Effects of microstructure and composition on spall fracture in aluminum

Spall strength was measured as a function of composition and microstructure in three Al materials: a high-purity Al (Al HP), a commercial-purity Al (AA1100) and an alloy of Al containing 3 wt.% Mg (Al–3Mg). The Al HP and AA1100 materials were tested as single-crystal sheets, and the Al–3Mg alloy was tested as polycrystalline sheets having a variety of controlled grain sizes. A high-intensity laser produced shock loadings to create tensile strain rates ranging from 2 × 106 s−1 to 5 × 106 s−1, which caused spall fracture. Crystallographic orientation, relative to the direction of shock propagation, does not discernibly affect spall strength in the Al-HP material. Intermetallic particles, associated with impurity elements, initiate microstructural damage during tensile shock loading and reduce spall strength of the AA1100 material below that of the Al-HP material. The spall strength of the Al–3Mg is lowest among the three materials, and this is a result of the decreased ductility during spall fracture caused by the Mg solid-solution alloying addition. Grain size affects fracture character of the Al–3Mg material, but does not discernibly affect spall strength; the fraction of ductile transgranular fracture, versus brittle intergranular fracture, increases with grain size.