Low quench sensitivity of superplastic 8090 AI-Li thin sheets
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INTRODUCTION
SUPERPLASTIC 8090 A1-Li alloys are being considered for aircraft structure applications due to their high specific strength, or modulus, together with their superplasticity nature. Low sensitivity to the cooling rate of 8090 alloys from either the superplastic forming or solution treatment temperature (both within 500 ~ to 540 ~ r~l has made them even more competitive to the conventional 2000 and 7000 series aluminum alloys. The precipitation of stable phases along grain boundaries has been considered to be the main reason for the high quench sensitivity of the 2000 and 7000 series alloys,t5-9] Hirano et al. t3] followed the same concept to explain the low quench sensitivity of the 8090 alloy from the stability of the metastable 6' (A13Li) phase. Stable 6 (AI Li) particles tend not to precipitate in the alloy during slow cooling because of the stability of 6' phase; thus, the mechanical response of the alloy remains unaffected even when subjected to slow cooling. Kanno et al., t41 on the other hand, claimed that the low quench sensitivity of the 8090 alloy arose from the coherency of/3' (A13Zr) dispersoids. The coherency of /3' dispersoids remains unchanged and cannot act as heterogeneous nucleation sites for the stable 6 phase. Therefore, the 8090 alloy is insensitive to different cooling rates. However, the stability of the metastable 6' phase depends essentially on the aging temperature. Also, precipitation of the stable 6 phase is not controlled by the nucleation sites alone. In contrast, chemical composition (Li content) and the aging processes (including aging temperature, aging time, and step) are more important factors that determine whether the stable 6 phase will be formed, or not. II~ Prasad et al.tm recently studied the precipitation of the 6
TSUNG-RONG CHEN and GUAN-JYE PENG, Graduate Students, and J.C. HUANG, Professor, are with the Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Talwan, 80424 Republic of China. Manuscript submitted January 16, 1996 METALLURGICAL AND MATERIALS TRANSACTIONS A
phase in the A1-Li-Cu-Mg-Zr alloy and showed that aging at higher temperatures or longer times (e.g., 350 ~ hr or 170 ~ hr) could promote the precipitation of the stable 6 phase extensively at grain boundaries, as well as in the matrix. The mechanical properties of the alloy will be consequently degraded even though the stable 6 phase is not formed during the quenching process. Besides, both Hirano et al. [31 and Kanno et al. t41 did not consider individual effects from the precipitation of various phases and their influence on the mechanical properties. In the current study, the origin of the low quench sensitivity of the superplastic 8090 alloy and its effects on mechanical properties are investigated. Effects other than the coherency of the dispersoids or the formation of stable phases are considered. Particular emphasis is placed on the relationship between the microstructure and mechanical properties developed by the different cooling rates. The
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