Microtexture and grain boundary evolution during microstructural refinement processes in SUPRAL 2004
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I.
INTRODUCTION
THE engineering development of aluminum alloys thermomechanically processed to achieve extensive finegrained superplasticity has generally followed one of two basic processing schemes. The first developed was the SUPRAL process, reported by Grimes[1] and Watts et al.[2] to involve dynamic recrystallization. The alloy, nominally Al 2 6.0 wt pct Cu 2 0.4 wt pct Zr, evolves to a microstructure capablez of superplastic flow at commercial forming strain rates, ε . ;1022 s21, only upon deformation and with a microstructural transition occurring early in the deformation process. The Rockwell process, developed later by Paton and Hamilton[3] and initially used for processing of aluminum alloy AA7475, has been reported[4] to involve the development of coarse particles through aging or thermomechanical processing (TMP) to assist in a discontinuous recrystallization during TMP or post-TMP annealing in a manner which may be described by the particle-stimulated nucleation theory of Humphreys.[5] The latter process is thought to involve conventional recrystallization mechanisms, as defined by Doherty et al.[6] to include the formation and migration of high-angle boundaries. The microstructural transition in the SUPRAL process has not been as well described. A comprehensive study was conducted by Bricknell and Edington using Xray diffraction (XRD) and the crystallite orientation distribution function to quantify textural changes associated with TERRY R. McNELLEY, Professor and Chairman, is with the Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5146. MICHAEL E. McMAHON, CDR, U.S. Navy, formerly Doctoral Student, Department of Mechanical Engineering, Naval Postgraduate School, is with the Department of the Navy, Naval Sea Systems Command, Arlington, VA 22242-5160. Manuscript submitted December 27, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
the microstructural evolution in this material.[7] Their study hypothesized that continuous recrystallization mechanisms involving the movement of dislocations and dislocation networks were associated with the microstructural evolution to a superplastic microstructure in the alloy. They followed this study with a similar one using transmission electron microscopy to examine the as-processed alloy and found evidence of a banded structure with a large number of lowangle boundaries (LABs, u , 5 deg) and moderately misoriented boundaries (MMBs, 5 deg ≤ u , 15 deg) to be associated with the process of microstructural refinement. Conclusive evidence regarding the involvement of the LABs and MMBs was lacking due to an absence of data on boundary evolution. The goal of the present study is to use an interactive electron backscatter pattern (EBSP) method to examine the microtextural changes and boundary evolution in SUPRAL 2004 from the as-processed state, during static annealing, and with increasing strain levels up to the onset of grain boundary sliding (GBS). Discrete boundary character data obtained from the accompanying orientation inform
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