17Al12, which precipitated along the grain boundaries, respectively. Fig. 3 shows the as-rolled microstructures of the Mg–9% Al–0.7%Si alloy with different reduction ratios from 0% to 30%. Fig. 3(a) shows the as-cast microstructure of the noncompressed alloy. It can be seen that the α-Mg dendrites distinctly exhibited the spontaneous growth character, which was formed during the metallurgical solidification process. After being compressed with a small ratio of 10%, most of primary α-Mg kept dendritic morphology, as illustrated in Fig. 3(b). But with the further increase of the reduction ratio imposed on the Mg–9%Al–0.7%Si alloy, the ascast microstructure presented a remarkable deformation character. When the reduction ratio reached 20% and 30%, the α-Mg dendrites oriented themselves even more heavily and almost presented a parallel character in the alloy; especially a fragment of Mg2Si occurs due to its brittleness,
Fig. 2 – (a) XRD pattern and (b) as-cast microstructure of Mg–9% Al–0.7%Si alloy.
362
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0–3 6 6
Fig. 3 – The as-rolled microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios: (a) 0%, (b) 10%, (c) 20% and (d) 30%.
Fig. 4 – The semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios after isothermal heat treatment at 565 °C for 20 min: (a) 0%, (b) 10%, (c) 20% and (d) 30%.
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0 –3 6 6
as illustrated in Fig. 3(c) and (d). The separation distance of the primary α-Mg dendrites decreased. In conclusion, with the increase of the reduction ratio, the α-Mg dendrites in the as-rolled microstructure gradually oriented themselves in the direction perpendicular to the compressive direction (indicated by arrows in Fig. 3) and obviously presented a directional character. Fig. 4 shows the microstructure evolution of Mg–9%Al– 0.7%Si alloy during semisolid isothermal treatment with different reduction ratios from 0% to 30%. It can be observed that the α-Mg grain of non-deformed alloy is very large and irregular (Fig. 4(a)), while the grains of the deformed alloys with 10% and 20% reduction ratios are small and globular (Fig. 4(b) and (c)). Comparing Fig. 4(a) with (d), it is found that the spheroidization and fining are particularly clear for higher reduction ratios, and its average size decreases from 76.24 μm to 52.16 μm, as shown in Fig. 5. Moreover, the amount of the liquid islands inside the grain becomes less with the increase in reduction ratios. It can be concluded that the marked microstructure difference is indeed induced by the deformation. This is mainly because the recrystallization attained prior to semisolid isothermal heat treatment can easily give rise to this favorable small initial grain that evolves into small spheroidal grains rapidly during isothermal heat treatment [25]. It is obvious that the more heavily deformed alloy experiences a higher strain than the slightly deformed alloy; therefore, the stored energy due to deformation is higher than the more heavily deformed alloy [26]. The higher stored energy results in the formation of newer grains via recovery and recrystallization [26]; consequently, the amount of grain increases and its size decreases. Furthermore, with the increasing of the reduction ratio, the liquid volume fraction fL increases gradually (from 37.26 vol.% to 48.28 vol.%) according to the results of a quantitative analysis, as shown in Fig. 6. It can be concluded that the marked microstructure difference is indeed induced by the deformation. The spheroidization
Fig. 5 – The effect of the reduction ratio on the average size of α-Mg grain after isothermal heat treatment at 565 °C for 20 min.
