Critical assessment of the extreme mechanical behavior of a stable nanocrystalline alloy under shock loading

A material's spall-strength and Hugoniot elastic limit (HEL), are measures of its ability to resist failure and plastic deformation under shock loading. An ideal single-crystal, i.e., a defect-free material, offers perfect lattice rectification, and therefore, provides an upper bound, or expected limit, which has yet to be exceeded by their polycrystalline counterparts. Toward this, we used a nanocrystalline (NC) copper-tantalum alloy, a model system, to probe the HEL and the spall strength of a stable NC alloy and the pertaining microstructural features that control failure. The results reveal significant increases in the HEL to about 2.0 GPa and spall strength of 1.19–1.67 GPa compared to polycrystalline Cu along with negligible changes in the residual hardness and microstructure of the shock recovered samples. The observed spall strength is approximately 2-times that of polycrystalline Cu. Further, advanced microstructural characterization using transmission electron microscopy (TEM) found no increase in dislocation density and/or mechanical twinning between the as-received and shock recovered samples, i.e., stabilized NC-alloys exhibit an unprecedented ability to resist high defect (such as dislocation) accumulation and damage. This anomalous behavior in stable NC-alloys is attributed to the elimination/limitation of defects formed under shock conditions coupled with a divergent strain-rate-insensitive behavior of its main microstructural features. The present work highlights, if designed properly, that some critical lower length-scale features including grain and phase boundaries may not contribute to the failure process. However, more fundamental research is needed to address the role processing parameters have on the resultant material that could result in spall strengths comparable to those attained for single crystals.