Microstructure Evolution in the Near-Surface Region During Homogenization of a Twin-Roll Cast AlFeMnSi Alloy
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RECENT investigations on thermo-mechanically processed aluminum alloys have revealed near-surface deformed layers[1–7] which have significant influences on electrochemical properties, corrosion performance, optical properties, etc.[4,6,8–18] The near-surface deformed layers are characterized by an ultrafine-grained microstructure, consisting of a large population of grain boundaries which represents high stored energy. At elevated temperatures, the ultrafine grains in the near-surface deformed layer are susceptible to grain growth. However, the thermal stability of near-surface layers on aluminum alloys is enhanced if a high population density of incorporated oxide particles [2,4,19] and/or dispersed intermetallic precipitates[11] is present at the grain boundaries. Both types of particles can effectively retard the movement of grain boundaries through the Zener drag pinning effect. Zhou et al.[2] reported that the near-surface layer comprising a high population density of rolled-in oxide particles in a rolled AA3005 alloy survived an annealing treatment at 623 K (350 °C) for 2 hours, while the near-surface layer free of oxides was annealed out. Scamans et al. [4] reported that the near-surface layers produced by mechanical grinding JUNJIE WANG, Visiting Academic, and XIAORONG ZHOU and GEORGE E. THOMPSON, Professors, are with the School of Materials, The University of Manchester, Manchester, M13 9PL, UK. Contact e-mail: [email protected] JOHN A. HUNTER, Group Leader, and YUDIE YUAN, Lead Surfaces Scientist and Corrosion Specialist, are with Novelis Global Research and Technology Center, Kennesaw, GA 30144. Manuscript submitted March 20, 2015. Article published online June 1, 2016 4268—VOLUME 47A, AUGUST 2016
were much less thermally stable than those generated by hot rolling due to the insufficient level of incorporated oxides for pinning the grain boundaries. A previous investigation on copper alloys reported that with sufficient closely spaced, thermally stable particles, the deformed/recovered microstructure could be preserved up to the melting point of the alloy matrix.[20] However, up to date, no work on whether or not the ultrafine-grained microstructure in the near-surface layer with a high population density of oxide particles and/or intermetallic precipitates in aluminum alloy can survive exposure to high temperatures, i.e., homogenization, has been reported. Further, the authors’ previous work revealed that the distribution of the oxide/lubricant particles within the near-surface layer of foil stock twin-roll cast (TRC) 8906 AlFeSi alloy was not uniform due to the varying extents of local tool/workpiece interaction.[21] The influence of the local population densities of the oxide/lubricant particles on the thermal stability of the near-surface layer has not been reported. Although elemental redistribution and intermetallic phase transformation in the bulk of continuously cast AlFeMnSi alloys during high temperature annealing has been reported previously,[22,23] the response of intermetallic precipitate
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