The role of interlayer gases and surface asperities in compression-induced intermetallic formation in Ni/Al nanocomposites.

Reactive composites comprising alternating nano- or microscale layers of Ni and Al are known to undergo self-sustaining alloying reactions under compression loading, however the effect of infiltrated gas within the microstructure of such reactive nanolaminates-as well as the presence of asperities on the free surfaces of such composites-is not well understood. This work presents atomistic molecular dynamics simulation and analysis of the mechanical dynamics and thermal evolution of planar Ni/Al nanolaminates under a variety of scenarios of layer dimensions, surface asperity shape and orientation, and interlayer gas identity and concentration. These simulations indicate that the rate of the alloying reaction is inversely correlated with the layer width of the nanolaminate, recapitulating experimental results. The presence of surface asperities of comparable scale to the nanolaminate layer thickness enhances short-term intermetallic mixing but has a marginal accelerant effect on the reaction. Interlayer argon gas acts as a mechanical interferent to reaction, whilst interlayer nitrogen gas-modelled here with a novel interatomic potential-is shown to enhance heat production. These calculations also characterise compression wave dynamics in compression-loaded Ni/Al nanolaminates in greater detail than prior studies and illustrate significant qualitative and quantitative differences between extant embedded atom model (EAM) parameterisations of Ni/Al. Speeds of sound for each metal and EAM are also reported. These differences have implications for the interpretation and comparability of EAM-based modelling of Ni/Al reactions moving forward, and may also have wider implications for EAM modelling of intermetallic systems in general.