Tensile and Fracture Properties of Cast and Forged Composite Synthesized by Addition of Al-Si Alloy to Magnesium
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INTRODUCTION
MAGNESIUM alloys, since lighter than aluminum alloys, have received considerable attention for application in automobile parts, which is anticipated to increase globally at an average rate of 15 pct per year.[1] The present study involves the Mg-Al-Si system in which the intermetallic phase of Mg2Si was generated in a matrix of Mg-9 wt pct Al alloy to result in composite by casting molten magnesium alloyed with Al-Si alloy as the source of silicon. Since solidification processing results in casting defects and segregation, the effect of secondary
H.M. NANJUNDA SWAMY, Research Scholar, and S.K. NATH, Professor, and S. RAY, Professors, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee-247 667, India. Contact e-mail: [email protected] Manuscript submitted March 28, 2009. Article published online October 16, 2009 3284—VOLUME 40A, DECEMBER 2009
processing such as hot forging on the mechanical properties was also investigated. The matrix alloy composition has similar aluminum content as in AZ91 alloy, which is the most widely used die casting magnesium alloy, offering a good combination of castability, corrosion resistance, and mechanical properties.[2,3] For the AZ91 alloy, the strengthening is primarily determined by the volume and morphology of intermetallic Mg17Al12 phase, and the alloy generally fails by brittle cleavage and quasi-cleavage fracture along (0001) crystal plane. Lu et al.[4] claimed that microcracks tend to initiate in the Mg/Mg17Al12 interface and even in the Mg17Al12 particles, leading to fracture. Magnesium and its alloys were strengthened further by reinforcing with intermetallic compounds such as TiB2 and ceramic such as SiC. Wang et al.[5] observed 41, 106, and 181 pct increases in hardness over that of cast magnesium by reinforcing magnesium with 10, 20, and 30 vol pct TiB2 particles, respectively, by powder METALLURGICAL AND MATERIALS TRANSACTIONS A
metallurgy. In cast AZ91 alloy reinforced with 2, 5, and 7.5 wt pct fine TiB2 (~7 lm) particles, Wang et al.[6] observed that hardness increases with increasing TiB2 content. However, no tensile properties of these composites were reported. In AZ91 alloy reinforced with 9.4 and 15.1 vol pct SiCp, the yield strengths observed by Lloyd[7] are 191 and 208 MPa, respectively, but the ultimate tensile strength (UTS) in both the composites is surprisingly the same, 236 MPa. However, the corresponding elongations decrease to 2 and 1 pct, respectively. Lu et al.[8] investigated the mechanical properties of Mg-9 wt pct Al alloy based composite reinforced by Mg2Si, formed in-situ by the mechanical milling process, and the composite shows increasing yield strength from 201 to 245 MPa when Mg2Si increases from 5 to 15 vol pct. The forming limit of Mg alloy shows an excellent workability at temperatures from 250 C to 400 C, but at temperatures lower than 200 C, it is brittle, and at temperatures higher than 400 C, there is heavy oxidation.[9] The forged magnesium alloys are reported
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