Available online at ScienceDirect. Procedia Materials Science 11 (2015 ) 38 43

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1 Available online at ScienceDirect Procedia Materials Science 11 (2015 ) th International Biennial Conference on Ultrafine Grained and Nanostructured Materials, UFGNSM15 Effect of Hot Extrusion on Microstructure and Tensile Properties of Ca Modified Mg-Mg 2 Si Composite M. Lotfpour a,, M. Emamy a *, S. H. Allameh a, B. Pourbahari a a School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran, Iran Abstract Effect of 0.05, 0.1, 0.5, 1 and 3 (wt. %) Ca addition and extrusion process on the microstructure and tensile properties of Mg-Mg2Si metal matrix composite has been studied by the use of optical microscopy (OM) and standard tensile testing. The average size of primary Mg2Si particles decreased from 34 µm to about 10 µm with the addition of 0.05 (wt.%) Ca and extrusion process and also the size of eutectic Mg2Si decreased from 20 µm to about 2 µm. The morphology of Mg2Si particles altered from octahedron to fine polygonal and more round shape and eutectic phases altered to well distuributed fragmented particles on the microstructure that was enhance the mechanical properties in comparison with as-cast specimens. Tensile test showed that UTS value increases with the addition of Ca. The maximum UTS value was achieved with 0.1 (wt.%) Ca addition Published The Authors. by Elsevier Published Ltd. by This Elsevier is an open Ltd. access article under the CC BY-NC-ND license Peer-review ( under responsibility of the organizing committee of UFGNSM15. Peer-review under responsibility of the organizing committee of UFGNSM15 Keywords: Extrusion process; Ca addition; Tensile test. 1. Introduction The attention to magnesium alloys and their composites in automotive and aerospace industries increases, because of reduction in weight and energy consumption, Luo (2004), Golmakaniyoon et al. (2011), Hou et al. (2011). But, because of difficulties during melting and casting and its reactivity with oxygen in atmosphere, their weak mechanical properties and corrosion properties, its usage has been restricted, Pekguleryuz and Avedesian (1992). Also, particulate metal matrix composites showed good inherent isotropic properties. In-situ particle composites showed better wettability with matrix, stability and distribution compared with ex-situ particle composites, Bahrami et al. (2012). * Corresponding author. Tel.: address: mehrablotfpour@yahoo.com Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of UFGNSM15 doi: /j.mspro

2 M. Lotfpour et al. / Procedia Materials Science 11 ( 2015 ) One of the most important elements in Mg alloys is Si, in which according to the Mg-Si binary phase diagram, at hypereutectic chemical composition after solidification, Mg 2Si has been produced in Mg matrix, Lu et al. (2001), Mabuchi and Higashi (1996), Mabuchi et al. (1996). This intermetallic has proper properties such as high melting point, low density, low thermal expansion coefficient and high elastic modulus, Li et al. (1993), Lu et al. (1998), Zhang et al. (1999), Arunachaleswaran et al. (2007), Srinivasan et al. (2005), Ding et al. (2009). In Mg-Si binary phase diagram, Mg-3 (wt.%)si alloy is a hypereutectic alloy, whereas the eutectic point is 1.34 wt.% Si.The solubility of Si in Mg at room temperature is very low (0.003 (wt.%) Si), so the achieved phases are (Mg+Mg 2Si) E + (Mg 2Si) P. The eutectic temperature is 640 C and melting starts to solidify from 720 C. By approaching to solidus line, the primary Mg 2Si is formed firstly, then after crossing the eutectic line, (Mg+Mg 2Si) E is formed. Thus, the microstructure consists of (Mg 2Si) P and (Mg 2Si + Mg) E. So, with simple casting procedure of Mg-Si alloy, primary and eutectic Mg 2Si phase is expected to have dendritic and plate-like morphology in Mg matrix that may debilitate the mechanical properties such as ductility and strength of the material. So, the microstructure of the in-situ composite must be modified to enhance its mechanical properties, Zhang et al. (2000), Jiang et al. (2005), Zhang et al. (2001), Kim et al. (1992). Several works have been carried out on modification of Mg-Si alloys, Kondoh et al. (2003), Lin et al. (2010), Lu et al. (2003). But, the simplest way to modify the microstructure is the use of chemical modifier, with normal casting procedure. In this case several works have been carried out in Mg-Si alloys. In this research, effects of hot extrusion on the microstructure and mechanical properties of modified Mg-Mg 2Si with Ca additions have been investigated to realize the effect of extrusion in mechanical and microstructure properties Mg-Mg 2Si metal matrix composite. 2. Experimental 2.1. Materials and processing Commercial pure Mg ingot (>99.9%) and Si (>99.8 %) were used to prepare Mg-10 wt.%si master alloy. The melting process was carried in graphite crucible located in an induction furnace protected by (pure Ar) cover gas. Pure Mg was melted at 720 C and then pure Si was added into the molten magnesium. Then, melt was hold for 10 min for complete dissolution of silicon. Mg-10 (wt.%) Ca master alloy was also prepared with pure Mg and Ca (>99.8 %) and with the same procedure. Then all master alloy ingots were broken for subsequent procedure. To prepare Mg-3 (wt.%) Si alloy with 0.05, 0.1, 0.5, 1 and 3 (wt.%) Ca. All broken master alloys were added to melt according to charge calculations. Then melt was kept at 750 C for 5 min to complete dissolution. Melt was stirred by silica rod for about 1 min then it was poured into a preheated cylindrical steel mold Extrusion process Cast billets in each addition were machined to final dimensions, using lathe in order to fit in extrusion container. These small billets were heated to 350 C and then extruded with ram speed of 1 mm/s and extrusion ratio of 1:12. The extrusion was performed on the 2MN maximum lead with extrusion machine diagrammed in Fig. 1. So all specimens were formed to rod shape( 10mm). Two specimens cut from the rods and changed to tensile test samples with milling machine according to ASTM E8-04 small size as shown in Fig. 2. For microstructural characterization, all specimens of were cut from the same position of each rod.then, they were ground and polished through standard routines and etched with Nital (5% HNO 3 and 95% alcohol solution) etchant.