363
Fig. 6 – The effect of the reduction ratio on the liquid volume fraction after isothermal heat treatment at 565 °C for 20 min.
mechanism of α-Mg dendrites agrees well with those reported previously [19,26,27]. Fig. 8 shows the morphology of the Mg2Si phase in semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios after isothermal heat treatment at 565 °C for 20 min. It is observed that the original Chinese script shaped Mg2Si is fragmented (as shown in Fig. 7 by the arrow), with the morphology of globular shape, instead of polygon shape during semisolid isothermal heat treatment [23,28], and most of the globular Mg2Si phases are present in the liquid phases distributed at the grains boundaries. With increasing of the reduction ratio, the morphology and size of Mg2Si do not change obviously. The liquid phase of Mg–9%Al–0.7%Si alloy with semisolid microstructures mainly consists of remelting of divorced eutectic Mg17Al12 and α-Mg + Mg2Si eutectic. According to the actual liquid volume fraction (without regarding the “liquid islands”) in semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios after treated at 565 °C for 20 min, the weight percentage of Al and Si in the semisolid liquid of the Mg–9%Al–0.7%Si alloy can be calculated and listed in Table 1. With increasing Al content, the eutectic composition (1.34 wt.%) shifted to lower silicon content distinctly in the modified Mg–Si phase diagram [10]; such Si content (Table 1) calculated in the semisolid microstructure is much bigger than that of the eutectic point, which led to forming more globular shape primary Mg2Si despite the hypoeutectic composition (0.76 wt.%) in the as-cast Mg–9%Al–0.7%Si alloy, which can be evidenced by the previous researches [23], the weight percentage of the Al and Si in the semisolid liquid phase of Mg–6Al–1Zn–0.7Si alloy can be calculated as 28.6 wt.% and 3.3 wt.% (corresponding to a 21 vol.% liquid content); thus the morphology of Mg2Si phases in the experimental alloy changes from the initial Chinese script shape to granule and/or polygon shapes. However, M. Zha et al. [29] reported that the eutectic Mg2Si in Mg–5Si–1Al alloy turns from granular shape into Chinese script morphology by
364
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0–3 6 6
Fig. 7 – The morphology of the Mg2Si phase in semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios of (a) 0%, (b) 10%, (c) 20% and (d) 30%.
Fig. 8 – The morphology of the Mg2Si in Mg–9%Al–0.7%Si alloy before isothermal heating after rolling with different reduction ratios: (a) 0%, (b) 10%, (c) 20% and (d) 30%.
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0 –3 6 6
Table 1 – The weight percentage of Al and Si in the semisolid liquid of the Mg–9%Al–0.7%Si alloy with different reduction ratios after being treated at 565 °C for 20 min. Reduction ratios (%) 0 10 20 30
Liquid volume Weight Weight fraction percentage of percentage of (vol.%) Al (wt.%) Si (wt.%) 37.26 39.42 43.64 48.28
25.2 23.6 21.4 19.3
2.1 1.9 1.7 1.6
partial remelting owing to the low Al content with increasing liquid volume fraction above 660 °C. In spite of the above, the modification mechanism of Chinese script shaped Mg2Si phases during the SIMA process is not completely clear, and further investigation needs to be carried out.
4.
Conclusions (1) The semisolid microstructure of Mg–9%Al–0.7%Si alloy is successfully produced by the SIMA process. With the reduction ratio increase from 0% to 30%, the morphology of α-Mg grain becomes more globular and its size decreases from 76.24 μm to 52.16 μm, and the liquid phase increases from 37.26 vol.% to 48.28 vol.%. Moreover, the liquid islands in the grains become less with the increasing of the reduction ratio. (2) The morphology of the eutectic Mg2Si phase transformed from the coarse Chinese script shape to globular shape in Mg–9%Al–0.7%Si alloy during the SIMA process. A possible reason of the modification of Mg2Si morphology is the shifting of the eutectic composition of the liquid towards lower silicon contents with increasing Al content.
Acknowledgements This research is supported by the Basic Research Foundation of Harbin Engineering University (No. HEUFT05038), the National High Technology Research and Development Program of China (863 Program) (No. 2009AA03Z423) and the National Natural Science Foundation of China (No. 51071055).
