Local melting in Al-Mg-Zn-alloys
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INTRODUCTION
W H E N an alloy within a eutectic alloy system has been brought to equilibrium in the two-phase (a + /3) field and subsequently is upquenched into the onephase a field, local melting will occur in the alloy if the /3-phase particles do not dissolve before the eutectic temperature is reached. The importance of this type of internal melting, which can take place in rather dilute alloys which contain no or very small concentration gradients, has been recognized quite recently. It now has been studied in the binary AI-Si ll'21 and AI-Cu alloy system t31 and also in the ternary A1-Mg-Si alloy system, t4j The mode of melting and the kinetics of the subsequent dissolution of the melt into the a matrix have been investigated in detail. This melting reaction will have a detrimental effect on the high-temperature mechanical properties of the material. Reiso t5'6] and Gjestland et al.[71 have demonstrated convincingly that one of the important limitations to the extrudability of the Al-Mg-Si-type alloys of industrial importance is closely related to the internal melting reaction. The detailed thermal treatments of the extrusion billets and the extrusion parameters thus become of great importance in an effort to optimize the productivity of the extrusion press. Quite substantial improvement in extrudability recently has been obtained, particularly in the case of fairly concentrated A1-Mg-Si alloys. 18.9] In addition to the AI-Mg-Si alloys, the AI-Mg-Zn alloys are another class of precipitation-hardened alloys with an interesting potential as extrusion alloys, tt~ The alloys of medium to low strength are known to have (generally) good extrudability, and these alloys can be obtained in the solution-treated condition directly after extrusion (T-5 condition). Also, several of the important engineering properties (ultimate strength, ductility, fatigue strength and weldability) suggest that the A1Mg-Zn alloys have potential as extrusion alloys, the major PER-ERIK DROENEN, Research Scientist, is with the Institutt for Energiteknikk, N-2007 Kjeller, Norway. NILS RYUM, Professor, is with the M e t a l l u r g i s k I n s t i t u t t , The N o r w e g i a n I n s t i t u t e of Technology, N-7034 Trondheim, Norway. Manuscript submitted December 16, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
exception being the tendency toward stress corrosion cracking. However, if the total amount of alloying elements is kept below approximately 7 to 8 wt pct, this problem for the most part is avoided, l~~ Also, special heat-treatment procedures have been developed to eliminate the tendency toward stress corrosion, ll~'121 In spite of this, at present alloys within this system occupy a comparatively small fraction of the extrusion market. As already stated, the hot formability of the AIMg-Zn alloys generally is good, even though they are considered inferior to A1-Mg-Si alloys of comparable strength. A limitation to the hot formability may be related to internal melting. The motivation of the present investigation was to charact
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