In recent years, new Al-Mg-Sc-Zr alloys developed from the conventional Al-Mg alloys have gained more attention due to its low density, high strength，good corrosion resistance, weldbility and excellent thermal stability. In this paper, precipitation and coarsening of the Al3(Sc, Zr) phase and its effects on the properties of the alloy have been investigated.
Precipitation of coherent Al3(Sc, Zr) particles and its effect on stabilizing the deformed microstructure were studied by tensile test and TEM, OM observation. The result shows that nanoscale coherent Al3(Sc, Zr) particles with excellent thermal stability were produced during decomposition of the solid solution. This particles could inhibit transformation of the deformed structure, increasing the recrystallization temperature of the alloy by 200℃.
Nanoscale coherent Al3(Sc, Zr) particles may have higher coasening rate in the deformed alloys during the intermediate annealing in the course of hot rolling. Factors influencing the coarsening rate were studied. Higher annealing temperature, longer annealing time and larger deformation rate in hot rolling could promote coasening of the Al3(Sc, Zr) particles. Coasening of the particles led to drop of the number density of the particles and lower coherent strains. Consequently, recrystallization resistance of the alloy would decrease.
Repeated deformation and intermediate annealing need to be applied during the cold work. Influence of the annealing temperature and annealing times on the properties of the cold worked alloy was studied. Higher intermediate annealing temperature and frequent annealing could lead to loss in strength of the cold worked alloy. Microstructure observation revealed that coherent Al3(Sc, Zr) phase coasened during the higher temperature annealing, some particles lost coherency with the matrix. Coasening of the particles not only weakened its precipitation strenthening effect but also the ability to stabilizing the deformed structure during intermediate annealing.
Primary precipitates formed during solidification could significantly refine grain size of the as-cast alloy. Influence of cooling rate of the melt alloy on the precipitation behavior of the primary precipitates were studied. With very slow cooling rate, big size Al3Sc, Al3Zr and Al3(Sc, Zr) with unregular shape were obtained; Rapid solidification could completely restrain formation of the primary precipitates; when using the iron mold, square particles of Al3Zr phase were formed. This kind of particles have a special structure with square shell wrapping the square shaped dendrite. Formation of such a structure was explained as a change in growth mode during the cooling of the melt metal.
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