Microprojectile Impact: From Metal Processing to Extreme Mechanics
The unique combination of high pressures, temperatures, and deformation rates generated during microprojectile impact can trigger a rich spectrum of material responses, ranging from solid-state metallic bonding to extreme deformation and failure. In this talk, I will highlight two major research thrusts in our group aimed at uncovering the fundamental mechanisms governing these phenomena. In the first part, I will focus on impact-induced solid-state bonding, in which metallic microparticles adhere to metallic substrates upon impact at sufficiently high velocities. This phenomenon underpins some emerging solid-state metal manufacturing and deposition technologies. Through in situ impact experiments, site-specific micromechanical measurements, and electron microscopy, we establish links among impact conditions, interfacial microstructure, and micromechanical response. Our studies reveal a gradient in bond strength along the interface, a non-monotonic dependence of bond strength on impact velocity, a significant role of the native oxide layer in governing interfacial strength, and strong mechanochemical coupling during impact bonding. We discuss the mechanistic origins of these phenomena and their implications for designing reliable, high-performance interfaces in solid-state metal processing. In the second part, we explore deformation mechanisms in metals and alloys subjected to ultrahigh strain rates. Under these conditions, thermally activated dislocation glide transitions to a regime dominated by ballistic dislocation transport, in which rapidly moving dislocations interact strongly with lattice vibrations, or phonons. Understanding this dislocation-phonon drag regime is essential for predicting and controlling material behavior under extreme loading conditions. We introduce a new impression-based experimental framework that combines laser-induced microprojectile impact with spherical nanoindentation. By comparing deformation produced under similar geometries but vastly different loading rates, this approach isolates the contribution of phonon drag to plastic flow. Using pure Cu and Cu-based alloys as model systems, we demonstrate how microstructural engineering can be used to control dislocation-phonon interactions and tailor mechanical response at extreme deformation rates.
Bio: Mostafa Hassani is an assistant professor jointly appointed in the Sibley School of Mechanical and Aerospace Engineering and the Department of Materials Science and Engineering at Cornell University. He began his tenure-track appointment at Cornell in July 2021. Prior to that, he held positions as a senior research associate and lecturer at Cornell and as a postdoctoral associate at MIT and earned his Ph.D. from Politecnico di Milano. Mostafa’s research focuses on the processing and mechanical behavior of structural materials. He is a recipient of the NSF CAREER Award and the TMS Early Career Leaders Professional Development Award.