REFERENCES [1] Wang L, Qin XY. The effect of mechanical milling on the formation of nanocrystalline Mg2Si through solid-state reaction. Scr Mater 2003;49:243–8. [2] Wang HY, Jiang QC, Ma BX, Wang Y, Wang JG, Li JB. Modification of Mg2Si in Mg–Si alloys with K2TiF6, KBF4 and KBF4+K2TiF6. J Alloys Compd 2005;387:105–8. [3] Zhang J, Fan Z, Wang YQ, Zhou BL. Microstructural development of Al–15wt.%Mg2Si in situ composite with mischmetal addition. Mater Sci Eng A 2000;281:104–12.
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[4] Pekguleryuz MO, Avedesian MM. Magnesium alloying, some potentials for alloy development. J Jpn Inst Light Met 1992;42:679–86. [5] Mabuchi M, Kubota K, Higashi K. High strength and high strain rate superplasticity in a Mg–Mg2Si composite. Scr Metall Mater 1995;33:331–5. [6] Mabuchi M, Kubota K, Higashi K. Elevated temperature mechanical properties of magnesium alloys containing Mg2Si. Mater Sci Technol 1996;12:35–9. [7] Raghunathan N, Sheppard T. Fabrication and properties of rapidly solidified magnesium and Mg–Si alloys. Mater Sci Technol 1990;6:629–40. [8] Frommeyer G, Beer S, Von Oldenburg K. Microstructure and mechanical properties of mechanically alloyed intermetallic Mg2Si–Al alloys. Metallkde 1994;85:372–7. [9] Lu YZ, Wang QD, Zeng XQ, Zhu YP, Ding WJ. Effects of silicon on microstructure, fluidity, mechanical properties, and fracture behaviour of Mg–6Al alloy. Mater Sci Technol 2001;17:207–14. [10] Kim JJ, Kim DH, Shin KS, Kim NJ. Modification of Mg2Si morphology in squeeze cast Mg–Al–Zn–Si alloys by Ca or P addition. Scr Mater 1999;41:333–40. [11] Yuan GY, Liu ZL, Wang QD, Ding WJ. Microstructure refinement of Mg–Al–Zn–Si alloys. Mater Lett 2002;56:53–8. [12] Yoo MS, Shin KS, Kim NJ. Effect of Mg2Si particles on the elevated temperature tensile properties of squeeze-cast Mg–Al alloys. Metall Mater Trans A 2004;35:1629–32. [13] Srinivasan A, Pillai UTS, Pai BC. Microstructure and mechanical properties of Si and Sb added AZ91 magnesium alloy. Metall Mater Trans A 2005;36:2235–43. [14] Nam KY, Song DH, Lee CW, Lee SW, Park YH, Cho KM, et al. Modification of Mg2Si morphology in as-cast Mg–Al–Si alloys with strontium and antimony. Mater Sci Forum 2006;510/511: 238–41. [15] Quimby PD, Lu SZ, Plichta R, Visser DK, Jacobe KP. In: Luo A, Neelameggham N, Beals R, editors. Effects of minor addition and cooling rate on the microstructure of cast magnesium–silicon alloys [A]. Magnesium technology[C], San Antonio, Texas, USA, TMS; 2006. p. 535–8. [16] Tang B, Li SS, Wang XS, Zeng DB, Wu R. An investigation on hot-crack mechanism of Ca addition into AZ91D alloy. J Mater Sci 2005;40:2931–6. [17] Flemings MC, Riek RG, Young KP. The rheology of a partially solid alloy. Metall Trans 1972;17:1925–32. [18] Wang JL, Su YH. Structural evolution of conventional cast dendritic and spray-cast non-dendritic structures during isothermal holding in the semi-solid state. Scr Mater 1997;37: 2003–7. [19] Tzimas E, Zavaliangos A. Evolution of near-equiaxed microstructure in the semisolid state. Mater Sci Eng A 2000;289:228–40. [20] Lee JI, Kim GH, Lee HI. Generation of dislocations and subgrains in stir cast Al–6.2Si alloy during semisolid state processing. Mater Sci Technol 1998;14:770–5. [21] Tzimas E, Zavaliangos A. A comparative characterization of near-equiaxed microstructures as produced by spray casting, magnetohydrodynamic casting and the stress induced, melt activated process. Mater Sci Eng A 2000;289:217–27. [22] Czerwinski F, Zielinska-Lipiec A. The melting behaviour of extruded Mg–8%Al–2%Zn alloy. Acta Mater 2003;51:3319–32. [23] Yang MB, Pan FS, Cheng RJ, Bai L. Effect of semi-solid isothermal heat treatment on the microstructure of Mg–6A1–1Zn–0.7Si alloy. J Mater Process Technol 2008;206:374–81. [24] Ma GR, Li XL, Li QF. Effect of holding time on microstructure of Mg–9Al–1Si alloy during semisolid isothermal heat treatment. Trans Nonferrous Met Soc China 2010;20:430–4. [25] Wang JG, Lu P, Wang HY, Jiang QC. Effect of predeformation on the semisolid microstructure of Mg–9Al–0.6Zn alloy. Mater Lett 2004;58:3852–6.
