Correlation between Electrical Resistivity, Particle Dissolution, Precipitation of Dispersoids, and Recrystallization Be
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INTRODUCTION
ONE of the primary objectives of the homogenization treatment applied to direct-chill cast aluminum alloys before hot deformation is to create small dispersoids that later act as recrystallization inhibitors during deformation and annealing treatments.[1] In industrial practice, the billet is homogenized at a temperature in a range from 450 C to 480 C, under strict control of the heating rate and time, in order to produce a distribution of fine dispersoids at grain boundaries.[1] In 3XXXseries aluminum alloys, for example, Mn in solid solution and Mn-containing dispersoids, formed during homogenization, play an important role in controlling the recrystallization behavior of the alloys.[2–4] In 6XXX-series aluminum alloys, the formation of different kinds of dispersoids, i.e., Zr-, Mn-, and Cr-containing dispersoids, which play the role of recrystallization inhibition, has been investigated.[5–9] In the case of 7XXX-series aluminum alloys, the formation of Zr- and Sc-containing dispersoids has been investigated.[10–16] For example, Robson et al.[10,11] investigated the effect of Zr addition on the dispersoid formation and recrystallization fraction after hot deformation. It was concluded that, when using an optimum two-step homogenization treatment, a smaller fraction of recrystallization could be obtained. Robson[12] also considered the effect of Sc on the formation of dispersoids, because Sc was expected to eliminate the dispersoid-free zones, A.R. EIVANI and H. AHMED, Postdoctoral Researchers, are with the Materials Innovation Institute, 2628 CD Delft, The Netherlands. J. ZHOU and J. DUSZCZYK, Associate Professors, are with the Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] Manuscript submitted January 5, 2009. Article published online August 12, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
observed in scandium-free 7050, thus greatly increasing the recrystallization resistance. Senkova et al.[13] and Senkov et al.[14] investigated the effect of minor additions of Sc and Zr on the tensile properties of two developmental Al-Zn-Mg-Cu alloys in the temperature range of 196 C to 300 C. They concluded that, due to the presence of Sc and Zr in a fine-dispersoid form, both low-temperature and elevated-temperature strengths of these alloys were much higher than those of similar 7XXX-series alloys that did not contain these elements. Morere et al.[15] investigated the influence of Al3Zr dispersoids on the static recrystallization of hotdeformed AA7010 alloys. It was found that the fraction recrystallized decreased with increasing Zr content, rising deformation temperature, and decreasing dispersoid size. However, finding a comprehensive investigation of the formation of Cr- and Mn-containing dispersoids in 7XXX-series aluminum alloys during homogenization and its effect on the recrystallization resistance is difficult. It is well known that the presence of alloying elements or impurities in solid solution increases th
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