3 40 M. Lotfpour et al. / Procedia Materials Science 11 ( 2015 ) Fig. 2. Schematic of tensile test standard (mm). Fig. 1. schematic of extrusion mashine. 3. Results and discussion 3.1. Microstructure Figure 3 depicts the optical micrographs of as-cast Mg-Mg2Si metal matrix composite with 0, 0.1 and 3 (wt. %) Ca additions. The average size of Mg2Si particles in Mg-Mg2Si without Ca addition is 50 μm. Fig. 4 shows optical micrograph images of Mg-Mg2Si with and without Ca content after extrusion process, prepared from radial direction (RD). It can be seen that Ca additions and extrusion process fragment the primary Mg2Si and change them to more spherical and smaller particles and distribute them uniformly in the matrix by fragmentation. The variation of average size of primary Mg2Si with Ca content (wt.%) is shown in Fig. 5. As could be understood, the average size of Mg2Si particles in Mg-Mg2Si without Ca addition and with extrusion process is 33 μm. The size of primary Mg2Si phase was reduced with Ca addition and extrusion process to about 10 µm. So, these processes could create more fine particles which are expected to enhance mechanical properties of the composite. Also, the size of eutectic Mg2Si decreased from 20 µm to about 2 µm with 0.1 (wt. %) Ca addition and extrusion process. the eutectic phase were broken to small fragmented particles with size of 2 µm that shown in Fig. 6. The best condition of microstructural properties is achieved from the addition of 0.1 (wt. %) Ca and applying extrusion process. But addition of 3 (wt. %) Ca, there were some needle-like particles as shown in Fig. 3(c) that destructed the morphology of Mg-Mg2Si MMC. (a) (b) (c) Fig. 3. Optimal micrographs of as-cast Mg-Mg 2Si MMC with 0, 0.1 and 3 (Wt. %) Ca additions.

4 M. Lotfpour et al. / Procedia Materials Science 11 ( 2015 ) Fig. 4. Optical microscopy images with Ca addition and extrusion process, (a) 0 (wt. %); (b) 0.05 (wt. %); (c) 0.1 (wt. %); (d) 0.5 (wt. %); (e) 1 (wt.%); and (f) 3 (wt. % ) Ca addition. Fig. 5. Relationship between average size of primary Mg 2Si and Ca content (wt. %) with extrusion process and Ca additions.

5 42 M. Lotfpour et al. / Procedia Materials Science 11 ( 2015 ) μm Fig. 6. Optical micrograph of Mg-Mg 2Si with 0.1 (wt. %) Ca addition and extrusion process Mechanical properties Figure 7 depicts tensile test results of Mg-Mg 2Si composite with extrusion process and Ca addition. Combining extrusion process and adding 0.1 (wt. %) Ca introduce maximum UTS and elongation values. Ca additions up to 3 (wt.%) does not change UTS values seriously, but elongation percentage was reduced slightly. It is important to note that Mg-Mg 2Si metal matrix composite normally show low strength and ductility. With hot extrusion mechanical working improves UTS values in compared with as-cast condition. From Fig. 4, it can be seen that the particle size is reduced in Ca added and extruded specimens. So, according to Griffith s theory high fracture stress is achieved when extensive microstrucural refinement is accessable, Zhang et al. (2000).Extrusion process cause the breakage of Mg-Mg 2Si eutectic phase and the introduction of well distuributed fragmented particles that can suppressed the propagating of cracks. (a) (b) Figure. 7. Tensile test results after extrusion and Ca additions, (a) UTS, (b) Elongation.