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[26] Qin QD, Zhao YG, Xiu K, Zhou W, Liang YH. Microstructure evolution of in situ Mg2Si/Al–Si–Cu composite in semisolid remelting processing. Mater Sci Eng A 2005;407:196–200. [27] Doherty RD, Lee H, Feest E. Microstructure of stir-cast metals. Mater Sci Eng 1984;65:181–9.
[28] Ma GR, Li XL, Xiao L, Li QF. Effect of holding temperature on microstructure of an AS91 alloy during semisolid isothermal heat treatment. J Alloys Compd 2010;496:577–81. [29] Zha M, Wang HY, Xue PF, Li LL, Liu B, Jiang QC. Microstructural evolution of Mg–5Si–1Al alloy during partial remelting. J Alloys Compd 2009;472:18–22.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Modification of Mg2Si morphology in Mg–9%Al–0.7%Si alloy by the SIMA process G.R. Ma, X.L. Li⁎, L. Li, X. Wang, Q.F. Li Center for Biomedical Materials and Engineering, College of Materials Science and Chemical Engineering, Harbin Engineering University, No. 145 Nantong Street, Harbin 150001, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
The effect of predeformation on the semisolid microstructure of Mg–9%Al–0.7%Si alloy
Received 22 July 2010
produced by the strain induced melt activation (SIMA) process was studied. Experimental
Received in revised form
results indicated that a non-dendritic microstructure could be obtained by the SIMA process
9 September 2010
in the alloy. With the reduction ratio varying from 0% to 30%, the volume fraction of liquid
Accepted 7 January 2011
gradually enlarged from 37.26 vol.% to 48.28 vol.%, and the morphology of α-Mg grains transformed from dendritic shape to spherical shape, with a size of 52.16 μm. Also, the liquid
Keywords:
islands in the grains become less with the increase of the reduction ratio. In addition, the
Predeformation
eutectic Mg2Si phase transformed from Chinese script shape to globular shape during the
Semisolid
isothermal heat treatment. A possible reason of the modification of Mg2Si morphology is the
SIMA
shifting of the eutectic composition of the liquid towards lower silicon contents with
Mg2Si phase
increasing Al content.
Modification
1.
Introduction
Magnesium alloys containing Mg2Si particles have received a wide attention since the intermetallic compound Mg2Si exhibits high melting temperature, low density, high hardness, low thermal expansion coefficient and reasonably high elastic modulus [1–3]. However, Mg2Si phases in the Mg–Al–Si alloys are prone to forming undesirable, coarse Chinese script shape under a low solidification rate resulting from the eutectic reaction, which will deteriorate the mechanical properties of the magnesium alloys [4]. In order to modify the Chinese script shaped Mg2Si phases in the Mg–Al–Si alloys, many methods have been studied, such as hot extrusion [5,6], rapid solidification [7], mechanical alloying [8,9] and micro-alloying (Sb, Ca, and P) [10–15]. But some investigations indicated that these methods may lead to the increase of the production cost and other problems [11,15,16].
