Metallurgical reactions in two industrially strip-cast aluminum-manganese alloys
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INTRODUCTION
ALUMINUMalloy sheet is produced via direct chill (DC) casting, block casting, or by twin-roll casting, with typical thicknesses of the castings between 300 and 600, 40, and 7 mm, respectively. The reduced casting thickness in the latter processes is accompanied by an increase in cooling rate during solidification and also by a decrease in hot-working deformation prior to cold rolling. In twin-roll casting, the cooling rate is commonly around 300 to 700 s -~, which leads to a cast structure characterized by a high degree of supersaturation and relatively small primary particles. This primary structure differs significantly from conventional DC-cast and hotrolled material and offers new possibilities for microstructure control during the shorter process route, which is a main feature of the twin-roll process, tlj Through cold rolling, a dense dislocation network is formed, since alloying elements retained in solid solution hinder dislocation movement. During subsequent heat treatment ("back annealing"), dispersoids will nucleate on this network and stabilize a subgrain structure, slow recovery, and delay recrystallization. The dispersoid distribution is thus perceived as an essential factor in the microstructure development; hence, it is of importance to understand in detail the formation, growth, and phase transformation of the dispersoid particles that are precipitated from the supersaturated solid solution, notably in the AI(Mn,Fe)Si alloys. The influence of the second-phase particles on other microstructure elements is usually described in terms of drag forces on the subgrain and grain boundaries (the socalled Zener drag); for a recent discussion of this effect, see Nes, et al.t21 The particle size is of prime importance here; following Hillert, t3j the dispersion ratio f / r , where V. HANSEN, Research Associate, is with the Center for Materials Research Department of Physics, University of Oslo, Norway. B. ANDERSSON, Chief Scientist, is with SINTEF, Oslo, Norway. J.E. TIBBALLS, Research Consultant, formerly with SINTEF, Oslo, Norway, is with Comalco Research Center, Victoria 3074 Australia. J. GJONNES, Professor, is with the Center for Materials Research/ Department of Physics, University of Oslo, Oslo, Norway. Manuscript submitted March 14, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
f is the volume fraction and r is the average particle radius, can be used to define a limiting subgrain size Dli m = (4/3) (r/3~. Subgrain coalescence and subgrain growth play an important role in recrystallization; a marked effect from the dispersoid distribution on kinetics as well as on the grain size obtained by recrystallization treatment of DC-cast material was demonstrated by Nes, E4] who applied Hillert's t3] theoretical treatment to subgrain development in DC-cast AI-0.9 wt pct Mn (Morris and Duggan[Sl). Close correspondence between recovery and precipitation kinetics during the annealing of twin-roll-cast alloys in the A1(MnFe)-S??i system was found by Andersson. t6j Annealing of the twin-roll-ca
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