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

在613-693 K和10^<-4>-10 ^<-2>/s温度下，研究了含Mg_2Si第二相粒子的Al-4.5Mg-0.09Si和Al-4.5Mg-0.2Si合金的变形行为和空泡特性。两种合金在653 K和10 ^ /s下的伸长率均超过350%<-3>。0.2Si合金比0.09Si合金表现出更多的空蚀发展。这表明，大量添加Si促进空腔形成。应该注意的是，微腔在基质/颗粒界面处成核。0.09Si合金的空洞生长速率与0.2Si合金相同。另一方面，0.2Si合金的空洞数量远大于0.09Si合金。这些结果最终表明，由于在界面处的应力集中，Si的添加促进空腔成核。对空穴形核位置的研究表明，实验得到的空穴形成的临界粒径与临界扩散长度一致。与沿晶颗粒相比，晶内颗粒倾向于成核空穴。在此基础上，研究了含Si_3N_4颗粒的高应变速率超塑性铝基复合材料孔洞形核的临界颗粒尺寸。结果表明，空洞形核的临界颗粒尺寸与由Stwell方程给出的临界应变速率和由Mabuchi方程给出的临界应力的理论值吻合较好。在少量液体存在下，实验临界粒径大于理论值。

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.