Microstructural evolution and energetic characteristics of TiZrHfTa0.7W0.3 high-entropy alloy under high strain rates and its application in high-velocity penetration

• A hierarchical metastable triple-phase structural TiZrHfTa 0.7 W 0.3 HEA. • Overcoming the trade-off between strength and ductility during dynamic loading. • Low reaction threshold during dynamic loading • Significant chemical energy release capacity and hole-enlarging capability Energetic structural materials (ESMs) integrated a high energy density and rapid energy release with the ability to serve as structural materials. Here, a novel triple-phase TiZrHfTa 0.7 W 0.3 high-entropy alloy (HEA) was fabricated and investigated as a potential ESM. A hierarchical microstructure was obtained with a main metastable body-centered-cubic (BCC) matrix with distributed Ta-W-rich BCC precipitates of various sizes and interwoven hexagonal close-packed (HCP) lamellar nano-plates. The compressive mechanical properties were tested across a range of strain rates and demonstrated a brittle-to-ductile transition as the strain rate increased while maintaining a high ultimate strength of approximately 2.5 GPa. This was due to the phase transformation from metastable matrix BCC to HCP structures. In addition, during the dynamic deformation, metal combustion originating from the failure surface was observed. Furthermore, the composition of the fragments was studied, and the results indicated that the addition of tungsten promoted combustion. Finally, the potential application of this HEA was evaluated by high-velocity penetration tests, and the results were compared to other typical structural materials for penetrators and bullets. A comparison was conducted by assessing the geometries of the penetration channel employing two dimensionless parameters normalized by the projectile size, representing longitudinal and lateral damage, respectively. The normalized depth of the TiZrHfTa 0.7 W 0.3 HEA projectile was comparable to those of the other investigated materials, but the normalized diameter was the largest, showing an excellent ability to deliver lateral damage.