6 M. Lotfpour et al. / Procedia Materials Science 11 ( 2015 ) Conclusions The effect of Ca addition and extrusion process on Mg-Mg 2Si MMC revealed that: With Ca additions could modify the Mg-Mg 2Si MMC and change its morphology from dendritic to polyhedral shape and also eutectic phase alters to finer rods. With extrusion process, primary Mg 2Si becomes more spherical and smaller and the eutectic phase is broken to small fragmented particles. UTS and elongation values increase with Ca addition up to 0.1 wt.% Ca, i.e. the optimum condition of tensile properties. References Arunachaleswaran, A., Pereira, I.M., Dieringa, H., Huang, Y., Hort, N., Dhindaw, B.K., Kainer, K.U., Creep behavior of AE42 based hybrid composites. Mater. Sci. Eng. A , Bahrami, A., Razaghian, A., Emamy, M., Khorshidi, R., The effect of Zr on the microstructure and tensile properties of hot-extruded Al Mg2Si composite. Materials and Design. 36, Ding, S.S., Ding, C., Hua, C.Z., Effects of Si addition on microstructure and mechanical properties of RS/PM AZ91 alloy. J. Alloys Compd. 470, L17 L20. Golmakaniyoon, S., Mahmudi, R., Microstructure and creep behavior of the rare-earth doped Mg 6Zn 3Cu cast alloy. Mater. Sci. Eng. A. 528, Hou, J., Li, C., Liu, X.F., Nucleating role of an effective in situ Mg3P2 on Mg2Si in Mg Al Si alloys. J. Alloys Compd. 509, Jiang, Q.C., Wang, H.Y., Wang, Y., Ma, B.X., Wang, J.G., Modification of Mg2Si in Mg Si alloys with yttrium. Mater. Sci. Eng. A. 392, Kim, J.J., Kim, D.H., Shin, K.S., Kim, N.J., Modification of Mg2Si morphology in squeeze cast Mg-Al-Zn-Si alloys by Ca or P addition. Scr. Mater. 41, Kondoh, K., Oginuma, H., Tuzuki, R., Aizawa, T., Magnesium matrix composite with solid-state synthesized Mg2Si dispersoids. Mater. Trans. 44, 611. Li, G.H., Gill, H.S., Varin, R.A., Magnesium silicide intermetallic alloys, Metall. Trans. A. 24, Lin, L.X., Bin, C.Y., Xiang, W., Rui, M.G., Effect of cooling rates on as-cast microstructures of Mg-9Al-xSi (x= 1, 3) alloys. Trans.Nonferrous Met. Soc. China. 20, s393. Lu, L., Lai, M.O., Hoe, M.L., Formaton of nanocrystalline Mg2Si and Mg2Si dispersion strengthened Mg-Al alloy by mechanical alloying. Nanostruct. Mater. 10, Lu, L., Thong, K.K., Gupta M., Mg-based composite reinforced by Mg2Si. Compos. Sci. Technol. 63, 627. Lu, Y.Z., Wang, Q.D., Zeng, X.Q., Zhu, Y.P., Ding, W.J., Behavior of Mg-6Al-xSi alloys during solution heat treatment at 420 Mater. Sci. Eng. A. 301, Luo, A.A., Recent magnesium alloy development for elevated temperature applications. Int. Mater. Rev. 49, Mabuchi M., Higashi K., Strengthening mechanisms of Mg2Si alloys. Acta Mater. 44, Mabuchi, M., Kubota, K., Higashi, K., Tensile strength, ductility and fracture of magnesium-silicon alloys. J. Mater. Sci. 31, Pekguleryuz, M.O., Avedesian M. M., Special issue" Magnesium and magnesium alloys". Magnesium alloying, some potentials for alloy development. J. Japan Inst. Light Metals. 42(12), 679. Srinivasan, A., Pillai, U.T.S., Pai, B.C., Microstructure and mechanical properties of Si and Sb added AZ91 magnesium alloy. Metall. Mater. Trans. 36, Zhang, J.H., Leng, Z., Liu, S.J., Zhang, M.L., Meng, J., Wu, R.Z., Structure stability and mechanical properties of Mg Al-based alloy modified with Y-rich and Ce-rich misch metals. J. Alloys Compd. 509, L187 L193. Zhang, J., Fan, Z., Wang, Y.Q., Zhou, B.L., Microstructural development of Al 15wt.% Mg2Si in situ composite with mischmetal addition. J Mater Sci Eng A. 281, Zhang, J., Fan, Z., Wang, Y.Q., Zhou, B.L., Microstructural development of Al 15wt.% Mg2Si in situ composite with mischmetal addition. Mater. Sci. Eng. A. 281, Zhang, J., Wang, Y.Q., Yang, B., Zhou, B.L., Effects of Si content on the microstructure and tensile strength of an in situal/mg2si composite. J. Mater. Res. 14,