Effect of Partial Substitution of Mn for Ni on Mechanical Properties of Friction Stir Processed Hypoeutectic Al-Ni Alloy
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THE hypoeutectic alloys based on the Al-Ni eutectic system (6.1 wt pct Ni and 640 °C)[1,2] exhibit excellent fluidity, great feeding ability, and very low susceptibility to hot-tearing.[3,4] Moreover, compared to conventional Al-Si-based alloys, Al-Ni alloys have higher solidus temperature, and due to the formation of thermally stable intermetallics like Al3Ni (melting point of 854 °C) in their microstructure, they exhibit higher mechanical properties at elevated temperatures (up to 500 °C).[5,6] In addition, contrary to weak interface of a-Al/eutectic Si particles in Al-Si alloys,[7] the interfacial bonding between the a-Al matrix and the Al3Ni phase is strong leading to a more efficient inter-phase load transfer and lower microcracking tendency.[8] Therefore, these alloys are good candidates for producing low- and high-temperature casting parts by casting processes such as sand molding, investment casting, permanent-mold gravity casting, squeeze casting, and high-pressure die casting.[4]
FATEMEH YOUSEFI, REZA TAGHIABADI, and SAEID BAGHSHAHI are with the Department of Materials Science and Metallurgy, Imam Khomeini International University (IKIU), Qazvin, Iran. Contact e-mail: [email protected] Manuscript submitted April 30, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
However, mechanical properties of hypoeutectic Al-Ni alloys are not high enough to meet the requirements of some engineering applications, especially, at high temperatures.[2,9] Therefore, alloying elements such as Mo, Fe, La, Zr, and V are usually added to increase their mechanical properties.[2,10–14] One of the most important and low-cost alloying element is Mn. Previous studies have shown that the addition of Mn to a binary Al-Ni alloy increases its strength, thereby comparable or even higher mechanical properties can be obtained as compared to widely used A356 alloy, especially at high temperatures.[1] Improved high-temperature mechanical properties of Al-Ni-Mn alloys are attributed to the formation of Mn-rich intermetallic compounds like Al6Mn, o-Al60Mn11Ni4, and j-Al80.3Mn17.5Ni2.2 in their microstructure. Compared to Al2Cu and Mg2Si intermetallics which exist in the microstructure of Al-Si-Mg(Cu) or Al-Cu alloys,[2,15] these Mn-rich phases exhibit lower tendency to be coarsened or dissolved in the matrix when the alloy is subjected to high service temperatures (more than 250 °C). This is thought to be due to the lower diffusivity of Mn atoms in Al than Si, Cu, and Mg.[2] The effect of Mn on mechanical properties of Al-Ni-Mn alloys at room and high temperature is, however, dependent on the size, morphology, distribution, and volume fraction of Mn-rich intermetallics. The formation of large and brittle intermetallics negatively affects the mechanical properties of these alloys,
especially their ductility and fracture toughness, making them unsuitable candidate for most engineering applications.[16] Therefore, some attempts have been made to improve the mechanical properties of Al-Ni-Mn alloys by refining the microstructural constitu
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