Microstructure Modification Of High Purity Aluminum And Its Effects On Spallation Behavior

Aluminum and its alloys have high specific strength,and are widely used in the automotive,aerospace and defense industries.And thus how to acquire aluminum materials of high strength and toughness by modifying microstructure is an important issue in material science and engineering.Equal channel angular pressing(ECAP)technology can modify the microstructure of materials by severe plastic deformation,and hence give rise to high strength and toughness.On the other hand,since aluminum and its alloys may subject to dynamic damage and fracture during their service,it is necessary to know more about the dynamic mechanical properties and their governing factors.In the present study,microstructure of high purity aluminum(99.999%)is modified by ECAP technique with different parameters,and then the effects of microstructure on their spall behavior are investigated.The results show that due to the different extrusion rates,the activated slip systems and work hardening extents are quite different,resulting in the different recrystallization extents and the ensuing discrepant grain-refining effects during ECAP process.The low impurity content and the low melting point of the high purity aluminum facilitate dynamic recovery and dynamic recrystallization,but the recrystallized grains grow larger consecutively during deformation.Therefore,it is difficult to obtain ultrafine grained high purity aluminum by ECAP technique.The relative finer grain structure can be obtained at low extrusion temperature.The spall experiments show that the grain size has a slight effect on the spall strength but an obvious influence on the pullback slope(damage).Electron backscatter diffraction characterization of the recovered samples shows that voids tend to nucleate at grain boundaries(GBs)and GB vicinities.Since abundant microscopic defects accumulate in GBs,GBs are the weak zones in the material,and thus intergranular voids are apt to nucleate at GBs;GBs can activate the multiple slips in their vicinities,resulting in nucleation of intragranular voids.Intergranular voids tend to nucleate earlier at GB and have sufficient growth time,giving rise tointergranular voids bigger than intragranular voids.And GB is two-dimensional defect and occupies less total volume,resulting in more intragranular voids than intergranular voids.Compared with grains with large size,small grains are suitable for the nucleation of both intergranular and intragranular voids,because the high density of GBs affords more nucleation spots for intergranular voids while promotes multiple slips which lead to the nucleation of intragranular voids.