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

Reactions at interfaces between solids are critical processing steps for applications including microelectronics, coatings on turbine blades, and reactive materials. It is established experimentally that the first phase to form through an interfacial reaction need not be the most stable phase. Understanding the thermodynamics and kinetics of the solid-state reaction at interface is crucial for device engineering.

We study the formation of the intermetallic NiAl in the Ni/Al multilayer system, focusing on how composition gradients at the interfaces impact intermetallic formation. High composition gradients (10⁸-10⁹ m^-1) were generated to mimics Ni/Al interface. Simulation temperatures below 800K were chosen so that both Ni and Al remain crystalline. We observed melting/amorphization at the interface region due to intermixing and stress. The intermetallic phase then forms at the interface from the melted/amorphous region through heterogeneous nucleation. Kinetics of the transformation follows the Johnson-Mehl-Avrami model. The NiAl formation is growth-controlled and the growth rate is found to increase with decreasing composition gradient. Our finding supports a growth-competition mechanism of phase selection for interfacial reactions.