© 2011 Elsevier Inc. All rights reserved.
Semisolid metal (SSM) processing has been developed rapidly since the 1970s [17]. SSM processing is regarded as an advanced forming technology for magnesium alloy. The methods to obtain a semisolid structure are mainly electromagnetism stirring, mechanical stirring, spray stirring and semisolid isothermal heat treatment [18–21]. Comparing with these methods, SIMA has several advantages. It omits the procedure of molten metal treatment, and is applicable for both low and high melting alloys [22]. Recent results indicate that the Chinese script shaped Mg2Si phases in Mg–6Al–1Zn– 0.7Si alloy can be modified to granule and/or polygon shapes by semisolid isothermal heat treatment [23]. Our previous research shows [24] that the eutectic Mg2Si phase changes from the initial Chinese script shape to granule and/or polygon shape in Mg–9Al–1Si alloy during the isothermal heat treatment. But less work has been done on the Mg–Al–Si alloy by the SIMA process.
⁎ Corresponding author. Tel.: +86 451 8251 8173; fax: +86 451 8251 8644. E-mail address: [email protected] (X.L. Li). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.01.006
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0 –3 6 6
In the present work, the microstructure evolution of Mg–9% Al–0.7%Si alloy during the SIMA process is studied, and the effects of the reduction ratio on the semisolid microstructure of Mg–9%Al–0.7%Si alloy are discussed.
2.
Experimental Procedure
Commercial pure Mg (> 99.9 wt.%, purity), Al (> 99.8 wt.%, purity) and Si (> 99.4 wt.%, purity) were used to prepare the experimental alloy. The actual compositions of the experimental alloy were measured by the ARL4460 Metals Analyzer, and given as following (wt.%): 89.92 Mg, 9.32 Al, and 0.76 Si; thus it can be termed Mg–9%Al–0.7%Si alloy. The alloy was melted in a graphite crucible electric resistance furnace and then poured into a preheated steel mold to form ingots. Subsequently, the ingots were machined into the samples with sizes of 60 mm × 12 mm × 12 mm, and then were predeformed with different reduction ratios from 0% to 30% by rolling at 350 °C. The eutectic temperature of (α-Mg + Mg2Si)E in the Mg–9%Al–0.7%Si alloy was 485.4 °C, determined by differential scanning calorimeter (DSC) (Pekin-Elmer, USA) with a heating rate of 5 °C/min (Fig. 1). Based on the DSC result, the semisolid holding temperature was selected at 565 °C, ranging between the eutectic temperature and the liquidus temperature of the alloys (596.6 °C). After deformation, the samples were holding at a temperature of 565 °C for 20 min in the electric resistance furnace with the protective atmosphere of CO2/SF6, and then were quenched in cold water. Microstructure analyses were carried out at the mid section of the sample by means of optical microscopy (OM) and X-ray diffraction (XRD). The samples were carefully ground and polished, and then were etched by 3% HNO3 in alcohol. In this paper, quantitative metallography analyses including the volume fraction and size of α-Mg grains were conducted using the quantitative analysis system (Omnimet Imaging Systems-Buehler, USA) to study the evolution of the microstructures during the SIMA process.
Fig. 1 – DSC curve of Mg–9%Al–0.7%Si alloy.
3.
