The Work Hardening Rate of an Aged and Recovered Al-Mg-Sc Alloy
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INTRODUCTION
THE mechanical response of recovered but unrecrystallized materials is a question of significant practical concern, as several precipitate containing aluminum[1] and ferrous[2,3] alloys can undergo significant recoveryinduced softening without recrystallization. Precipitates play an important role in this given that they can be effective in pinning of high-angle boundaries. While there were some attempts to experimentally measure and model the evolution of the yield strength of aluminum alloys with recovery,[4,5] there were many fewer attempts to correlate the evolution of work hardening rate during recovery.[4,6] In the case of two-phase alloys, where very substantial recovery can occur without recrystallization,[7,8] there were even fewer studies. Understanding the work hardening behavior of such materials is important from a practical perspective, since knowledge of the rate of work hardening, along with the yield strength, allows for prediction of the ultimate strength and uniform elongation of the material in its recovered state. Such an understanding is complicated by the multiple contributions to the work hardening response coming from solute, subgrain boundaries and precipitates. In a detailed study of the effect of recovery on the yield strength of binary Al-Mg alloys by Verdier et al.,[4] a single internal state variable model based on the dislocation density, presented within the context of the Kocks–Mecking[9–11] approach, was applied to explain the evolution of work hardening rate at different levels of static recovery. In their modeling, Verdier et al.[4] considered that beyond statistical dislocation trapping, subgrain boundaries would contribute to dislocation storage due to local strain gradients as well as to enhanced dynamic recovery. This approach has several R. ROUMINA, Postdoctoral Fellow, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S 4L7, Canada. Contact e-mail: [email protected] C.W. SINCLAIR, Associate Professor, is with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Manuscript submitted September 21, 2009. Article published online December 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
similarities to the more detailed work hardening model developed by Nes and co-workers,[12,13] where the number of internal state parameters included in the evolution equations is expanded to include subgrain size and subgrain boundary misorientation. We have recently sought to develop a unified picture for the yield strength and work hardening rate of an aged[14] and aged and recovered[15] Al-Mg-Sc alloy containing Al3Sc precipitates. Despite their high cost, Sc containing aluminum alloys have found a niche application in aerospace and sporting goods products.[16,17] In both of these fields, the ability of a fine dispersion of Al3Sc to suppress recrystallization provides opportunities for increased strength, while the addition of Sc also benefits corrosion resistance and weldabi
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