Alloying reaction mechanism of shocked Ni/Al nanolaminates regulated via atomic diffusion

The Ni/Al nanolaminates represent cutting-edge functional materials that exhibit alloying reactions and release substantial energy when subjected to shock loading. However, the extremely short timeframes of the shock loading and the induced reactions surpass the resolving capability of state-of-the-art monitoring techniques, rendering the alloying reaction mechanism of Ni/Al nanolaminates a challenging multi-physical problem. To address this issue, we conducted extensive molecular dynamics simulations on large-scale models of Ni/Al nanolaminates at varying shock velocities to investigate their in situ thermodynamics response and shock-induced kinetic evolution related to phase transitions and chemical reactions. Our simulations revealed that atomic diffusion plays a pivotal role in accelerating the activation and intensifying the alloying reaction. For a self-sustaining reaction to occur, the shock-induced pressure must surpass a threshold, triggering global atomic diffusion that overcomes lattice trapping barriers or fluid viscosity, facilitating the formation of a sufficient number of Ni-Al intermetallic bonds to store energy. Subsequently, interfacial and bulk atomic diffusion becomes unstoppable, leading to a uniform distribution of mixed atoms and a steady energy release accompanied by continuous temperature rise, thereby triggering self-sustaining alloying reactions akin to an avalanche. Our findings not only offer a valuable baseline for understanding reactions in real defective composites but also establish a lower bound on the required shock intensity for future experiments using new high-quality samples.