361
Results and Discussion
Fig. 2 shows the XRD pattern and microstructure of the as-cast Mg–9%Al–0.7%Si alloy. The XRD result reveals that the components of the alloy consist of α-Mg, Mg2Si and Mg17Al12. As seen from Fig. 2(b), the as-cast microstructure of Mg–9%Al– 0.7%Si alloy contains three constituents: white primary α-Mg dendrites, Chinese script shaped eutectic Mg2Si and gray Mg17Al12, which precipitated along the grain boundaries, respectively. Fig. 3 shows the as-rolled microstructures of the Mg–9% Al–0.7%Si alloy with different reduction ratios from 0% to 30%. Fig. 3(a) shows the as-cast microstructure of the noncompressed alloy. It can be seen that the α-Mg dendrites distinctly exhibited the spontaneous growth character, which was formed during the metallurgical solidification process. After being compressed with a small ratio of 10%, most of primary α-Mg kept dendritic morphology, as illustrated in Fig. 3(b). But with the further increase of the reduction ratio imposed on the Mg–9%Al–0.7%Si alloy, the ascast microstructure presented a remarkable deformation character. When the reduction ratio reached 20% and 30%, the α-Mg dendrites oriented themselves even more heavily and almost presented a parallel character in the alloy; especially a fragment of Mg2Si occurs due to its brittleness,
Fig. 2 – (a) XRD pattern and (b) as-cast microstructure of Mg–9% Al–0.7%Si alloy.
362
MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0–3 6 6
Fig. 3 – The as-rolled microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios: (a) 0%, (b) 10%, (c) 20% and (d) 30%.
Fig. 4 – The semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios after isothermal heat treatment at 565 °C for 20 min: (a) 0%, (b) 10%, (c) 20% and (d) 30%.
M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 3 6 0 –3 6 6
as illustrated in Fig. 3(c) and (d). The separation distance of the primary α-Mg dendrites decreased. In conclusion, with the increase of the reduction ratio, the α-Mg dendrites in the as-rolled microstructure gradually oriented themselves in the direction perpendicular to the compressive direction (indicated by arrows in Fig. 3) and obviously presented a directional character. Fig. 4 shows the microstructure evolution of Mg–9%Al– 0.7%Si alloy during semisolid isothermal treatment with different reduction ratios from 0% to 30%. It can be observed that the α-Mg grain of non-deformed alloy is very large and irregular (Fig. 4(a)), while the grains of the deformed alloys with 10% and 20% reduction ratios are small and globular (Fig. 4(b) and (c)). Comparing Fig. 4(a) with (d), it is found that the spheroidization and fining are particularly clear for higher reduction ratios, and its average size decreases from 76.24 μm to 52.16 μm, as shown in Fig. 5. Moreover, the amount of the liquid islands inside the grain becomes less with the increase in reduction ratios. It can be concluded that the marked microstructure difference is indeed induced by the deformation. This is mainly because the recrystallization attained prior to semisolid isothermal heat treatment can easily give rise to this favorable small initial grain that evolves into small spheroidal grains rapidly during isothermal heat treatment [25]. It is obvious that the more heavily deformed alloy experiences a higher strain than the slightly deformed alloy; therefore, the stored energy due to deformation is higher than the more heavily deformed alloy [26]. The higher stored energy results in the formation of newer grains via recovery and recrystallization [26]; consequently, the amount of grain increases and its size decreases. Furthermore, with the increasing of the reduction ratio, the liquid volume fraction fL increases gradually (from 37.26 vol.% to 48.28 vol.%) according to the results of a quantitative analysis, as shown in Fig. 6. It can be concluded that the marked microstructure difference is indeed induced by the deformation. The spheroidization
Fig. 5 – The effect of the reduction ratio on the average size of α-Mg grain after isothermal heat treatment at 565 °C for 20 min.
