Precipitation Strengthening in Al-Ni-Mn Alloys
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TRODUCTION
IT has been shown that aluminum alloys that contain appreciable amounts of the aluminum-nickel eutectic structure exhibit excellent fluidity and very good resistance to hot-tearing.[1] Moreover the Al3Ni eutectic phase, which is typically in the form of thin rods, adds significant strength to aluminum by the well-known Orowan looping mechanism.[2] Furthermore, the Al3Ni eutectic phase is chemically and thermally stable, and resists coarsening up to 773 K (500 C).[3] For these reasons, alloys based on this eutectic system are potential replacements for traditional aluminum alloys in high temperature applications. The yield strength of this binary eutectic does not exceed 100 MPa at room temperature and 50 MPa at 573 K (300 C),[4] which falls short of the current requirements of many engineering applications. Hence, it is necessary to add to aluminum and nickel other strength-inducing alloying elements—preferably ones that allow precipitation hardening by thermally stable precipitates. Many of the currently available age-hardenable aluminum alloys are precipitation hardened by Al2Cu, and/or Mg2Si precipitates. However, because of the high diffusivity of copper, magnesium, and silicon in aluminum, and the low thermal stability of the Al2Cu and Mg2Si phases, these precipitates tend to coarsen and dissolve in the aluminum matrix when the alloy is used at temperatures exceeding 523 K (250 C).[5] Al6Mn precipitates, which can form in aluminum by aging at temperatures between 673 K and 723 K (400 C and 450 C), are stable at significantly higher temperatures,[6] and are thus an attractive alternative. This is mainly because the diffusivity in aluminum of manganese is much smaller than that of copper, magnesium, and silicon [at 673 K (400 C)], DMn = YANGYANG FAN, Post Doctoral Fellow, KAI HUANG, Graduate Student, and MAKHLOUF M. MAKHLOUF, Professor, are with the Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609. Contact e-mail: [email protected] Manuscript submitted December 4, 2014. Article published online September 11, 2015 5830—VOLUME 46A, DECEMBER 2015
5.20 9 1019 m2/seconds, DCu = 2.31 9 1015 m2/seconds, DMg = 1.1 9 1014 m2/seconds, and DSi = 3.68 9 1015 m2/seconds).[7] Consequently, Al6Mn precipitates coarsen at a much slower rate than Al2Cu and Mg2Si precipitates. For example, at 513 K (240 C), the coarsening kinetics constant of Al2Cu is 690 nm3/seconds compared to 0.00234 nm3/seconds for Al6Mn at 773 K (500 C).[8] Unfortunately, because the equilibrium solubility of manganese in solid aluminum is small (max. solubility = 1.2 wt pct. At 932 K (659 C)[9]), the maximum volume fraction of Al6Mn precipitate that may form in aluminum by a typical heat treatment regimen is only 5.8 pct. Consequently, the strengthening increment attained by the presence of the Al6Mn phase in aluminum is limited. However, the volume fraction of Al6Mn precipitate may be significantly increased by adopting a non-traditional heat treatment regimen in which the molten alloy is cast at a high
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