The influence of Al 3 Zr dispersoids on the recrystallization of hot-deformed AA 7010 alloys
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NTRODUCTION
THE AA7010 (Al-Zn-Mg-Cu) alloy is widely used for structural components in critical aerospace applications such as wing spars. The essential properties that combine high yield stress and good fracture toughness are obtained by a combination of thermomechanical processing and age hardening. The processing includes hot rolling and cooling to room temperature followed by a solution treatment, quenching, and aging. The aging reactions and their influence on the mechanical properties of these alloys have been widely studied.[1,2,3] However, it is now recognized that the critical fracture toughness properties can be strongly influenced by the amount of recrystallization that occurs during the solution treatment as a consequence of the energy stored during hot rolling. In general, the higher the degree of recrystallization, the lower the fracture toughness, so that manufacturers try to inhibit recrystallization by the addition of Zr, which forms fine Al3Zr dispersoids. Clearly, both the hot rolling conditions and the alloy content have to be optimized to obtain suitable microstructures for the required properties. In this article, the specific influences of the size and volume fraction of Al3Zr dispersoids are analyzed by controlled hot deformation tests on a series of 7010 alloys of varying Zr contents (0.05 to 0.12 pct). The hot rolling conditions are simulated by instrumented high-temperature plane strain channel-die compression (PSC) tests under conditions of constant temperature and strain rate. This deformation method gives relatively homogeneous deformations and direct flow stress data and enables the sample to be quenched. The microstructures are characterized by quantitative optical and electron metallography, including transmission electron microscopy (TEM) and scanning electron microscopy–electron backscattered diffraction (SEM-EBSD). Particular attention is paid to the size and spatial distribution of the Al3Zr dispersoids, which tend BRUCE MORERE, Research Engineer, and RAVI SHAHANI, R & D Program Leader, are with the Pechiney Centre de Recherche de Voreppe, 38341 Voreppe Cedex, France. CLAIRE MAURICE, CNRS Scientist, and JULIAN DRIVER, Research Director, are with the Microstructures and Processing Department, Ecole des Mines, 42023 Saint Etienne, France. Manuscript submitted February 22, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
to be distributed very heterogeneously within the grains. This heterogeneous dispersoid distribution has a pronounced effect on the fraction of material that is capable of recrystallizing for a given set of conditions.
II. EXPERIMENTAL PROCEDURE Four experimental (10 kg) casts of different Zr contents, denoted A, B, C, and D, were prepared at the Pechiney CRV research center. Their compositions are given in Table I, along with that of a standard 7010 alloy denoted R. Apart from the Zr contents, the other alloying elements have levels close to those of the reference. The compression samples cut out from the ingots were homogenized 24 hours at 480 ⬚C, using one of t
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