Melting of secondary-phase particles in Al-Mg-Si alloys
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INTRODUCTION
H O M O G E N I Z A T I O N heat treatment of alloys is a process of great industrial importance. Usually it is carfled out on castings in order to improve the properties of the casting or to facilitate subsequent processing steps to final products. In such cases, the homogenization treatment is carried out as a well-defined operation with rather strict temperature control. A similar process often takes place during hot deformation, such as extrusion, rolling, and forging, where generally a multiphase alloy is plastically deformed in the single-phase region. Homogenization then takes place during the heating period and during the deformation process. In these cases, the homogenization reaction is much less accurately controlled. One problem associated with the homogenization process is incipient melting. If the composition of a binary alloy is higher than Cmax (Figure 1) and the alloy is annealed at a temperature higher than T~ut, the alloy starts to melt. The plastic deformation of an alloy in this condition is problematic due to low ductility, but the phenomenon may also be utilized--for example, during thixocasting. In alloys with segregation of the alloying elements, the composition may locally exceed the critical composition C . . . . even though the mean composition is lower. Also in this case, incipient melting occurs when T is higher than Te,t, but this is a transient phenomenon. Recent experiments tl-SJ have demonstrated that incipient melting may take place during homogenization heat treatment of aluminum alloys even when the alloy is ODDVIN REISO, Senior Research Metallurgist, and JAN STRID, Senior Research Metallurgist, are with the Metallurgical R & D Centre, Hydro Aluminium a.s, N-6600 SunndalsCra, Norway. NILS RYUM, Professor, is with the Division of Metallurgy, The Norwegian Institute of Technology, University of Trondheim, NTH, 7034 Trondheim, Norway. Manuscript submitted September 22, 1992. METALLURGICAL TRANSACTIONS A
close to equilibrium at a low temperature and the mean, as well as the local matrix concentration of the alloy, is far below Cmax- Reiso t1'2] arrived at this conclusion in a rather indirect way by investigating the extrudability of several aluminum alloys. It was shown that one of the limiting conditions for extrusion speed probably is incipient melting of secondary phases, and it was indicated that the most likely reaction was the formation of a liquid of eutectic composition. Similar results were also obtained by Gjestland e t al. ~3~ Lohne and Ryum t41 subsequently studied the binary alloy AI-Si at mean compositions below Cmax and clearly demonstrated that local melting occurred in this alloy when the annealing temperature was higher than Te,t. The incipient melting reaction of secondary phases was further studied by Reiso e t al. tSJ in a binary A1-Cu alloy, where a more detailed characterization of the reaction is given. In the present investigation, the study of this incipient melting reaction is extended to ternary A1-Mg-Si alloys. This alloy system is of
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