A study of superplasticity in a modified 5083 Al-Mg-Mn alloy
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I.
INTRODUCTION
THE Al-Mg-Mn 5083 alloy has been the focus of several recent studies[1–6] investigating its superplastic (SP) properties. It is of interest for superplastic forming (SPF) because it combines adequate SP ductility with moderate strength, corrosion resistance, and weldability.[7,8] Commercial grades of this alloy have been-reported to give elongations ranging from around 200 pct[2] to over 300 pct.[1] Higher elongations in excess of 500 pct have been achieved with special SPF-grade 5083 alloys, which have a lower Fe and Si impurity content.[3–6] The fine-grained structure required for superplasticity is usually achieved by heavy cold reductions (about 80 pct) of the material that has alloying additions of Mn, Cr, Cu, and Zr.[9] These, along with the Fe and Si impurities, form intermetallic particles with Al. The second-phase particles help in grain refinement by serving as sites for nucleation of recrystallization or by pinning migrating grain boundaries, depending on the particle size. The effect of increasing the amount of Mn on the asrecrystallized grain size and its stability with prolonged high-temperature exposure was the subject of an earlier study by the authors.[10] Rapid grain growth was observed in an Al-4.75 wt pct Mg alloy with no Mn, for which the grain size was observed to vary from about 9 mm in the as-recrystallized state to about 60 mm after a 24-hour anneal at 525 7C. In contrast, the same alloy with 1.6 wt pct Mn exhibited negligible grain growth upon a similar anneal. The present work is a continuation of studies on this alloy, with the intent of evaluating its potential for exhibiting superplasticity. This study consists of an evaluation of the SP properties of this alloy in uniaxial tension, including flow properties and tensile elongations, along with microstructural characterization of the grain size and cavity evolution with strain. An attempt is also made to obtain a constitutive equation to represent its flow behavior. K. KANNAN, Postdoctoral Research Associate, and C.H. HAMILTON, Professor, are with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920. C.H. JOHNSON, Assistant Professor, is with the Industrial Engineering Technology Department, Central Washington University, Ellensburg, WA 98926. Manuscript submitted September 18, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENT
The alloy used in our study was an experimental version of the 5083 aluminum alloy, with the composition given in Table I (this is the same alloy as in our previous study,[10] but from a later heat). The starting material was in the form of 2-mm-thick sheets with an initial grain size (d0) of 8.7 mm (measured along the longitudinal direction by a linear intercept method). Tensile testing and microstructural characterization were performed under different test conditions, in the temperature range of 500 7C to 550 7C and constant strain rates of 5 3 1024 to 1021 s21. Tensile samples of a gage length of 25.4 mm and a width 6.3 mm we
