Solid-State Transformation of Iron-Rich Intermetallic Phases in Al-Cu 206 Cast Alloys During Solution Heat Treatment
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TRODUCTION
THE aim of the solution heat treatment (SHT) for aging-hardenable aluminum alloys is to allow alloying elements that bonded by constituents entering the matrix and form a supersaturated solid solution through rapid cooling. During the SHT, the iron-rich intermetallics are usually hard to dissolve into the matrix because of the high melting temperatures.[1–5] However, solid-state phase transformations between the iron-rich intermetallics may happen in aluminum alloys. For example, it was reported that the metastable iron-rich intermetallics such as Al6Fe, AlmFe, and a-AlFeSi can be transformed into stable Al3Fe in AA1xxx alloys during the SHT.[6–8] Similarly, the solid-state phase transformation from Alm(FeMn) to Al3(FeMn) was observed in AA5182 alloy after SHT at 793 K (520 C) for 7 hours.[9] It was also observed that the Al3(FeMn) was transformed into a-Al(FeMn)Si after 48-hour holding at 743 K (470 C) in AA5182 alloy.[9] In AA3xxx alloys, Al6(FeMn) was found to transform into aAl(FeMn)Si after SHT at 773 K (500 C) for 3 hours.[10] In addition, Chinese script Al8Fe2Si and p-Al9Mg3FeSi5 K. LIU, Ph.D. Candidate, and X.-G. CHEN, Professor, are with the Department of Applied Science, University of Quebec at Chicoutimi, Chicoutimi, QC G7H 2B1, Canada. Contact e-mail: [email protected] X. CAO, Adjunct Professor, is with the Department of Applied Science, University of Quebec at Chicoutimi, and also a Research Officer, Aerospace Manufacturing Technology Centre, Institute for Aerospace Research, National Research Council Canada, 5145 Decelles Avenue, Montreal, QC H3T 2B2, Canada. This statement applies only to author Cao: This manuscript is published with the permission of the National Research Council of Canada, by right of the Crown in Right of Canada. Manuscript submitted November 30, 2012. Article published online April 10, 2013 3494—VOLUME 44A, AUGUST 2013
were also observed to transform into platelet b-Al5FeSi during SHT in Al-Si cast alloys.[11,12] In Al-Si-Cu alloys, the platelet b-Al5FeSi phase was found to transform into platelet Al7Cu2Fe during the SHT.[13,14] Although there has been a good agreement with regard to the fact that the platelet iron-rich intermetallics are detrimental to the mechanical properties because of their brittle nature and stress concentrating potential,[15,16] considerable inconsistencies have been reported for their evolution during the SHT. For instance, no significant variation was noted in the size, number, or morphology of the platelet b-Al5FeSi during the T6 heat treatment for Al-Si cast alloys.[17,18] In A319 alloy, however, platelet b-Al5FeSi was found to fragmentize and dissolve after SHT at 788 K (515 C) for 8 hours and thus to improve the mechanical properties.[19] In their study, Eshaghi et al.[5] also found that the volume fraction and the average length of the platelet b-Al5FeSi decreased after SHT at 793 K (520 C) for 20 hours. Precipitation-hardened Al-Cu 206 cast alloys have been widely used in aerospace and automotive industries because of the high strength a
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