Grain Refinement of the Al-Cu-Mg-Ag Alloy with Er and Sc Additions
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TRODUCTION
GRAIN refinement is an important technique to improve the cast quality of the aluminum alloys. It reduces the size of primary a-Al grains in the casting. The refined grain restrained coarse columnar grain structure. A fine equiaxed grain structure leads to several benefits, such as high yield strength, high toughness, improved machinability, and excellent deep drawability of the products.[1] There are many methods to refine grain, whereas the microalloying or trace elements added to aluminum alloys is the most economical one. The common grain refiners of casting aluminum alloys include Al-Ti, Al-B , Al-Ti-C, and AlTi-B master alloys.[2–6] An effective method to refine the grain of heat-treatable alloys was micro-alloying with transition metals or rare earth metals, such as additions of Sc, Zr, and Er as alloying elements.[7–10] Research on the addition with scandium element in aluminum alloys has been carried out for many years. As an efficient grain refiner of casting alloys, Sc forms a Al3Sc phase in aluminum alloys. The Al3Sc phase is a thermostable L12-type (Cu3Au) one, which is characterized by a structural and dimensional matching with the aluminum matrix.[11,12] It acts as a nucleation site efficiently for heterogeneous nucleation of a-Al grain growth in the melt. When Sc was added to Al-Cu serial alloys, a ternary W phase formed. According to the Al-Cu-Sc ternary phase diagram,[13,14] Al3Sc and a ternary W phase are predominant phases.[15–17] When W primary particles formed in the melt, they made it respond poorly to grain refinement.[16] However, Yu[18] has found that addition of scandium and zirconium to 2618 alloy resulted in a primary Al3(Sc,Zr) phase, which refined grain signifiYUNTAO LI, QINGKUN XIA, and YANBIN LIU, Doctoral Candidates, and ZHIYI LIU, Professor, are with the College of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, PeopleÕs Republic of China. Contact e-mail: [email protected] Manuscript submitted January 5, 2007. Article published online October 5, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
cantly. The Sc added to 2024 alloy has also been discovered to improve the cast microstructure.[19] Recently, several publications have been concerned with the addition of rare earth Er to aluminum alloys.[16,20,21] The Er addition formed a thermostable L12-type (Cu3Au) Al3Er phase similar to alloying element Sc. It refined grain significantly. Zhao[22] studied the Al-Zn-Mg-Cu alloy with addition of Er and found that Er in this alloy mainly forms Al3Er phase. The dispersed Al3Er phase acts as the site for heterogeneous nucleation of g(MgZn2) phase and enhances the mechanical properties. Fu and Yang[23–25] have studied the high pure aluminum and the Al-Mg alloy with addition of Er and found the existence of Al3Er phase. The study carried out by Nie revealed that the addition of Er in Al-4Cu alloy mainly forms a Al8Cu4Er phase.[26] Al-Cu-Mg-Ag-(Zr) alloys are the promising candidates for future use for high-speed aircraft.[27–30] The research carried out by
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