363
Fig. 6 – The effect of the reduction ratio on the liquid volume fraction after isothermal heat treatment at 565 °C for 20 min.
mechanism of α-Mg dendrites agrees well with those reported previously [19,26,27]. Fig. 8 shows the morphology of the Mg2Si phase in semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios after isothermal heat treatment at 565 °C for 20 min. It is observed that the original Chinese script shaped Mg2Si is fragmented (as shown in Fig. 7 by the arrow), with the morphology of globular shape, instead of polygon shape during semisolid isothermal heat treatment [23,28], and most of the globular Mg2Si phases are present in the liquid phases distributed at the grains boundaries. With increasing of the reduction ratio, the morphology and size of Mg2Si do not change obviously. The liquid phase of Mg–9%Al–0.7%Si alloy with semisolid microstructures mainly consists of remelting of divorced eutectic Mg17Al12 and α-Mg + Mg2Si eutectic. According to the actual liquid volume fraction (without regarding the “liquid islands”) in semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios after treated at 565 °C for 20 min, the weight percentage of Al and Si in the semisolid liquid of the Mg–9%Al–0.7%Si alloy can be calculated and listed in Table 1. With increasing Al content, the eutectic composition (1.34 wt.%) shifted to lower silicon content distinctly in the modified Mg–Si phase diagram [10]; such Si content (Table 1) calculated in the semisolid microstructure is much bigger than that of the eutectic point, which led to forming more globular shape primary Mg2Si despite the hypoeutectic composition (0.76 wt.%) in the as-cast Mg–9%Al–0.7%Si alloy, which can be evidenced by the previous researches [23], the weight percentage of the Al and Si in the semisolid liquid phase of Mg–6Al–1Zn–0.7Si alloy can be calculated as 28.6 wt.% and 3.3 wt.% (corresponding to a 21 vol.% liquid content); thus the morphology of Mg2Si phases in the experimental alloy changes from the initial Chinese script shape to granule and/or polygon shapes. However, M. Zha et al. [29] reported that the eutectic Mg2Si in Mg–5Si–1Al alloy turns from granular shape into Chinese script morphology by
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Fig. 7 – The morphology of the Mg2Si phase in semisolid microstructures of Mg–9%Al–0.7%Si alloy with different reduction ratios of (a) 0%, (b) 10%, (c) 20% and (d) 30%.
Fig. 8 – The morphology of the Mg2Si in Mg–9%Al–0.7%Si alloy before isothermal heating after rolling with different reduction ratios: (a) 0%, (b) 10%, (c) 20% and (d) 30%.
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Table 1 – The weight percentage of Al and Si in the semisolid liquid of the Mg–9%Al–0.7%Si alloy with different reduction ratios after being treated at 565 °C for 20 min. Reduction ratios (%) 0 10 20 30
Liquid volume Weight Weight fraction percentage of percentage of (vol.%) Al (wt.%) Si (wt.%) 37.26 39.42 43.64 48.28
25.2 23.6 21.4 19.3
2.1 1.9 1.7 1.6
partial remelting owing to the low Al content with increasing liquid volume fraction above 660 °C. In spite of the above, the modification mechanism of Chinese script shaped Mg2Si phases during the SIMA process is not completely clear, and further investigation needs to be carried out.
4.
Conclusions (1) The semisolid microstructure of Mg–9%Al–0.7%Si alloy is successfully produced by the SIMA process. With the reduction ratio increase from 0% to 30%, the morphology of α-Mg grain becomes more globular and its size decreases from 76.24 μm to 52.16 μm, and the liquid phase increases from 37.26 vol.% to 48.28 vol.%. Moreover, the liquid islands in the grains become less with the increasing of the reduction ratio. (2) The morphology of the eutectic Mg2Si phase transformed from the coarse Chinese script shape to globular shape in Mg–9%Al–0.7%Si alloy during the SIMA process. A possible reason of the modification of Mg2Si morphology is the shifting of the eutectic composition of the liquid towards lower silicon contents with increasing Al content.
Acknowledgements This research is supported by the Basic Research Foundation of Harbin Engineering University (No. HEUFT05038), the National High Technology Research and Development Program of China (863 Program) (No. 2009AA03Z423) and the National Natural Science Foundation of China (No. 51071055).
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