Spall behavior of aluminum with varying microstructures

A series of plate-impact spall experiments was conducted to study the spall strength of seven microstructural conditions of aluminum, including three grain sizes of 6061 Al alloy, both ultrapure and commercially pure (1060) polycrystalline aluminum, and single-crystal Al with two different orientations, over the stress range of 4-22 GPa. The pullback velocity, which is a characteristic signature of spall strength, is observed to depend on initial microstructure, impact stress, pulse duration, and loading rate. The pullback velocity generally increases over the stress range of 4-14 GPa and achieves a maximum as the impact stress approaches 22 GPa. The pullback velocity of [100] single-crystal Al is higher than that for both polycrystalline samples and [111] single-crystal samples, indicating that grain orientation strongly affects material response. Experimental results also show that the spall behavior is strongly dependent on sample thickness, while the effect of shock pulse duration was observed to be less significant. Comparison among three 6061 materials indicates that the observed differences depend on initial yield strength. The results also show that initial microstructures and impurities have a diminishing effect on the pullback velocity at stresses near 22 GPa. However, initial properties are observed to have a pronounced effect on the detailed structure of the pullback velocity profile at all stress levels. In particular, an interesting feature, i.e., a sharp slope during pullback followed by a distinct transition to a slower slope, is consistently observed. The occurrence of this change in slope is observed to depend on impact stress, loading rate, and grain size.