Correlation between former alpha boundary growth kinetics and superplastic flow in Zn-22 pct Al
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I. INTRODUCTION
MICROGRAIN superplasticity refers to the ability of fine-grained materials to exhibit a large elongation to failure (usually greater than 300 pct) during elevated-temperature deformation (T . 0.5 Tm , where Tm is the melting point). When Zn-22 pct Al, a model material used extensively in the study of micrograin superplasticity, is tested isothermally under creep conditions, the relationship between applied stress and steady-state strain rate is often sigmoidal.[1,2] This sigmoidal relationship is manifested by the presence of three regions: region I (the low-stress region), region II (the intermediate-stress region; associated with maximum ductility), and region III (the high-stress region). As reported elsewhere,[1,2] the values of the stress exponent (at constant temperature and grain size) in regions I and III are higher than those in region II (the superplastic region). In agreement with the concept[3] that the origin of region I is related to impurity segregation at boundaries, recent experimental data have revealed[4–11] that the presence of impurities in Zn-22 pct Al strongly influences both the deformation and cavitation behavior of the alloy in region I. In general, direct examination of boundary segregation requires the application of spectroscopic techniques, such as Auger spectroscopy, in which samples are fractured in vacuum and exposed grain boundaries are examined for segregation. In the case of materials that exhibit a ductileto-brittle transition, in situ fracture is produced by a combination of sample cooling and impact fracture. However, since superplastic Zn-22 pct Al samples do not exhibit low-temperature brittleness, the procedure is not feasible to propagate intergranular failure. Due to the difficulty of obtaining direct information (using conventional methods) regarding boundary segregation in Zn-22 pct Al, an alternative approach AHMADALI YOUSEFIANI, Graduate Research Assistant, and FARGHALLI A. MOHAMED, Professor, are with the Chemical and Biochemical Engineering and Materials Science Department, University of California, Irvine, CA 92697-2575. Manuscript submitted February 12, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
needs to be considered. The very recent microstructural observations described subsequently provide the basis for such an approach. The occurrence of micrograin superplasticity in metallic systems requires a fine and stable grain size of less than ,10 mm. Such a condition is normally achieved in Zn22 pct Al through solution treatment above the eutectoid temperature, followed by rapid quenching. The procedure results in a fine microduplex structure that is subsequently annealed below the eutectoid temperature to achieve the desired grain/phase size. Recent microstructural observations on Zn-22 pct Al[11–14] have indicated the presence of residual grain boundaries (referred to as former a boundaries (FaBs)) that form as a consequence of solution treatment. These FaBs represent domains consisting of fine elongated a grains, which encompass groups of fi
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