Atomistic Simulations of the Shock and Spall Behavior of the Refractory High-Entropy Alloy HfNbTaTiZr

Using molecular dynamics simulation, we study the effect of a shock wave on the refractory high-entropy alloy HfNbTaTiZr. A single-crystalline sample, shocked along the [001] direction, is considered. The initial compression leads to only weak dislocation activity and a bcc \(\rightarrow\) $\to$ hcp transformation in some regions of the sample. After the shock wave is reflected from the free back surface of the sample, hcp transforms back to bcc, and twins are formed in the bcc phase. The sample spalls under the high tensile pressures developing after wave reflection. In this stage, we observe dislocation activity from the twin boundaries and inside the nanograins generated by twinning. Under the large tensile stresses, some fcc phase appears together with disordered amorphous regions where voids nucleate and lead to spall. The fracture surfaces follow the twin boundaries set up in the compression phase. The spall strength is similar to the one found in simulations of other bcc metals at similar strain rates. Similar simulations for the equiatomic HfNbTaZr HEA show the same qualitative behavior, with twins and reduced dislocation activity, but without phase transformations.