Computer simulation of annealing and recovery effects on serrated flow in some Al-Mg alloys
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INTRODUCTION
IN an effort to optimize formability and retain high strength in 5XXX-series (Al-Mg) aluminum alloys, a considerable amount of experimental work has been performed.[1,2] Formability was quantified using several parameters and experimental techniques: strain to the onset of serrated flow (εc), Olsen cup height, bulge test, guided bend test, and standard uniaxial tension tests.[1] Hollinshead showed, in his detailed study of formability of 5182 end stock, that Olsen cup heights, tensile, ultimate tensile strength-yield strength, strain to onset of serrated flow, and serrated-flow characteristics correlated to the amount of Mg in b'-precipitate form or in solid solution.[2] Several other experimental techniques were used to shed additional light on the formability of these materials (e.g., a technique based upon CODF (crystal orientation distribution function) measurements to predict anisotropic yield surfaces, forming-limit diagrams, acoustic emissions etc.). As a result of these experimental efforts, two microstructural factors were identified as affecting formability: precipitate and defect (dislocation) populations, both their interactions and their densities. It is an established fact that an increased amount of Mg in the form of Mg5Al3 (rather than in solid solution) correlates with formability.[1,2] However, in the course of the thermal exposure which is necessary to achieve an appreciable density of precipitates, the dislocation population decreases significantly as a result of MICHAEL V. GLAZOV, Senior Scientist, DANIEL J. LEGE, Manager, FREDERIC BARLAT, Technology Specialist, and OWEN RICHMOND, Director, are with the Alcoa Technical Center, Alcoa Center, PA 15069-0001. Manuscript submitted April 10, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
recovery. This also is generally thought of as a favorable process for enhancing formability. However, because it is virtually impossible in a laboratory setting (not to mention in field tests and in plant practices) to separate these two metallurgical factors (increased density of precipitate and decreased dislocation density), decisive evidence in favor of either metallurgical factor was unavailable. Recent dramatic developments in nonlinear dynamics, in general, and in dislocation density–related constitutive modeling of metallic materials, in particular,[3,7] now provide a theoretical framework and computational tools for assessing the influence of these two metallurgical factors separately and independently.* The reason is that the inter*Of course, it is important to state clearly that, in its present form, the current article does not address the problem of differing populations of precipitates.
nal-variable models which can be used for this assessment can be fine-tuned to account for the precipitate population. At the same time, by changing only the initial values of the internal variables which are associated with the densities of different dislocation populations (mobile, immobile, and Cottrell-type dislocations with atmospheres), it becomes
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