MICROSTRUCTURAL INVESTIGATION ON CAVITY NUCLEATION AT PRECIPITATIONS IN SUPERPLASTIC ALUNIMUM ALLOYS.

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The deformation behavior and cavitation characteristics were investigated at 613-693 K and at 10^<-4>-10^<-2>/s in Al-4.5Mg-0.09Si and Al-4.5Mg-0.2Si containing second phase particles of Mg_2Si. Both alloys showed elongation above 350% at 653 K and at 1O^<-3>/s. The 0.2Si alloy showed more cavitation development than the 0.09Si alloy. This indicates that the large addition of Si promotes cavity formation. It should be noted that microcavities were nucleated at the matrix/particle interfaces. The cavity growth rate of the 0.09Si alloy was the same as that of the 0.2Si alloy. On the other hand, the number of cavities for the 0.2Si alloy was much larger than that for the 0.09Si alloy. These results conclusively demonstrate that the addition of Si promotes cavity nucleation because of stress concentrations at the interfaces. Investigation of the sites for cavity nucleation revealed that the experimental critical particle size for cavity formation was in agreement with the critical diffusion length. The intragranular particles tended to nucleate cavities compared to the intergranular particles. Furthermore, the critical particle size for cavity nucleation was investigated using high-strain-rate superplastic aluminum matrix composite containing Si_3N_4 particles. As results, the critical particle size for cavity nucleation was in a good agreement with theoretical value estimated from Stwell's equation giving the critical strain rate and from Mabuchi's equation giving the critical stress. In a presence of small amount of liquid, however, the experimental critical particle size was larger than the theoretical values.