Intermetallic formation at deeply supercooled Ni/Al multilayer interfaces: A molecular dynamics study

NiAl intermetallic formation occurs along the interfaces in the Ni/Al multilayer system during molecular dynamics simulations of deep (>50%) supercooling. The simulations begin with a crystalline solid solution at the Ni/Al interfaces that melts at 800 K, a supercooling of 56% of NiAl's simulated melting temperature (1800 K), and undergoes solid-state amorphization at 650 K, a supercooling of 64%. The intermetallic phase, NiAl, then forms at the interface from the melted/amorphous region through heterogeneous nucleation followed by growth in both lateral and normal directions. Upon nucleation, the intermetallic phase retains a fraction of the composition gradient present within the initial solid solution, and that fraction is always larger at 650 K, compared to 800 K, for the same initial composition gradient. Kinetics of the transformation follows the Johnson-Mehl-Avrami model, and an Avrami exponent of 0.5 was extracted at 800 K and 0.1 at 650 K. The NiAl formation is growth-controlled and the growth rate is found to increase with the decreasing initial composition gradient. Our finding supports a growth-competition mechanism of phase selection for interfacial reactions.