Aging behavior of an Al-Li-Cu-Mg-Zr alloy
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I.
INTRODUCTION
ALTHOUGH the aging processes in solution-treated AlLi–based alloys have been studied extensively, there are still many arguments about the initial stage of ordering. In the earlier transmission electron microscopy studies,[1,2,3] the reactions have been interpreted simply as supersaturated solid solution → d' (Al3Li) → d (AlLi). Electrical resistivity[4] and calorimetric studies,[5,6] however, revealed various precipitation reactions, the increase in resistivity and the exothermic reaction prior to d' precipitation being attributed to either cluster formation or short-range ordering. Sigli and Sanchez[7] calculated the Al-Li phase diagram containing metastable d' phase by means of a cluster variation method. In their calculation, however, the d' phase in equilibrium with a solid solution is assumed to be in stoichiometric composition, but the experimental data so far obtained are far from stoichiometry. Khachaturyan et al.[8] then investigated the transformation paths from a solid solution to an L12 phase by means of the mean-field approximation. They suggested that, at lower temperatures, a solid solution decomposes into an ordered structure without any compositional change (congruent ordering), and this ordered structure is further separated into the Li-rich ordered d' regions and the Li-lean disordered regions via a spinodal decomposition. Such a theoretical prediction is in keeping with some of the results obtained by high-resolution transmission electron microscopy.[9,10] For example, Radmilovic YASUYA OHMORI, Professor, and KIYOMICHI NAKAI, Associate Professor, are with the Department of Materials Science and Engineering, Ehime University, Ehime Prefecture 790-8577, Japan. SADAYOSHI ITO, formerly Graduate Student, Department of Materials Science and Engineering, Ehime University, is with Matsushita-Kotobuki Electronics Co., Ltd., Ehime Prefecture 793-0035, Japan. Manuscript submitted May 1, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
et al.[9] did not observe discrete d' (Al3Li) precipitates at higher magnification, but obtained diffraction patterns always containing L12 superlattice reflections. In addition to this, they recognized a modulated structure of ordered domains separated by disordered regions. Sato and Kamio[10] also observed a uniformly ordered structure in the asquenched specimen. Mahadev et al.[11] however, examined the early stage of d' formation by means of an X-ray diffuse scattering technique and concluded that the data obtained are consistent with a classical nucleation and growth mechanism. Schmitz et al.[12,13] also supported a nucleation and growth model based on their results by high-resolution transmission electron microscopy. They observed small L12 ordered domains with somewhat diffuse interfaces in the as-quenched specimens and reported that spherical d' particles nucleate, with an incubation time longer than 4 minutes at 463 K, and grow via Ostwald ripening.[12] They determined that there is no positive evidence for the congruent ordering.[13] The aim of th
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