Structural Transformations and Localized Amorphization in Al/Ni Nano-lamellar Composite Under Microballistic Indentation

Al/Ni nano-lamellar composites (NLCs) are nanomaterials with unique properties, including high-energy density, tunable reaction rates, large interfacial density, and substantial mechanical contrast. Exhibiting self-propagating exothermic reactions upon initiation, these NLCs are known for their applications in joining, welding, energetic systems, and microelectromechanical devices. This study investigates the high-strain rate (HSR) dynamic behavior and impact-induced amorphization of Al/Ni NLCs using the Laser-Induced Projectile Impact Test (LIPIT) method as microballistic indentation. In the LIPIT, an 8 ns infrared laser pulse ablated a gold layer on a launch pad, propelling an alumina microsphere (20 µm diameter) toward a Ni/Al NLC. The collision kinetics are recorded from ultrafast stroboscopic imaging, capturing the entire trajectory in one frame. This technique enabled the calculation of impact dynamics, including impact and rebound velocities (Vi and Vr) and the coefficient of restitution (CoR), defined as the ratio of the residual momentum of the μP, based on inter-μP distances and time intervals. Al/Ni NLCs undergo microballistic impacts at Vi ranging from 200 to 800 m/s, inducing HSR from 10⁵ to 10⁸ s^-1. Under HSR microscopic deformation, adiabatic heating models, incorporating deformation-induced and exothermic Al/Ni reaction heat, predicted temperature spikes in impacted regions. These extreme physical conditions facilitate a complex, multi-stage structural transformation in the Al/Ni nanolamellar composite system. This transformation encompasses several processes, such as the amorphization of fcc-Al, exothermic dissolution of Ni into the amorphous Al, temperature elevation to Al's melting point, amorphization of fcc-Ni, formation of intermetallic compounds, localized metallic glass formation in regions of severe deformation. The structural transformation is driven by multiple synergistic mechanisms involving strain-induced crystalline phase instabilities, thermal effects resulting from adiabatic dynamic nonlinearity, structural disordering by the self-propagating exothermic reaction, and high mixing enthalpy of the Al-Ni system. In the post-impact analysis, these alterations are examined using scanning electron microscopy. Impact crater profiles are measured using laser profilometry, and the relationship between hardness-Vi for Al/Ni NLCs is determined to characterize the plastic deformation response during the ballistic loading. Nanostructural changes and phase transitions under the impact craters resulting from the collision are analyzed through cross-sectional electron images after focused ion beam milling. These comprehensive analyses provided detailed insights into its mechanical response under extreme strain and impact-induced localized metallic glass formation. The research elucidates the intricate interplay between HSR adiabatic deformation, high mixing enthalpy, and amorphization in driving structural transitions in Al/Ni NLCs, with significant implications for a new path in metallic glass formation.